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USGS2022

Geologic map of the South Boston 30' × 60' quadrangle, Virginia and North Carolina

This 1:100,000-scale geologic map of the South Boston 30 ’ × 60 ’ quadrangle, Virginia and North Carolina, provides geologic information for the Piedmont along the I–85 and U.S. Route 58 corridors and in the Roanoke River watershed, which includes the John H. Kerr Reservoir and Lake Gaston. The Raleigh terrane (located on the eastern side of the map) contains Neoproterozoic to early Paleozoic(?) polydeformed, amphibolite-facies gneisses and schists. The Carolina slate belt of the Carolina terrane (located in the central part of the map) contains Neoproterozoic metavolcanic and metasedimentary rocks at greenschist facies. Although locally complicated, the slate-belt structure mapped across the South Boston map area is generally a broad, complex anticlinorium of the Hyco Formation (here called the Chase City anticlinorium) and is flanked to the west and east by synclinoria, which are cored by the overlying Aaron and Virgilina Formations. The western flank of the Carolina terrane (located in the western-central part of the map) contains similar rocks at higher metamorphic grade. This terrane includes epidote-amphibolite-facies to amphibolite-facies gneisses of the Neoproterozoic Country Line complex, which extends north-northeastward across the map. The Milton terrane (located on the western side of the map) contains Ordovician amphibolite-facies metavolcanic and metasedimentary gneisses of the Cunningham complex. Crosscutting relations and fabrics in mafic to felsic plutonic rocks constrain the timing of Neoproterozoic to late Paleozoic deformations across the Piedmont. In the eastern part of the map, a 5- to 9-kilometer-wide band of tectonic elements that contains two late Paleozoic mylonite zones (Nutbush Creek and Lake Gordon) and syntectonic granite (Buggs Island pluton) separates the Raleigh and Carolina terranes. Amphibolite-facies, infrastructural metaigneous and metasedimentary rocks east of the Lake Gordon mylonite zone are generally assigned to the Raleigh terrane. In the western part of the map area, a 5- to 8-kilometer-wide band of late Paleozoic tectonic elements includes the Hyco and Clover shear zones, syntectonic granitic sheets, and amphibolite-facies gneisses along the western margin of the Carolina terrane at its boundary with the Milton terrane. This band of tectonic elements is also the locus for early Mesozoic extensional faults associated with the early Mesozoic Scottsburg, Randolph, and Roanoke Creek rift basins. The map shows fluvial terrace deposits of sand and gravel on hills and slopes near the Roanoke and Dan Rivers. The terrace deposits that are highest in altitude are the oldest. Saprolite regolith is spatially associated with geologic source units and is not shown separately on the map. Mineral resources in the area include gneiss and granite quarried for crushed stone, tungsten-bearing vein deposits of the Hamme district, and copper and gold deposits of the Virgilina district. Surface-water resources are abundant and include rivers, tributaries, the John H. Kerr Reservoir, and Lake Gaston. Groundwater flow is concentrated in saprolite regolith, along fractures in the crystalline bedrock, and along fractures and bedding-plane partings in the Mesozoic rift basins.

North Carolina, Virginia↗

USGS2005

Use of a ground-water flow model to delineate contributing areas to the Puchack Well Field, Pennsauken township and vicinity, Camden county, New Jersey

The New Jersey Department of Environmental Protection (NJDEP) Well Head Protection Program, developed in response to the 1986 Federal Safe Drinking Water Act Amendments, requires delineation of Well Head Protection Areas (WHPA's), commonly called contributing areas, for all public and non-community water-supply wells in New Jersey. Typically, WHPA's for public community water-supply wells in New Jersey are delineated using a two-dimensional ground-water flow model incorporating the regional hydraulic gradient; however, NJDEP guidelines allow for the use of a three-dimensional flow model to delineate contributing areas to wells in complex hydrogeologic settings. The Puchack well field in Pennsauken Township, Camden County, N.J., is an area of strong hydraulic connection between the Lower aquifer of the Potomac-Raritan-Magothy aquifer system and the Delaware River. Interactions among and within the public-supply well fields in the area are complex. To delineate the contributing area to the Puchack well field, the U.S. Geological Survey, in cooperation with the NJDEP, developed an 11-layer ground-water flow model of the Potomac-Raritan-Magothy aquifer system in the Pennsauken Township area to simulate flow in the vicinity of the well field. The model incorporates the interaction between the aquifer system and the Delaware River, and includes boundary flows from an existing regional model of the Camden area. Recharge used in the model ranged from 4.5 to 14 inches per year, and horizontal hydraulic conductivity ranged from 50 to 250 feet per day. Values of vertical hydraulic conductivity ranging from 0.001 to 0.5 feet per day were assigned to zones created on the basis of variations in hydrogeologic conditions observed in geophysical logs from wells. A steady-state simulation was used to calibrate the model to synoptic water-level data collected in March 1998. Near the Puchack well field, simulated heads generally were within 1 foot of the measured heads in both the Middle and Lower aquifers. Simulated water-level differences across the confining units at most of the nested wells were within ? 0.5 feet of the differences calculated from measured water levels. The existing flow model was modified to meet NJDEP guidelines for delineating contributing areas in complex hydrogeologic settings. These modifications included rediscretizing the model grid to a finer grid and preparing the water-use data set for use in the rediscretized model. The contributing area to the Puchack well field was delineated by means of particle tracking. An uncertainty analysis was conducted in which 36 model-input parameters were both increased and decreased until the resulting change in simulated heads exceeded the model-calibration criterion of ? 5 feet at any model cell. Porosity most affected the size and shape of the contributing area. The distribution of withdrawals at the Morris/Delair well field and variations in recharge affected both the size and shape of contributing area to the Puchack well field and the source of water to the Puchack wells. The results of the uncertainty analysis were combined to determine the 'aggregate' contributing area to the Puchack well field--a composite of areas on the land surface that contributed flow to the Puchack well field in less than 12 years in any uncertainty simulation. The shape of the aggregate contributing area was most similar to that associated with a reduction in porosity, which indirectly affected the size and shape of the contributing areas by changing travel time.

Scientific Investigations Report↗

USGS2004

Death Valley regional ground-water flow system, Nevada and California -- hydrogeologic framework and transient ground-water flow model

A numerical three-dimensional (3D) transient ground-water flow model of the Death Valley region was developed by the U.S. Geological Survey for the U.S. Department of Energy programs at the Nevada Test Site and at Yucca Mountain, Nevada. Decades of study of aspects of the ground-water flow system and previous less extensive ground-water flow models were incorporated and reevaluated together with new data to provide greater detail for the complex, digital model. A 3D digital hydrogeologic framework model (HFM) was developed from digital elevation models, geologic maps, borehole information, geologic and hydrogeologic cross sections, and other 3D models to represent the geometry of the hydrogeologic units (HGUs). Structural features, such as faults and fractures, that affect ground-water flow also were added. The HFM represents Precambrian and Paleozoic crystalline and sedimentary rocks, Mesozoic sedimentary rocks, Mesozoic to Cenozoic intrusive rocks, Cenozoic volcanic tuffs and lavas, and late Cenozoic sedimentary deposits of the Death Valley Regional Ground-Water Flow System (DVRFS) region in 27 HGUs. Information from a series of investigations was compiled to conceptualize and quantify hydrologic components of the ground-water flow system within the DVRFS model domain and to provide hydraulic-property and head-observation data used in the calibration of the transient-flow model. These studies reevaluated natural ground-water discharge occurring through evapotranspiration and spring flow; the history of ground-water pumping from 1913 through 1998; ground-water recharge simulated as net infiltration; model boundary inflows and outflows based on regional hydraulic gradients and water budgets of surrounding areas; hydraulic conductivity and its relation to depth; and water levels appropriate for regional simulation of prepumped and pumped conditions within the DVRFS model domain. Simulation results appropriate for the regional extent and scale of the model were provided by acquiring additional data, by reevaluating existing data using current technology and concepts, and by refining earlier interpretations to reflect the current understanding of the regional ground-water flow system. Ground-water flow in the Death Valley region is composed of several interconnected, complex ground-water flow systems. Ground-water flow occurs in three subregions in relatively shallow and localized flow paths that are superimposed on deeper, regional flow paths. Regional ground-water flow is predominantly through a thick Paleozoic carbonate rock sequence affected by complex geologic structures from regional faulting and fracturing that can enhance or impede flow. Spring flow and evapotranspiration (ET) are the dominant natural ground-water discharge processes. Ground water also is withdrawn for agricultural, commercial, and domestic uses. Ground-water flow in the DVRFS was simulated using MODFLOW-2000, a 3D finite-difference modular ground-water flow modeling code that incorporates a nonlinear least-squares regression technique to estimate aquifer parameters. The DVRFS model has 16 layers of defined thickness, a finite-difference grid consisting of 194 rows and 160 columns, and uniform cells 1,500 m on each side. Prepumping conditions (before 1913) were used as the initial conditions for the transient-state calibration. The model uses annual stress periods with discrete recharge and discharge components. Recharge occurs mostly from infiltration of precipitation and runoff on high mountain ranges and from a small amount of underflow from adjacent basins. Discharge occurs primarily through ET and spring discharge (both simulated as drains) and water withdrawal by pumping and, to a lesser amount, by underflow to adjacent basins, also simulated by drains. All parameter values estimated by the regression are reasonable and within the range of expected values. The simulated hydraulic heads of the final calibrated transient model gener

Scientific Investigations Report↗

USGS2006

Annual ground-water discharge by evapotranspiration from areas of spring-fed riparian vegetation along the eastern margin of Death Valley, 2000-02

Flow from major springs and seeps along the eastern margin of Death Valley serves as the primary local water supply and sustains much of the unique habitat in Death Valley National Park. Together, these major spring complexes constitute the terminus of the Death Valley Regional Ground-Water Flow System--one of the larger flow systems in the Southwestern United States. The Grapevine Springs complex is the least exploited for water supply and consequently contains the largest area of undisturbed riparian habitat in the park. Because few estimates exist that quantify ground-water discharge from these spring complexes, a study was initiated to better estimate the amount of ground water being discharged annually from these sensitive, spring-fed riparian areas. Results of this study can be used to establish a basis for estimating water rights and as a baseline from which to assess any future changes in ground-water discharge in the park. Evapotranspiration (ET) is estimated volumetrically as the product of ET-unit (general vegetation type) acreage and a representative ET rate. ET-unit acreage is determined from high-resolution multi-spectral imagery; and a representative ET rate is computed from data collected in the Grapevine Springs area using the Bowen-ratio solution to the energy budget, or from rates given in other ET studies in the Death Valley area. The ground-water component of ET is computed by removing the local precipitation component from the ET rate. Two different procedures, a modified soil-adjusted vegetation index using the percent reflectance of the red and near-infrared wavelengths and land-cover classification using multi-spectral imagery were used to delineate the ET units within each major spring-discharge area. On the basis of the more accurate procedure that uses the vegetation index, ET-unit acreage for the Grapevine Springs discharge area totaled about 192 acres--of which 80 acres were moderate-density vegetation and 112 acres were high-density vegetation. ET-unit acreage for two other discharge areas delineated in the Grapevine Springs area (Surprise Springs and Staininger Spring) totaled about 6 and 43 acres, respectively; and for the discharge areas delineated in the Furnace Creek area (Nevares Springs, Cow Creek-Salt Springs, Texas Spring, and Travertine Springs) totaled about 29, 13, 11, and 21 acres, respectively. In discharge areas other than Grapevine Springs, watering and spring diversions have altered the natural distribution of the vegetation. More...

Scientific Investigations Report↗

USGS2007

Hydrogeologic Setting and Ground-Water Flow in the Leetown Area, West Virginia

The Leetown Science Center is a research facility operated by the U.S. Geological Survey that occupies approximately 455-acres near Kearneysville, Jefferson County, West Virginia. Aquatic and fish research conducted at the Center requires adequate supplies of high-quality, cold ground water. Three large springs and three production wells currently (in 2006) supply water to the Center. The recent construction of a second research facility (National Center for Cool and Cold Water Aquaculture) operated by the U.S. Department of Agriculture and co-located on Center property has placed additional demands on available water resources in the area. A three-dimensional steady-state finite-difference ground-water flow model was developed to simulate ground-water flow in the Leetown area and was used to assess the availability of ground water to sustain current and anticipated future demands. The model also was developed to test a conceptual model of ground-water flow in the complex karst aquifer system in the Leetown area. Due to the complexity of the karst aquifer system, a multidisciplinary research study was required to define the hydrogeologic setting. Geologic mapping, surface- and borehole-geophysical surveys, stream base-flow surveys, and aquifer tests were conducted to provide the hydrogeologic data necessary to develop and calibrate the model. It would not have been possible to develop a numerical model of the study area without the intensive data collection and methods developments components of the larger, more comprehensive hydrogeologic investigation. Results of geologic mapping and surface-geophysical surveys verified the presence of several prominent thrust faults and identified additional faults and other complex geologic structures (including overturned anticlines and synclines) in the area. These geologic structures are known to control ground-water flow in the region. Results of this study indicate that cross-strike faults and fracture zones are major avenues of ground-water flow. Prior to this investigation, the conceptual model of ground-water flow for the region focused primarily on bedding planes and strike-parallel faults and joints as controls on ground-water flow but did not recognize the importance of cross-strike faults and fracture zones that allow ground water to flow downgradient across or through less permeable geologic formations. Results of the ground-water flow simulation indicate that current operations at the Center do not substantially affect either streamflow (less than a 5-percent reduction in annual streamflow) or ground-water levels in the Leetown area under normal climatic conditions but potentially could have greater effects on streamflow during long-term drought (reduction in streamflow of approximately 14 percent). On the basis of simulation results, ground-water withdrawals based on the anticipated need for an additional 150 to 200 gal/min (gallons per minute) of water at the Center also would not seriously affect streamflow (less than 8 to 9 percent reduction in streamflow) or ground-water levels in the area during normal climatic conditions. During drought conditions, however, the effects of current ground-water withdrawals and anticipated additional withdrawals of 150 to 200 gal/min to augment existing supplies result in moderate to substantial declines in water levels of 0.5-1.2 feet (ft) in the vicinity of the Center's springs and production wells. Streamflow was predicted to be reduced locally by approximately 21 percent. Such withdrawals during a drought or prolonged period of below normal ground-water levels would result in substantial declines in the flow of the Center's springs and likely would not be sustainable for more than a few months. The drought simulated in this model was roughly equivalent to the more than 1-year drought that affected the region from November 1998 through February 2000. The potential reduction in streamflow is a result of capture of ground water tha

Scientific Investigations Report↗

USGS2008

An evaluation of selected extraordinary floods in the United States reported by the U.S. Geological Survey and implications for future advancement of flood science

Thirty flood peak discharges determine the envelope curve of maximum floods documented in the United States by the U.S. Geological Survey. These floods occurred from 1927 to 1978 and are extraordinary not just in their magnitude, but in their hydraulic and geomorphic characteristics. The reliability of the computed discharge of these extraordinary floods was reviewed and evaluated using current (2007) best practices. Of the 30 flood peak discharges investigated, only 7 were measured at daily streamflow-gaging stations that existed when the flood occurred, and 23 were measured at miscellaneous (ungaged) sites. Methods used to measure these 30 extraordinary flood peak discharges consisted of 21 slope-area measurements, 2 direct current-meter measurements, 1 culvert measurement, 1 rating-curve extension, and 1 interpolation and rating-curve extension. The remaining four peak discharges were measured using combinations of culvert, slope-area, flow-over-road, and contracted-opening measurements. The method of peak discharge determination for one flood is unknown. Changes to peak discharge or rating are recommended for 20 of the 30 flood peak discharges that were evaluated. Nine floods retained published peak discharges, but their ratings were downgraded. For two floods, both peak discharge and rating were corrected and revised. Peak discharges for five floods that are subject to significant uncertainty due to complex field and hydraulic conditions, were re-rated as estimates. This study resulted in 5 of the 30 peak discharges having revised values greater than about 10 percent different from the original published values. Peak discharges were smaller for three floods (North Fork Hubbard Creek, Texas; El Rancho Arroyo, New Mexico; South Fork Wailua River, Hawaii), and two peak discharges were revised upward (Lahontan Reservoir tributary, Nevada; Bronco Creek, Arizona). Two peak discharges were indeterminate because they were concluded to have been debris flows with peak discharges that were estimated by an inappropriate method (slope-area) (Big Creek near Waynesville, North Carolina; Day Creek near Etiwanda, California). Original field notes and records could not be found for three of the floods, however, some data (copies of original materials, records of reviews) were available for two of these floods. A rating was assigned to each of seven peak discharges that had no rating. Errors identified in the reviews include misidentified flow processes, incorrect drainage areas for very small basins, incorrect latitude and longitude, improper field methods, arithmetic mistakes in hand calculations, omission of measured high flows when developing rating curves, and typographical errors. Common problems include use of two-section slope-area measurements, poor site selection, uncertainties in Manning’s n -values, inadequate review, lost data files, and insufficient and inadequately described high-water marks. These floods also highlight the extreme difficulty in making indirect discharge measurements following extraordinary floods. Significantly, none of the indirect measurements are rated better than fair, which indicates the need to improve methodology to estimate peak discharge. Highly unsteady flow and resulting transient hydraulic phenomena, two-dimensional flow patterns, debris flows at streamflow-gaging stations, and the possibility of disconnected flow surfaces are examples of unresolved problems not well handled by current indirect discharge methodology. On the basis of a comprehensive review of 50,000 annual peak discharges and miscellaneous floods in California, problems with individual flood peak discharges would be expected to require a revision of discharge or rating curves at a rate no greater than about 0.10 percent of all floods. Many extraordinary floods create complex flow patterns and processes that cannot be adequately documented with quasi-steady, uniform one-dimensional analyses. These floods are most accurately described by multidimensional flow analysis. Within the U.S. Geological Survey, new approaches are needed to collect more accurate data for floods, particularly extraordinary floods. In recent years, significant progress has been made in instrumentation for making direct discharge measurements. During this same period, very little has been accomplished in advancing methods to improve indirect discharge measurements. Greater use of paleoflood hydrology could fill many shortcomings of U.S. Geological Survey flood science today, such as enhanced knowledge of flood frequency. Additional links among flood runoff, storm structure, and storm motion would provide more insight to flood hazards. Significant improvement in understanding flood processes and characteristics could be gained from linking radar rainfall estimation and hydrologic modeling. Additionally, more could be done to provide real-time flood-hazard warnings with linked rainfall/runoff and flow models. Several important recommendations are made to improve the flood-documentation capability of the U.S. Geological Survey. When very large discharges are measured by current meter or hydroacoustics, water-surface slope should be measured as well. This measurement would allow validation of roughness values that can significantly extend the discharge range of verified Manning’s n for 1-dimensional and 2-dimensional flow analyses. At least two of the floods investigated may have had flow so unstable that large waves affected the interpretation of high-water marks. Instability criteria should be considered for hydraulic analysis of large flows in high-gradient, smooth channels. The U.S. Geological Survey needs to modernize its toolbox of field and office practices for making future indirect discharge measurements. These practices could include, first and foremost, a new peak-flow file database that allows greater description and interpretation of flow events, such as stability criteria in high-gradient, smooth channels, debris flow documentation, and details of flood genesis (hurricane, snowmelt, rain-on-snow, dam failure, and the like). Other modernized practices could include (a) establishment of calibrated stream reaches in chronic flash flood basins to expedite indirect computation of flow; (b) development of process-based theoretical rating curves for streamflow-gaging stations; (c) adoption of step-backwater models as the standard surface-water modeling tool for U.S. Geological Survey field offices; (d) development and support for multidimensional flow models capable of describing flood characteristics in complex terrain and high-gradient channels; (e) greater use of the critical-depth method in appropriate locations; (f) deployment of non-contact instruments to directly measure large floods, rather than attempting to reconstruct them; (g) increased use of paleoflood hydrology; and (h) assurance that future collection of hydro-climatic data meets the needs of more robust watershed models.

Scientific Investigations Report↗

USGS2010

Development of an Environmental Flow Framework for the McKenzie River Basin, Oregon

The McKenzie River is a tributary to the Willamette River in northwestern Oregon. The McKenzie River is approximately 90 miles in length and has a drainage area of approximately 1,300 square miles. Two major flood control dams, a hydropower dam complex, and two hydropower canals significantly alter streamflows in the river. The structures reduce the magnitude and frequency of large and small floods while increasing the annual 7-day minimum streamflows. Stream temperatures also have been altered by the dams and other anthropogenic factors, such as the removal of riparian vegetation and channel simplification. Flow releases from one of the flood control dams are cooler in the summer and warmer in the fall in comparison to unregulated flow conditions before the dam was constructed. In 2006, the Oregon Department of Environmental Quality listed a total of 112.4, 6.3, and 55.7 miles of the McKenzie River basin mainstem and tributary stream reaches as thermally impaired for salmonid rearing, salmonid spawning, and bull trout, respectively. The analyses in this report, along with previous studies, indicate that dams have altered downstream channel morphology and ecologic communities. In addition to reducing the magnitude and frequency of floods, dams have diminished sediment transport by trapping bed material. Other anthropogenic factors, such as bank stabilization, highway construction, and reductions of in-channel wood, also have contributed to the loss of riparian habitat. A comparison of aerial photography taken in 1939 and 2005 showed substantial decreases in secondary channels, gravel bars, and channel sinuosity, particularly along the lower alluvial reaches of the McKenzie River. In addition, bed armoring and incision may contribute to habitat degradation, although further study is needed to determine the extent of these processes. Peak streamflow reduction has led to vegetation colonization and stabilization of formerly active bar surfaces. The large flood control dams on Blue River and South Fork McKenzie River likely have had the greatest effect on downstream habitats because these sediment and flood-rich tributaries historically contributed a disproportionate volume of bed material, wood, and peak flows in comparison with the spring-fed tributaries of the upper McKenzie River basin. The ecological effects of the dams were examined by focusing on nine exemplar aquatic and terrestrial species, including spring Chinook salmon, bull trout, Oregon chub, Pacific and western brook lamprey, red-legged frog, western pond turtle, alder, and cottonwood. The changes caused by the dams to streamflow hydrograph affect all these and other species in complex ways, although a few commonalities are apparent. A loss of channel complexity in the McKenzie River basin, which is associated with the reduction in flood events and widespread channel stabilization, is the primary factor related to the observed population declines for all nine exemplar species. The dams also have caused direct ecological effects by blocking access to habitat, changing the amount and timing of available critical habitat, and changing water temperature during important seasons for different life stages.

Scientific Investigations Report↗

USGS2013

Sediment distribution and hydrologic conditions of the Potomac aquifer in Virginia and parts of Maryland and North Carolina

Sediments of the heavily used Potomac aquifer broadly contrast across major structural features of the Atlantic Coastal Plain Physiographic Province in eastern Virginia and adjacent parts of Maryland and North Carolina. Thicknesses and relative dominance of the highly interbedded fluvial sediments vary regionally. Vertical intervals in boreholes of coarse-grained sediment commonly targeted for completion of water-supply wells are thickest and most widespread across the central and southern parts of the Virginia Coastal Plain. Designated as the Norfolk arch depositional subarea, the entire sediment thickness here functions hydraulically as a single interconnected aquifer. By contrast, coarse-grained sediment intervals are thinner and less widespread across the northern part of the Virginia Coastal Plain and into southern Maryland, designated as the Salisbury embayment depositional subarea. Fine-grained intervals that are generally avoided for completion of water-supply wells are increasingly thick and widespread northward. Fine-grained intervals collectively as thick as several hundred feet comprise two continuous confining units that hydraulically separate three vertically spaced subaquifers. The subaquifers are continuous northward but merge southward into the single undivided Potomac aquifer. Lastly, far southeastern Virginia and northeastern North Carolina are designated as the Albemarle embayment depositional subarea, where both coarse- and fine-grained intervals are of only moderate thickness. The entire sediment thickness functions hydraulically as a single interconnected aquifer. A substantial hydrologic separation from overlying aquifers is imposed by the upper Cenomanian confining unit. Potomac aquifer sediments were deposited by a fluvial depositional complex spanning the Virginia Coastal Plain approximately 100 to 145 million years ago. Westward, persistently uplifted granite and gneiss source rocks sustained a supply of coarse-grained sand and gravel. Immature, high-gradient braided streams deposited longitudinal bars and channel fills across the Norfolk arch subarea. By contrast, across the Salisbury and Albemarle embayment subareas, mature, medium- to low-gradient meandering streams deposited medium- to coarse-grained channel fills and point bars segregated from fine-grained overbank deposits. The Virginia depositional complex merged northward across the Salisbury embayment subarea with another complex in Maryland. Here, additional sediments were received from schist source rocks that underwent three cycles of initial uplift and rapid erosion followed by crustal stability and erosional leveling. Because of the predominance of coarse-grained sediments, transmissivity, hydraulic conductivity, and regional velocities of lateral flow through the Potomac aquifer are greatest across the Norfolk arch depositional subarea, but decrease progressively northward with increasingly fine-grained sediments. Confining units hydraulically separate the Potomac aquifer from overlying aquifers, as indicated by large vertical hydraulic gradients. By contrast, most of the Potomac aquifer internally functions hydraulically as a single interconnected aquifer, as indicated by uniformly small vertical gradients. Most fine-grained sediments within the aquifer do not hydraulically separate overlying and underlying coarse-grained sediments. Across the Salisbury embayment depositional subarea, however, hydraulic separation among the vertically spaced subaquifers is imposed by the intervening confining units. The Potomac aquifer is the largest and most heavily used source of groundwater in the Virginia Coastal Plain. Water-level declines as great as 200 feet create the potential for saltwater intrusion. Conventional stratigraphic correlation has been generally ineffective at accurately characterizing complexly distributed fluvial sediments that compose the Potomac aquifer. Consequently, the aquifer’s internal hydraulic connectivity and overall hydrologic function have not been well understood. Water-supply planning and development efforts have been hampered, and interpretations of regulatory criteria for allowable water-level declines have been ambiguous. An investigation undertaken during 2010–11 by the U.S. Geological Survey, in cooperation with the Virginia Department of Environmental Quality, provides a comprehensive regional description of the spatial distribution of Potomac aquifer sediments and their relation to hydrologic conditions. Altitudes and thicknesses of 2,725 vertical sediment intervals represent the spatial distribution of Potomac aquifer sediments in the Virginia Coastal Plain and adjacent parts of Maryland and North Carolina. Sediment intervals are designated as either dominantly coarse or fine grained and were determined by interpretation of geophysical logs and ancillary information from 456 boreholes. Sediment-interval and borehole summary statistical data indicate regional trends in sediment lithology and stratigraphic continuity, upon which three structurally based and hydrologically distinct sediment depositional subareas are designated. Broad patterns of sediment deposition over time are inferred from published sediment pollen-age data. Discrepancies in previously drawn hydrostratigraphic relations between southeastern Virginia and northeastern North Carolina are partly resolved based on borehole geophysical logs and a recently documented geologic map and corehole. A conceptual model theorizes the depositional history of the sediments and geologically accounts for their distribution. Documented pumping tests of the Potomac aquifer at 197 locations produced 336 values of transmissivity and 127 values of storativity. Based on effective aquifer thicknesses, 296 values of sediment hydraulic conductivity and 113 values of sediment specific storage are calculated. Vertical hydraulic gradients are calculated from 9,479 pairs of water levels measured between November 17, 1953, and October 4, 2011, in 129 closely spaced pairs of wells. Borehole sediment-interval and related data provide a means to achieve high yielding production wells in the Potomac aquifer by site-specific targeting of drilling operations toward water-bearing coarse-grained sand and gravel. Advance knowledge of the potential of different parts of the aquifer also aids in planning optimal groundwater-development areas. Depositional subareas further provide a possible context for resource management. Current (2013) regulatory limits on water-level declines are relative to top surfaces of subdivided upper, middle, and lower Potomac aquifers across the entire Virginia Coastal Plain, but have the potential to exceed the same limit relative to a single undivided Potomac aquifer. By contrast, designation of the sediments as a single aquifer in the Norfolk arch and Albemarle embayment subareas—and as a series of vertically spaced subaquifers and intervening confining units in the Salisbury embayment subarea—best reflects understanding of the Potomac aquifer and can avoid the potential for excessive water-level declines. Simulation modeling to evaluate effects of groundwater withdrawals could be designed similarly, including vertical discretization and (or) zonation of the Potomac aquifer based on depositional subareas and a geostatistical distribution of aquifer properties derived from borehole sediment-interval data. Further resource-management information needs extend beyond the developed part of the Potomac aquifer, particularly across the Northern Neck and Middle Peninsula where only the shallowest part of the aquifer is known, and include structural aspects such as faults, basement bedrock, and the Chesapeake Bay impact crater.

Maryland, North Carolina, Virginia↗

USGS2014

U.S. Geological Survey Karst Interest Group Proceedings, Carlsbad, New Mexico, April 29-May 2, 2014

Karst aquifer systems are present throughout parts of the United States and some of its territories, and have developed in carbonate rocks (primarily limestone and dolomite) that span an interval of time encompassing more than 550 million years. The depositional environments, diagenetic processes, post-depositional tectonic events, and geochemical weathering processes that form karst aquifers are varied and complex, and involve biological, chemical, and physical changes. These factors, combined with the diverse climatic regimes under which karst development in these rocks has taken place, result in the unique dual- or triple-porosity nature of karst aquifers. These complex hydrogeologic systems typically represent challenging and unique conditions to scientists attempting to study groundwater flow and contaminant transport in these terrains. The dissolution of carbonate rocks and the subsequent development of distinct and beautiful landscapes, caverns, and springs has resulted in the most exceptional karst areas of the United States being designated as national or state parks; commercial caverns and known privately owned caves number in the tens of thousands. Both public and private properties provide access for scientists to study the flow of groundwater in situ. Likewise, the range and complexity of landforms and groundwater flow systems associated with karst terrains are enormous, perhaps more than for any other aquifer type. Karst aquifers and landscapes that form in tropical areas, such as the cockpit karst along the north coast of Puerto Rico, differ greatly from karst landforms in more arid climates, such as the Edwards Plateau in west-central Texas or the Guadalupe Mountains near Carlsbad, New Mexico, where hypogenic processes have played a major role in speleogenesis. Many of these public and private lands also contain unique flora and fauna associated with these karst hydrogeologic systems. As a result, numerous federal, state, and local agencies have a strong interest in the study of karst terrains. Many of the major springs and aquifers in the United States have developed in carbonate rocks, such as the Floridan aquifer system in Florida and parts of Alabama, Georgia, and South Carolina; the Ozark Plateaus aquifer system in parts of Arkansas, Kansas, Missouri, and Oklahoma; and the Edwards-Trinity aquifer system in west-central Texas. These aquifers, and the springs that discharge from them, serve as major water-supply sources and as unique ecological habitats. Competition for the water resources of karst aquifers is common, and urban development and the lack of attenuation of contaminants in karst areas can impact the ecosystem and water quality of these aquifers. The concept for developing a platform for interaction among scientists within the U.S. Geological Survey (USGS) working on karst-related studies evolved from the November 1999 National Ground-Water Meeting of the USGS. As a result, the Karst Interest Group (KIG) was formed in 2000. The KIG is a loose-knit, grass-roots organization of USGS and non-USGS scientists and researchers devoted to fostering better communication among scientists working on, or interested in, karst science. The primary mission of the KIG is to encourage and support interdisciplinary collaboration and technology transfer among scientists working in karst areas. Additionally, the KIG encourages collaborative studies between the different mission areas of the USGS as well as other federal and state agencies, and with researchers from academia and institutes. The KIG also encourages younger scientists by participation of students in the poster and oral sessions. To accomplish its mission, the KIG has organized a series of workshops that are held near nationally important karst areas. To date (2014) six KIG workshops, including the workshop documented in this report, have been held. The workshops typically include oral and poster sessions on selected karst-related topics and research, as well as field trips to local karst features. Proceedings of the workshops are published by the USGS and are available online at http://water.usgs.gov/ogw/karst/kig . The first KIG workshop was held in St. Petersburg, Florida, February 13–16, 2001, in the vicinity of the large springs and other karst features of the Floridan aquifer system. The second KIG workshop was held August 20–22, 2002, in Shepherdstown, West Virginia, in proximity to the carbonate aquifers of the northern Shenandoah Valley and highlighted an invited presentation on karst literature by the late Barry F. Beck of P.E. LaMoreaux and Associates. The third KIG workshop was held September 12–15, 2005, in Rapid City, South Dakota, nearby to karst features in evaporites and limestones of the Madison Group in the Black Hills of South Dakota, including Wind Cave National Park and Jewel Cave National Monument. The workshop also included a featured presentation by Thomas Casadevall, Central Region Director, USGS, on the status of earth science at the USGS and evening trips to Jewel Cave led by Mike Wiles, National Park Service (NPS) and Wind Cave led by Rod Horrocks, NPS. The fourth KIG workshop was held May 27–29, 2008, and hosted by the Hoffman Environmental Research Institute and Center for Cave and Karst Studies at Western Kentucky University in Bowling Green, Kentucky, near Mammoth Cave National Park and karst features of the Chester Upland and Pennyroyal Plateau. The workshop featured a late-night field trip into Mammoth Cave with Rickard Toomey and Rick Olsen, NPS. The fifth workshop was held April 26–29, 2011, and was a joint meeting of the USGS KIG and University of Arkansas HydroDays, hosted by the Department of Geosciences at the University of Arkansas in Fayetteville. The workshop featured an outstanding field trip to the unique karst terrain along the Buffalo National River of the southern Ozarks and a keynote presentation on paleokarst in the United States by Art and Peggy Palmer. This sixth and current 2014 KIG workshop is hosted by the National Cave and Karst Research Institute (NCKRI) in Carlsbad, New Mexico, with Director of NCKRI, George Veni, serving as co-chair of the workshop with Eve Kuniansky, USGS. The session planning committee for this sixth workshop includes Van Brahana, USGS retired and University of Arkansas Professor Emeritus; Tom Byl, USGS and Tennessee State University; Zelda Bailey, former Director of NCKRI and retired Director, National Institute of Standards and Technology, Boulder Laboratory, Colorado; Patrick Tucci, USGS retired; and Mike Bradley, Allan Clark, Geoff Delin, Daniel Doctor, James Kaufmann, Eve Kuniansky, Randy Orndorff, Larry Spangler, and Dave Weary of the USGS. The karst hydrology field trip on Thursday will be led by Lewis Land (NCKRI karst hydrologist) and the optional Friday field trip on the geology of Carlsbad Caverns National Park will be led by George Veni. The keynote speaker is Dr. Penelope Boston, Director of Cave and Karst Studies at New Mexico Tech, Socorro, and Academic Director at NCKRI, who will address the future of karst research. Additionally, there is a featured presentation “Irish karst and its management,” by Caoimhe Hickey, The Geological Survey of Ireland, preceding a panel discussion on “Collaboration During Times of Limited Resources.” The extended abstracts of USGS authors were peer reviewed and approved for publication by the U.S. Geological Survey. Articles submitted by university researchers and other federal and state agencies did not go through the formal USGS peer review and approval process, and therefore may not adhere to our editorial standards or stratigraphic nomenclature and is not research conducted or data collected by the USGS. However, all articles had at a minimum of two peer reviews, and all articles were edited for consistency of appearance in the published Proceedings. The use of trade, firm or product names in any article is for descriptive purposes only and does not imply endorsement by the U.S. Government. The USGS, Office of Groundwater, provides technical support for the Karst Interest Group website and public availability of the Proceedings from these workshops, and the USGS Groundwater Resources Program funds the publication costs. Finally, the cover illustration is the work of Ann Tihansky, USGS, used since the first KIG workshop in 2000.

Scientific Investigations Report↗

USGS2014

Hydrogeologic framework and groundwater/surface-water interactions of the upper Yakima River Basin, Kittitas County, central Washington

The hydrogeology, hydrology, and geochemistry of groundwater and surface water in the upper (western) 860 square miles of the Yakima River Basin in Kittitas County, Washington, were studied to evaluate the groundwater-flow system, occurrence and availability of groundwater, and the extent of groundwater/surface-water interactions. The study area ranged in altitude from 7,960 feet in its headwaters in the Cascade Range to 1,730 feet at the confluence of the Yakima River with Swauk Creek. A west-to-east precipitation gradient exists in the basin with the western, high-altitude headwaters of the basin receiving more than 100 inches of precipitation per year and the eastern, low-altitude part of the basin receiving about 20 inches of precipitation per year. From the early 20th century onward, reservoirs in the upper part of the basin (for example, Keechelus, Kachess, and Cle Elum Lakes) have been managed to store snowmelt for irrigation in the greater Yakima River Basin. Canals transport water from these reservoirs for irrigation in the study area; additional water use is met through groundwater withdrawals from wells and surface-water withdrawals from streams and rivers. Estimated groundwater use for domestic, commercial, and irrigation purposes is reported for the study area. A complex assemblage of sedimentary, metamorphic, and igneous bedrock underlies the study area. In a structural basin in the southeastern part of the study area, the bedrock is overlain by unconsolidated sediments of glacial and alluvial origin. Rocks and sediments were grouped into six hydrogeologic units based on their lithologic and hydraulic characteristics. A map of their extent was developed from previous geologic mapping and lithostratigraphic information from drillers’ logs. Water flows through interstitial space in unconsolidated sediments, but largely flows through fractures and other sources of secondary porosity in bedrock. Generalized groundwater-flow directions within the unconfined part of the aquifers in unconsolidated sediments indicate generalized groundwater movement toward the Yakima River and its tributaries and the outlet of the study area. Groundwater movement through fractures within the bedrock aquifers is complex and varies over spatial scales depending on the architecture of the fracture-flow system and its hydraulic properties. The complexity of the fracturedbedrock groundwater-flow system is supported by a wide range of groundwater ages determined from geochemical analyses of carbon-14, sulfur hexafluoride, and tritium in groundwater. These geochemical data also indicate that the shallow groundwater system is actively flushing with young, isotopically heavy groundwater, but isotopicallylight, Pleistocene-age groundwater with a geochemicallyevolved composition occurs at depth within the fracturedbedrock aquifers of upper Kittitas County. An eastward depletion of stable isotopes in groundwater is consistent with hydrologically separate subbasins. This suggests that groundwater that recharges in one subbasin is not generally available for withdrawal or discharge into surface-water features within other subbasins. Water budget components were calculated for 11 subbasins using a watershed model and varied based on the climate, land uses, and geology of the subbasin. Synoptic streamflow measurements made in August 2011 indicate that groundwater discharges into several tributaries of the Yakima River with several losses of streamflow measured where the streams exit bedrock uplands and flow over unconsolidated sediments. Profiles of stream temperature during late summer suggest cool groundwater inflow over discrete sections of streams. This groundwater/surfacewater connection is further supported by the stable-isotope composition of stream water, which reflects the local stableisotope composition of groundwater measured at some wells and springs. Collectively, these hydrogeologic, hydrologic, and geochemical data support a framework for evaluating the potential effects of future groundwater appropriations on senior surface-water and groundwater rights and streamflows. Although total pumping rates in upper Kittitas County of about 3.5 cubic feet per second are small relative to other components of the water budget, the magnitude, timing, and location of withdrawals may have important effects on the hydrologic system. The heterogeneous and variably fractured bedrock in the study area precluded a detailed evaluation of localized effects of pumping, but several generalizations about the groundwater and surface-water systems can be made. These generalizations include evidence for the continuity between the groundwater and surface-water system apparent from synoptic streamflow measurements, stream-temperature profiles, and stable-isotope data of groundwater and surface waters.

Washington↗

USGS2015

Assessing geomorphic change along the Trinity River downstream from Lewiston Dam, California, 1980-2011

The Trinity River Restoration Program, one of the nation’s largest adaptively managed river restoration programs, requires periodic assessment to determine the effectiveness of management actions in restoring channel dynamics and habitat features. This study documents riparian and channel changes along an intensively managed 65-kilometer reach of the Trinity River in California, downstream from Lewiston Dam. The two primary periods of interest, from 1980 to 2001 and from 2001 to 2011, are separated by a shift in restoration activities mandated by the U.S. Department of the Interior December 2000 Record of Decision. The post-2001 restoration strategy increased managed-flow releases, gravel augmentation, watershed restoration, and mechanical channel rehabilitation. We assessed the nature and extent of geomorphic change and a series of ecological performance measures (channel complexity, shoreline length, and channel–floodplain connectivity) by using a series of maps digitized from available rectified orthophotography acquired during low-flow conditions in 1980, 1997, 2001, 2006, 2009, and 2011. Lateral changes in riparian and channel features were used to quantify alluvial processes, and a review of existing streamflow, sediment, and restoration records was used to assess causal mechanisms. During the study period, natural bank erosion and mechanical rehabilitation of channel margins converted riparian features to channel features and expanded the active-channel area. The primary period of bank erosion and expansion of the active channel was from 1980 to 1997. Subsequent bar accretion from 1997 to 2001, followed by slightly greater bar scour from 2001 to 2006, took place primarily in the central and lower reaches of the study area, downstream of Indian Creek. In comparison, post‑2006 bank and bar changes were spatially limited to reaches that had sufficient local transport capacity or sediment supply supported by gravel augmentation, mechanical channel rehabilitation, and tributary contributions. The highest rates of change in the areal extents of channel and riparian features were observed during the pre‑2001 period, which was longer and relatively wetter than the post-2001 period. A series of tributary floods in 1997, 1998, and 2006 increased channel complexity and floodplain connectivity. During the post-2006 period, managed-flow releases, in the absence of tributary flooding, combined with gravel augmentation and mechanical restoration, caused localized increases in sediment supply and transport capacity that led to smaller, but measurable, increases in channel complexity and floodplain connectivity in the upper river near Lewiston Dam. Extensive pre-2001 channel widening and the muted geomorphic response of channel rehabilitation sites to post-2001 managed flows highlight the need for continued monitoring and assessment of the magnitude, duration, and timing of prescriptive flows and associated geomorphic responses.

California↗

USGS2015

Hydrology of and Current Monitoring Issues for the Chicago Area Waterway System, Northeastern Illinois

The Chicago Area Waterway System (CAWS) consists of a combination of natural and manmade channels that form an interconnected navigable waterway of approximately 90-plus miles in the metropolitan Chicago area of northeastern Illinois. The CAWS serves the area as the primary drainage feature, a waterway transportation corridor, and recreational waterbody. The CAWS was constructed by the Metropolitan Water Reclamation District of Greater Chicago (MWRDGC). Completion of the Chicago Sanitary and Ship Canal (initial portion of the CAWS) in 1900 breached a low drainage divide and resulted in a diversion of water from the Lake Michigan Basin. A U.S. Supreme Court decree (Consent Decree 388 U.S. 426 [1967] Modified 449 U.S. 48 [1980]) limits the annual diversion from Lake Michigan. While the State of Illinois is responsible for the diversion, the MWRDGC regulates and maintains water level and water quality within the CAWS by using several waterway control structures. The operation and control of water levels in the CAWS results in a very complex hydraulic setting characterized by highly unsteady flows. The complexity leads to unique gaging requirements and monitoring issues. This report provides a general discussion of the complex hydraulic setting within the CAWS and quantifies this information with examples of data collected at a range of flow conditions from U.S. Geological Survey streamflow gaging stations and other locations within the CAWS. Monitoring to address longstanding issues of waterway operation, as well as current (2014) emerging issues such as wastewater disinfection and the threat from aquatic invasive species, is included in the discussion.

Illinois↗

USGS2017

Field-trip guide to Columbia River flood basalts, associated rhyolites, and diverse post-plume volcanism in eastern Oregon

The Miocene Columbia River Basalt Group (CRBG) is the youngest and best preserved continental flood basalt province on Earth, linked in space and time with a compositionally diverse succession of volcanic rocks that partially record the apparent emergence and passage of the Yellowstone plume head through eastern Oregon during the late Cenozoic. This compositionally diverse suite of volcanic rocks are considered part of the La Grande - Owyhee eruptive axis (LOEA), an approximately 300-kilometer-long (185 mile), north - northwest - trending, middle Miocene to Pliocene volcanic belt located along the eastern margin of the Columbia River flood basalt province. Volcanic rocks erupted from and preserved within the LOEA form an important regional stratigraphic link between the (1) flood basalt - dominated Columbia Plateau on the north, (2) bimodal basalt - rhyolite vent complexes of the Owyhee Plateau on the south, (3) bimodal basalt-rhyolite and time-transgressive rhyolitic volcanic fields of the Snake River Plain - Yellowstone Plateau, and (4) the High Lava Plains of central Oregon. This field-trip guide describes a 4 - day geologic excursion that will explore the stratigraphic and geochemical relationships among mafic rocks of the Columbia River Basalt Group and coeval and compositionally diverse volcanic rocks associated with the early “Yellowstone track” and High Lava Plains in eastern Oregon. Beginning in Portland, the Day 1 log traverses the Columbia River gorge eastward to Baker City, focusing on prominent outcrops that reveal a distal succession of laterally extensive, large-volume tholeiitic flood lavas of the Grande Ronde, Wanapum, and Saddle Mountains Basalt formations of the CRBG. These “great flows” are typical of the well-studied flood basalt-dominated Columbia Plateau, where interbedded silicic and calc-alkaline lavas are conspicuously absent. The latter part of Day 1 will highlight exposures of middle to late Miocene silicic ash-flow tuffs, rhyolite domes, and calc-alkaline lava flows overlying the CRBG across the northern and central parts of the LOEA. The Day 2 field route migrates to southern parts of the LOEA, where rocks of the CRBG are associated in space and time with lesser known and more complex silicic volcanic stratigraphy associated with middle Miocene, large-volume, bimodal basalt-rhyolite vent complexes. Key stops will provide a broad overview of the structure and stratigraphy of the middle Miocene Mahogany Mountain caldera and middle to late Miocene calc-alkaline lavas of the Owyhee basalt. Stops on Day 3 will progress westward from the eastern margin of the LOEA, examining a transition linking the Columbia River Basalt-Yellowstone province with a northwestward-younging magmatic trend of silicic volcanism that underlies the High Lava Plains of eastern Oregon. Initial field stops on Day 3 will examine key outcrops demonstrating the intercalated nature of middle Miocene tholeiitic CRBG flood basalts, prominent ash-flow tuffs, and “Snake River-type” large-volume rhyolite lava flows exposed along the Malheur River. Subsequent stops on Day 3 will focus upon the volcanic stratigraphy northeast of the town of Burns, which includes regional middle to late Miocene ash-flow tuffs, and lava flows assigned to the Strawberry Volcanics. The return route to Portland on Day 4 traverses across the western axis of the Blue Mountains, highlighting exposures of the widespread, middle Miocene Dinner Creek Tuff and aspects of Picture Gorge Basalt flows and northwest-trending feeder dikes situated in the central part of the CRBG province.

Oregon↗

USGS2017

U.S. Geological Survey Karst Interest Group Proceedings, San Antonio, Texas, May 16–18, 2017

Introduction and Acknowledgments Karst aquifer systems are present throughout parts of the United States and some of its territories, and have developed in carbonate rocks (primarily limestone and dolomite) and evaporites (gypsum, anhydrite, and halite) that span an interval of time encompassing more than 550 million years. The depositional environments, diagenetic processes, post-depositional tectonic events, and geochemical weathering processes that form karst aquifers are varied and complex. These factors involve biological, chemical, and physical changes that when combined with the diverse climatic regimes in which karst development has taken place, result in the unique dual- or triple-porosity nature of karst aquifers. These complex hydrogeologic systems typically represent challenging and unique conditions to scientists attempting to study groundwater flow and contaminant transport in these terrains. The dissolution of carbonate rocks and the subsequent development of distinct and beautiful landscapes, caverns, and springs have resulted in the most exceptional karst areas being designated as national or state parks. Tens of thousands of similar areas in the United States have been developed into commercial caverns and known privately owned caves. Both public and private properties provide access for scientists to study the flow of groundwater in situ . Likewise, the range and complexity of landforms and groundwater flow systems associated with karst terrains are enormous, perhaps more than for any other aquifer type. Karst aquifers and landscapes that form in tropical areas, such as the cockpit karst along the north coast of Puerto Rico, differ greatly from karst landforms in more arid climates, such as the Edwards Plateau in west-central Texas or the Guadalupe Mountains near Carlsbad, New Mexico, where hypogenic processes have played a major role in speleogenesis. Many of these public and private lands also contain unique flora and fauna associated with these karst hydrogeologic systems. As a result, numerous federal, state, and local agencies have a strong interest in the study of karst terrains. Many of the major springs and aquifers in the United States have developed in carbonate rocks, such as the Floridan aquifer system in Florida and parts of Alabama, Georgia, and South Carolina; the Ozark Plateaus aquifer system in parts of Arkansas, Kansas, Missouri, and Oklahoma; and the Edwards-Trinity aquifer system in west-central Texas. These aquifers, and the springs that discharge from them, serve as major water-supply sources and form unique ecological habitats. Competition for the water resources of karst aquifers is common, and urban development and the lack of attenuation of contaminants in karst areas due to dissolution features that form direct pathways into karst aquifers can impact the ecosystem and water quality associated with these aquifers. The concept for developing a platform for interaction among scientists within the U.S. Geological Survey (USGS) working on karst-related studies evolved from the November 1999 National Groundwater Meeting of the USGS. As a result, the Karst Interest Group (KIG) was formed in 2000. The KIG is a loose-knit, grass-roots organization of USGS and non-USGS scientists and researchers devoted to fostering better communication among scientists working on, or interested in, karst science. The primary mission of the KIG is to encourage and support interdisciplinary collaboration and technology transfer among scientists working in karst areas. Additionally, the KIG encourages collaborative studies between the different mission areas of the USGS as well as with other federal and state agencies, and with researchers from academia and institutes. To accomplish its mission, the KIG has organized a series of workshops that have been held near nationally important karst areas. To date (2017) seven KIG workshops, including the workshop documented in this report, have been held. The workshops typically include oral and poster sessions on selected karst-related topics and research, as well as field trips to local karst areas. To increase non-USGS participation an effort was made for the workshops to be held at a university or institute beginning with the fourth workshop. Proceedings of the workshops are published by the USGS and are available online at the USGS publications warehouse https://pubs.er.usgs.gov/ by using the search term “karst interest group.” The first KIG workshop was held in St. Petersburg, Florida, in 2001, in the vicinity of the large springs and other karst features of the Floridan aquifer system. The second KIG workshop was held in 2002, in Shepherdstown, West Virginia, in proximity to the carbonate aquifers of the northern Shenandoah Valley, and highlighted an invited presentation on karst literature by the late Barry F. Beck of P.E. LaMoreaux and Associates. The third KIG workshop was held in 2005, in Rapid City, South Dakota, near evaporite karst features in limestones of the Madison Group in the Black Hills of South Dakota. The Rapid City KIG workshop included field trips to Wind Cave National Park and Jewel Cave National Monument, and featured a presentation by Thomas Casadevall, then USGS Central Region Director, on the status of Earth science at the USGS. The fourth KIG workshop in 2008 was hosted by the Hoffman Environmental Research Institute and Center for Cave and Karst Studies at Western Kentucky University in Bowling Green, Kentucky, near Mammoth Cave National Park and karst features of the Chester Upland and Pennyroyal Plateau. The workshop featured a late-night field trip into Mammoth Cave led by Rickard Toomey and Rick Olsen, National Park Service. The fifth KIG workshop in 2011 was a joint meeting of the USGS KIG and University of Arkansas HydroDays, hosted by the Department of Geosciences at the University of Arkansas in Fayetteville. The workshop featured an outstanding field trip to the unique karst terrain along the Buffalo National River in the southern Ozarks, and a keynote presentation on paleokarst in the United States was delivered by Art and Peggy Palmer. The sixth KIG workshop was hosted by the National Cave and Karst Research Institute (NCKRI) in 2014, in Carlsbad, New Mexico. George Veni, Director of the NCKRI, served as a co-chair of the workshop with Eve Kuniansky of the USGS. The workshop featured speaker Dr. Penelope Boston, Director of Cave and Karst Studies at New Mexico Tech, Socorro, and Academic Director at the NCKRI, who addressed the future of karst research. The field trip on evaporite karst of the lower Pecos Valley was led by Lewis Land (NCKRI karst hydrologist), and the field trip on the geology of Carlsbad Caverns National Park was led by George Veni. This current seventh KIG workshop is being held in San Antonio at the University of Texas at San Antonio (UTSA). This 2017 workshop is being hosted by the Department of Geological Sciences’ Student Geological Society (SGS), and student chapters of the American Association of Petroleum Geologists (AAPG) and Association of Engineering Geologists (AEG), with support by the UTSA Department of Geological Sciences and Center for Water Research. The UTSA student chapter presidents, Jose Silvestre (SGS), John Cooper (AAPG), and Tyler Mead (AEG) serve as co-chairs of the 2017 workshop with Eve Kuniansky of the USGS. The technical session committee is chaired by Eve Kuniansky, USGS, and includes Michael Bradley, Tom Byl, Rebecca Lambert, John Lane, and James Kaufmann, all USGS, and Patrick Tucci, retired USGS. The logistics committee includes Amy Clark, Yongli Gao, and Lance Lambert (Department Chair), UTSA Department of Geological Sciences; and Ryan Banta and Allan Clark, USGS, San Antonio, Texas. The field trip committee is chaired by Allan Clark and includes Amy Clark, Yongli Gao, and Keith Muehlestein, UTSA; Marcus Gary, Edwards Aquifer Authority and University of Texas at Austin; Ron Green, Southwest Research Institute; Geary Schindel, Edwards Aquifer Authority; and George Veni, NCKRI. Additionally, two organizations have assisted the UTSA student chapters in hosting the meeting by donating funds to the chapters: the Edwards Aquifer Authority, San Antonio, Texas, and the Barton Springs Edwards Aquifer Authority, Austin, Texas. Additionally, Yongli Gao, Center for Water Research and Department of Geological Sciences, UTSA, helped develop sessions on cave and karst research in China for this workshop. These proceedings could not have been accomplished without the assistance of Lawrence E. Spangler as co-editor who not only has subject matter expertise, but also serves as an editor with the USGS Science Publishing Network. We sincerely hope that this workshop continues to promote future collaboration among scientists of varied and diverse backgrounds, and improves our understanding of karst aquifer systems in the United States and its territories. The extended abstracts of USGS authors were peer reviewed and approved for publication by the USGS. Articles submitted by university researchers and other federal and state agencies did not go through the formal USGS peer review and approval process, and therefore may not adhere to USGS editorial standards or stratigraphic nomenclature. However, all articles had a minimum of two peer reviews and were edited for consistency of appearance in the proceedings. The use of trade, firm or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. The USGS Water Availability and Use Science Program funded the publication costs of the proceedings.

Scientific Investigations Report↗

USGS2017

A review of surface energy balance models for estimating actual evapotranspiration with remote sensing at high spatiotemporal resolution over large extents

Many approaches have been developed for measuring or estimating actual evapotranspiration ( ET a ), and research over many years has led to the development of remote sensing methods that are reliably reproducible and effective in estimating ET a . Several remote sensing methods can be used to estimate ET a at the high spatial resolution of agricultural fields and the large extent of river basins. More complex remote sensing methods apply an analytical approach to ET a estimation using physically based models of varied complexity that require a combination of ground-based and remote sensing data, and are grounded in the theory behind the surface energy balance model. This report, funded through cooperation with the International Joint Commission, provides an overview of selected remote sensing methods used for estimating water consumed through ET a and focuses on Mapping Evapotranspiration at High Resolution with Internalized Calibration (METRIC) and Operational Simplified Surface Energy Balance (SSEBop), two energy balance models for estimating ET a that are currently applied successfully in the United States. The METRIC model can produce maps of ET a at high spatial resolution (30 meters using Landsat data) for specific areas smaller than several hundred square kilometers in extent, an improvement in practice over methods used more generally at larger scales. Many studies validating METRIC estimates of ET a against measurements from lysimeters have shown model accuracies on daily to seasonal time scales ranging from 85 to 95 percent. The METRIC model is accurate, but the greater complexity of METRIC results in greater data requirements, and the internalized calibration of METRIC leads to greater skill required for implementation. In contrast, SSEBop is a simpler model, having reduced data requirements and greater ease of implementation without a substantial loss of accuracy in estimating ET a . The SSEBop model has been used to produce maps of ET a over very large extents (the conterminous United States) using lower spatial resolution (1 kilometer) Moderate Resolution Imaging Spectroradiometer (MODIS) data. Model accuracies ranging from 80 to 95 percent on daily to annual time scales have been shown in numerous studies that validated ET a estimates from SSEBop against eddy covariance measurements. The METRIC and SSEBop models can incorporate low and high spatial resolution data from MODIS and Landsat, but the high spatiotemporal resolution of ET a estimates using Landsat data over large extents takes immense computing power. Cloud computing is providing an opportunity for processing an increasing amount of geospatial “big data” in a decreasing period of time. For example, Google Earth Engine TM has been used to implement METRIC with automated calibration for regional-scale estimates of ET a using Landsat data. The U.S. Geological Survey also is using Google Earth Engine TM to implement SSEBop for estimating ET a in the United States at a continental scale using Landsat data.

Scientific Investigations Report↗

USGS2017

Correlation between basalt flows and radiochemical and chemical constituents in selected wells in the southwestern part of the Idaho National Laboratory, Idaho

Wastewater discharged to wells and ponds and wastes buried in shallow pits and trenches at facilities at the Idaho National Laboratory (INL) have contributed contaminants to the eastern Snake River Plain (ESRP) aquifer in the southwestern part of the INL. This report describes the correlation between subsurface stratigraphy in the southwestern part of the INL with information on the presence or absence of wastewater constituents to better understand how flow pathways in the aquifer control the movement of wastewater discharged at INL facilities. Paleomagnetic inclination was used to identify subsurface basalt flows based on similar inclination measurements, polarity, and stratigraphic position. Tritium concentrations, along with other chemical information for wells where tritium concentrations were lacking, were used as an indicator of which wells were influenced by wastewater disposal. The basalt lava flows in the upper 150 feet of the ESRP aquifer where wastewater was discharged at the Idaho Nuclear Technology and Engineering Center (INTEC) consisted of the Central Facilities Area (CFA) Buried Vent flow and the AEC Butte flow. At the Advanced Test Reactor (ATR) Complex, where wastewater would presumably pond on the surface of the water table, the CFA Buried Vent flow probably occurs as the primary stratigraphic unit present; however, AEC Butte flow also could be present at some of the locations. At the Radioactive Waste Management Complex (RWMC), where contamination from buried wastes would presumably move down through the unsaturated zone and pond on the surface of the water table, the CFA Buried Vent; Late Basal Brunhes; or Early Basal Brunhes basalt flows are the flow unit at or near the water table in different cores. In the wells closer to where wastewater disposal occurred at INTEC and the ATR-Complex, almost all the wells show wastewater influence in the upper part of the ESRP aquifer and wastewater is present in both the CFA Buried Vent flow and AEC Butte flow. The CFA Buried Vent flow and AEC Butte flow are also present in wells at and north of CFA and are all influenced by wastewater contamination. All wells with the AEC Butte flow present have wastewater influence and 83 percent of the wells with the more prevalent CFA Buried Vent flow have wastewater influence. South and southeast of CFA, most wells are not influenced by wastewater disposal and are completed in the Big Lost Flow and the CFA Buried Vent flow. Wells southwest of CFA are influenced by wastewater disposal and are completed in the Big Lost flow and CFA Buried Vent flow at the top of the aquifer. Basalt stratigraphy indicates that the CFA Buried Vent flow is the predominant flow in the upper part of the ESRP aquifer at and near the RWMC as it is present in all the wells in this area. The Late Basal Brunhes flow, Middle Basal Brunhes flow, Early Basal Brunhes flow, South Late Matuyama flow, and Matuyama flow are also present in various wells influenced by waste disposal. Some wells south of RWMC do not show wastewater influence, and the lack of wastewater influence could be due to low hydraulic conductivities. Several wells south and southeast of CFA also do not show wastewater influence. Low hydraulic conductivities or ESRP subsidence are possible causes for lack of wastewater south of CFA. Multilevel monitoring wells completed much deeper in the aquifer show influence of wastewater in numerous basalt flows. Well Middle 2051 (northwest of RWMC) does not show wastewater influence in its upper three basalt flows (CFA Buried Vent, Late Basal Brunhes, and Middle Basal Brunhes); however, wastewater is present in two deeper flows (the Matuyama and Jaramillo flows). Well USGS 131A (southwest of CFA) and USGS132 (south of RWMC) both show wastewater influence in all the basalt flows sampled in the upper 600 feet of the aquifer. Wells USGS 137A, 105, 108, and 103 completed along the southern boundary of the INL all show wastewater influence in several basalt flows including the G flow, Middle and Early Basal Brunhes flows, the South Late Matuyama flow and the Matuyama flow; however, the strongest wastewater influence appears to be in the South Late Matuyama flow. The concentrations of wastewater constituents in deeper parts of these wells support the concept of groundwater flow deepening in the southwestern part of the INL.

Idaho↗

USGS2018

Santa Barbara and Foothill groundwater basins Geohydrology and optimal water resources management—Developed using density dependent solute transport and optimization models

Groundwater has been a part of the city of Santa Barbara’s water-supply portfolio since the 1800s; however, since the 1960s, the majority of the city’s water has come from local surface water, and the remainder has come from groundwater, State Water Project, recycled water, increased water conservation, and as needed, seawater desalination. Although groundwater from the Santa Barbara and Foothill groundwater basins only accounts for a small percentage of the long-term supply, it is an important source of supplemental water during times of surface-water shortages. During the late 1980s and early 1990s, production wells extracted additional groundwater to compensate for drought related water-delivery shortfalls from other sources; in response, water levels declined substantially in the Santa Barbara and Foothill groundwater basins (below sea level in the Santa Barbara groundwater basin). In coastal basins that have groundwater extraction near shore, seawater intrusion is often a problem. Seawater intrusion in the Santa Barbara groundwater basin is thought to be more limited than in other coastal basins because of an offshore fault that acts as a partial barrier to groundwater flow. During the late 1980s and early 1990s, seawater intrusion was observed in the Santa Barbara groundwater basin, as indicated by increased chloride concentrations at several monitoring wells that ranged from 200 ft to 1,300 ft from the ocean and as close as 2,900 ft to the nearest pumping well. This demonstrated that seawater can intrude into the Santa Barbara groundwater basin when groundwater levels fall below sea level near the coast. The city of Santa Barbara is interested in developing a better understanding of the sustainability of its groundwater supplies. In 2014, California adopted historic legislation to manage its groundwater: the Sustainable Groundwater Management Act (SGMA). The SGMA requires the development and implementation of “Groundwater Sustainability Plans” in 127 priority groundwater basins; although Santa Barbara was not a designated priority basin, the city is taking steps to achieve sustainability. Sustainability was defined in the SGMA in terms of avoiding undesirable results: significant and unreasonable groundwater-level declines, reduction in groundwater storage, seawater intrusion, water-quality degradation, land subsidence, and surface-water depletion. In this project, a cooperative study between the U.S. Geological Survey (USGS) and the city of Santa Barbara, sustainable yield is defined as the volume of groundwater that can be pumped from storage without causing water-level drawdowns and the associated increases in seawater intrusion (as indicated by increases in measured chloride concentrations) at selected wells. In order to estimate the sustainability of Santa Barbara’s groundwater basins, a three-dimensional density-dependent groundwater-flow and solute-transport model (the Santa Barbara Flow and Transport Model, or SBFTM) was developed on the basis of an existing groundwater-flow model. To simulate seawater intrusion to the Santa Barbara Basin under various management strategies, the SBFTM uses the USGS code SEAWAT to simulate salinity transport and variable-density flow. The completed SBFTM was coupled with a management optimization tool, in this case a multi-objective evolutionary algorithm, to determine optimal pumping strategies that maximize the sustainable yield and at the same time satisfy user-defined drawdown and chloride-concentration constraints. As part of this study, a three-dimensional hydrogeologic framework model was developed to quantify the extent and hydrogeologic characteristics of the Santa Barbara and Foothill groundwater basins and to help define the discretization and hydraulic properties used in the SBFTM. The development of the hydrogeologic framework model required the collection and reconciliation of geologic and geophysical data from existing maps, reports, and databases, along with geologic and hydrologic data from recently drilled wells. These data were integrated into a three-dimensional hydrogeologic framework model that defines the stratigraphy and geometry of the aquifer zones and the major geologic structures in the basin. The hydrogeologic framework model also quantifies the variation in sediment grain size within each aquifer zone as the percentage of coarse-grained sediment. Previous studies indicated that there are two principal water-producing zones in the Santa Barbara groundwater basin, the upper and lower producing zones; an additional thin, productive zone was identified as part of this study. This “middle producing zone” is not as areally extensive as the upper and lower producing zones and only exists in the coastal part of Storage Unit I. These producing zones are bounded at depth by less productive shallow, middle, and deep zones. Two versions of the SBFTM were constructed: an initial-condition model and a modern transient model. The initial-condition model is a long-term transient model that simulates flow and solute-transport conditions during a period with limited anthropogenic influences preceeding the modern transient model. The simulation-transient model simulates flow and transport conditions from 1929 through 2013; however, because of data availability, the focus of the model calibration was 1972–2013. The SBFTM was calibrated to measured groundwater levels and drawdown, as well as measured chloride concentrations and change in concentrations, using a combination of automated and trial-and-error parameter-estimation techniques. A sensitivity analysis indicated that, in general, the SBFTM was most sensitive to recharge- and pumping-distribution parameters, specifically those controlling the amount of small-catchment recharge and the distribution of water extraction by hydrogeologic layer for production wells. The model was also sensitive to parameters controlling stream-recharge rates, horizontal and vertical hydraulic conductivity, and porosity. From 1929 to 1971, most of the water entering the area represented by the SBFTM was from creek and small-catchment recharge, and the majority of water leaving the SBFTM area was from pumping, discharge to creeks, and drains. In addition, about 37 percent of the total pumpage came from a net reduction in groundwater storage. From 1972 to 2013, the amount of water entering and leaving the SBFTM was fairly similar as that from 1929 to 1971, except the reduction in pumpage added about 17,000 acre-ft of water to storage. During this later period, there were also times of storage loss. For example, during July 1990, a month when approximately 705 acre-ft of groundwater was pumped in the study area, the pumpage was much greater than all sources of recharge combined, and about 382 acre-ft of water was removed from groundwater storage. Simulated hydraulic heads replicated the observed data to an acceptable matching of the measured water-level, flow direction, and vertical gradients. Simulated hydrographs for selected wells were in good agreement with the measured data, with an average residual of -2.7 ft and a standard deviation of 14.5 ft, indicating that the simulated heads, on average, underestimated the observed water levels. An examination of the model fit indicated that most of the discrepancies were lower simulated heads at wells proximal to production well sites. The simulated chloride concentrations reasonably matched the rising limbs of the measured breakthrough curves in terms of timing and magnitude; however, the simulation overestimated the chloride concentrations on the falling limbs. The overestimation of low chloride concentrations was attributed to the model overestimating the advance of the chloride front during periods of heavy pumping and underestimating the retreat of the chloride front during periods of low pumping. These simulation errors would result in a conservative response by local water managers to seawater intrusion. The SBFTM was used to develop a collection of predictive simulations optimized to produce pumping schedules that maximize yield, subject to a set of constraints and competing objectives. The simulations were grouped as scenarios that differed in their time horizon, initial conditions for groundwater levels and chloride concentrations, as well as precipitation, which was incorporated into the model through simulated recharge. Overall, five scenarios were developed in a multi-objective framework to obtain optimal pumping rates for all of the wells managed by the city, while minimizing excessive drawdown and seawater intrusion. For the current study, complexities in the simulation model and the optimization formulation required additional considerations. Incorporating the solute-transport equations to simulate chloride transport added a highly nonlinear process that is solved iteratively in each time step of the groundwater-flow model. These nonlinearities, coupled with the highly refined grid in the current model, creates challenges for many traditional optimization methods. Therefore, an optimization method was needed that could address nonlinear relationships as well as a very large problem size. Lastly, the optimization problem was reformulated to include multiple objectives without requiring convergence to a single solution. This approach, guided by the city’s objectives, allowed the maximum extraction of information from the complex simulation. Borg, a multi-objective evolutionary algorithm, was chosen as the optimization algorithm for this study for several reasons: (1) it is very computationally efficient; (2) it can run in parallel; (3) it requires little user input; and (4) it can solve for multiple competing objectives. The first three points allow the algorithm to proceed toward the optimal solutions at the fastest possible rate. The fourth point is advantageous for large, complex optimization problems because it is difficult to formulate the optimization problem in a way that produces only one optimal solution. The problem formulation consisted of four competing objectives and a constraint set in accordance with the main concerns of the city. The objectives were maximizing total pumpage, minimizing seawater intrusion, minimizing total drawdown in production wells, and minimizing the maximum drawdown. The constraints were pump capacity, meeting drinking-water standards for chloride, maintaining a specified minimum flowrate to a groundwater treatment plant, and maintaining minimum water levels in pumping wells. The decision variables either were quarterly pumpage by well or total pumpage by basin. Five optimization scenarios were developed that allow the decision makers to evaluate a range of optimal solutions for a variety of water levels and chloride concentrations as well as potential future climatic conditions. Three scenarios (1, 2, and 5) were multi-objective optimization formulations that allowed for variations in management preferences and climatic conditions. The other two scenarios (3 and 4) were designed to examine the optimization results to answer specific questions. Scenario 1 described the best-case sustainable yield assuming a “full” basin (that is, high initial water levels) and typical climate conditions for 10 years. Scenario 2 also started with a “full” basin; however, this was followed by a 10-year drought. Scenario 3 determined if an “empty” basin (that is, low initial water levels) would recover to full conditions (1998 conditions) given climate assumptions and optimal pumping schedules from scenarios 1 and 2. Scenario 4 was designed to produce decision rules that can be used by water managers to help choose an optimal pumping schedule based on measured water-level or chloride data. Scenario 5 identified future pumping schedules based on short-term climate variations during a 2-year management horizon. The results from scenarios 1 and 2 described the differences in maximum pumpage in the basin under typical and dry long-term climate projections, respectively. The scenario 1 results indicated the maximum 10-year pumpage of the basin was about 31,300 acre-ft under typical conditions and controlling simulated seawater intrusion and drawdowns. For scenario 2, less recharge over the 10-year dry climate produced a maximum pumpage estimate of 30,000 acre-ft to control seawater intrusion and drawdowns. The larger pumpage for scenario 1 resulted in more seawater intrusion, but less total drawdown, compared to that of scenario 2. Results for scenarios 3 and 4 showed the basin’s response to management actions combined with climate projections. Both scenarios used the optimal pumping schedules and the 10-year climates from scenarios 1 and 2. The scenario 3 results showed that under minimal pumping, the basin did not fully recover to 1998 water levels within 10 years under either climate scenario. The relatively larger recharge from the typical climate resulted in less drawdown at coastal monitoring wells after the 10-year recovery period than that from the dry climate. The location of the seawater intrusion front was not appreciably different between the scenarios, however. Scenario 4 used the optimal results from scenarios 1 and 2 to produce decision-rule curves that illustrated the pumpage for each basin, given measured levels of chloride concentration or drawdown. This allowed the use of additional measurements at monitoring wells to assess future management decisions on the basis of the sensitivity of observations of drawdown and seawater intrusion to various pumping rates. Scenario 5 allowed managers to investigate the effects of short-term climate variations on optimal pumping schedules. Three specific 2-year simulations were optimized: typical-to-dry (scenario 5A), dry-to-typical (scenario 5B), and dry-to-dry (scenario 5C). The most noteable result from scenario 5 was the overall reduction in optimal pumpage for most schedules in scenario 5C, when the climate is simulated as dry-to-dry. There are also many optimal pumping schedules that produced an overall increase in waterlevels over the two-year simulation period, regardless of climatic condition. Similar to scenario 2, the scenario 5C results represents conservative yield estimates under a minimal-precipitation climatic condition.

California↗

USGS2018

Mercury methylation and bioaccumulation in Sinclair Inlet, Kitsap County, Washington

The U.S. Geological Survey evaluated the transformation of mercury to bioavailable methylmercury in Sinclair Inlet, Kitsap County, Washington, and assessed the effect of the transformation processes on the mercury burden in marine organisms and sediment. In August 2008, samples of sediment, water, and biota from six sites in Sinclair Inlet and three bays representative of Puget Sound embayments were collected. The extensive sediment sampling included analysis of methylmercury in sediment and porewater, estimates of methylation production potential, and analyses of ancillary constituents associated with organic carbon and reduction-oxidation (redox) conditions to assist in interpreting the mercury results. Analyses of methylmercury in water overlying incubated cores provided an estimate of the release of methylmercury to the water column. Collection of samples for mercury species in the aqueous, particulate (suspended solids), and biological phases, and for ancillary carbon and nitrogen constituents in surface water, continued, on about a monthly schedule, at four stations through August 2009. In February, June, and August 2009, seasonal sediment samples were collected at 20 stations distributed between greater Sinclair Inlet and Operable Unit B Marine of the Bremerton naval complex, Bremerton, Washington, to examine geographical and seasonal patterns of mercury biogeochemistry of sediment in Sinclair Inlet. At six of these seasonal sediment stations, porewater was collected and triplicate core incubation experiments were done. Median sediment-methylmercury concentrations were not statistically different between the representative bays and Sinclair Inlet. The percentage of sediment methylmercury (relative to total mercury) was actually lower in the Sinclair Inlet sites compared with the representative bays, reflecting the higher sediment total mercury concentration for the Sinclair Inlet stations compared with the representative bays. Likewise, median sediment methylmercury concentrations were not statistically different between the greater Sinclair Inlet stations and the Bremerton naval complex stations; whereas the percentage of sediment methylmercury to total mercury was lower in the Bremerton naval complex due to higher sediment total mercury concentrations than the greater Sinclair Inlet stations. The biogeochemical characteristics of each station, measured by redox, organic carbon, and the seasonal availability of nutrients controlled methylmercury biogeochemistry. Mercury methylation production potential was a function of temperature, concentration of total mercury in sediment, and the percentage of ferrous iron (relative to total measured iron) across all sites. Methylmercury porewater concentrations were best described by using concentrations of dissolved organic carbon and reduction-oxidation conditions. Likewise, the variable fluxes of methylmercury from incubated cores were best described using dissolved organic carbon and reduction-oxidation conditions. Sinclair Inlet exhibited the classic Puget Sound biological cycle, with spring and autumn phytoplankton blooms resulting in depletion of nitrate, orthophosphate, and silicate in the surface water. Although variable in timing between 2008 and 2009, a strong corresponding seasonal trend of increased availability, incorporation, and bioaccumulation of methylmercury into the food web of Sinclair Inlet occurred during the early spring and summer growing season.

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