Geology topics

David L. Nelms

Publications and source records attributed to David L. Nelms.

At least 19 recordsLinked to original sources

USGS2018

Variability of hydrological droughts in the conterminous United States, 1951 through 2014

Spatial and temporal variability in the frequency, duration, and severity of hydrological droughts across the conterminous United States (CONUS) was examined using monthly mean streamflow measured at 872 sites from 1951 through 2014. Hydrological drought is identified as starting when streamflow falls below the 20th percentile streamflow value for 3 consecutive months and ending when streamflow remains above the 20th percentile streamflow value for 3 consecutive months. Mean drought frequency for all aggregated ecoregions in CONUS is 16 droughts per 100 years. Mean drought duration is 5 months, and mean drought severity is 39 percent on a scale ranging from 0 percent to 100 percent (with 100% being the most severe). Hydrological drought frequency is highest in the Western Mountains aggregated ecoregion and lowest in the Eastern Highlands, Northeast, and Southeast Plains aggregated ecoregions. Hydrological drought frequencies of 17 or more droughts per 100 years were found for the Central Plains, Southeast Coastal Plains, Western Mountains, and Western Xeric aggregated ecoregions. Drought duration and severity indicate spatial variability among the sites, but unlike drought frequency, do not show coherent spatial patterns. A comparison of an older period (1951–82) with a recent period (1983–2014) indicates few sites have statistically significant changes in drought frequency, drought duration, or drought severity at a 95-percent confidence level.

Scientific Investigations Report↗

USGS2018

Shallow bedrock limits groundwater seepage-based headwater climate refugia

Groundwater/surface-water exchanges in streams are inexorably linked to adjacent aquifer dynamics. As surface-water temperatures continue to increase with climate warming, refugia created by groundwater connectivity is expected to enable cold water fish species to survive. The shallow alluvial aquifers that source groundwater seepage to headwater streams, however, may also be sensitive to seasonal and long-term air temperature dynamics. Depth to bedrock can directly influence shallow aquifer flow and thermal sensitivity, but is typically ill-defined along the stream corridor in steep mountain catchments. We employ rapid, cost-effective passive seismic measurements to evaluate the variable thickness of the shallow colluvial and alluvial aquifer sediments along a headwater stream supporting cold water-dependent brook trout ( Salvelinus fontinalis ) in Shenandoah National Park, VA, USA. Using a mean depth to bedrock of 2.6 m, numerical models predicted strong sensitivity of shallow aquifer temperature to the downward propagation of surface heat. The annual temperature dynamics (annual signal amplitude attenuation and phase shift) of potential seepage sourced from the shallow modeled aquifer were compared to several years of paired observed stream and air temperature records. Annual stream water temperature patterns were found to lag local air temperature by ∼8–19 d along the stream corridor, indicating that thermal exchange between the stream and shallow groundwater is spatially variable. Locations with greater annual signal phase lag were also associated with locally increased amplitude attenuation, further suggestion of year-round buffering of channel water temperature by groundwater seepage. Numerical models of shallow groundwater temperature that incorporate regional expected climate warming trends indicate that the summer cooling capacity of this groundwater seepage will be reduced over time, and lower-elevation stream sections may no longer serve as larger-scale climate refugia for cold water fish species, even with strong groundwater discharge.

Limnologica - Ecology and Management of Inland Wat↗

USGS2017

Modeling summer month hydrological drought probabilities in the United States using antecedent flow conditions

Climate change raises concern that risks of hydrological drought may be increasing. We estimate hydrological drought probabilities for rivers and streams in the United States (U.S.) using maximum likelihood logistic regression (MLLR). Streamflow data from winter months are used to estimate the chance of hydrological drought during summer months. Daily streamflow data collected from 9,144 stream gages from January 1, 1884 through January 9, 2014 provide hydrological drought streamflow probabilities for July, August, and September as functions of streamflows during October, November, December, January, and February, estimating outcomes 5-11 months ahead of their occurrence. Few drought prediction methods exploit temporal links among streamflows. We find MLLR modeling of drought streamflow probabilities exploits the explanatory power of temporally linked water flows. MLLR models with strong correct classification rates were produced for streams throughout the U.S. One ad hoc test of correct prediction rates of September 2013 hydrological droughts exceeded 90% correct classification. Some of the best-performing models coincide with areas of high concern including the West, the Midwest, Texas, the Southeast, and the Mid-Atlantic. Using hydrological drought MLLR probability estimates in a water management context can inform understanding of drought streamflow conditions, provide warning of future drought conditions, and aid water management decision making.

Journal of the American Water Resources Associatio↗

USGS2016

Comparative mitogenomic analyses of three North American stygobiont amphipods of the genus Stygobromus (Crustacea: Amphipoda)

The mitochondrial genomes of three North American stygobiont amphipods Stygobromus tenuis potomacus , S. foliatus and S. indentatus collected from Caroline County, VA, were sequenced using a shotgun sequencing approach on an Illumina NextSeq500 (Illumina Inc., San Diego, CA). All three mitogenomes displayed 13 protein-coding genes, 22 tRNAs and two rRNAs typical of metazoans. While S. tenuis and S. indentatus displayed identical gene orders similar to the pancrustacean ground pattern, S. foliatus displayed a transposition of the trnL2 - cox2 genes to after atp8-atp6 . In addition, a short atp8 gene, longer rrnL gene and large inverted repeat within the Control Region distinguished S. foliatus from S. tenuis potomacus and S. indentatus . Overall, it appears that gene order varies considerably among amphipods, and the addition of these Stygobromus mitogenomes to the existing sequenced amphipod mitogenomes will prove useful for characterizing evolutionary relationships among various amphipod taxa, as well as investigations of the evolutionary dynamics of the mitogenome in general.

Mitochondrial DNA Part B↗

USGS2016

Hydrogeology of Virginia

The hydrogeology of Virginia documented herein is in two parts. Part 1 consists of an overview and description of the hydrogeology within each regional aquifer system in the Commonwealth. Part 2 includes discussions of hydrogeologic research topics of current relevance including: 1. the Chesapeake Bay impact structure, 2. subsidence/compaction in the Coastal Plain, 3. groundwater age and aquifer susceptibility, 4. the occurrence of groundwater at depth in fractured-rock and karst terrains, and 5. hydrologic response of wells to earthquakes around the world.

Virginia↗

USGS2015

Hydrologic budget and conditions of Permian, Pennsylvanian, and Mississippian aquifers in the Appalachian Plateaus physiographic province

In response to challenges to groundwater availability posed by historic land-use practices, expanding development of hydrocarbon resources, and drought, the U.S. Geological Survey Groundwater Resources Program began a regional assessment of the Appalachian Plateaus aquifers in 2013 that incorporated a hydrologic landscape approach to estimate all components of the hydrologic system: surface runoff, base flow from groundwater, and interaction with atmospheric water (precipitation and evapotranspiration). This assessment was intended to complement other Federal and State investigations and provide foundational groundwater-related datasets in the Appalachian Plateaus. A regional Soil-Water-Balance model was constructed for a 160,000-square-mile study area that extended to the topographic divide of all streams originating outside but flowing into areas underlain by Appalachian Plateaus aquifers. The model incorporated soil, landscape, and climate variables to estimate an annual water budget for the 32-year period from 1980 to 2011 and was calibrated using base-flow data estimated by hydrograph separation techniques from 20 streamflow gaging stations across the study area. Over this period, an average of 47 inches per year (in/yr) of precipitation fell on Appalachian Plateaus aquifers. Simulations from the regional Soil-Water-Balance model indicate that only 19 percent of the precipitation or an average 9 in/yr recharged aquifers, and 19 percent resulted in surface runoff to streams. The remaining 62 percent, an average of 27 in/yr of water, was returned to the atmosphere via evapotranspiration. Because withdrawals from aquifers due to pumping equated to less than 1 percent of the water budget, differences in predevelopment and postdevelopment regional water budgets of the Appalachian Plateaus were minimal. Storage changes caused by filling of abandoned coal-mine aquifers and long-term differences in aquifer storage resulting from climate fluctuations constitute a small portion of the overall water budget. The percentage of precipitation that results in recharge, runoff, or evapotranspiration from the landscape varies annually by up to a factor of two depending on temporal changes in prevailing climate conditions and spatial changes in basin characteristics, precipitation patterns, and sources of atmospheric moisture over a large study area. A comparison of water-budget estimates from the regional Soil-Water-Balance model for a dry year (1988) and wet year (2004) showed that evapotranspiration accounts for most of the annual differences in precipitation. As a portion of annual precipitation, evapotranspiration ranged from 69 percent (dry year) to 52 percent (wet year), a range four times greater than the 15 percent (dry year) to 18 percent (wet year) range estimated for recharge. Evapotranspiration as a percentage of precipitation peaks during dry periods, whereas base flow and runoff tend to reach minimum values. During wet periods, this relationship is reversed and base flow and runoff as a percentage of precipitation generally peak while evapotranspiration percentages reach minimum values. Annual recharge in the Appalachian Plateaus reaches a maximum at near 20 percent of annual precipitation, regardless of the severity of wet conditions. Hydrograph separation data from 849 streamflow gaging stations in the study area were used to assess trends in streamflow, base flow, surface runoff, and base-flow index, or ratio of base flow to streamflow, in the Appalachian Plateaus for the period from 1930 to 2011. Annual data anomalies for each of the four variables were individually defined as the annual standard deviation from the mean at all 849 streamflow gaging stations. Annual data anomalies confirm the close relation of annual precipitation to both base flow and runoff components of streamflow, and both components increased during the period of analysis. Around 1970, conditions shifted streamflow from values generally below to above long-term means. At a regional scale, increases in base flow account for most of these observed increases in mean annual streamflow. The independence of the base-flow index to annual climate trends indicate that changes in the components of streamflow of the Appalachian Plateaus are probably in response to shifts in seasonal precipitation or widespread land-use practices. A subset of 77 index streamgages, defined as having 60 or more years of complete record between the years 1930 and 2011 with no more than 20 percent missing data, was selected to show spatial patterns of change in the water budget. Data from the index streamgages showed that the overall trends in base flow are dependent upon the period of evaluation. Long-term (1930–2011) increases in base flow were observed throughout the study area. For two shorter periods (1930–1969 and 1970–2011) trends in base flow were largely negative. In general, spatial patterns of change in streamflow, base flow, and runoff were mixed but generally consistent with prevailing climate patterns and land-use changes.

Alabama, Kentucky, Maryland, Ohio, Pennslyvania, V↗

USGS2015

Annual and average estimates of water-budget components based on hydrograph separation and PRISM precipitation for gaged basins in the Appalachian Plateaus Region, 1900-2011

As part of the U.S. Geological Survey’s Groundwater Resources Program study of the Appalachian Plateaus aquifers, annual and average estimates of water-budget components based on hydrograph separation and precipitation data from parameter-elevation regressions on independent slopes model (PRISM) were determined at 849 continuous-record streamflow-gaging stations from Mississippi to New York and covered the period of 1900 to 2011. Only complete calendar years (January to December) of streamflow record at each gage were used to determine estimates of base flow, which is that part of streamflow attributed to groundwater discharge; such estimates can serve as a proxy for annual recharge. For each year, estimates of annual base flow, runoff, and base-flow index were determined using computer programs—PART, HYSEP, and BFI—that have automated the separation procedures. These streamflow-hydrograph analysis methods are provided with version 1.0 of the U.S. Geological Survey Groundwater Toolbox, which is a new program that provides graphing, mapping, and analysis capabilities in a Windows environment. Annual values of precipitation were estimated by calculating the average of cell values intercepted by basin boundaries where previously defined in the GAGES–II dataset. Estimates of annual evapotranspiration were then calculated from the difference between precipitation and streamflow.

Appalachian Plateaus Region↗

USGS2015

Widespread groundwater-level offsets caused by the M w 5.8 Mineral, Virginia, earthquake of 23 August 2011

Groundwater levels were offset in bedrock observation wells, measured by the U.S. Geological Survey or others, as far as 553 km from the M w 5.8 Mineral, Virginia (USA), earthquake on 23 August 2011. Water levels dropped as much as 0.47 m in 34 wells and rose as much as 0.15 m in 12 others. In some wells, which are as much as 213 m deep, the water levels recovered from these deviations in hours to days, but in others the water-level offset may have persisted. The groundwater-level offsets occurred in locations where the earthquake was at least weakly felt, and the maximum water-level excursion increased with felt intensity, independent of epicentral distance. Coseismic static strain from the earthquake was too small and localized to have contributed significantly to the groundwater-level offsets. The relation with intensity is consistent with ground motion from seismic waves leading to the water-level offsets. Examination of the hydrographs indicates that short-period ground motion most likely affected the permeability of the bedrock aquifers monitored by the wells.

GSA Special Papers↗

USGS2014

A 19-year record of chemical and isotopic composition of water from springs of the Shenandoah National Park, Virginia, 1995-2014

During October 1995 through March 2014, the U.S. Geological Survey in cooperation with the National Park Service, Luray, Virginia Station collected and analyzed samples of selected springs, air and unsaturated-zone gases in Shenandoah National Park, Virginia. The 19-year record of measurements of chemical and isotopic composition of water discharging from 34 springs located along the crest of the Blue Ridge Mountains in Shenandoah National Park, Virginia, is reported. These data include field measurements of water temperature, specific conductance, concentrations of dissolved oxygen (O 2 ), and pH. Laboratory measurements included major-, minor-, and trace-element chemistry; concentrations of dissolved gases (nitrogen, [N 2 ] argon [Ar], oxygen, and carbon dioxide [CO 2 ]); concentrations of dissolved trace atmospheric gases, including trichlorofluoromethane (CFC-11), dichlorodifluoromethane (CFC-12), and trichlorotrifluoroethane (CFC-113) and sulfur hexafluoride (SF 6 ); and hydrogen stable isotopic composition (δ 2 H) and oxygen isotopic composition (δ 18 O) of water. The data include an up to 14-year time series record of monthly sampling at five springs collected between 1995 and 2013. The measurements included temperature, specific conductance, pH, and discharge recorded at 30-minute intervals. Atmospheric mixing ratios of CFC-11, CFC-12, CFC-113, trifluorobromomethane (CF 3 Br), SF 6 , and trifluoromethyl sulfur pentafluoride (SF 5 CF 3 ) in air from the Big Meadows Air Monitoring Station, Shenandoah National Park, were measured at approximately weekly intervals from September 1995 through March 2014. Additional data include monthly (between May 2001 and August 2003) measurements of temperature, N 2 , O 2 , Ar, CO 2 , CFC-12, CFC-11, CFC-113, and SF 6 concentrations in unsaturated-zone air from seven multilevel piezometers in Shenandoah National Park and at the U.S. Geological Survey National Center in Reston, Virginia. All samples were analyzed at the U.S. Geological Survey Laboratories in Reston, Virginia.

Virginia↗

USGS2013

Comparison of age distributions estimated from environmental tracers by using binary-dilution and numerical models of fractured and folded karst: Shenandoah Valley of Virginia and West Virginia, USA

Measured concentrations of environmental tracers in spring discharge from a karst aquifer in the Shenandoah Valley, USA, were used to refine a numerical groundwater flow model. The karst aquifer is folded and faulted carbonate bedrock dominated by diffuse flow along fractures. The numerical model represented bedrock structure and discrete features (fault zones and springs). Concentrations of 3 H, 3 He, 4 He, and CFC-113 in spring discharge were interpreted as binary dilutions of young (0–8 years) water and old (tracer-free) water. Simulated mixtures of groundwater are derived from young water flowing along shallow paths, with the addition of old water flowing along deeper paths through the model domain that discharge to springs along fault zones. The simulated median age of young water discharged from springs (5.7 years) is slightly older than the median age estimated from 3 H/ 3 He data (4.4 years). The numerical model predicted a fraction of old water in spring discharge (0.07) that was half that determined by the binary-dilution model using the 3 H/ 3 He apparent age and 3 H and CFC-113 data (0.14). This difference suggests that faults and lineaments are more numerous or extensive than those mapped and included in the numerical model.

Virginia;West Virginia↗

USGS2012

Quantifying components of the hydrologic cycle in Virginia using chemical hydrograph separation and multiple regression analysis

This study by the U.S. Geological Survey, prepared in cooperation with the Virginia Department of Environmental Quality, quantifies the components of the hydrologic cycle across the Commonwealth of Virginia. Long-term, mean fluxes were calculated for precipitation, surface runoff, infiltration, total evapotranspiration (ET), riparian ET, recharge, base flow (or groundwater discharge) and net total outflow. Fluxes of these components were first estimated on a number of real-time-gaged watersheds across Virginia. Specific conductance was used to distinguish and separate surface runoff from base flow. Specific-conductance data were collected every 15 minutes at 75 real-time gages for approximately 18 months between March 2007 and August 2008. Precipitation was estimated for 1971–2000 using PRISM climate data. Precipitation and temperature from the PRISM data were used to develop a regression-based relation to estimate total ET. The proportion of watershed precipitation that becomes surface runoff was related to physiographic province and rock type in a runoff regression equation. Component flux estimates from the watersheds were transferred to flux estimates for counties and independent cities using the ET and runoff regression equations. Only 48 of the 75 watersheds yielded sufficient data, and data from these 48 were used in the final runoff regression equation. The base-flow proportion for the 48 watersheds averaged 72 percent using specific conductance, a value that was substantially higher than the 61 percent average calculated using a graphical-separation technique (the USGS program PART). Final results for the study are presented as component flux estimates for all counties and independent cities in Virginia.

Virginia↗

USGS2010

Hydrogeology and groundwater availability in Clarke County, Virginia

The prolonged drought between 1999 and 2002 drew attention in Clarke County, Virginia, to the quantity and sustainability of its groundwater resources. The groundwater flow systems of the county are complex and are controlled by the extremely folded and faulted geology that underlies the county. A study was conducted between October 2002 and October 2008 by the U.S. Geological Survey, in cooperation with Clarke County, Virginia, to describe the hydrogeology and groundwater availability in the county and to establish a long-term water monitoring network. The study area encompasses approximately 177 square miles and includes the carbonate and siliciclastic rocks of the Great Valley section of the Valley and Ridge Physiographic Province and the metamorphic rocks of the Blue Ridge Physiographic Province (Blue Ridge). High-yielding wells generally tend to cluster along faults, within lineament zones, and in areas of tight folding throughout the county. Water-bearing zones are generally within 250 feet (ft) of land surface; however, median depths are slightly deeper for the hydrogeologic units of the Blue Ridge than for those of the Great Valley section of the county. Total water-level fluctuations between October 2002 and October 2008 ranged from 2.86 to 87.84 ft across the study area, with an average of 24.15 ft. Generally, water-level fluctuations were greatest near hydrologic divides, in isolated elevated areas, and in the Opequon Creek Basin. Seasonally, water-level highs occur in the early spring at the end of the major groundwater recharge period and lows occur in late autumn when evapotranspiration rates begin to decrease. An overall downward trend in water levels between 2003 and 2008, which closely follows a downward trend in annual precipitation over the same period, was observed in a majority of wells in the Great Valley and in some of the wells in the Blue Ridge. Water-level fluctuations in the Blue Ridge tend to follow current meteorological conditions, and seasonal highs and lows tend to shift in response to the current conditions. Springs generally are present along faults and fold axes, and discharges for the study period ranged from dry to 10 cubic feet per second. A similar downward trend in discharges correlates with the trend in water levels and is indicative of an aquifer system that, over time, drains to a base level controlled by springs and streams. Point discharge from springs can occur as the start of flows of streams and creeks, along banks, and as discrete discharge through streambeds in the Great Valley. For the most part, streams, creeks, and rivers in the Great Valley function as aqueducts. Springs in the Blue Ridge have relatively low discharge rates, have small drainage areas, and are susceptible to current meteorological conditions. Estimates of effective groundwater recharge from 2001 to 2007 ranged from 6.4 to 23.0 inches per year (in/yr) in the Dry Marsh Run and Spout Run Basins with averages of 11.6 and 11.9 in/yr, respectively. Base flow accounted for between 80 and 97 percent of mean streamflow and averaged about 90 percent in these basins. The high base-flow index values (percent of streamflow from base flow) in the Dry Marsh Run and Spout Run Basins indicate that groundwater is the dominant source of streamflow during both wet and drought conditions. Between 46 and 82 percent of the precipitation that fell on the Dry Marsh Run and Spout Run Basins from 2001 to 2007 was removed by evapotranspiration, and an average of approximately 30 percent of the precipitation reached the water table as effective recharge. The high permeability of the rocks and low relief in these basins are not conducive for runoff; therefore, on average, only about 3 to 4 percent of the precipitation becomes runoff. Groundwater flow systems in the county are extremely vulnerable to current climatic conditions. Successive years of below-average effective recharge cause declines in water levels, spring discha

Scientific Investigations Report↗

USGS2010

Preliminary Assessment of the Hydrogeology and Groundwater Availability in the Metamorphic and Siliciclastic Fractured-Rock Aquifer Systems of Warren County, Virginia

Expanding development and the prolonged drought from 1999 to 2002 drew attention to the quantity and sustainability of the groundwater resources in Warren County, Virginia. The groundwater flow systems of the county are complex and are controlled by the extremely folded and faulted geology that underlies the county. A study was conducted between May 2002 and October 2008 by the U.S. Geological Survey, in cooperation with Warren County, Virginia, to describe the hydrogeology of the metamorphic and siliciclastic fractured-rock aquifers and groundwater availability in the county and to establish a long-term water monitoring network. The study area encompasses approximately 170 square miles and includes the metamorphic rocks of the Blue Ridge Physiographic Province and siliciclastic rocks of the Great Valley section of the Valley and Ridge Physiographic Province. Well depths tend to be shallowest in the siliciclastic rock unit (predominantly in the Martinsburg Formation) where 75 percent of the wells are less than 200 feet deep. Median depths to bedrock are generally less than 40 feet across the county and vary in response to the presence of surficial deposits, faults, siliciclastic rock type, and topographic setting. Water-bearing zones are generally within 200 feet of land surface; median depths, however, are slightly deeper for the hydrogeologic units of the Blue Ridge Province than for those of the Great Valley section of the county. Median well yields for the different rock units generally range from 10 to 20 gallons per minute. High-yielding wells tend to cluster along faults, along the eastern contact of the Martinsburg Formation, and within potential lineament zones. Specific capacity is relatively low and ranges from 0.003 to 1.43 gallons per minute per foot with median values from 0.12 to 0.24 gallon per minute per foot. Transmissivity values derived from specific capacity data range over four orders of magnitude from 0.6 to 380 feet squared per day. Estimates of effective groundwater recharge from 2001 to 2007 ranged from 2.4 to 29.4 inches per year in the Gooney Run, Manassas Run, and Crooked Run Basins, with averages of 15.3, 14.2, and 5.3 inches per year, respectively. Base flow accounted for between 57 and 86 percent of mean streamflow in the Gooney Run and Manassas Run Basins and averaged about 70 percent in these Blue Ridge Province basins. In the siliciclastic rock-dominated Crooked Run Basin of the Great Valley, base flow accounted for between 33 and 65 percent of mean streamflow and averaged about 54 percent. The high base-flow index values (percentage of streamflow from base flow) in these basins indicate that groundwater is the dominant source of streamflow during wet and drought conditions. About 50 percent of the precipitation that fell on the Blue Ridge basins from 2001 to 2007 was removed by evapotranspiration, and between 33 and 36 percent of the precipitation reached the water table as effective recharge. Nearly 76 percent of the precipitation was removed by evapotranspiration in the Crooked Run Basin, and effective recharge averaged about 12 percent of precipitation between 2001 and 2007. Average values of runoff in all three basins were less than 15 percent of precipitation. Groundwater flow systems in the county are extremely vulnerable to current climatic conditions. Successive years of below-average effective recharge cause declines in water levels, spring discharges, and streamflows. However, these systems can recover quickly because effective recharge increases with increasing precipitation. Lack of precipitation, especially snow, during the critical recharge period (January-April) can have an effect on the amount of recharge to the groundwater system and eventual stream base flow. Estimated values of annual mean base flow have approached and have been below the average regression-derived recharge rates during a period classified as having above-average precipitation. This relation is indicative

Virginia↗

USGS2009

Simulation of Groundwater-Level and Salinity Changes in the Eastern Shore, Virginia

Groundwater-level and salinity changes have been simulated with a groundwater model developed and calibrated for the Eastern Shore of Virginia. The Eastern Shore is the southern part of the Delmarva Peninsula that is occupied by Accomack and Northampton Counties in Virginia. Groundwater is the sole source of freshwater to the Eastern Shore, and demands for water have been increasing from domestic, industrial, agricultural, and public-supply sectors of the economy. Thus, it is important that the groundwater supply be protected from overextraction and seawater intrusion. The best way for water managers to use all of the information available is usually to compile this information into a numerical model that can simulate the response of the system to current and future stresses. A detailed description of the geology, hydrogeology, and historical groundwater extractions was compiled and entered into the numerical model. The hydrogeologic framework is composed of a surficial aquifer under unconfined conditions, a set of three aquifers and associated overlying confining units under confined conditions (the upper, middle, and lower Yorktown-Eastover Formation), and an underlying confining unit (the St. Marys Formation). An estimate of the location and depths of two major paleochannels was also included in the framework of the model. Total withdrawals from industrial, commercial, public-supply, and some agricultural wells were compiled from the period 1900 through 2003. Reported pumpage from these sources increased dramatically during the 1960s and 70s, up to currently about 4 million gallons per day. Domestic withdrawals were estimated on the basis of population census districts and were assigned spatially to the model on the assumption that domestic users are located close to roads. A numerical model was created using the U.S. Geological Survey (USGS) code SEAWAT to simulate both water levels and concentrations of chloride (representing salinity). The model was calibrated using 605 predevelopment and transient water-level observations that are associated predominantly with 20 observation nests of wells sited across the study area. Sampling for groundwater chemistry at these sites revealed that chloride has not increased significantly in the last 20 years. Environmental tracers in the samples also indicated that the water in the surficial aquifer is typically years to decades old, whereas water in the confined aquifers is typically centuries to millennia old. The calibration procedure yielded distributions of hydraulic conductivity and storage coefficients of the aquifers and confining units that are based on 21 pilot points, but vary smoothly across the study area. The estimated values are consistent with other measurements of these properties measured previously on cores and during hydraulic tests at various well fields. Simulations performed with the model demonstrated that the calibrated model can reproduce the observed historical water levels fairly well (R2 = 0.93). The chloride concentrations were also simulated, but a match with chloride concentrations was more difficult to achieve (R2 = 0.16) because of the lack of sufficient data and the unknown exact behavior of the entire transition zone in the millennia leading up to the present day. Future pumping scenarios were simulated through 2050, with pumping set to either 2003 rates or total permitted withdrawal rates. Water levels in 2050 are predicted to be lower than current levels by a few feet where stresses are currently heaviest but potentially by tens of feet if total permitted withdrawals are extracted at current low-stressed sites. Simulations of chloride concentrations through 2050 revealed some potential for seawater intrusion in the areas of Cape Charles, Chincoteague, east of the town of Exmore, and east of the town of Accomac, but precise estimates of concentration increases are highly uncertain. Simulation results were also used to estimate that the down

Scientific Investigations Report↗

USGS2008

Fiber‐optic distributed temperature sensing: A new tool for assessment and monitoring of hydrologic processes

Fiber‐optic distributed temperature sensing (FO DTS) is an emerging technology for characterizing and monitoring a wide range of important earth processes. FO DTS utilizes laser light to measure temperature along the entire length of standard telecommunications optical fibers. The technology can measure temperature every meter over FO cables up to 30 kilometers (km) long. Commercially available systems can measure fiber temperature as often as 4 times per minute, with thermal precision ranging from 0.1 to 0.01 °C depending on measurement integration time. In 2006, the U.S. Geological Survey initiated a project to demonstrate and evaluate DTS as a technology to support hydrologic studies. This paper demonstrates the potential of the technology to assess and monitor hydrologic processes through case‐study examples of FO DTS monitoring of stream‐aquifer interaction on the Shenandoah River near Locke's Mill, Virginia, and on Fish Creek, near Jackson Hole, Wyoming, and estuary‐aquifer interaction on Waquoit Bay, Falmouth, Massachusetts. The ability to continuously observe temperature over large spatial scales with high spatial and temporal resolution provides a new opportunity to observe and monitor a wide range of hydrologic processes with application to other disciplines including hazards, climate‐change, and ecosystem monitoring.

Conference Paper↗

USGS2008

Bedrock structural controls on the occurrence of sinkholes and springs in the Northern Great Valley Karst, Virginia and West Virginia

Recent geologic mapping at a scale of 1:24,000 has enabled a qualitative correlation of the occurrence of springs and sinkholes with bedrock structures and ground-water conditions in the northern Great Valley of Virginia and West Virginia. Sinkholes tend to be concentrated in zones of faulting, local minor folding, and clustered within susceptible bedrock units at the noses and axes of large plunging folds. Alignment of sinkholes mainly occurs along strike of bedding. Enhanced rock solution and conduit formation correlates with carbonate units of greater limestone purity and finer grain size, suggesting some lithologic control on karst formation. In addition, there is an apparent topographic correlation, with sinkholes usually formed in elevated and flat (<5 degrees slope) areas, as well as frequent sinkhole occurrence proximal to entrenched surface streams. Density of sinkhole occurrence tends to increase in areas where water-table fluctuations are large. Large perennial springs occur along faults, and may lie above base level streams indicating upward flow gradients particularly where cross-strike faults and joints intersect bedding planes and strike-parallel faults. Sinkhole formation also frequently occurs in areas proximal to entrenched surface streams, reflecting subsequent vadose-zone modification and excavation of sediment-filled conduits where downward ground-water flow gradients are most steep. Geologic evidence indicates that deep karst development may have taken place by rising fluids under confined (hypogenic) conditions in the distant geologic past in this region.

Virginia, West Virginia↗

USGS2005

Hydrogeology and ground-water availability in the carbonate aquifer system of Frederick County, Virginia

The carbonate aquifer system of the northern Shenandoah Valley provides an important water supply to local communities, including Frederick County, Va., which depends on ground water as a source of water supply. The county and surrounding area are undergoing increased urbanization, and increased demands on the carbonate aquifer system are expected. A study was conducted between October 2000 and March 2004 by the U.S. Geological Survey (USGS), in cooperation with the County of Frederick, Va., to describe the hydrogeology and ground-water availability in the carbonate aquifer system underlying the county. The study area encompasses about 25 percent (105 square miles) of the county that is underlain by carbonate bedrock. The carbonate aquifer system of Frederick County is in the Shenandoah Valley region of the Valley and Ridge Physiographic Province. Approximately 10,000 feet of folded and fractured Middle Cambrian to Upper Ordovician sedimentary rocks are exposed and are overlain by Pleistocene (?) and Holocene surficial deposits. All geologic units in the study area are considered to be aquifers. The geologic units are generally unconfined, fractured-rock aquifers that are recharged by precipitation and discharge locally to streams and springs, and by evapotranspiration. Stream density in the carbonate study area is less than in the remainder of the county, which is underlain by siliciclastic rock units. Most streams flow normal to strike (from the northwest towards the southeast) across the study area. These streams are characterized by shallow incisement and are usually limited to a single stream channel. In the southern third of the study area, streams flow parallel to strike (from the northeast towards the southwest) towards the deeply intrenched Cedar Creek. Springs are commonly located at the start of flows for all streams in the carbonate study area, and spring discharges are often a large portion of the streamflow (especially during drought conditions). The general direction of ground-water flow is from the hills in the west of the study area into and across the carbonate valley. A ground-water divide may occur north of Round Hill in the vicinity of the Apple Pie Ridge fault where the North Mountain fault zone cuts out the resistant Silurian and Devonian sandstone units and results in surface drainage from the carbonate rocks toward the west and out of the carbonate valley. Estimates of effective ground-water recharge for 2001-02 range from 5.8 to 6.2 inches in the Cedar Creek Basin, with base flow accounting for between 60 and 64 percent of streamflow, and from 3.2 to 3.8 inches in the Opequon Creek Basin, with base flow accounting for between 86 and 92 percent of streamflow. Water budgets calculated for 2001, a year of below-normal precipitation (33.1 inches), and 2002, a year of above-normal precipitation (41.2 inches), include a streamflow of 9.0 inches in 2001 and 9.2 inches in 2002 in Cedar Creek. Evapotranspiration ranged from 25.9 to 30.7 inches, and ground-water storage decreased 1.8 inches in 2001 and increased 1.3 inches in 2002. Streamflow was 3.7 inches in 2001 and 2002 in Opequon Creek. Evapotranspiration ranged from 29.8 to 37.5 inches, and ground-water storage decreased 0.4 inch in 2001 and did not change in 2002.

Scientific Investigations Report↗

USGS2003

Aquifer Susceptibility in Virginia: Data on Chemical and Isotopic Composition, Recharge Temperature, and Apparent Age of Water from Wells and Springs, 1998-2000

The determination of aquifer susceptibility to contamination from near-surface sources by the use of ground-water dating techniques is a critical part of Virginia's Source Water Assessment Program. As part of the Virginia Aquifer Susceptibility study, water samples were collected between 1998 and 2000 from 145 wells and 6 springs in various hydrogeologic settings across the Commonwealth. Samples were analyzed to determine water chemistry?including nitrate (NO3), dissolved organic carbon (DOC), and radon-222 (222Rn), major dissolved and noble gases?nitrogen (N2), argon (Ar), oxygen (O2), carbon dioxide (CO2), methane (CH4), helium (He), and neon (Ne), environmental tracers?chlorofluorocarbons (CFCs), sulfur hexafluoride (SF6), tritium (3H), and tritium/helium-3 (3H/3He), carbon isotopes?carbon-14 (14C) and carbon-13 (d13C), and stable isotopes of oxygen (d18O) and hydrogen (d2H). The chemical and isotopic composition, recharge temperatures, and apparent ages of these water samples are presented in this report. Data collected between 1999 and 2000 from 18 wells in Virginia as part of two other studies by the U.S. Geological Survey also are presented. Most of the sites sampled serve as public water supplies and are included in the comprehensive Source Water Assessment Program for the Commonwealth.

Open-File Report↗