Geology Reports⌕ Search

SEARCH · Geology Reports

Results for “Southeastern Geology”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 559 records · Page 31Linked to original sources
USGS1972

Geological and geophysical investigations of an Apollo 9 photo anomaly near Point of Pines, Arizona

An infrared photograph of southeastern Arizona, taken during the Apollo 9 multispectral terrain photography experiment in 1969, reveals a ringlike feature, some 3-4 miles (5-6 kin) in diameter, on the Natanes Plateau, 35 miles (56 kin) north of the town of Safford. Because the feature occurs in an area of nearly flat lying Tertiary volcanic rocks, the possibilities of its being a small collapse caldera or an exposed circular intrusive body were considered. Geological and geophysical studies of the area were made to test these hypotheses. The local stratigraphic section consists of approximately 1,500 feet (457 m) of Oligocene and perhaps older volcanic rocks, resting on a moderately irregular basement surface carved from nearly flat lying trending Basin-and-Range faults define a broad horst within which two arcuate cross faults, with 300-600 feet (91-183 m) of displacement, bound a downdropped area. Deep erosion along these faults has created a polygonal network of canyons which constitutes the 'ring' seen on the photograph. A mild arching of the volcanic rocks within the ring is suggested by structure contours on the base of the youngest flows. A sharp 350-gamma positive aeromagnetic anomaly is centered within the ring. In its southwest quadrant the anomaly has an elongate extension that trends northwest along an adjoining Basin-and-Range fault. Associated with both is a subtle gravity low. The geophysical data thus suggest the presence of a small blind silicic pluton, possibly of middle Tertiary or younger age. Although it can be argued that the arcuate faults and mild arching of the volcanic pile are related to this postulated pluton, no evidence of hydrothermal alteration or thermal metamorphism of the country rocks was seen. Thus if a pluton is present and of postvolcanic age, it must have been emplaced as a relatively cool dry body; or alternatively, it is older than the surface volcanic rocks. In either instance, its magnetic expression contrasts with that of the known mineralized Laramide porphyry intrusive bodies of the region.

Arizona↗
USGS1997

Geologic studies in Alaska by the U.S. Geological Survey, 1995

This collection of 20 papers continues the annual series of U.S. Geological Survey (USGS) reports on geologic investigations in Alaska1 . Contributions cover a broad spectrum of earth science topics and report results from all parts of the State (fig. 1). USGS activities in Alaska include studies of environment and climate, hazards, resources, and geologic framework. Five papers in this volume discuss aspects of environment and climate. Environmental geochemistry of parts of southwestern and south-central Alaska is the focus of four articles; a fifth study, of emergent postglacial lake shorelines in southwestern Alaska, contributes to ongoing investigations of paleoclimate. Two papers address geologic hazards. The first assesses ground deformation in Katmai National Park at Novarupta dome, site of the largest volcanic eruption in this century, and the second evaluates earthquake risks related to the "Twin Peak fault" near Anchorage. Resources, including metallic minerals in northern, southwestern, and southeastern Alaska and coal in south-central Alaska, are discussed in four articles. Nine geologic framework studies apply a variety of techniques to a wide range of subjects throughout Alaska, including tectonics, geophysics, geochronology, bio stratigraphy, sedimentology, paleogeography, and paleomagnetism. Two bibliographies at the end of the volume list reports about Alaska in USGS publications released in 1995 and reports about Alaska by USGS authors in non-USGS publications in 1995.

Alaska↗
USGS1996

Assessment of saltwater intrusion in southern coastal Broward County, Florida

Of the counties in southeastern Florida, Broward County has experienced some of the most severe effects of saltwater intrusion into the surficial Biscayne aquifer because, before 1950, most public water-supply well fields in the county were constructed near the principal early population centers located less than 5 miles from the Atlantic Ocean. The construction of major regional drainage canals in the early 20th century caused a lowering of the water table and a gradual inland movement of the saltwater front toward the well fields. The U.S. Geological Survey began field investigations of saltwater intrusion in the Biscayne aquifer of southeastern Broward County in 1939. As part of the present study, the positions of the saltwater front in 1945, 1969, and 1993 were estimated using chloride concentrations of water samples collected between 1939 and 1994 from various monitoring and exploratory wells. The data indicate that, between 1945 and 1993, the saltwater front has moved as much as 0.5 mile inland in parts of the study area. The position and movement of the saltwater front were simulated numerically to help determine which of the various hydrologic factors and water-management features characterizing the coastal subsurface environment and its alteration by man are of significance in increasing or decreasing the degree of saltwater intrusion. Two representational methods were applied by the selection and use of appropriate model codes. The SHARP code simulates the position of the saltwater front as a sharp interface, which implies that no transition zone (a zone in which a gradational change between freshwater and saltwater occurs) separates freshwater and saltwater. The Subsurface Waste Injection Program (SWIP) code simulates a two-fluid, variable-density system using a convective-diffusion approach that includes a representation of the transition zone that occurs between the freshwater and saltwater bodies. The models were applied to: (1) approximately replicate predevelopment and current positions of the interface in the study area; and (2) study the relative importance of various factors affecting the interface position. The model analyses assumed a conceptual model of uniform easterly flow in the aquifer toward points of offshore discharge to tidewater. Measurements of water-table altitude and the depth to the interface in the study area exhibit an interrelation that differes substantially from the classical Ghyben-Herzberg relation. However, both model codes simulated water-table altitudes and interface positions that were generally consistent with the Ghyben-Herzberg relation but differed substantially from observed data. The simulate interface positions were inland of the known positions, and simulate water-table altitudes were higher than measured ones. The SHARP and SWIP simulations were in general agreement with each other when a low value of longitudinal dispersivity was specified in the SWIP simulation and also for higher values of longitudinal dispersivity when modified dispersion algorithms were used in SWIP that greatly reduced the simulated degree of vertical dispersion. Sensitivity analyses performed using the SHARP code indicated simulation results to be relatively insensitive to a substantial change in the specified slope of the base of the aquifer and moderately sensitive to a 150-percent change in net atmospheric recharge to the aquifer (rainfall minus evapotranspiration). Representing well-field pumping by the City of hallandale had only a minor, localized influence on the simulated regional interface position. Using various cross-sectional grid designs in applications of the SWIP code, near convergence of all lines of equal concentrations in the transition zone was achieved within a simulation time of 10 years. The simulated equilibrium interface location was sensitive to substantial spatial variations in the specified hydraulic conductivity values, but was relatively insensitive to seasonal varying

Florida↗
USGS1977

Geological and geochemical investigations of uranium occurrences in the Arrastre Lake area of the Medicine Bow Mountains, Wyoming

Metasedimentary rocks of Precambrian X age in and near the Snowy Range wilderness study area of southeastern Wyoming are lithologically and chronologically similar to those on the north shore of Lake Huron in Canada. The rocks in Canada contain major deposits of uranium in quartz-pebble conglomerates near the base of the metasedimentary sequence. Similar conglomerates in the Deep Lake Formation in the Medicine Bow Mountains of southeastern Wyoming are slightly radioactive and may contain deposits of uranium and other valuable heavy metals. During the summer of 1976, a geological and geochemical pilot study was conducted in the vicinity of Arrastre Lake in the Medicine Bow Mountains to determine the most effective exploration methods for evaluating the uranium potential of the Snowy Range wilderness study area. The area around Arrastre Lake was selected because of the presence of a radioactive lens within a quartz-pebble conglomerate of the Deep Lake Formation. The results of the survey indicate possible uranium mineralization in the subsurface rocks of this formation. The radon content of the dilute waters of the area is much higher than can be accounted for by the uranium content of the surface rocks. Two sources for the high content of the radon are possible. In either case, the high values of radon obtained in this study are a positive indication of uranium mineralization in the subsurface rocks. The determination of the radon content of water samples is the recommended geochemical technique for uranium exploration in the area. The determination of uranium in water and in organic-rich bog material is also recommended.

Wyoming↗
USGS1999

Field trip guide to selected studies of the Southwest Mineral and Environmental Investigations Project in southeastern Arizona

The Southwest Mineral and Environmental Investigations Project is designed to address issues raised by rapid urban development in the basins of the southwestern U.S. These issues require objective geoscientific data that can be used by land managers and stakeholders to develop informed land and water use strategies. The project integrates new and existing geologic, geophysical, and geochemical data, and imagery to provide three-dimensional visualizations of the basins of southeastern Arizona. Emphasis is on developing better knowledge of the aquifer systems of both the basins and the ranges, on acquiring background and baseline information, and on determining the distribution of metals related to mineralization and the fate of these metals in surface and subsurface environments. The products of the project will be used in resolving issues of water quality and quantity, in understanding environmental impacts such as riparian ecosystem maintenace, and in evaluating mineral resources beneath and within the basins. The field trip highlights three topics and areas (figs. 1 and 2): (1) geology and geophysics of the upper San Pedro and upper Santa Cruz basins (M.E. Gettings, M. W. Bultman, and B.B. Houser), (2) geology, geophysics, and mineral resource potential of the San Rafael basin (M.W. Bultman), and (3) hydrology and aqueous geochemistry of the Red Mountain and Sonoita Creek drainage system (Floyd Gray). The trip guide, which begins and ends in Tucson, Arizona, also includes commentary on the cultural and mining history of the area.

Arizona↗
USGS1995

Dissolved-solids contribution to the Colorado River from public lands in southeastern Nevada, through September 1993

The Bureau of Land Management administers about 9,300 square miles of public lands in southeastern Nevada that are part of the Colorado River Basin. The U.S. Geological Survey, in cooperation with the Bureau of Land Management, began a 5-year program in October 1988 to assess the contribution of dissolved solids to the fiver from those lands. About 6,200 square miles of public lands are in the Muddy River subbasin in Nevada. The estimated average dissolved-solids load contributed to the Colorado River from those lands was 28,000 tons per year from October 1988 through September 1993. Subsurface flow contributed about 86 percent (24,000 tons per year) of that load. About 730 square miles of public lands in the Las Vegas Wash subbasin contribute dissolved-solids load to the Colorado River. (About 120 square miles of public lands do not contribute to the river.) The estimated average dissolved-solids load contributed to the river from those lands was about 1,300 tons per year from October 1988 through September 1993. Subsurface flow contributed almost all of that load. About 1,100 square miles of public lands are in the Virgin River subbasin in Nevada. The estimated average dissolved- solids load contributed to the Colorado River from Nevada public lands in the subbasin was 8,700 tons per year. Subsurface flow contributed almost the entire load. About 1,200 square miles of Nevada public lands are in ephemeral tributaries that drain direcfly to the Colorado River or its impoundments (Lake Mead and Lake Mobave). The estimated average dissolved-solids load contributed to the river from those lands was 50 tons per year from surface runoff; however, the dissolved-solids load contributed by subsurface flow was not estimated. From October 1992 to September 1993, the Colorado River carried about 6,600,000 tons of dissolved solids past a streamflow gaging station 0.3 mile downstream from Hoover Dam. In contrast, surface runoff and subsurface flow contribute an estimated average dissolved-solids load of 38,000 tons per year from public lands in southeastern Nevada to the Colorado River. Land-management practices probably would not substantially reduce this contribution.

Water-Resources Investigations Report↗
USGS1989

Processing and attenuation of noise in deep seismic-reflection data from the Gulf of Maine

The U.S. Geological Survey deep crustal studies reflection profile across the Gulf of Maine off southeastern New England was affected by three sources of noise: side-scattered noise, multiples, and 20-Hz whale sounds. The special processing most effective in minimizing this noise consisted of a combination of frequency-wavenumber (F-K) filtering, predictive deconvolution, and spectral whitening, each applied in the shot domain (prestack). Application of the F-K filter to remove side-scatter noise in the poststack domain resulted in a much poorer quality profile. The prestack noise suppression processing techniques resulted in a reflection profile with good signal-to-noise ratios and reliable strong reflections, especially at depths equivalent to the lower crust (24-34 km). Certain geologic features, such as a buried rift basin and a crustal fault are resolved much better within the upper crust after this processing. Finite difference migration of these data using realistic velocities produced excellent results. Migration was essential to distinguish between abundant dipping and subhorizontal reflections in the lower crust as well as to show an essentially transparent upper mantle. ?? 1989 Kluwer Academic Publishers.

Marine Geophysical Research↗
USGS2014

Three-dimensional geologic mapping of the Cenozoic basin fill, Amargosa Desert basin, Nevada and California

Understanding the subsurface geologic framework of the Cenozoic basin fill that underlies the Amargosa Desert in southern Nevada and southeastern California has been improved by using borehole data to construct three-dimensional lithologic and interpreted facies models. Lithologic data from 210 boreholes from a 20-kilometer (km) by 90-km area were reduced to a limited suite of descriptors based on geologic knowledge of the basin and distributed in three-dimensional space using interpolation methods. The resulting lithologic model of the Amargosa Desert basin portrays a complex system of interfingered coarse- to fine-grained alluvium, playa and palustrine deposits, eolian sands, and interbedded volcanic units. Lithologic units could not be represented in the model as a stacked stratigraphic sequence due to the complex interfingering of lithologic units and the absence of available time-stratigraphic markers. Instead, lithologic units were grouped into interpreted genetic classes, such as playa or alluvial fan, to create a three-dimensional model of the interpreted facies data. Three-dimensional facies models computed from these data portray the alluvial infilling of a tectonically formed basin with intermittent internal drainage and localized regional groundwater discharge. The lithologic and interpreted facies models compare favorably to resistivity, aeromagnetic, and geologic map data, lending confidence to the interpretation.

California;Nevada↗
USGS1957

Pegmatite geology of the Shelby district, North Carolina

The Shelby district is divided into a northwestern and a southeastern province. The rocks in the southeastern province include various units in the Battleground schist formation and the Yorkville granodiorite. Those in the northwestern province include the Carolina gneiss, with its Shelby gneiss member, and the Toluca quartz monzonite. The Cherryville quartz monzonite forms a batholith that is just west of the boundary between the two provinces. Pegmatites related to both the Toluca and the Cherryville quartz monzonites lie in the Carolina gneiss and many dikes of pegmatite that are related to the Cherryville quartz monzonite are in the tin-spodumene belt that lies along the boundary between provinces. The rocks of the southeastern province have been bent into steep isoclinal folds; those of the northwestern province were bent into open folds and gently-dipping isoclinal folds. The rocks to the southeast have been metamorphosed in the epidote-amphibolite facies whereas the rocks to the northwest represent the amphibolite or granulite facies. The pegmatites related to the Toluca quartz monzonite form sills, dikes, and concordant lenses in the Carolina gneiss, as well as dikes in the Toluca quartz monzonite. The bodies are unzoned and consist mainly of gneissic microcline-plagioclase-quartz pegmatite. The pegmatites related to the Cherryville quartz monzonite form dikes and disconformable lenses in the Carolina gneiss and the Toluca quartz monzonite. These pegmatites range widely in composition and many are zoned. The dikes west of the Cherryville batholith are rich in muscovite and plagioclase and may contain no microcline or only a moderate amount of microcline. Quartz cores and microcline-rich intermediate zones are common. Similar pegmatite forms dikes along the west edge of the tin-spodumene belt. The tin-spodumene belt containes albite-microcline-spodumene-quartz pegmatite. These dikes of albitic pegmatite are largest and most nearly parallel to one another south of Kings Mountain. Farther north they lie in fractures of several sets. The main factors that affected the development of zones in the pegmatite dikes are composition of magma, temperatures of magma and wall rock, and deformation during crystallization. For best development of zones and the growth of the largest muscovite books the rate of cooling and the rate of crystal growth must be rather fast. Complete consolidation probably was reached in most dikes within a gew years after the start of crystallization.

Open-File Report↗
USGS1989

Paleomagnetism and tectonic rotation of the lower Miocene Peach Springs Tuff: Colorado Plateau, Arizona, to Barstow, California

We have determined remanent magnetization directions of the lower Miocene Peach Springs Tuff at 41 localities in western Arizona and southeastern California. An unusual northeast and shallow magnetization direction confirms the proposed geologic correlation of isolated outcrops of the tuff from the Colorado Plateau to Barstow, California, a distance of 350 km. The Peach Springs Tuff was apparently emplaced as a single cooling unit about 18 or 19 Ma and is now exposed in 4 tectonic provinces west of the Plateau, including the Transition Zone, Basin and Range, Colorado River extensional corridor, and central Mojave Desert strike-slip zone. As such, the tuff is an ideal stratigraphic and structural marker for paleomagnetic assessment of regional variations in tectonic rotations about vertical axes. From 4 sites on the stable Colorado Plateau, we have determined a reference direction of remanent magnetization (I = 36.4°, D = 33.0°, α 95 = 3.4°) that we interpret as a representation of the ambient magnetic field at the time of eruption. A steeper direction of magnetization (I = 54.8°, D = 22.5°, α 95 = 2.3°) was observed at Kingman where the tuff is more than 100 m thick, and similar directions were determined at 7 other thick exposures of the Peach Springs Tuff. The steeper component is presumably a later-stage magnetization acquired after prolonged cooling of the ignimbrite. When compared to the Plateau reference direction, tilt-corrected directions from 3 of 6 sites in the central Mojave strike-slip zone show localized rotations up to 13° in the vicinity of strike-slip faults. The other three sites show no significant rotations with respect to the Colorado Plateau. Both clockwise and counterclockwise rotations were measured, and no systematic regional pattern is evident. Our results do not support kinematic models which require consistent rotation of large regions to accommodate the cumulative displacement of major post-middle Miocene strike-slip faults in the central Mojave Desert. Most of our sites in the Transition Zone and Basin and Range province have had no significant rotation, although small counterclockwise rotation in the McCullough and New York Mountains may be related to sinistral shear along en echelon faults southwest of the Lake Mead shear zone. The larger rotations occur in the Colorado River extensional corridor, where 8 of 14 sites show rotations ranging from 37° clockwise to 51° counterclockwise. These rotations occur in allochthonous tilt blocks which have been transported northeastward above the Chemehuevi-Whipple Mountains detachment fault. Upper-plate blocks within 1 km of the exposed detachment unexpectedly show no significant rotation. From this relation, we infer that rotations are accommodated along numerous low-angle faults at higher structural levels above the detachment surface.

Arizona, California↗
USGS2006

Regional surficial geochemistry of the northern Great Basin

The regional distribution of arsenic and 20 other elements in stream-sediment samples in northern Nevada and southeastern Oregon was studied in order to gain new insights about the geologic framework and patterns of hydrothermal mineralization in the area. Data were used from 10,261 samples that were originally collected during the National Uranium Resource Evaluation (NURE) Hydrogeochemical and Stream Sediment Reconnaissance (HSSR) program in the 1970s. The data are available as U.S. Geological Survey Open-File Report 02-0227. The data were analyzed using traditional dot maps and interpolation between data points to construct high-resolution raster images, which were correlated with geographic and geologic information using a geographic information system (GIS). Wavelength filters were also used to deconvolute the geochemical images into various textural components, in order to study features with dimensions of a few kilometers to dimensions of hundreds of kilometers. The distribution of arsenic, antimony, gold, and silver is different from distributions of the other elements in that they show a distinctive high background in the southeast part of the area, generally in areas underlain by the pre-Mesozoic craton. Arsenic is an extremely mobile element and can be used to delineate structures that served as conduits for the circulation of metal-bearing fluids. It was used to delineate large crustal structures and is particularly good for delineation of the Battle Mountain-Eureka mineral trend and the Steens lineament, which corresponds to a post-Miocene fault zone. Arsenic distribution patterns also delineated the Black Rock structural boundary, northwest of which the basement apparently consists entirely of Miocene and younger crust. Arsenic is also useful to locate district-sized hydrothermal systems an d clusters of systems. Most important types of hydrothermal mineral deposit in the northern Great Basin appear to be strongly associated with arsenic; this is less so for low-sulfidation epithermal deposits. In addition to individual elements, the distribution of factor scores that resulted from principal component studies of the data was used. The strongest factor is characterized by Fe, Ti, V, Cu, Ni, and Zn and is used to map the distribution of distinctive basalts that are high in Cu, Ni, and Zn and that appear to be related to the Steens Basalt. The other important factor is related to hydrothermal precious metal mineralization and is characterized by Sb, Ag, As, Pb, Au, and Zn. The map of the distribution of this factor is similar in appearance to the one for arsenic, and we used wavelength filters to remove regional variations in the background for this factor score. The resulting residual map shows a very strong association with the most significant precious metal deposits and districts in the region. This residual map also shows a number of areas that are not associated with known mineral deposits, illustrating the utility of the method as a regional exploration tool. A number of these prospective areas are distant from known significant mineral deposits. The deconvolution of the spatial wavelength structure of geochemical maps, combined with the use of large regional geochemical data sets and GIS, permits new possibilities for the use of stream-sediment geochemistry in the study of large-scale crustal features as well as the isolation of mineral-district scale anomalies. ?? 2006 Society of Economic Geologists, Inc.

Economic Geology↗
USGS1971

Hydrogeologic characteristics of the valley-fill aquifer in the Arkansas River Valley, Bent County, Colorado

The investigation on which this report is based is a part of a comprehensive evaluation of the water resources of the Arkansas River valley undertaken by the U.S. Geological Survey in cooperation with the Colorado Water Conservation Board and the Southeastern Colorado Water Conservancy District. The study reach extends 150 miles from Pueblo to the Kansas State line (see fig. 1 index map). The water-resources investigation of the stream aquifer system in the Arkansas River valley, which began in 1963, is being made to provide information about the water resources for planning, management, and administration of the supply. The objectives of the investigation are to define the effects of present water use, to determine the relation between ground and surface water, and to evaluate the effects of proposed changes in water law and management. The major steps in the study are: (1) Inventory the water resources, (2) describe the hydrogeologic character of the aquifer, (3) document and evaluate the effects of development, (4) construct and verify models to aid in the evaluation of the hydrology, and (5) develop models to test water-management plans and to optimize water use.

Colorado↗
USGS1972

Hydrogeologic characteristics of the valley-fill aquifer in the Arkansas River Valley, Bent County, Colorado

The investigation on which this report is based is a part of a comprehensive evaluation of the water resources of the Arkansas River valley undertaken by the U.S. Geological Survey in cooperation with the Colorado Water Conservation Board and the Southeastern Colorado Water Conservancy District. The study reach extends 150 miles from Pueblo to the Kansas State line. The water-resources investigation of the stream aquifer system in the Arkansas River valley, which began in 1963, is being made to provide information about the water resources for planning, management, and administration of the supply. The objectives of the investigation are to define the effects of present water use, to determine the relation between ground and surface water, and to evaluate the effects of proposed changes in water law and management. The major steps in the study are: (1) Inventory the water resources, (2) describe the hydrogeologic character of the aquifer, (3) document and evaluate the effects of development, (4) construct and verify models to aid in the evaluation of the hydrology, and (5) develop models to test water-management plans and to optimize water use.

Colorado↗
USGS1984

Maps showing selected geology and phosphate resources of the Sage Valley quadrangle, Caribou County, Idaho

This report summarizes information on the thickness, grade, lateral continuity, phosphate resources, and ownership of phosphate-bearing units in the Meade Peak Phosphatic Shale Member of the Phosphoria Formation in the Sage Valley quadrangle. This report is one of a series of quadrangle reports prepared by the Idaho Bureau of Mines and Geology under U.S. Geological Survey costsharing contract #1408-0001-17925 to calculate phosphate resources in southeastern Idaho (fig. 1).

Idaho↗
USGS2018

Relations between total phosphorus and orthophosphorus concentrations and rainfall, surface-water discharge, and groundwater levels in Big Cypress Seminole Indian Reservation, Florida, 2014–16

The Seminole Tribe of Florida (the Tribe) is partnering with the U.S. Environmental Protection Agency to develop a numeric phosphorus criterion for the 52,000-acre Big Cypress Seminole Indian Reservation (BCSIR), which is located downgradient of the Everglades Agricultural Area, and of other public and private lands, in southeastern Hendry County and northwestern Broward County in southern Florida. The U.S. Geological Survey (USGS), in cooperation with the Tribe, used water-quality data collected between October 2014 and September 2016 by the Tribe and the South Florida Water Management District (SFWMD), along with data from rainfall gages, surface-water stage and discharge gages, and groundwater monitoring wells, to (1) examine the relations between local hydrology and measured total phosphorus (TP) and orthophosphorus (OP) concentrations and (2) identify explanatory variables for TP concentrations. Of particular concern were conditions when TP exceeded 10 parts per billion (ppb) (0.01 milligram per liter [mg/L]) given that the State of Florida and the Miccosukee Tribe of Indians Alligator Alley Reservation (located downstream of the BCSIR) have adopted a 10-ppb maximum TP criterion for surface waters. From October 2014 to September 2016, the Tribe collected 47–52 samples at each of nine water-quality sites for analysis of TP and OP, except at one site where 28 samples were collected. For all sites sampled, concentrations of TP (as phosphorus [P]) ranged from less than 0.002 mg/L (2 ppb) to a maximum of nearly 0.50 mg/L (500 ppb), whereas concentrations of OP (as P), the reactive form of inorganic phosphorus readily absorbed by plants and (or) abiotically absorbed, ranged from less than 0.003 mg/L (3 ppb) to a maximum of 0.24 mg/L (240 ppb). The median and interquartile ranges of concentrations of TP and OP in the samples collected in 2014–16 by the Tribe were similar to the median and interquartile ranges of concentrations in samples collected by the SFWMD at nearby sites during the same period. Differences in concentrations can likely be explained by differences in sample collection methods, sampling locations, sample collection time, and the hydrology during sampling or by the number of samples collected. A major limitation of this study was the short duration of sample collection, which covers a limited range of hydrologic conditions within the BCSIR. The effect of surface-water and groundwater hydrologic conditions on TP and OP concentrations was assessed by using rainfall data and surface-water stage and discharge records. The highest TP and OP concentrations occurred during peak surface-water flows in the canals following long dry periods. Concentrations of TP and OP increased internal to the BCSIR in the western half of the BCSIR during wet periods, but increased concentrations tended to lag behind rainfall events, likely because control structures upstream of sampling sites do not release flows until the water levels in the canals reach predetermined levels. This pattern may indicate that bed sediments in the canals contain high concentrations of phosphorus that becomes resuspended during high flows or that phosphorus salts that had accumulated on dry land during dry periods are carried into the canals by runoff. The largest TP spikes usually occurred at the beginning of high-flow events, but then quickly tapered off even when flows remained high. Groundwater flows were assessed in the BCSIR by using groundwater level observations from two preexisting USGS monitoring well clusters, each characterized by a shallow well installed in the surficial aquifer system and a deeper well installed in the intermediate aquifer system. Groundwater levels were evaluated with respect to surface-water levels and discharge in the BCSIR during the period of surface-water sampling. During dry conditions water levels in canals were often higher than groundwater levels in the surficial aquifer, indicating the potential for surface water to recharge the surficial aquifer. During wetter conditions, this trend reversed, and there was potential for shallow groundwater discharge into the canals. From October 2014 to September 2016, concentrations of TP tended to decrease as surface-water inflows moved across the BCSIR from north to south. In both the western and eastern halves of the reservation, the mean concentration of TP was lower in the surface-water outflows from the BCSIR than in the inflows. The mean concentration of TP in the inflows to the western reservation was 0.04 mg/L (40 ppb), whereas the mean concentration of TP in the outflows was 0.03 mg/L (30 ppb). In the eastern reservation, the mean concentration of TP in the inflows was 0.07 mg/L (70 ppb), whereas the mean concentration of TP in the outflows was 0.04 mg/L (40 ppb). TP and OP concentrations were evaluated relative to other water-quality parameters, including turbidity, suspended solids, nitrate plus nitrite, dissolved oxygen, pH, and specific conductance, to determine if any relations existed between TP and other variables. Weak relations were indicated for turbidity and suspended solids at two sites, which indicates that there may be a relation of increased TP to mobilization of sediment.

Florida↗
USGS2019

Groundwater-flow model and analysis of groundwater and surface-water interactions for the Big Sioux aquifer, Sioux Falls, South Dakota

The city of Sioux Falls, in southeastern South Dakota, is the largest city in South Dakota. The U.S. Geological Survey (USGS), in cooperation with the city of Sioux Falls, completed a groundwater-flow model to use for improving the understanding of groundwater-flow processes, estimating hydrogeologic properties, and analyzing groundwater and surface-water interactions for the Big Sioux aquifer in the model area. The model area includes the Big Sioux aquifer and the underlying hydrogeologic units from Dell Rapids, South Dakota, to the confluence of the Big Sioux River and the outlet of the Sioux Falls Diversion Channel in eastern Sioux Falls, S. Dak. The Big Sioux aquifer is the primary aquifer in the model area and the focus of the groundwater-flow model. The Big Sioux River is the largest stream in the model area and is in hydraulic connection with the Big Sioux aquifer. A conceptual model for the area was constructed and includes a characterization of the hydrogeologic framework, analysis and construction of potentiometric surfaces, and summary of estimated water budget components in the model area. The primary hydrogeologic units in the model area consist of (1) the Big Sioux aquifer, (2) a glacial till confining unit, and (3) bedrock aquifers (Split Rock Creek and Sioux Quartzite aquifers). Sources of groundwater recharge included infiltration of precipitation, stream seepage, and groundwater exchanges among the hydraulically connected Big Sioux aquifer, glacial till confining unit, and bedrock aquifers. Groundwater losses included evapotranspiration, groundwater discharge to streams, and groundwater withdrawal to supply water-use needs. A numerical groundwater-flow model (numerical model) was constructed and was used to simulate all aspects of the conceptual model for predevelopment (steady-state) and time-varying (transient) monthly conditions for 1950–2017. The numerical model was constructed using the USGS modular hydrologic simulation program, MODFLOW–6, and was calibrated using the Parameter ESTimation software, PEST++. The transient numerical model was calibrated for steady-state and transient monthly conditions for 1950–2017. Calibration targets were observations of hydraulic head, changes in hydraulic head, monthly mean streamflow (as a rate), and cumulative monthly stream discharge (as a volume). Parameters adjusted during model calibration were horizontal and vertical hydraulic conductivity, specific storage, specific yield, recharge and evapotranspiration multipliers, and streambed hydraulic conductivity. Horizontal and vertical hydraulic conductivity were estimated at pilot points distributed within the model area; specific storage and specific yield were assigned to uniform values in each layer in the model area; recharge and evapotranspiration multipliers were assigned uniformly for every stress period in the numerical model; and streambed hydraulic conductivity values were assigned uniformly between stream confluences. The final calibrated parameter values of horizontal and vertical hydraulic conductivity, specific yield, specific storage, streambed hydraulic conductivity, recharge, and evapotranspiration were considered reasonable for the hydrogeologic materials and conditions in the model area for 1950–2017. Overall, simulated hydraulic head altitudes had a linear regression coefficient of determination (R 2 ) of 0.48. Hydraulic head altitude residuals for the glacial till confining unit and bedrock aquifers were typically greater in magnitude when compared to residuals in the Big Sioux aquifer, but simulated hydraulic head altitudes in the Big Sioux aquifer compared favorably with mean observed hydraulic head altitudes and had a linear regression R 2 of 0.93. Simulated streamflow hydrographs matched the general trends of observed increases and decreases in streamflow for USGS streamgages 06482000 (Big Sioux River at Sioux Falls, S. Dak.) and 06482020 (Big Sioux River at North Cliff Avenue at Sioux Falls, S. Dak.), but larger streamflows were overestimated at the first streamgage and underestimated at the second streamgage. The numerical model reasonably estimated cumulative monthly stream discharge for the first 10–15 years of available streamflow records at both USGS streamgages. After the first 10–15 years of available streamflow record, cumulative monthly stream discharge was closely estimated for USGS streamgage 06482000 and underestimated at USGS streamgage 06482020. Composite sensitivities without regularization were calculated by PEST++ for the calibrated numerical model parameters and were averaged by parameter group. The parameter group with the highest mean composite sensitivity was the recharge multiplier parameter group. Model simplifications, assumptions, and limitations were necessary for construction of the conceptual and numerical models and for calibration efficiency. Spatial simplification of hydraulic properties could cause the numerical model to misrepresent reactions to changes in localized stresses, such as additional demands for groundwater withdrawal. The numerical model was temporally discretized into monthly periods and required scaling daily rates into representative monthly rates for model input and calibration targets. Based on the comparison between the observed and simulated groundwater levels, monthly mean streamflow and cumulative monthly stream discharge, and general groundwater distribution and flow, the numerical model favorably simulated the flow in the Big Sioux aquifer. Eventual capture was calculated in the model area using a steady-state numerical groundwater-flow model. The eventual capture map shows areas of higher streamflow capture adjacent to the Big Sioux River north of the city of Sioux Falls and along the lower part of the Sioux Falls Diversion Channel, and areas of lower streamflow capture along aquifer boundaries and near the southern Sioux Quartzite barrier. The timing of capture was determined using a transient numerical groundwater-flow model to determine the likely captured water sources for 30 years of groundwater withdrawal at three hypothetical wells using three continuous withdrawal rates (112.5, 450.0, and 900.0 gallons per minute). Supply for all three hypothetical wells became capture-dominated after only a short period of continuous withdrawal. Capture stabilized after about 10–15 years for well A, and after 20–25 years for well B, and after about 10–15 years for well C. The groundwater-flow model is a suitable tool to use for improving the understanding of groundwater-flow processes, estimating hydrogeologic properties, and analyzing groundwater and surface-water interactions for the Big Sioux aquifer near Sioux Falls, S. Dak. The numerical model can be used to simulate hydrologic scenarios, advance understanding of groundwater budgets, compute system response to stress, and determine likely sources of water supplied to wells.

South Dakota↗