The U.S. Geological Survey, in cooperation with the U.S. Air Force Civil Engineering Center, investigated the use of surface geophysical methods to delineate the top of the Cretaceous Pierre Shale along survey transects in selected areas within and near Ellsworth Air Force Base, South Dakota. Two complementary geophysical methods-electrical resistivity and passive seismic-were used along 26 co-located transect surveys within and near Ellsworth Air Force Base for a total of 12.7 line-kilometers. Electrical resistivity results were analyzed using EarthImager2D electrical resistivity tomography processing and inversion software. Two-dimensional earth models showing the electrical properties of the subsurface were evaluated by directly comparing the high and low subsurface resistivity values to a surficial geologic map and nearby wells with driller logs. Passive seismic data were analyzed using the horizontal-to-vertical spectral ratio method to determine the depth to the Pierre Shale at each survey point. The electrical resistivity and passive seismic results were compared to driller logs from nearby wells to delineate the top of the Pierre Shale. The depth to the Pierre Shale along the transects ranged from about 2.4 to 20.3 meters, and mean and median depths were about 9.2 and 9.0 meters, respectively. The elevation of the Pierre Shale and thickness of unconsolidated deposits generally increased with land-surface elevation from south to north; however, some transects displayed topographically high and low areas that sometimes did not correlate with land-surface topography and may affect local groundwater flow.
Samples from rain collectors were collected when enough daily precipitation accumulated in the collectors. Sampled precipitation events during the study period were in May, June, July, August, and September 2018. Physical properties of sampled water-temperature, pH, and specific conductance-were measured in the field. Water samples were analyzed for stable isotopes of oxygen and hydrogen and for chloride concentration. Recharge rates for aquifers supplying springs were determined using precipitation volume and chloride concentrations for a 12-day period before the sample-collection date. Multivariate statistical analysis methods used on water-chemistry data included principal component analysis, cluster analysis, and end-member mixing analysis. Water composition was used to determine the spring type and contributing aquifers for 11 springs in the North and South Units of Theodore Roosevelt National Park from analyses of water-chemistry data between May and September 2018. In the North Unit, Achenbach Spring was classified as a filtration spring with water from an unconfined part of the upper Fort Union aquifer and infiltration of precipitation. Hagen Spring, Mandal Spring, and Stevens Spring were classified as contact springs supplied by semiconfined parts of the upper Fort Union aquifer. Overlook Spring at one time may have been a natural spring or seep but now is a developed spring that behaves like a flowing artesian well completed in a confined part of the upper Fort Union aquifer. In the South Unit, six springs were classified into two spring types: filtration and contact springs. Boicourt Spring and Sheep Butte Spring were classified as filtration springs that have water supplied by unconfined parts of the upper Fort Union aquifer and infiltrated precipitation. Big Plateau Spring, Lone Tree Spring, Sheep Pasture Spring, and Southeast Corner Spring were classified as contact springs that receive waters from a semiconfined part of the upper Fort Union aquifer. from the ground and flows or forms pools (Bryan, 1919). Springs commonly form when the side of a hill, a valley bottom, or other excavations intersect flowing groundwater at or below the local water table (U.S. Geological Survey, 2019). Seeps are a type of spring in which the water flows from pores in the ground over considerable areas (Bryan, 1919). Seeps occur from a slow movement of water through Unconfined aquifer water table Unconfined aquifer water table Rotational slump or landslide Low permeability unit Low permeability unit EXPLANATION Spring Groundwater level Groundwater flow direction Filtration Confined aquifer hydraulic head Confined aquifer hydraulic head spring or seep Flowing artesian spring Upper contact spring Lower contact spring Figure 3. Types of springs in the study area.
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and historical spills and releases in the area. The spatial and temporal variability in the BTEX compounds and naphthalene concentrations from samples collected from 2014 to 2018 do not indicate a consistent pattern of subsurface flow or contaminate movement that would be expected if a contaminant plume migrated with the flow and movement of groundwater.Gasoline-range organic and diesel-range organic compounds were detected in most of the groundwater samples collected in the study area around Fuels Area C in 2014-18; however, concentrations were often less than the laboratory reporting level. Median gasoline-range organic compound concentrations were greater than the laboratory reporting level at sites 1, 5, 9, 10, and 11. The highest concentrations of gasoline-range organic and diesel-range organic compounds generally were observed in samples collected from sites 10 and 11. Gasoline-range organic compound concentrations ranged from 1,500 to 9,700 micrograms per liter at site 10 and from less than 100 to 13,000 micrograms per liter at site 11. Diesel-range organic compound concentrations ranged from 9,600 to 55,000 micrograms per liter at site 10 and from 560 to 7,300 micrograms per liter at site 11. Study Area DescriptionFuels Area C (fig. 1) was constructed in about 1953 and has operated nearly continuously since construction (Tetra Tech Incorporated, 2008). The area is west of Ellsworth Street and covers about 0.3 square kilometer (79 acres; fig. 1). A small unnamed ephemeral stream is east of Fuels Area C, and Gateway Lake (not shown on figure) is northeast of the area. Prairie Ridge Golf Course is about 265 meters (m; 870 feet [ft]) east of Fuels Area C.
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