Purpose and Scope

The purpose of this report is to describe the development of a three-dimensional (3D) model created with GeoScene3D (I-GIS, 2021) to visualize the subsurface, particularly the Dakota Sandstone, in western Sarpy County (fig. 1). This 3D model incorporates geologic, groundwater-level, and water-quality data and can be used by PMRNRD managers and staff to (1) identify areas where groundwater and surface water are hydrologically connected, (2) visually understand existing (and potential) areas of groundwater-quality concern, and (3) identify areas where additional groundwater-level and water-quality monitoring will be beneficial to the PMRNRD. A corresponding USGS data release includes raster and contour files of the top of the Dakota Sandstone and the top of the Pennsylvanian-age units that support the GeoScene3D model (Cherry and others, 2023).

Geologic Setting

The surficial Quaternary-age Pleistocene and Holocene geologic units in eastern Nebraska consist of complex sequences of clay, silt, till, sand, and gravel overlying bedrock units. These materials are capped by loess deposits in many locations throughout eastern Nebraska. The geologic and hydrogeologic features from Verstraeten and Ellis (1995), Jorgensen and others (1993), Helgesen and others (1993), McGuire and others (2012), and Miller and Appel (1997) are summarized in figure 2. The surficial deposits are predominantly glacial (clay, silt, and gravel), alluvial (clay, silt, sand, and gravel), and eolian (wind deposited silt) deposits of Quaternary age. These deposits are unconsolidated and range from relatively permeable sand and gravel to poorly permeable silts and clays.

The surficial deposits directly overlay consolidated bedrock that consists of sedimentary rocks of Cretaceous and Pennsylvanian age (fig. 2). The rocks of Cretaceous age consist primarily of massive to interbedded sandstone/sand with ironstone and shale; some zones are argillaceous to slightly sandy and are collectively known as the Dakota Sandstone (Condra and Reed, 1959). In eastern Nebraska, the Dakota Sandstone reflects a complex pattern of deposition and erosion consistent with the alluvial, deltaic, and shoreline sedimentary environments of the eastern margin of the Cretaceous Western Interior seaway (Karl, 1976). The Dakota Sandstone makes up the oldest Cretaceous-age rock in western Sarpy County and is covered by Quaternary-age alluvium and glacial till except where the Dakota Sandstone crops out along the bluffs near the Platte River to the south (Burchett and others, 1975).

Unexposed older sedimentary rocks of Mississippian, Devonian, Silurian, Ordovician, and Cambrian ages consist mostly of dolomite, limestone, shale, and sandstone. Most rocks that compose the confining unit of Paleozoic age in the PMRNRD are Pennsylvanian in age. They are generally impermeable shale and dense limestone. None of these formations can be considered a significant source of water (Verstraeten and Ellis, 1995). Undifferentiated igneous, metamorphic, and sedimentary rocks form the basement units (Verstraeten and Ellis, 1995). Within these bottom units lies the Western Interior Plains aquifer system, which is a confined saline aquifer with a large (1,000–1,500 feet thick) saturated thickness (Jorgensen and others, 1993).

The geologic history relevant to this study began 112 million years ago during the Early Cretaceous period when the deposition of the Dakota Sandstone occurred. During this deposition, the environment was a coast/estuary system bisected by meandering rivers. As the mouth of the rivers rose with the formation of the Cretaceous seaway, the river gradients dropped, causing sediment deposition. The point of deposition slowly moved eastward as the seaway rose, and the Dakota Sandstone Formation was deposited along the coast (Condra and Scherer, 1939). The repeated advance and retreat of glaciers, which began about 2 million years ago, resulted in a complex framework of largely Pleistocene-age sediments overlying the Dakota Sandstone (Wayne, 1985). The patterns that formed the shape of the current Dakota Sandstone are related to erosional episodes including glaciation, fluvial processes, and recent erosion in the Platte River, Buffalo Creek, and Springfield Creek drainages (Wayne, 1985). In Sarpy County, the Dakota Sandstone is discontinuous (fig. 1). The overlying Quaternary layers are thin or absent (Verstraeten and Ellis, 1995; McGuire and others, 2012), making the Dakota Sandstone an important groundwater resource.

Previous Studies

Several groundwater and geologic studies have been completed in Sarpy County (Verstraeten and Ellis, 1995; Korus and others, 2012; McGuire and others, 2012; Cannia and others, 2017; Hobza and Flynn, 2021). These studies have primarily been groundwater-quality focused. Nitrate is a common groundwater contaminant throughout Nebraska, and the PMRNRD has been monitoring nitrate concentrations extensively since 1992 by sampling domestic, irrigation, municipal, and monitoring wells. Verstraeten and Ellis (1995) reviewed groundwater-quality data to describe the water quality of the principal aquifers in the PMRNRD. Their report stated that the highest nitrate concentrations were in the Upland area alluvial aquifers and the Dakota aquifer. They also tested for several pesticides, but none were detected at concentrations greater than the U.S. Environmental Protection Agency (EPA) maximum contaminant level (MCL) for drinking water (U.S. Environmental Protection Agency, 2022). McGuire and others (2012) and Hobza and Flynn (2021) also examined groundwater quality and groundwater age in the primary and secondary bedrock aquifers in the area. Groundwater-recharge dates ranged from older than 1940 in samples from wells screened in the Missouri River Valley alluvial aquifer to as recent as the early 1980s in samples from wells screened in the Dakota aquifer (McGuire and others, 2012). Hobza and Flynn (2021) detected mixed-age (older and modern, post-1950) and modern-age water in the Dakota aquifer in western Sarpy County, which was younger than water from other Dakota aquifer samples taken outside of Sarpy County. During 2012, the USGS, in cooperation with the PMRNRD, sampled domestic wells and determined that many of the wells screened in the Dakota aquifer had elevated nitrate values (greater than 5 milligrams per liter [mg/L]) and some had nitrate values that exceeded the EPA nitrate MCL (10 mg/L) for drinking water (U.S. Geological Survey, 2021b; U.S. Environmental Protection Agency, 2022).

In 2017, an AEM survey was completed in western Sarpy County to improve understanding of the extent, thickness, and characteristics of the Dakota Sandstone, overlying Quaternary-age units, and other bedrock units of Pennsylvanian age or older. The AEM data were collected in a grid near the communities of Springfield and Gretna, and lines were collected about every 300 m (Cannia and others, 2017). The PMRNRD requested these surveys to improve the understanding of the hydrogeologic framework so that they could better manage the drinking water supplies in that area.

To illustrate the geologic units in eastern Nebraska, geologic cross sections were created from test-hole and other borehole data. One of these cross sections was developed for a section near the western boundary of Sarpy County. These cross sections were developed primarily to characterize the unconsolidated Quaternary deposit and the bedrock deposits directly below them (Korus and others, 2012).

Airborne Electromagnetic Surveys

AEM surveys are a method for measuring electrical resistivity in subsurface material. Lower electrical resistivity generally corresponds to subsurface materials with higher porosity and smaller grain sizes, whereas higher electrical resistivity generally corresponds to subsurface materials with larger grain sizes. These measurements can be affected by the presence of clays, dissolved solids, or other more conductive materials. When these AEM surveys are used in conjunction with data from lithologic logs, the two data sources can be used to identify and characterize different subsurface geologic formations (Hobza and Steele, 2020).

AEM surveys are usually done using either time-domain electromagnetic systems or frequency-domain electromagnetic systems. The AEM surveys in Sarpy County used a time-domain electromagnetic system called SkyTEM304M (SkyTEM, 2022). For this system, a helicopter with a large wire loop with multiple turns attached to the underside by a long cable is flown. An electrical current is passed through the wire loop, creating an electromagnetic field. This field is turned off and on, and the electromagnetic response of subsurface materials is measured by receiver coils connected to the loop. These measured magnetic responses are recorded and can be used to create a model of subsurface resistivity. A more indepth explanation of these methods is documented in Fitterman and Labson (2005) and Cannia and others (2017).

The AEM data collection for Sarpy County was completed in 2016 (Cannia and others, 2017). Individual flight lines were 16–20 kilometers (km) in length for east-west lines and 13–16 km for north-south lines. North-south lines were stationed about 1 km apart whereas east-west lines were stationed about 200–250 m apart (fig. 3). These lines were positioned in areas identified as of primary concern for the PMRNRD in relation to water quality and quantity (Cannia and others, 2017). A few additional AEM flight lines were flown in the northwestern part of the study area near Gretna, but the distance between these lines was much greater (1.77 km in the east-west direction and 1.62 km in the north-south direction). Flight lines in the northwestern part of the study area were less dense because development of additional domestic wells was less prevalent in that area, and the area has more urban-related flight obstructions.

Digital Elevation Model

A 10-m DEM of the area was downloaded from the USGS National Map web service (U.S. Geological Survey, 2021a). This DEM was then converted to an ASCII file and uploaded in GeoScene3D.

Geologic Test Holes

The University of Nebraska Conservation and Survey Division (CSD) drills test holes around the State of Nebraska to provide detailed lithologic and stratigraphic information (University of Nebraska, 2022). Of these CSD test holes, 23 are within the study area (table 1 and fig. 3). These test holes were used by Cannia and others (2017) to compare resistivity values to collocated test-hole data to guide AEM interpretation.

In addition to these test holes, driller logs for wells registered with the Nebraska Department of Natural Resources (Nebraska Department of Natural Resources, 2021) were compiled and inspected. Driller logs tend to be less detailed than CSD test-hole logs, but in many cases, the driller logs provide adequate descriptions of the various materials detected during well installation. Most of the driller logs were associated with domestic wells, but logs for other wells (irrigation, industrial, commercial, observational, monitoring, geothermal, and livestock) also were used (fig. 3). Any well logs that had sufficient information for identifying individual stratigraphic layers and that were from wells in or near the study area were input into GeoScene3D (I-GIS, 2021).

Groundwater-Level Data

Groundwater-level data in the study area were limited and temporally disconnected because systematic groundwater-level monitoring in the area is minimal. The data available were downloaded from the USGS National Water Information System (NWIS) database (U.S. Geological Survey, 2021b). The measurement dates for the groundwater levels in NWIS ranged from January 8, 2020, to August 23, 2021 (table 2).

The USGS used the available groundwater measurements in NWIS and a 10-m DEM (U.S. Geological Survey, 2021a) to create an interpolated water surface for western Sarpy County for input into GeoScene3D (I-GIS, 2021). Because groundwater-level data were not spatially comprehensive or precisely associated with the aquifer(s) being measured, the resulting interpolated water surface should be considered a generalized estimate. Because all the groundwater-level data were much coarser than the 10-m DEM (U.S. Geological Survey, 2021a) incorporated into GeoScene3D, adjustments to the groundwater level at land surface in stream valleys and other low-lying areas were needed.

Groundwater-level altitude for each groundwater-level measurement was calculated by subtracting depth to water, in meters, from land surface altitude at the well, in meters, from the project DEM. An additional source of groundwater-level data in western Sarpy County was created by establishing a network of DEM control points where the groundwater level was interpreted to be at land surface based on aerial photography (Esri, 2021). The network of DEM control points was established within the Platte River Valley at river, pond, and sand pit lake locations. Additional control points were added near the edge of the Platte River Valley for tributaries to the Platte River, including Buffalo and Springfield Creeks. The 5-m groundwater-level contours were manually drawn in ArcGIS (Esri, 2017) using the groundwater-level measurements and DEM control points as guides. The contours and groundwater-level measurements were converted to a raster of the groundwater-level surface using the Topo to Raster command in the ArcGIS toolbox (Esri, 2017). The resulting raster was checked against the project DEM to ensure that the groundwater-level surface did not exceed the DEM in locations other than the Platte River Valley or at the lower end of major drainages. It should be noted that localized errors in the groundwater-level surface are likely because of the temporal variation of the groundwater-level measurements, uncertainty about the aquifer measured, and in some cases, the large distances between known groundwater levels. If additional groundwater-level data become available in the future, those data could be used to refine the current (2024) groundwater-level surface, which was incorporated into GeoScene3D (I-GIS, 2021).

Nitrate Sample Results in Groundwater

Samples have been collected by the USGS for nitrate analysis at various wells within the study area. The samples considered in this study were collected between July 29, 1992, and September 26, 2020 (U.S. Geological Survey, 2021b). Only wells that had well depth information were included for this report so that each result could be associated with a specific geologic layer. If a well was sampled more than once, the most recent sample result was retrieved. The data for the selected samples were compiled and incorporated into GeoScene3D (I-GIS, 2021) to show nitrate levels in reference to the various geologic layers (fig. 4). Most of these samples were collected from domestic wells. The wells were color coded in GeoScene3D to identify various magnitudes of nitrate concentrations. Wells with concentrations greater than the EPA MCL of 10 mg/L were coded as red, wells with concentrations of greater than 7.5–10 mg/L were coded as orange, wells with concentrations of greater than 5–7.5 mg/L were coded as yellow, and wells with concentrations of 0–5 mg/L were coded as green (fig. 4). Additional nitrate sample results for wells that had insufficient well-construction information may be available in NWIS (U.S. Geological Survey, 2021b) by searching for wells in Sarpy County.

Hydrogeologic Profile 16120201

Just west of the community of Springfield is a south to north profile of about 11,500 m along AEM line 16120201 (figs. 4 and 5). Groundwater flow is generally north to south, and the groundwater level is generally 5–10 m above the top of the Dakota Sandstone. A small stream is fed by groundwater around 2,300 m on the profile line (fig. 5). The vadose zone in the northern section consists of less permeable glacial till (nonaquifer and marginal aquifer in fig. 5), whereas in the southern section, the vadose zone consists of more permeable aquifer and coarse aquifer material. The more permeable vadose zone in the southern part of the profile allows for a higher likelihood of nitrate contamination to the aquifer, which may be the reason for the elevated nitrate concentration (8.36 mg/L) in well U232 (fig. 5). The most recent nitrate sample results from a well (DO22B) drilled upgradient from U232, beneath the less permeable glacial till, had a lower nitrate concentration (6.2 mg/L) than U232 (fig. 5).

Hydrogeologic Profile 201001

Profile 201001 stretches about 14,000 m from north to south in the eastern third of the study area (figs. 4 and 6). The north boundary begins just west of Papillion, and it traverses south; the southwestern segment of the profile covers a part of the Platte River Valley. Groundwater flow in this area is generally north to south towards the Platte River Valley. The groundwater level ranges from 5 m below the top of the Dakota Sandstone to 10 m above it. Along this profile, the Dakota Sandstone is consistently around 25 m thick until it approaches the Platte River Valley alluvium (about 9,200 m along the profile) where it has been eroded and is not present. The Quaternary-age deposits along this profile are more permeable than the previous profile (fig. 5) and become thin in the stream valleys (for example near 1,200 m, 3,800 m, and 4,400 m in fig. 6). Another detail of interest that can be observed in this profile is a small stream (not shown in fig. 6) intersecting the groundwater level around the 6,500-m location. The profile indicates that the tributary has eroded some of the Dakota Sandstone in that area. Within the Platte River Valley segment of this profile, the AEM data indicate that the alluvial aquifer in this area has a coarse layer that is as thick as 20 m in some places. Based on visual inspections of other profiles within the Platte River Valley, there seems to be fine and coarse material throughout the area. Surficial coarse material is present to the south (about 10,500 to 11,500 m), indicating likely interaction between the Platte River Valley alluvial aquifer and Platte River surface water, especially during higher streamflows.

Two wells with nitrate sample results are near profile 201001. The well near Papillion (D131) had high nitrate concentrations (12.0 mg/L). Because this well is screened in the Dakota aquifer and the overlying vadose zone is composed of low-permeability units, this may be an area of the Dakota aquifer that could be investigated further. The well towards the southern end of the profile (P022) is screened in the Platte River Valley alluvial aquifer and had a lower nitrate concentration of 6.7 mg/L. The vadose zone in this area has a thin low-permeability layer, which indicates that this may be an area susceptible to surface contamination. The lack of high nitrate concentrations in this well indicates that the thickness of low-permeability layers in the vadose zone is not the only predictive factor for nitrate concentrations but that other factors such as land use and geochemical conditions may be important.

Hydrogeologic Profile 162601

Profile 162601 is about 10,000 m and runs north to south from the southern end of Papillion to near the southern border of Sarpy County. A part of this hydrologic profile underlies the Papillion area (figs. 4 and 7). The groundwater level along this profile is generally 5–10 m above the top of the Dakota Sandstone. Groundwater flow in this area is generally north to south towards the Platte River Valley. At the Platte River Valley, the groundwater level is within 5 m of the surface. At the northern end of the profile, the vadose zone consists of a substantial amount (30–35 m) of less permeable glacial till. Where it is present, the Dakota aquifer in that area is therefore more protected from infiltration of surface contaminants. Towards the southern end of the profile, the Dakota Sandstone is not present, and the Platte River Valley alluvium directly overlies Pennsylvanian-age units. Both wells with nitrate information near this profile (P022 and P022C) are screened within the Platte River Valley alluvial aquifer. Nitrate concentrations in these wells are elevated (from 5 to 10 mg/L) but are not above the EPA MCL. At these well locations, little low-permeability material is in the vadose zone to prevent surface contamination into the groundwater. Coarse surficial material to the south of these wells indicates that, like profile 201001, water within the Platte River Valley alluvial aquifer may be mixing with Platte River surface water, especially during high streamflow events.

Hydrogeologic Profile 141501

Profile 141501 is about 7,800 m long, oriented west to east, and follows AEM line 141501. The profile starts within the Platte River Valley and extends to about 2,500 m east of Gretna, Nebraska, into the loess uplands (figs. 4 and 8). Part of this profile intersected with the town of Gretna, so the AEM coverage was less dense than most of the other profiles, which were rural. Groundwater flow in this area is generally east to west towards the Platte River Valley. Within the Platte River Valley, the groundwater level is within 2–3 m of the surface.

At the western end of the profile (from 0 to about 3,000 m in fig. 8), the Dakota Sandstone is overlain by the Platte River Valley alluvial aquifer. Farther east along the profile (near 3,250 m in fig. 8), the Dakota Sandstone thins out as the profile line moves towards the eroded southern edge of the northern primary remnant (inset map in fig. 8). The Dakota Sandstone eventually disappears completely around 5,800 m (fig. 8). As the profile moves east away from the Platte River Valley and up into the loess hills, the Quaternary deposits thicken and transition from Platte River alluvial to Upland alluvial. Beginning around 3,800 m, the Quaternary layer is overtopped by a sizeable amount (about 30–40 m) of nonaquifer material (likely glacial till and eolian loess). Nitrate sample results from a well cluster just outside of Gretna (well identifiers Ash03108, Ash03162, Ash03187, and Ash03250) indicate that nitrate levels are not greater than 5.3 mg/L, which likely is related to the large amount of nonaquifer material in the upper segment of this profile. Sample results from another well outside of Gretna, Nebr., about 750 m north of the profile line and about 2 km east of the well cluster (U033; fig. 4) also had low nitrate concentrations (less than 0.05 mg/L). Based on the AEM interpretation, the Dakota aquifer is not present in the area of U033, which indicates this well was screened in the Upland area alluvial aquifers. Although it is not within the Dakota aquifer, the AEM data indicate that the Dakota aquifer and Upland area alluvial aquifers are connected in this area. One well near this profile (P004A; fig. 8) is screened in the Platte River Valley alluvial aquifer and also had low nitrate concentrations (less than 0.05 mg/L). The AEM data indicate that the Platte River alluvial aquifer is connected to the Dakota aquifer in this area.

These profiles provide examples of how the GeoScene3D (I-GIS, 2021) model of western Sarpy County can provide groundwater managers with a tool to integrate detailed hydrogeologic, groundwater-level, and water-quality information. This model could be used to select areas for future groundwater studies, such as targeting areas where the Dakota aquifer is absent or where the Dakota aquifer is intersecting surface water features. The model could be used to identify areas that are more susceptible to contamination, such as where the Quaternary-age deposits are thin or more permeable. The model can allow users to visualize what wells may be affected by changes in surrounding water uses by examining flow paths. If contaminant concerns exist, such as high nitrate concentrations, the model could be used to guide further investigation and mitigation efforts. Also, new data can be added into the model to enhance understanding of the groundwater resources in western Sarpy County.