Hydrogeology

Verstraeten and Ellis (1995) identified five primary aquifers within the PMRNRD—the Platte River alluvial, Elkhorn River alluvial, Missouri River alluvial, upland alluvial, and the Dakota Sandstone (fig. 1). These aquifers are the surficial aquifers in the respective area, except generally the Dakota Sandstone (Verstraeten and Ellis, 1995). In the current PMRNRD groundwater management plan (PMRNRD, 2018), the Platte River alluvial and Elkhorn River alluvial aquifers are labeled as a single aquifer: the Platte/Elkhorn alluvial aquifer. However, the distinction of the Platte River alluvial aquifer and the Elkhorn River alluvial aquifer will be used in this report to facilitate comparison to historical groundwater quality data (Verstraeten and Ellis, 1995; McGuire and others, 2012), as discussed in the “Nitrate-N Time Series Analysis, 1992–2020” section of this report.

Land and Water Use

Land use within the PMRNRD is predominantly agriculture (cropland and rangeland) and includes minimal urban land (mainly in the Omaha metro area) (McGuire and others, 2012). Urban areas within towns or cities generally have municipal wastewater collection and treatment systems, and rural regions are dominated by individual septic systems to treat household wastewater. Groundwater and surface water are substantial water sources within the PMRND, and overall groundwater usage is higher than surface-water usage for all sources other than power generation (McGuire and others, 2012).

Groundwater Sample Collection

All groundwater samples were collected following the USGS National Field Manual for the Collection of Water-Quality Data (NFM; USGS, variously dated). All sampling equipment was cleaned at the Nebraska Water Science Center laboratory (Lincoln, Nebraska) before sampling. Cleaning for all sampling equipment followed a sequence of nonphosphate laboratory soap and tap water solution soak, tap water rinse, deionized water rinse, methanol spray, and organic-free blank water final rinse as specified in the NFM. On the day of CEC sampling, the two-person crews refrained from using any potential contaminants, such as caffeine or nicotine, to avoid contamination of the samples during collection.

Before samples were collected at each location, water was purged from the well to ensure the sample collected was representative of the aquifer. The pumps for monitoring wells owned by the PMRNRD that were used for CEC and regular sampling were dedicated stainless steel submersible positive pressure Grundfos pumps, which were already installed in the wells. Other wells included in the regular sampling used either a spigot or hose fitting from an existing pump in the selected domestic, irrigation, industrial, and monitoring wells or, for monitoring wells without a dedicated pump, a portable pump. Plastic splitters with Teflon O-rings were fitted to the spigots on the wells (the spigots were brass for the PMRNRD-owned wells and varied material on other wells), which then split the discharge into two streams. One side of the splitter accepted a garden hose running to a bucket for collection of physical properties of groundwater, including pH, specific conductance, DO, water temperature, and turbidity. The physical properties pH, specific conductance, DO, and water temperature were measured with a multiparameter meter (YSI ProDSS, YSI, Inc., Yellow Springs, Ohio); turbidity was measured with a portable turbidimeter (Hach 2100P, Hach, Loveland, Colorado); and all physical properties were recorded according to guidance in the NFM. The other side of the splitter held a Teflon adapter to a polytetrafluoroethylene tube running to a lab truck for collection of groundwater samples. Once the wells were purged, hormone samples were collected (Foreman and others, 2012) by filtering the water through a 0.45-micrometer (μm) filter into a 0.5-liter (L) high-density polyethylene bottle. Pharmaceutical samples were collected using a syringe to capture unfiltered sample water, and then 10 milliliters (mL) of sample water were processed through a 0.45-μm filter into a 20-mL baked glass container. Nitrate samples were collected using the same field setup and were filtered through a 0.45-μm filter into a 125-mL amber bottle. Iron samples were filtered (though the same filter used for nutrients) into a 250-mL bottle and acidified in the field with nitric acid. After collection, the samples were packed with ice and shipped overnight for analysis by the NWQL.

Laboratory Analytical Methods

All laboratory analyses were performed at the NWQL. Analytical details for pharmaceutical, hormone, and nitrate-N methods are discussed in brief and are covered in depth in the method development reports (Patton and Kryskalla, 2011; Foreman and others, 2012; Furlong and others, 2014)912. The reporting limits for some CEC analytes changed during the period of data analysis at the NWQL. For these analytes, the higher reporting limit is given in table 2.

The pharmaceutical method (Furlong and others, 2014) determines the concentration to a nanogram per liter level for 113 compounds consisting of pharmaceutical chemicals, pharmaceutical metabolites, and a few select chemically similar compounds of environmental interest, including the herbicide atrazine that is included in the analysis to compare performance to other analysis methods. Chemicals within the sample are separated using high-performance liquid chromatography, ionized using a positive ion electrospray ionization source, and detected and quantified using tandem mass spectrometers.

The hormone method (Foreman and others, 2012) determines the concentration to a nanogram per liter level of 20 steroidal hormone compounds. Chemicals within the sample are separated and isolated twice using solid-phase extraction, and eludation solvents are evaporated to dryness. The chemicals are then reacted to form ester derivatives for analysis using a gas chromatograph and tandem mass spectrometry.

The nitrate-N method (Patton and Kryskalla, 2011) determines the concentration of nitrite plus nitrate as nitrogen to a milligram per liter level and has a reporting level of 0.04 milligram per liter (mg/L). Nitrate is reduced to nitrite and then colorimetric reagents are used to determine the combined nitrite plus nitrate as nitrogen concentration using automated discrete analysis. Nitrite is also measured independently through a different method (Fishman, 1993), and nitrite as nitrogen concentration can be subtracted from nitrate plus nitrite as nitrogen to get the nitrate-N concentration. Because nitrite was detected in less than 25 percent of samples during the 2010–19 sampling, and the maximum concentration was 0.035 mg/L as nitrogen (less than the detection limit for the nitrate plus nitrite analysis), the results of the nitrate plus nitrite analysis were taken as the nitrate-N concentration without correction for nitrite; and the abbreviation nitrate-N is used herein when discussing nitrogen results.

Quality Assurance

Several QA measures were incorporated into the study design of the CEC study to ensure reliable results. Field QA samples collected for this study included replicate and blank samples and were collected during the June 2019 sampling efforts for the CEC sampling and annually for the regular sampling. Laboratory QA steps are regularly completed at the NWQL for pharmaceutical and hormone analyses, and laboratory QA checks for all methods and instrumentation are also completed regularly (Maloney, 2005). Field and laboratory QA steps and results for the regular sampling are checked regularly but are not discussed in this report.

Data Censoring Criteria

The NWQL’s analytical results are reported without qualification for values above the laboratory reporting level, and as estimated values, or “E” coded values, for concentrations between the laboratory reporting level and the detection level. Non-detects are reported as less than the laboratory reporting level (Foreman and others, 2021). Reporting and detection limits are set independently for each analyte and vary throughout time because of updated information from the ongoing laboratory QA measures, changing equipment, and new analytical techniques. The reporting limits for some CEC analytes changed during the period of data analyses at NWQL. For these analytes the higher reporting limit is given in table 2.

Nitrate Analysis Methods

For this study, nitrate-N concentrations in groundwater trends were investigated in two ways. The first was to examine changes in the median nitrate-N concentrations in each aquifer over time in 4-year periods. The second was to quantify these trends using a Mann-Kendall test. Trend analyses were performed considering the Platte River and Elkhorn River alluvial aquifers separately because differing trends were noticed between them when separated. Where multiple samples were collected at the same well within the same calendar year, the median of those concentrations was used. The median nitrate-N concentration for each year was calculated for each aquifer, and then a Mann-Kendall test (Helsel and others, 2020) was used to determine if the nitrate-N concentration had a significant trend over the period of study. The wells sampled more frequently had enough data to test each well for trends in nitrate-N concentration over time. For 10 wells where there were more than 10 years with sampling data, a Mann-Kendall test was used to determine if nitrate-N concentration significantly increased, decreased, or had no trend in the individual wells.

Physical Properties of Groundwater

Physical properties (water temperature, specific conductance, DO, pH, and turbidity) were collected in the field for most samples using a multiparameter meter. These results are included for each well sampled for selected CECs in table 5 and are summarized in table 6 for the wells sampled for selected CEC by aquifer (for wells screened in the Elkhorn and Platte River alluvial aquifers) or by well (for wells screened in the Dakota Sandstone, upland alluvial, and Missouri River alluvial aquifers). The physical properties for these wells varied minimally during the 5 months between CEC samples. Well depth and physical properties, which were collected during the regular sampling from 2010 to 2019, are summarized by aquifer in table 7. Comparing average well depths by the aquifer, the Platte and Elkhorn River alluvial aquifer samples were collected from the wells with the shallowest depth (60–62 ft below ground surface), the Missouri River alluvial aquifer samples were collected from wells with a middle depth (108 ft below ground surface), and the Dakota Sandstone and upland alluvial aquifer samples were collected from wells with the deepest depth (170–174 ft below ground surface.). Temperature, pH, and turbidity did not greatly differ between the aquifers. Specific conductance was lowest in the Platte and Elkhorn River alluvial aquifers and highest in the Missouri River alluvial aquifer. The Elkhorn and Missouri River alluvial aquifers had the lowest average DO concentrations that were just above 1 mg/L. These aquifers had many samples under the anoxic threshold of 0.5 mg/L, and anoxic conditions also were present in some samples from the other three aquifers.

Presence of Contaminants of Emerging Concern in Groundwater

Seven of the 132 contaminants in the CEC analysis were detected during this study (U.S. Geological Survey, 2020). In total, 6 different CECs were detected 11 times, and atrazine was detected 15 times among the 34 samples collected from 17 wells (table 8). Surrogate recovery percentages for the four analytes with IDS or surrogates in the laboratory QA procedure ranged from 95 to 106 percent (table 4), giving added confidence to the reliability of these results. No hormones were detected in the 18 samples collected from 9 of the 17 wells sampled for CEC analysis. The detected pharmaceuticals, not including atrazine, were found in samples from 6 of the 17 wells (fig. 2). The pharmaceutical results varied for some wells from the June to the October or November sampling event. The deep well in the Venice cluster (E-V1) had no detections in the June 2019 sample and one analyte detected in the October 2019 sample. The shallow well in the Ashland cluster (P-A3) had one detected analyte in the June 2019 sample and no detections in the October 2019 sample. The deep well in the Elkhorn Crossing cluster (E-EC1) had no detections in June 2019 and three analytes detected in the October 2019 sample. Three wells, two in the Springfield cluster (P-S1 and P-S2) and one in the Valley cluster (P-Va1), had one analyte detected in both the June 2019 and October or November 2019 samples. A total of 25 samples did not have any detected pharmaceutical or hormone CECs in either the June 2019 or October or November 2019 samples. All pharmaceutical and hormone CECs detected in this study were from wells in the Platte River and Elkhorn River alluvial aquifers (fig. 2), which are unconfined aquifers with shallow depth to water (table 1).

The most frequently detected CEC in this study was the antibiotic sulfamethoxazole. It was detected in the summer and fall samples from the deep (P-S1) and medium-depth (P-S2) wells in the Springfield cluster but was not detected in the shallow well (P-S3). The concentration of sulfamethoxazole was higher in the fall samples collected from both wells (table 8). Nicotine was detected in two samples from different wells (P-A3 and E-EC1) at a concentration below the reporting level but above the detection limit. The other CEC detected in both samples from one well was methyl-1H-benzotiazole, an industrial product with a variety of applications. It was detected in the summer and fall samples from the shallow well of the Valley cluster (P-Va1). The concentration decreased in the 5 months between samples. In the second sample, the concentration was lower than the reporting level but above the detection limit. Pharmaceutical analytes detected in only one sample during this study were acetaminophen (analgesic), caffeine, and metformin (diabetes medicine).

Atrazine was detected in 15 samples from 8 wells in 4 well clusters in the Platte River and Elkhorn River alluvial aquifers (table 8). Seven of the wells, in three clusters, had atrazine detected in the summer and fall samples. Previous sampling for atrazine has been performed at six of the sites with detections in this study (USGS, 2020). Data analysis for the previous testing used different analytical methods with a higher detection level than this study (50 ng/L for previous testing as compared to 10 ng/L for this study), but all six sites had low levels of atrazine detected at least once since 1999.

The only wells with a detection of CECs and nitrate-N concentration above 0.2 mg/L were at the Springfield (Buffalo Road) deep (P-S1) and medium-depth (P-S2) wells (table 8). The samples from these wells (P-S1 and P-S2) were also the only CEC detections with a DO concentration above 1.2 mg/L (table 8), suggesting that anoxic conditions and denitrification were present in all other wells with CEC detections.

March 2019 brought unprecedented flooding to eastern Nebraska, including much of the PMRNRD and the Platte and Elkhorn Rivers along the western and southern boundaries of the PMRNRD (Nebraska Department of Natural Resources, 2019). Several of the PMRNRD monitoring well clusters were inundated during the flooding, and all CEC sampling occurred after the flooding. The ground at 10 of the 17 wells sampled for CECs (in 4 clusters) was inundated during the flooding (table 1). Five of the 6 detected CECs (from 4 of 6 wells with detections) were from wells that were inundated during the flood. Additionally, 14 of the 15 atrazine detections (from 7 of the 8 wells with detections) were from wells inundated during the flooding. All detections in non-inundated wells (4 sulfamethoxazole and 1 atrazine detection) were from one cluster that was very near, but not within, the area inundated by the flood. Because the sampling only occurred 3–8 months after the flooding, and there are no data about CECs in the floodwater, the effect of the flooding on the CEC detection in this study cannot be determined. However, specific conductance and DO concentrations were similar during regular PMRNRD groundwater sampling before and after the flood at all wells with CEC detection (USGS, 2020), suggesting that dilution of the local groundwater was not a major effect from the flood.

Presence of Nitrate-N in Groundwater, 2010–19

From 2010 to 2019, 473 samples were collected for nitrate-N analysis from 150 wells (including the wells sampled for CECs) as part of the regular PMRNRD groundwater sampling program (fig. 3) (USGS, 2020). Nitrate-N was detected at a level above 4 mg/L in 92 samples (19 percent) from 33 wells, and results exceeded 10 mg/L in 36 samples (7.6 percent) from 15 wells (table 9). The Dakota Sandstone aquifer had the highest percentage of samples with nitrate-N concentrations above 10 mg/L, whereas the Platte River alluvial aquifer had the highest percentage of samples with concentrations above 4 mg/L. Groundwater nitrate-N concentrations decrease with depth with little to no nitrate occurring deeper than 300 ft (fig. 4). Naturally occurring nitrate exists in ecosystems at concentrations usually below 2 mg/L (Nolan and others, 2002) as part of the nitrogen cycle, whereas non-naturally occurring nitrate is introduced into the ecosystem through anthropogenic inputs such as commercial fertilizers.

DO concentrations were taken for 1,913 of the 2,095 samples collected since 1992. Nitrate-N concentrations in groundwater were lowest at DO concentrations less than 0.5 mg/L, indicating redox conditions appropriate for nitrate removal through denitrification, but nitrate-N ranged from 0 to 24.7 mg/L for all DO concentrations from 2010 to 2019 (512 samples) (fig. 5). DO concentrations less than 0.5 mg/L indicate potential denitrifying conditions; however, denitrification may occur at DO concentrations as high as 2 mg/L (Erickson and others, 2021). Low nitrate-N concentrations at high (>2 mg/L) DO concentrations do not indicate denitrification, but rather a lack of non-naturally occurring nitrate, which is why there is a range of nitrate-N concentrations at these DO concentrations. Median DO concentrations were lowest in the Elkhorn and Missouri River alluvial aquifers (table 9). The Elkhorn River alluvial aquifer also had the highest percentage of wells and samples with DO less than 0.5 mg/L, indicating potential denitrification throughout this aquifer. Previous work in this area has quantified the amount of denitrification for 17 wells throughout the study area (McGuire and others 2012). The upland alluvial aquifer and Missouri River alluvial aquifer have the highest median iron concentrations (table 9). Groundwater iron concentrations greater than 100 μg/L are also an indicator of potential denitrification (Erickson and others, 2021).

Nitrate-N Time Series Analysis, 1992–2020

Nitrate-N concentrations in groundwater have varied in the samples collected as part of the regular PMRNRD groundwater sampling program from 1992 to 2020. When looking at the median nitrate-N concentration in 4-year periods from the entire sampling period of 1992–2019 (calendar year) (table 10), the Platte River alluvial aquifer median nitrate-N concentration has been increasing from 0.36 mg/L in 1992 to 8.33 mg/L in 2015; however, concentration was lower (4.52 mg/L) during 2016–19. Median nitrate-N concentrations in the Elkhorn River alluvial aquifer have been decreasing since 1992 and have been below the detection limit since 2008–11; however, some wells sampled previously have not been resampled since 2010 for all aquifers in this study area (USGS, 2020). After 2010, sampling shifted from mostly irrigation wells with long well screens to sampling mostly domestic wells with shorter well screens. This change may have influenced nitrate trends in the aquifers. The long well screens of irrigation wells make it difficult to pinpoint where the nitrate-N contamination is occurring in the aquifer, and some wells are screened across multiple aquifers. Sampling domestic wells gives a more precise measurement of the nitrate-N concentrations at a point in the aquifer because of the smaller screened interval. The median nitrate-N concentration in the Missouri River alluvial aquifer was below the detection limit for the period of study. The nitrate-N concentrations in the upland alluvial and Dakota Sandstone aquifers have varied throughout the sampling period.

The significance of trends over the study period was determined for each aquifer using the Mann-Kendall test (Helsel and others, 2020) (table 11). The Missouri River alluvial, upland alluvial, and Dakota Sandstone aquifers did not show a trend in groundwater nitrate-N concentrations in the overall period of study. The Elkhorn River alluvial aquifer had a negative trend in groundwater nitrate-N concentration over time, whereas the Platte River alluvial aquifer had a positive trend in nitrate-N concentration over time. However, three well nests—one in the Platte River alluvial (wells P-S1, P-S2, and P-S3), one in the Elkhorn River alluvial (wells E-V1, E-V2, and E-V3), and one in the Dakota Sandstone (wells D-T1, D-T2, and D-T3)—are sampled on an annual basis because they are located in a groundwater management area. The wells in all the aquifers are sampled on an about 4-year rotation; however, the same wells may not be sampled each time, and sampling between these 4-year rotations is minimal. These annually sampled well nests all had one or more wells with a strong positive or negative trend. The Mann-Kendall test was run again on these three aquifers excluding the results from those well nests to see if the trend was driven by those specific wells or the aquifer as a whole. For example, if all the wells in the aquifer except these stayed constant while these specific wells increased, the entire aquifer would seem to be increasing when it is a localized occurrence. When those wells were excluded from the trend analysis, trends were not significant (p-value less than 0.05) in groundwater nitrate-N concentrations for any of the five aquifers in the PMRNRD.

Ten wells, sampled more frequently than the 4-year rotation, had 10 years of sampling data and were evaluated to determine if the concentration trend was significant within them (table 12). These wells represent 3 well nests and 1 stand-alone well in 3 different aquifers. The nest of three wells (P-S1, P-S2, and P-S3) in the Platte River alluvial aquifer is within a Phase 2 management area (Papio-Missouri River Natural Resources District, 2018) and had trends of increasing nitrate-N concentration at all depths. The increasing nitrate-N concentrations in the Platte River alluvial aquifer well nest seems to be driving the trend analysis for the whole aquifer when the wells were included, evidenced by the lack of a trend in the aquifer when those wells were excluded from analysis (table 11). In the Elkhorn River alluvial aquifer, the stand alone well (E004) had no trend, whereas the nest of three wells (E-V1, E-V2, and E-V3) had trends of decreasing nitrate-N concentration at all depths. The nest of three wells (D-T1, D-T2, and D-T3) in a Phase 2 management area (Papio-Missouri River Natural Resources District, 2018) within the Dakota Sandstone aquifer had a decreasing trend in the deepest well and an increasing trend in the shallow and medium wells.

The increase in nitrate-N in any of these wells does not necessarily reflect on current nitrate-N management strategies. Previous studies done on these wells looked at apparent groundwater age based on age dating studies (McGuire and others 2012). The well nest in the Platte River alluvial aquifer had recharge dates from the early to mid-1970s, and the well nests in the Elkhorn River alluvial and Dakota Sandstone aquifers had recharge dates from the early to mid-1960s (McGuire and others 2012). These apparent ages show a considerable time lag between current nitrate-N management practices and groundwater nitrate-N concentrations.

Comparison of Concentrations of Anthropogenic Compounds with Health-Based Water-Quality Standards and Benchmarks

Only 5 of the 132 analytes included in the CEC analysis for pharmaceuticals and hormones (table 2) have regulatory or health-based standards in Nebraska, including 1 for atrazine, which was included in this method to compare performance between compounds and between methods (table 13) (Nebraska Department of Environmental Quality, 2006; Furlong and others, 2014; Norman and others, 2018). The four CECs with health-based standards were not detected in this study.

The agricultural herbicide atrazine has a groundwater maximum contaminant level (MCL) of 3 micrograms per liter (μg/L) set by the U.S. Environmental Protection Agency (2009) and the State of Nebraska (Nebraska Department of Environmental Quality, 2006). Atrazine was detected in 15 samples in the Platte/Elkhorn alluvial aquifer at a maximum concentration of 0.0947 μg/L, substantially below the MCL.

A nitrate-N threshold of 4 mg/L was used in a previous study to represent the anthropogenic effect of groundwater when using a background nitrate-N level of 2 mg/L. (Nolan and others, 2002). The use of the 4 mg/L threshold for anthropogenic effect indicates some non-naturally occurring nitrate likely is present (Nolan and others. 2002). Additionally, Ward and others (1996) found a positive association between nitrate-N concentrations greater than 4 mg/L in Nebraska community water systems and cancer rates. In a review of published studies of nitrate in drinking water, Ward and others (2018) reported adverse health effects from drinking water nitrate levels as low as 2.46 mg/L and found the strongest evidence for adverse health effects related to elevated nitrate levels (>5.0 mg/L) for colorectal cancer, thyroid disease, and neural tube defects. The State of Nebraska’s MCL for nitrate in groundwater and for the U.S. Environmental Protection Agency in drinking water is 10 mg/L as N (U.S. Environmental Protection Agency, 2009; Nebraska Department of Environmental Quality, 2006). Nitrate samples exceeding the 2- and 4-mg/L thresholds were collected throughout the PMRNRD and in all aquifers except the Elkhorn River alluvial aquifer, which had no samples with nitrate-N above 4 mg/L and only 3 samples between 2 and 4 mg/L (table 9). Dissolved oxygen was less than or equal to 0.5 mg/L in over 60 percent of the samples from the Elkhorn River alluvial aquifer for which it was analyzed, indicating that denitrification is a possible partial explanation for the lower nitrate-N concentrations in that aquifer.

Environmental Implications

Only one of the CECs detected during this study has been reported to have effects upon environmental processes. Underwood and others (2011) determined that low concentrations of sulfamethoxazole inhibited the nitrate reduction capabilities of an aquifer-sourced bacterial culture in anoxic conditions. The lowest tested sulfamethoxazole concentrations (1 μg/L) were about 100 times higher than the sulfamethoxazole detected in this study in the deep (P-S1) and medium-depth (P-S2) wells at the well nest Springfield (Buffalo Road) (table 8; fig. 2). Underwood and others (2011) determined that sulfamethoxazole at low concentrations of 1 μg/L decreased nitrate reduction potential by 47 percent. Nutrient samples were only collected with CEC sampling from each well in June 2019, and sample results for nitrate-N from both wells with sulfamethoxazole detections (P-S1 and P-S2, fig. 2) were greater than 10 mg/L. The shallower well from the cluster did not have any CECs detected and had a nitrate-N value less than 5 mg/L. Dissolved-oxygen concentrations from both wells with a sulfamethoxazole detection (P-S1 and P-S2) were greater than 5.4 mg/L for all samples collected from 2010 to 2019 (U.S. Geological Survey, 2020), indicating oxic conditions (no denitrification expected) within the deep and medium-depth sampled regions of the aquifer.

CECs were detected in the Platte River and Elkhorn River alluvial aquifers that were targeted with sampling. These detections demonstrate that groundwater supplies of the PMRNRD are affected by anthropogenic contamination from nonagricultural and agricultural sources. Although the extent and degree of contamination related to agricultural activities has been characterized from many years of district-wide sampling, CEC monitoring in the PMRNRD, including in the Platte River and Elkhorn River alluvial aquifers targeted by this study, has been minimal. CECs were detected in six wells screened in the Platte River and Elkhorn River alluvial aquifers (Ashland Shallow, P-A3; Venice Deep, E-V1; Valley shallow, P-Va1; Springfield [Buffalo Road] medium, P-S2, and deep, P-S1; and Elkhorn Crossing deep, E-EC1) with a depth to water ranging from 3.3 to 12.9 feet and well depths ranging from 21 to 98 feet; and none had a pattern between physical properties and CEC occurrence (U.S. Geological Survey, 2020). CEC concentrations were not detected in samples from the Missouri River alluvial aquifer, upland alluvial aquifer, or Dakota Sandstone aquifer. However, because samples were collected from only four wells screened in the Missouri River alluvial (D-D2), the upland alluvial (U-A3), and the Dakota Sandstone aquifers (D-Sp2 and D-T3), the presence of CECs in other parts of these aquifers cannot be precluded.