Spring and Well Selection and Identification

Spring sampling locations were chosen from the mapped focused groundwater discharge locations interpreted by Hobza and Schepers (2018). The spring sampling locations are in an area of groundwater discharge from Pliocene-age sand and gravel deposits along a 62-mi reach of the South Loup River downstream from the Arnold streamgage (fig. 1). Along this reach, groundwater discharge to the South Loup River is dominated by discharge at focused points (Hobza and Schepers, 2018). Monitoring wells were selected for sampling based on their proximity to the South Loup River. Eight of the monitoring wells were clustered monitoring wells where a shallow and deep well are screened in two different geologic units at one location (fig. 1; table 1). Samples also were collected from a group of four shallow monitoring wells screened in Quaternary-age deposits upstream from the Arnold streamgage.

This report uses three different methods of site identification. The first method uses the USGS station number, which is a 15-digit number such as 412340100074901 (table 1). The 15-digit USGS station number is particularly useful for accessing historic water-quality or water-level data for a given well in the USGS National Water Information System (NWIS; U.S. Geological Survey, 2021b). The next identification method is the USGS station identification. This station identification name uses the legal description of the well or spring location and a local well or spring name, such as 17N 24W31C1 SLoup Spr 35. The last method of site identification is the field name, which is the local spring or well name typically used by participating NRDs, such as SPR 35 or SLR-2 (table 1).

Sample Collection

Samples were collected from 12 wells and 8 springs in the South Loup River Basin in Logan and Custer Counties in August and September 2019 (fig. 1; table 1) using the guidelines and protocols described in the USGS National Field Manual (U.S. Geological Survey, variously dated). Groundwater samples were collected from monitoring wells and springs using a stainless-steel submersible pump. All samples were collected in-line, where Tygon tubing was connected to the discharge port of the stainless-steel submersible pump to ensure groundwater samples were not exposed to the atmosphere before collection. Before sampling monitoring wells, the water level was measured using methods described in Cunningham and Schalk (2011). The well casing volume and flow rate were calculated to determine the length of time needed to purge at least three well volumes before samples were collected. Groundwater sampled from springs and wells was pumped into a flow-through chamber to facilitate the monitoring of the physical properties of the water to ensure the collection of a representative sample. Specific conductance, pH, water temperature, and dissolved oxygen (DO) were measured in the field at 3-minute intervals by a multiparameter sonde (Xylem Analytics, 2020). Representative groundwater samples were collected after sequential readings stabilized within limits described by the USGS National Water Quality Field Manual (U.S. Geological Survey, variously dated) and for monitoring wells after three well volumes had been pumped from the well.

Groundwater samples were collected from springs by creating a temporary sampling point emplaced within unconsolidated sediments. Samples were collected by first determining the location where the groundwater discharge was greatest within the exposed sand and gravel. At this location, a 2.5-in. diameter column of sediment was removed using a streambed sediment coring tube. Immediately after removing the column of sediment, a 2.5-in. diameter polyvinyl chloride well screen and casing with a solid endpoint was manually pushed into the sand and gravel deposits to create a temporary sampling point. The temporary sampling point was pushed to a minimum depth of 2 ft, ensuring that the screened interval was beneath the sediment-water interface. Following emplacement of the temporary sampling point, sediments were allowed to collapse against the screen and casing. The water level within the temporary sampling point rose higher than the water level outside the spring, indicating a higher hydraulic head within the sediments of the spring. In some cases, groundwater readily flowed out of the temporary sampling point effectively creating an artesian well. As with samples collected from monitoring wells, spring samples were collected using a stainless-steel submersible pump. The stainless-steel submersible pump was used in part to develop the temporary sampling point and ensure the sampled groundwater was free of fine-grained sediments. All samples were collected in-line, where Tygon tubing was connected to the discharge port of the stainless-steel submersible pump to ensure groundwater samples were not exposed to the atmosphere prior to collection. When groundwater samples were collected from springs, the flow rate of the submersible pump was adjusted so that the water level inside the casing of the temporary sampling point remained above the level of the sediment water interface to avoid entraining air into the sample or pumping water that had already been exposed to the atmosphere.

Dissolved gas and age tracer samples were collected with specific protocols to ensure representative samples. Dissolved gas samples were collected in septum stopper glass bottles using methods described in U.S. Geological Survey (2020a). Samples were analyzed by the USGS Reston Groundwater Dating Lab in Reston, Virginia, for argon (Ar), nitrogen (N₂), oxygen (O₂), carbon dioxide (CO₂), and methane (CH₄) as described in U.S. Geological Survey (2020a). Stoppered N₂/Ar bottles were chilled to halt biological activity after sampling, but continued gas transformation could not be assured. Unfiltered samples for tritium (³H) were collected in 1-liter (L) polyethylene bottles, sealed with no air space in the container, and analyzed by the USGS Tritium Laboratory in Menlo Park, California, by electrolytic enrichment (U.S. Geological Survey, 2020b). Samples for sulfur hexafluoride (SF₆) were collected unfiltered in duplicate 1-L amber glass bottles with polyseal cone-lined caps. The samples were collected by submerging the bottle and using a short section of copper tubing to fill from the bottom of the bottle to minimize exposure to the atmosphere. Each sample was allowed to overflow for at least three bottle volumes in order to rinse the bottles while minimizing contact with the air. Samples were analyzed by the USGS Age Dating Laboratory in Reston, Va., according to methods described by Busenberg and Plummer (2000). The USGS Age Dating Laboratory reported that analytical precision of the water samples is about 20 percent at the detection limit of less than (<) 0.01 fentamol per liter (fmol/L) and about 3 percent or better at concentrations from 0.2 fmol/L to a maximum reporting level of 20 fmol/L. Samples of carbon-14 (¹⁴C) and stable isotopes of carbon (delta carbon-13 [δ¹³C]) were filtered with a capsule filter (poresize 0.45 micron) and collected in 1-L glass bottles. The bottles were filled from the bottom and allowed to overflow for several volumes in order to rinse the bottles while minimizing contact with the air, sealed with polyseal caps, and analyzed at the National Ocean Sciences Accelerator Mass Spectrometry Facility at the Woods Hole Oceanographic Institution in Woods Hole, Massachusetts, by accelerator mass spectrometry (Woods Hole Oceanographic Institution, 2020). Analyses of ¹⁴C are reported as percent modern carbon (pmC) with “modern” carbon being defined as 95 percent of the activity of Nation Bureau of Standards oxalic acid from 1950 (Stuiver and Polach, 1977) and converted to non-normalized values following Mook and van der Plicht (1999). Delta carbon-13 (δ¹³C) refers to the abundance of ¹³C to carbon-12 (¹²C) in the sample relative to the standard Vienna Pee Dee Belemite (Coplen and others, 2006) and analysis results are reported as per mil. Analytical error for ¹⁴C was less than 0.5 pmC; for δ¹³C, it was greater than or equal to 0.3 per mil (‰) (Woods Hole Oceanographic Institution, 2020).

Except for an initial equipment blank, all environmental and quality control samples were collected and processed on site. Samples that required filtration (major ions, trace elements, nutrients, and ¹⁴C) were filtered using a disposable 0.45-micron filter. Field equipment was cleaned immediately following sample collection according to procedures described in the USGS National Water Quality Field Manual (U.S. Geological Survey, variously dated). Samples were sent to analytical laboratories within 3 days of sample collection. Samples that were preserved by cooling (nutrients, dissolved gases, and ¹⁴C) were packed on ice to ensure the sample temperature did not exceed 4 degrees Celsius (°C) prior to delivery to the analytical laboratory. Other samples were placed into coolers without ice and delivered to the appropriate analytical laboratories.

Water-Quality Sample Analysis and Reporting

Water samples from all sites were analyzed for major ions, trace elements, nutrients, stable isotopes, dissolved gases, and selected age tracers (³H, SF₆, and ¹⁴C). The analytes, analyzing laboratories, references to methods used, and field preservation procedures are provided in table 2. Sampling results from all analyses are available online from the USGS NWIS (U.S. Geological Survey, 2021b).

For this report, conventional nomenclature was used to describe analyses of water samples for stable isotopes. The composition of stable isotopes of low mass (light) of O and H commonly are reported as “δ” (del) values, which indicate parts per thousand or per mil (‰). The reported value is compared to the isotopic ratio of the Vienna Standard Mean Ocean Water (VSMOW; Clark and Fritz, 1997). The general expression for the δ value is calculated by the following equation (Clark and Fritz, 1997):

A negative δ value indicates that the sample is depleted of the rare isotope relative to the standard; that is, the sample is isotopically “light.” A positive δ value indicates that the sample is enriched in the rare isotope relative to the standard; that is, the sample is isotopically “heavy.” Understanding and interpreting the stable isotopic composition of groundwater samples are aided by understanding hydrologic processes that can affect isotopic ratios. Isotopic fractionation occurs where the isotopic composition is altered by chemical, biological, or physical processes that may result in the preferential enrichment or depletion of one isotope over another. This fractionation process partitions isotopes as a function of the differences in the masses of the isotopes. Because the heavier isotope has a stronger molecular bond (deuterium [²H] has a stronger molecular bond than hydrogen-1 [¹H], and oxygen-18 [¹⁸O] has a stronger molecular bond than oxygen-16 [¹⁶O]), the liquid phase of water generally is “heavier” than the gaseous phase (Kendall and Caldwell, 1998). As a result, evaporation is a major fractionation process in which the lighter isotope is concentrated in water vapor, whereas the heavier isotope enriches the aqueous phase. Additional information on isotopes and their presence in the environment can be found in Clark and Fritz (1997).

Water-Quality Data Analysis Procedures

Data-analysis procedures include general statistical analyses, creation of Piper diagrams (Piper, 1944), scatter plots, statistical testing, and summary tables. This report only contains selected plots and tables necessary to support the interpretations and conclusions presented. Analyses of stable isotope data and age tracer results, including radiocarbon dating procedures, are also described in this section of the report.

Correlation coefficients were calculated to determine the strength of correlation between two constituents or physical parameters for a set of samples or sample group. Measures of correlation are dimensionless values and range from −1 and 1 where values close to −1 or 1 show a very strong inverse and direct correlations, respectively, and a value of zero indicates no correlation (Helsel and others, 2020). Monotonic correlation occurs when one variable increases or decreases as another variable increases. A scatterplot is a useful diagnostic tool that can indicate if data are correlated linearly or nonlinearly. One type of monotonic correlation is linear correlation where a scatterplot of the two variables will produce a linear pattern (Helsel and others, 2020).

The strength of correlations of different variables or constituents was examined in this study. Pearson’s r test (Helsel and others, 2020) was used to measure the strength of linear correlation and Spearman’s rho (ρ) test (Helsel and others, 2020) was used the measure the strength of nonlinear, monotonic correlation. Pearson’s r is not resistant to outliers because it is computed using means and standard deviations, which are nonresistant measures. If the data lie precisely along a straight line with a positive slope, then r=1, and if the line has a negative slope then r=−1. A low value for Pearson’s r may be calculated even though the two variables are highly correlated because of skewness or outliers. The null hypothesis for this test is that r is not significantly different from zero, which is to say no correlation. The alternative hypothesis is that r is significantly different from zero and the variables are linearly correlated (Helsel and others, 2020). Spearman’s ρ is a rank-based correlation coefficient that depends on the ranks of the data rather than the observed values. Spearman’s ρ is resistant to outliers and is particularly useful for examining water-quality data where concentrations are known to be less than the detection limit. The statistical significance of ρ is tested under the null hypothesis that ρ is not significantly different from zero, or there is no correlation, and the alternate hypothesis is that the test statistic is significantly different from zero (Helsel and others, 2020).

The Wilcoxon rank-sum test (Helsel and others, 2020) was used to determine if differences between distributions of the selected values from the two subsets of samples collected from Pliocene deposits were significant. Water samples collected from the Pliocene-age deposits were divided into two groups based on geographic location and water-quality characteristics. The null hypothesis for the test was that the distributions for the two groups are identical, and the alternative hypothesis was that the distributions between the two groups are different (Helsel and others, 2020). The null hypothesis was accepted when the calculated p-value was less than an alpha value (α) of 0.05, which indicates a 95-percent confidence level that the probability value from the statistical test generated an accurate representation of the populations being tested (Helsel and others, 2020). A p-value is defined as the measure of the probability that an observed difference occurred by random chance (Helsel and others, 2020). If a calculated p-value is less than 0.05, then it can be inferred there is a significant difference between the medians of the sample groups with 95-percent confidence.

Groundwater Age Tracers

Estimating the age of groundwater is an increasingly common approach to inform water resource management strategies. Groundwater age is often used to estimate groundwater recharge rates, examine the sustainability of groundwater resources, and assist in groundwater model calibration (Kazemi and others, 2006). For this report, ³H, SF₆, and ¹⁴C samples were collected from selected wells and springs. Data analysis procedures including radiocarbon adjustments of ¹⁴C data are discussed below.

Lumped Parameter Modeling

Groundwater age is described in this study by the estimated mean age and the age distribution. In most cases, a groundwater sample is a mixture of flow paths with varying ages, and the age distribution represents the probability of a water parcel with a given estimated age occurring in the sample. Narrow age distributions indicate a narrow range of groundwater ages captured in the sample mixture, whereas a broad distribution indicates a wide range of captured ages. Mean ages, age distributions, and the susceptibility index (SI; described in the “Susceptibility Index and Fraction Modern” section of this report) were determined by calibrating lumped parameter models (LPMs) to the measured tracer concentrations. Tracers used for age determination are assumed to be conservative or have a predictable decay rate. The LPMs simulate tracer concentrations in accordance with the model’s distribution type and parameters (Jurgens and others, 2012). In groundwater studies, fitted distributions are interpreted to represent the aquifer dimensions, mean age, and extent of flow path mixing. The LPMs are optimized by minimizing the misfit between measured and LPM-calculated tracer concentrations. A modified version of the USGS program TracerLPM (Jurgens and others, 2012) was used for calibration of LPMs. The approach for LPM selection and interpretation was dictated by the measured tracer concentrations and the conceptual hydrogeology. Estimated age distributions describe the advective age of groundwater and provide a maximum estimate of the response time to hydraulic perturbations (for example, the propagation of pressure driven by changing water levels); that is, groundwater levels at wells and discharge at springs will respond to changes in aquifer hydraulic conditions over time scales less than the mean age.

The LPMs used in this study were the dispersion model (DM), the exponential mixing model (EMM), and the binary mixing model (BMM; Jurgens and others, 2012). The DM is a solution to the advection-dispersion equation and describes the amount of mixing along and between flow paths being captured in a sample. The DM is parametrized by the mean age and the dispersion parameter (DP), which is the inverse of the Peclet number (Gelhar and others, 1992). Based on field scale studies of dispersion in groundwater (Gelhar and others, 1992), the DP varies between 0.001, which approximates a piston-flow model with very little mixing and a narrow age distribution, to 3, which approximates the EMM with a broad age distribution. The EMM is used to represent groundwater age distribution in a sample capturing flow paths across the full depth of an aquifer receiving areally distributed recharge. Exponentially mixed samples capture the full range of groundwater flow paths and respective varying ages with the relative contributions of the flow paths determined by the mean age, which is the single fitting parameter of the EMM. The BMM describes mixtures of two distinct water sources. The two components of the BMMs, the young and old components, were modeled by two DMs parameterized based on results from other samples in this study and a previous study of groundwater mixing parameters (for example, Bexfield and others, 2012; Eberts and others, 2012; Stackelberg and others, 2018; Solder and others, 2020). The dispersion parameter and mean age for the young component DM was estimated from fully constrained LPM solutions for nearby samples. For the old component DM, a dispersion parameter of 0.1 was assumed, consistent with kilometer scale longitudinal dispersion (Gelhar and others, 1992). Measured tracer concentrations were fit by varying the mean age of the old component DM and the mixing fraction of the two BMM components. Full description of the LPMs used in this study are provided by Jurgens and others (2012).

Susceptibility Index and Fraction Modern

The SI and fraction modern metrics are used in this study as complimentary measures of groundwater susceptibility to changing hydrologic conditions, such as drought, during somewhat short time periods. The SI is most sensitive to relative contributions of very young water less than 10 years old. Conversely, the fraction modern is more sensitive than the SI to contributions of groundwater that are greater than about 20 years old (Solder and others, 2020). Combined, the two metrics provide a useful means of summarizing the groundwater age distribution.

The SI provides a quantitative estimate of susceptibility of a well to land surface contamination. More specifically, it is a relative measure of how soon a conservative contaminant in the recharge area would arrive at the well by groundwater transport and does not include unsaturated zone travel time. The SI is calculated as the normalized difference between the LPM-derived cumulative distribution function (CDF) of age and a reference CDF (Solder and others, 2020) as described by the following equations:

The SI was initially conceptualized to examine the movement of contaminants from the land surface; however, in this study it was used as a proxy for determining the susceptibility to short-term drought. Groundwater discharging from a spring or sampled from a well with an SI close to 1 indicates relatively young groundwater with a narrow age distribution. Groundwater in this scenario is sourced from recent recharge and would likely be affected by short-term drought conditions. In contrast, groundwater samples with an SI that approaches zero indicates old groundwater with a broad age distribution. Groundwater in this scenario is not as reliant on recent recharge and is likely more resilient to short-term drought conditions. The SI is a relative measure for comparison between sites in the same study area under the assumption of similar aquifer properties, such as transmissivity and porosity, that control the hydraulic response to changing hydrologic conditions such as drought.

The LPM fraction modern describes the relative contribution of recharge post-1950 to the full LPM estimated age distribution. The fraction modern is a standardized measure to understand the relative contribution of modern recharge to a sample and for comparison between samples. The calculated value of the fraction modern is extracted directly from the LPM-estimated cumulative age distribution as the cumulative fraction of water recharged after 1950.

Quality Assurance and Quality Control

Quality control samples were collected for this study and included an equipment blank, field blanks, and field replicates. Quality control samples were collected to evaluate and determine if samples have been contaminated or if the data were biased by the sampling equipment, collection, processing, storage, or laboratory analysis. For this report, one equipment blank, two field blanks, and two field replicate samples were collected (table 3). One field blank and one replicate were collected during each of the two field campaigns (August and September 2019). Equipment blanks were collected in the laboratory before sampling to determine if the samples were biased by the sampling equipment and cleaning procedures. Field blank samples were collected to determine the occurrence and magnitude of sample contamination from collection, processing, storage, and analysis. Blank samples were collected using water that has been certified to be free of inorganic constituents.

Blank samples were analyzed for major ions, trace elements, and nutrients. Relatively few concentrations were greater than the reporting limit in the field blanks. Most of the concentrations were within the range of environmental samples for specific constituents (table 3); however, there were some notable exceptions. Concentrations of copper were an order of magnitude greater than the reporting level in the equipment blank and one field blank and were within an order of magnitude with the other field blank (table 3). Manganese had concentrations exceeding an order of magnitude of the lower detection limit for one of the two field blanks (table 3). The source(s) of copper and manganese in blank samples is currently (2021) unknown.

The relative percent difference was calculated for all sample pairs for each constituent using the following formula:

The relative percent differences between replicates for major ions and trace elements were 5 percent or less, with the exception of higher differences of fluoride, chromium, cobalt, and nickel; however, the differences between the replicates and the samples were near the reporting limit (table 3). For one replicate sample (East shallow), the relative percent difference of lithium was nearly 19 percent and the difference in concentration between the environmental sample concentration and the replicate was 2.75 micrograms per liter (μg/L), which is an order of magnitude greater than the reporting limit (table 3). For nutrients, relative percent differences between samples were less than 5 percent. For the East shallow sample, the relative percent difference for ammonia and nitrite were 12 percent and 143 percent, respectively (table 3). Although the relative percent differences between the environmental and replicate samples were large, actual differences in concentration were small; 0.00913 mg/L for ammonia and <0.206 mg/L for nitrite (table 3).

Water-Quality Characteristics

In general, groundwater sampled from wells and springs in the South Loup River Basin in 2019 was relatively fresh, with total dissolved solids (TDS) generally less than about 500 mg/L (table 6) and generally calcium-bicarbonate type (fig. 5). Spring sites from Pliocene-age units had a higher relative proportion of sulfate (SO₄), whereas SLR-1 and two wells in Logan County (Mann shallow and Mann deep) had a higher relative proportion of sodium and potassium. The difference in dissolved ions indicates differing flow paths and subsequent geochemical evolution of groundwater or differences in groundwater recharge chemistry. Groundwater samples are oxic for 15 of the 20 sites, with measured DO greater than or equal to 0.5 mg/L (table 6). Oxic conditions are consistent with very low amounts of dissolved iron and aluminum in solution for most of the samples (table 6).

Local land use appears to be a dominant control on nitrate concentrations within the study area. Nitrate concentrations of samples are below the background concentration of 2 mg/L (Mueller and Helsel, 1996) for 10 of the 20 samples and below 3 mg/L for 17 of the 20 samples. The U.S. Environmental Protection Agency (EPA) maximum contaminant level of 10 mg/L (U.S. Environmental Protection Agency, 2018) was exceeded in two samples (SPR 25 and SPR 35). Elevated nitrate concentrations in groundwater indicate a portion of the sample collected contains groundwater recharged within the past 50 years, approximately the time when commercial fertilizers were widely available. An anthropogenic source of nitrate is supported by generally higher nitrate concentrations in samples with higher ³H and ¹⁴C, suggestive of younger water, as discussed in the “Groundwater Age Tracers” section of this report. The effects of land use on groundwater quality are evident when examining nitrate concentrations from springs and wells in Pliocene deposits. The nitrate concentrations of springs SPR 24, SPR 25, and SPR 35 were the highest for this study with nitrate concentrations of 6.51, 14.3, and 16.5 mg/L, respectively. The chloride concentrations for these samples were among the highest for the 20 samples collected (table 6). The nitrate concentration of Connor shallow well was below the background concentration of 2 mg/L; however, the DO concentration was 0.2 mg/L, indicating anoxic conditions and possible denitrification. The four samples (SPR 24, SPR 25, SPR 35, and Connor shallow) are located approximately 3.5 mi downstream from the Arnold streamgage and downgradient from groundwater-irrigated row crops (fig. 6). The general groundwater flow direction in this area is northeast, towards the South Loup River (Summerside and others, 2001). Other springs in Pliocene-age deposits (SPR 27, SPR 28, SPR 29, SPR 30, and SPR 31) sampled 7 mi downstream are downgradient from rangeland and appear to be unaffected by agricultural activities (fig. 6). Nitrate concentrations for Pliocene-age samples downgradient of rangeland were less than 3 mg/L and chloride concentrations were all less than 3 mg/L (table 6). Because of the differences in groundwater chemistry, sites in Pliocene-age deposits have been subdivided into two groups: Western Pliocene (SPR 24, SPR 25, SPR 35, and Connor shallow) and Pliocene (SPR 27, SPR 28, SPR 29, SPR 30, SPR 31).

Groundwater Age Characteristics

This section of the report describes the groundwater age and recharge characteristics of 20 springs and wells sampled in August and September 2019. Isotopic ratios, dissolved gas concentrations, and tracer concentrations were used to examine groundwater recharge sources, characterize recharge conditions and seasonality, and estimate groundwater ages. The “Isotope Tracers” subsection compares the concentrations of isotope tracers to concentrations of other sampled constituents to assist with hydrogeologic conceptualization. The interpretation of stable isotope data is also presented in the “Isotope Tracers” subsection. The “Dissolved Gases” subsection discusses concentrations of dissolved gases to estimate groundwater recharge temperature, assess denitrification, and support interpretations of age tracer data. The “Lumped Parameter Models and Groundwater Age Metrics” subsection discusses the age metrics estimated by calibration of LPMs to concentrations of ³H and ¹⁴C.

Isotope Tracers

Isotope and age tracer data indicate a wide range of possible groundwater ages with a stable recharge source. Groundwater samples contain ³H ranging from about zero to 7 TUs and ¹⁴C ranging from less than 20 pmC to greater than 100 pmC (table 7), which is indicative of the presence of premodern and modern waters. In general, groundwater with concentrations of ³H greater than 3 TUs is likely modern. Conversely, the stable isotopes δ²H and δ¹⁸O are consistent with local precipitation (fig. 7A) and relatively invariant across the sample sites, with the notable exception of SLR-1, indicating a temporally stable recharge source across the sites. Carbon-13 isotopic ratios (δ¹³C) are relatively invariable and range from about −11 and −6 ‰ with no clear consistent relation between δ¹³C and ¹⁴C.

Table 7.    

Tracer concentrations, modeled seasonal precipitation recharge contributions, and modeled age metrics for groundwater sampled in the South Loup River Basin, Nebraska, 2019.

[USGS, U.S. Geological Survey; ID, identification; MM, month; DD, day; YYYY, year; δ¹⁸O, ratio of ratio of oxygen-18 to oxygen-16 in sample to ratio of oxygen-18 to oxygen-16 in reference; per mil, parts per thousand; δD, ratio of ratio of deuterium or hydrogen-2 to hydrogen-1 in sample to ratio of deuterium or hydrogen-2 to hydrogen-1 in reference; LC, line conditioned; TU, tritium units; δ¹³C, ratio of ratio of carbon-13 to carbon-12 in sample to ratio of carbon-13 to carbon-12 in reference; pmC, percent modern carbon; SF₆, sulfur hexafluoride; pptv, parts per trillion, volume; LPM, lumped parameter model; χ2, chi-squared; BMM, binary mixing model modeled with young and old components each modeled with a dispersion model; ~, approximately; ³H, tritium; ¹⁴C, carbon-14; <, less than; DM, dispersion model; EMM, exponential mixing model]

Table 7.    Tracer concentrations, modeled seasonal precipitation recharge contributions, and modeled age metrics for groundwater sampled in the South Loup River Basin, Nebraska, 2019.

USGS station	Station ID	Field name	Sample group	Sample date (MM/DD/YYYY)	δ18O (per mil)	δD (per mil)	LC-excess (unitless)	Fraction winter recharge (unitless)	Tritium (TU)	Tritium error (TU)	δ13C (per mil)	Carbon-14 (pmC)	Carbon-14 error (pmC)	SF6 (pptv)	LPM name	χ2	Tracers modeled	Estimated mean age (years)	Susceptibility index (unitless)	Fraction modern
411126099422502	14N 21W 15AB 02 Pressey 2	Pressey shallow	Pliocene	9/11/2019	−10.1	−70.85	1.63	0.39	0.33	0.1	−8	51	0.23	0.91	BMM	~0	3H, 14C	6,700	0.028	0.04
411126099422501	14N 21W 15AB Pressey	Pressey deep	Ogallala	9/11/2019	−9.65	−69.55	-0.48	0.33	<0.12	0.12	−10.18	19.2	0.22	1.07	DM	5.72E-17	14C	18,000	0.014	0
412244100013501	16N 24W 2DDDD1	East shallow	Quaternary	9/11/2019	−10.4	−74.29	0.46	0.43	6.96	0.14	−8.92	102.6	0.24	1.72	EMM	3.79	3H, 14C	27	0.374	0.93
412244100013502	16N 24W 2DDDD2	East deep	Ogallala	9/11/2019	−9.55	−69.12	-0.81	0.31	<0.15	0.15	−9.63	40.8	0.2	0.78	DM	4.36E-17	14C	8,700	0.02	0
412213100051001	16N 24W 8D1 Sloup Spr 31	SPR 31	Pliocene	8/14/2019	−9.89	−72.19	-1.3	0.36	1.9	0.14	−8	78.2	0.28	1.22	BMM	5.28E-29	3H, 14C	2,900	0.064	0.23
412145100055601	16N 24W17B1 Sloup Spr 27	SPR 27	Pliocene	8/13/2019	−9.77	−70.68	-0.7	0.34	0.96	0.14	−8.4	77.1	0.26	0.98	BMM	1.11E-25	3H, 14C	2,400	0.054	0.12
412149100060401	16N 24W17B2 Sloup Spr 28	SPR 28	Pliocene	8/14/2019	−9.89	−70.94	-0.05	0.36	<0.13	0.13	−8.45	78.1	0.31	0.95	DM	8.63E-17	14C	2,000	0.041	0
412146100055701	16N 24W17B3 Sloup Spr 30	SPR 30	Pliocene	8/13/2019	−9.87	−69.93	0.81	0.36	1.84	0.11	−8.51	78.8	0.28	1.08	BMM	5.21E-29	3H, 14C	2,700	0.065	0.23
412148100061501	16N 24W18A1 Sloup Spr 29	SPR 29	Pliocene	8/13/2019	−9.83	−70.16	0.28	0.35	1.11	0.16	−9.03	81.1	0.31	1.26	BMM	2.67E-19	3H, 14C	1,900	0.062	0.14
412407100080501	16N 24W31B1 Sloup Spr 24	SPR 24	Western Pliocene	8/12/2019	−9.55	−67.76	0.55	0.31	1.14	0.13	−7.51	94.6	0.32	2.29	DM	0.18	3H, 14C	74	0.15	0.27
412406100080401	16N 24W31B2 Sloup Spr 25	SPR 25	Western Pliocene	8/12/2019	−9.45	−67.22	0.34	0.3	3.02	0.15	−7.46	106.5	0.33	5.07	DM	0.01	3H, 14C	18	0.257	1
412340100074901	17N 24W31C1 Sloup Spr 35	SPR 35	Western Pliocene	8/12/2019	−9.48	−66.56	1.22	0.3	3.12	0.2	−9.21	108.1	0.31	3.66	DM	2.5E-15	3H, 14C	20	0.253	1
412429100080302	17N 24W30CCDD2	Conner shallow	Western Pliocene	9/9/2019	−9.67	−68.7	0.52	0.33	0.58	0.15	−7.83	86.3	0.23	1.15	DM	1.81	3H, 14C	77	0.148	0.16
412429100080303	17N 24W30CCDD3	Conner deep	Ogallala	9/9/2019	−9.67	−68.68	0.54	0.33	<0.17	0.17	−10.43	13.2	0.2	0.99	DM	4.59E-17	14C	23,000	0.012	0
412717100150701	17N 25W 7CC (SLR-3)	SLR-3	Quaternary	9/9/2019	−9.95	−70.59	0.75	0.37	<0.15	0.15	−9.11	77.3	0.24	1.47	DM	5.28E-17	14C	2,100	0.04	0
412620100132101	17N 25W 17DC (SLR-1)	SLR-1	Quaternary	9/12/2019	−12.74	−92.5	-0.03	0.75	2.95	0.12	−11.14	93.8	0.21	6.44	DM	0.66	3H, 14C	70	0.154	0.44
412628100150701	17N 25W 18CB (SLR-2)	SLR-2	Quaternary	9/9/2019	−10.65	−74.62	2.03	0.47	1.44	0.16	−9	97	0.24	4.63	DM	0.05	3H, 14C	75	0.151	0.26
412547100125601	17N 25W 20DA (SLR-4)	SLR-4	Quaternary	9/12/2019	−9.94	−70.72	0.55	0.37	0.34	0.11	−7.44	79.1	0.24	0.67	BMM	6.91E-18	3H, 14C	1,900	0.05	0.04
412611100384401	17N 29W22ABBB1	Mann shallow	Quaternary	9/10/2019	−9.88	−69.85	0.96	0.36	<0.15	0.15	−6.44	89.7	0.26	0.58	DM	8.8E-16	14C	840	0.062	0
412611100384402	17N 29W22ABBB2	Mann deep	Pliocene	9/10/2019	−9.48	−67.01	0.77	0.3	<0.14	0.14	−6.1	85.7	0.25	0.55	DM	2.25E-17	14C	1,200	0.053	0

Values of δ²H and δ¹⁸O plotted near the LMWL (fig. 7A, B). Line conditioned excess (LC-excess; Landwehr and Coplen, 2006) ranges from −1.3 to about 2, indicating local deviations from the LMWL. The most negative values of LC-excess, indicating additional evaporation prior to recharge, occur along the central portion of the study reach, with the lowest values of LC-excess north of the South Loup River (East deep, SPR 31). The highest values of LC-excess, indicating a more depleted recharge source relative to local precipitation, are located south of the South Loup River (Pressey shallow, SPR 30, SPR 35, SLR-3, SLR-2, Mann shallow, Mann deep).

Precipitation-weighted seasonal δ¹⁸O end members (winter and summer) in precipitation are statistically different with a high level of confidence (p-value much less than 0.001) based on the Wilcoxon rank-sum test. Calculated fraction of winter precipitation from δ¹⁸O data ranged from 0.3 to 0.47 (excluding the result from SLR-1 of 0.75), indicating that summer precipitation is the dominant source of recharge (table 7). These results are consistent with 30-year (1981–2010) average precipitation records from Broken Bow Municipal Airport (fig. 1), which show 76 percent of annual precipitation falls between April and September (National Centers for Environmental Information, 2021). At the time this report was written (2021), it is unknown why sample SLR-1 is heavily biased towards wintertime recharge. Samples collected from springs in the Western Pliocene sample group (SPR 24, SPR 25, and SPR 35) are more isotopically enriched and have lower fractions of winter recharge compared to the rest of the springs sampled (table 7). This difference indicates that a larger proportion of recharge to these springs occurs during summer months and is likely caused by the recharge of groundwater applied to row crops or irrigation return flow. This interpretation is supported by the elevated concentrations of nitrate and chloride in these samples (table 6).

Relevant to the use of ¹⁴C for age dating, the youngest groundwaters (³H greater than about 3 TUs) are near equilibrium with calcite (saturation indices close to zero; table 6) based on PHREEQC model results, which is suggestive of a solid carbonate phase in the unsaturated zone and shallow aquifer. The shallow carbonate and soil zone CO₂ in isotopic equilibrium with the atmosphere and soil zone sources (plant and microbial respiration) are likely the primary sources of DIC in groundwater. Values of measured δ¹³C in groundwater reflect the mixture of C3 and C4 photosynthetic pathways of vegetation in the Sand Hills with a bias in DIC δ¹³C toward C4 (warm season) plant activity, which results in soil gas CO₂ δ¹³C of about −10 ‰ relative to C3 (cool season) plants which have a δ¹³C signature of about −25 ‰ (Cerling, 1984). The DIC δ¹³C bias toward C4 plants could be a result of C4 plants preference to more sandy soil, which allows faster infiltration and higher relative recharge; generally lower rates of transpiration from C4 plants, resulting in more relative recharge; and deeper rooting depths of C4 plants, which may result in greater influence on soil CO₂ near the water table (Barnes and Harrison, 1982). Accumulation of groundwater DIC in the unsaturated zone from a single isotopic source is supported by the relative invariance of δ¹³C between samples; a lack of clear relation between δ¹³C and age tracers (³H and ¹⁴C); and the bulk aquifer mineralogy, which does not contain a major carbonate source. For these reasons, the uncorrected ¹⁴C is likely a reasonable tracer of groundwater age for the older samples.

Finchville Monitoring Site

The Finchville monitoring site is immediately below the convergence of flow from focused groundwater discharge points within an exposed sheet of Pliocene-age sand and gravel, including SPR 27 and SPR 30 (table 4; fig. 3). Seven discharge measurements were collected at the Finchville monitoring site with a handheld acoustic Doppler velocimeter and ranged from 0.84 to 1.12 cubic feet per second (ft³/s; table 5). All discharge measurements, except one were rated as poor because the narrow channel only allowed for the collection of eight or fewer stations for a given discharge measurement. The poor measurement rating indicates that the difference between measured and true discharge may exceed 8 percent. The gage height record varied relatively little during the monitoring period (May 2019 to October 2019; fig. 9A) and measured specific conductance ranged between 75 and 220 microsiemens per centimeter (μS/cm) during the deployment period (fig. 9B). The gage height record shows several sharp upward spikes (typically 0.1–0.2 ft) coincident with sharp downward spikes in specific conductance (about 30–125 μS/cm) during periods of intense precipitation, after which the spring appears to immediately return to approximate baseline conditions after rainfall ceased. Nearly all rapid increases in gage height correspond with a decrease in specific conductance. Pronounced response in gage height and specific conductance to precipitation events is likely explained by capture of overland runoff from an exposed sheet of sand and gravel at the monitoring site. Overland runoff resulted in increased gage height and decreased specific conductance by dilution with low conductance rainwater. The measured data indicate a rapid return to the pre-storm event gage height and specific conductance of 200–220 µS/cm. Water temperature indicates diurnal variations and followed seasonal air temperature patterns (fig. 9B), indicating some influence of surface conditions and that observed temperature is not solely representative of groundwater temperature.

Mills Valley Monitoring Site

The Mills Valley monitoring site is located upstream from the Arnold streamgage where groundwater discharges from silty Quaternary-age deposits (fig. 3; table 4). The monitoring site is located within a well-defined topographic draw that captures surface runoff during precipitation events. The seven discharge measurements recorded ranged from 0.01 to 0.07 ft³/s (table 5, fig. 10A). Two measurements were collected with a handheld acoustic Doppler velocimeter, and five measurements were collected with a 3-in. Parshall flume (Turnipseed and Sauer, 2010; table 5). The measurements collected with the handheld acoustic Doppler velocimeter were rated poor to fair, and the measurements collected with the 3-in. Parshall flume were rated as fair to excellent (table 5). The discharge measurement collected on July 11 was at the end of a 10-day period of greater than 3 in. of precipitation recorded at nearby weather stations (National Centers for Environmental Information, 2021). Gage height ranged from 1.8 to 6.05 ft (fig. 10A) and specific conductance from 64 to about 560 µS/cm (fig. 10B). Water temperature shows diurnal variations and followed seasonal air temperature changes (fig. 10B), indicating the influence of surface conditions and that observed temperature is not representative of groundwater temperature.

During precipitation events, the increase in gage height and decrease in specific conductance occurred during a short period of time, which is suggestive of the capture of overland flow at the monitoring site. When changes to gage height were large, changes in specific conductance often persisted several days before returning to the base-flow range of 450–550 μS/cm (figs. 10B and 11B), which is consistent with discharge of shallow subsurface flow. The frequency and intensity of the largest precipitation events during the period of record made continuous data collection and interpretation difficult. Above average precipitation during the study period resulted in “backwater” conditions at the Mills Valley monitoring site. Elevated river stage resulted in gage height measurements that were likely not representative of upgradient discharge from the monitoring site and created the potential for discharge to bypass the measurement weir, such that discharge could not be reliably quantified from those periods. Observed gage height greater than 5.5 ft at the downstream Arnold streamgage was used to identify potentially affected gage-height data. Based on the discharge measurement collected on July 11, 2019 (table 5), the affected gage-height data from Mills Valley monitoring site likely still does represent an increase in discharge. At the end of a 10-day period of precipitation, during which greater than 3 in. fell (fig. 9C), the discharge measured at the Mills Valley monitoring site was the highest discharge measured (table 5). The gage height at the Arnold streamgage was stable during the preceding 10-hour period, indicating discharge measured on that day was a reasonable estimate of actual discharge from the monitoring site and not draining of backwater conditions. Further, the monitoring site was positioned such that increases in discharge from the monitoring site could be reliably identified at the onset of the precipitation events before increased river stage resulted in backwater conditions. In regard to water-quality parameters, a comparison of specific conductance and water temperature records from the Mills Valley monitoring site to nearby Hoagland monitoring site during backwater events indicated the continuous water-quality parameters remained a reliable representation of spring complex discharge; that is, elevated river stage altered base levels such that gage height data from the Mills Valley monitoring site was affected, but there was no evidence of mixing of stream water with the spring discharge at the monitoring location.

Comparison of Finchville and Mills Valley

Comparisons of discharge measurements and gage height records from the Finchville and Mills Valley monitoring sites indicate that each monitoring site responds differently to precipitation events. The gage height at the Finchville monitoring site was slower to respond to precipitation events and returned to base-flow (pre-storm event) levels shortly after precipitation ceased, whereas the Mills Valley monitoring site responded more immediately and changes in gage height persisted much longer (figs. 9A and 10A). The Finchville monitoring site showed a much smaller relative variation in measured discharge compared to the Mills Valley monitoring site during the period of record (May 2019 to October 2019; table 5). Specific conductance records from the two sites show a similar behavior as that of gage height: a delayed response and rapid recovery with overall less variation at the Finchville monitoring site, and rapid response and prolonged recovery with overall greater variation at the Mills Valley monitoring site (figs. 9B and 10B).

An example of the difference in gage height and specific conductance in response to precipitation events at the two monitoring sites is shown in figure 11. During the period from July 18 to July 23, a total of 0.35 in. of precipitation fell at the Stapleton weather station (National Centers for Environmental Information, 2021). Although larger precipitation events occurred during the period of record, backwater conditions at the Mills Valley monitoring site prevented a direct interpretation of the relative responses between the two monitoring sites for larger precipitation events. Gage height at the Finchville monitoring site was relatively invariant from May to July 21 (fig. 9A), whereas gage height at the Mills Valley monitoring site responds to smaller precipitation events before July 21 (fig. 10A). The spike in gage height at the Finchville monitoring site lasted approximately 1 hour, whereas the gage height at the Mills Valley monitoring site remained elevated for about 30 hours before returning to base flow (fig. 11A). A similar pattern is observed in the specific conductance record (fig. 11B), with the Finchville monitoring site showing rapid recovery and the Mills Valley monitoring site showing a prolonged recovery. The rapid response and recovery of the Finchville monitoring site to precipitation events shows overland flow is captured at the monitoring site but overall, the effect is relatively small and short lived. The stability at the Finchville monitoring site indicates that groundwater discharge from Pliocene-age sand and gravel deposits changes little in response to recent precipitation and likely short-term climatic changes. This interpretation is supported by mean groundwater ages of 2,400 and 2,700 years for Pliocene sites SPR 27 and SPR 30, respectively, indicating a larger storage volume that buffers discharge from short-term variations in recharge (table 7). At the Mills Valley monitoring site, the relation between gage height and specific conductance response varied between precipitation events. Gage height increased in response to larger precipitation events, such as July 21, and resulted in decreased specific conductance, consistent with overland flow. Conversely, gage height increased in response to relatively smaller precipitation events, such as those on July 18–19, and resulted in increased specific conductance (fig. 11B), which is suggestive of shallow groundwater flow path activation that mobilizes soil salinity. In both cases of response to precipitation, the prolonged recovery of gage height and specific conductance was consistent with capture of a component of shallow subsurface groundwater that is sensitive to short-term precipitation variability. The Quaternary-age deposits attenuated the hydrologic response to precipitation events, manifesting as a prolonged period of increased discharge as the system drained the recent recharge. Capture of shallow groundwater sensitive to short-term precipitation changes is consistent with younger ages observed in Quaternary-age deposits (table 7). Furthermore, during low-flow conditions the range in specific conductance measured at the Mills Valley monitoring site was two times greater than the range at the Finchville monitoring site (figs. 9B and 10B). Sampling results from this study indicated that groundwater with higher specific conductance generally had higher SI values and fraction modern, and younger estimated mean ages (tables 6 and 7), which further supports the relatively greater vulnerability of groundwater in the Quaternary-age deposits groundwater to short-term changes in precipitation patterns.

The measured water temperature for Mills Valley and Finchville monitoring sites displayed a diurnal temperature signal in response to daytime warming and nighttime cooling (figs. 9B and 10B). At the Finchville monitoring site, the daily minimum temperature recorded through the summer months was similar to the ambient groundwater temperature measured during sampling, indicating that measured water temperatures are not dominated solely by air temperature (fig. 9B; table 6). During the summer months at the Mills Valley monitoring site, the minimum measured water temperatures were much higher than the ambient groundwater temperature recorded in nearby wells, indicating influence of runoff and exposure to warmer ambient air temperatures prior to measurement (fig. 10B). 

Hoagland Monitoring Site

The Hoagland monitoring site is located on the main stem of the South Loup, near the headwaters, approximately 750 ft downstream from the confluence with the North Fork South Loup River (figs. 3 and 4). The seven discharge measurements made from May 2019 through October 2019 ranged between 20.5 and 57.1 ft³/s (table 5, fig. 12A). Discharge measurements were generally consistent, with the exception of elevated discharge on July 11 (57.1 ft³/s; table 5), measured at the end of a 10-day period of greater than 3 in. of precipitation recorded at nearby weather stations (National Centers for Environmental Information, 2021). Discharge was also measured on the South Loup above the confluence with the North Fork South Loup River (413053100221401) and calculated for the North Fork South Loup River (413055100221101) (table 5, fig. 12A). At the Hoagland monitoring site, gage height ranged from 2.42 to 5.58 ft and specific conductance ranged from 148 to 329 µS/cm (fig. 12B). Water temperature shows diurnal variations and a general trend upward then downward (fig. 12C) consistent with seasonal air temperature changes. A low temperature spike recorded in late May is associated with decrease in gage height and was likely a result of increased groundwater contributions with a cooler temperature. Cool temperatures in September and October did not correspond with decreased gage heights and are more likely a result of cooler air temperatures.

The gage height record at the Hoagland monitoring site is highly variable and indicates frequent increases in discharge in response to precipitation events (fig. 12A, B). The gage height recorded at the Hoagland monitoring site closely resembles the gage height record at the Arnold streamgage with the exception of two sharp increases in gage height at Arnold streamgage in response to two specific intense precipitation events (fig. 12A). These precipitation events are recorded as slower gage height increases at the Hoagland monitoring site indicating a slower response to overland runoff, as compared to the Arnold streamgage. Much of the drainage area within the headwaters of the South Loup River represented by the Hoagland monitoring site is nearly flat lying rangeland or grass-covered eolian sand dunes. The topography and high permeability soils likely reduce overland runoff to streams and maximize infiltration and recharge to the shallow groundwater system (Dugan and Zelt, 2000). However, the Hoagland monitoring site does capture overland runoff, as indicated by sharp decreases in specific conductance at the start of a gage height increase (fig. 12B). Overall, specific conductance at the Hoagland monitoring site tended to increase with an increase in discharge and gage height (fig. 12A, B), likely the result of activation of shallow subsurface flow that discharges to the river. Precipitation events and subsequent recharge elevated water tables, resulting in dissolution and mobilization of salts accumulated in the root zone (Steele and others, 2014). Similar capture of shallow subsurface flow and the resulting increase in specific conductance during relatively small precipitation events are observed at the Mills Valley monitoring site (fig. 11B). For many runoff events at the Hoagland monitoring site, there is a lag between gage height peaks and specific conductance peaks (fig. 12B), which supports the interpretation that rises in groundwater levels trigger increases in shallow groundwater discharge and measured specific conductance in the South Loup River. Prolonged recovery period of gage height and specific conductance (fig. 12B) similarly observed at the Mills Valley monitoring site (fig. 11B) is consistent with attenuation of the recharge signal in gage height and dropping water table signal and decreasing specific conductance. Recharge to the shallow groundwater system drains to the river during a period of weeks after the precipitation event.

In addition to the sharp short-lived decreases in specific conductance at the start of intense precipitation events, there were decreases in specific conductance near the peak of responses to larger precipitation events. This behavior was most clearly observed during the largest (late May 2019) and a relatively intermediate (late August 2019) response to precipitation events (fig. 12B). Decreases in specific conductance were associated with gage height increases, indicating overland flow, but the timing indicates these events were likely caused by a different mechanism of overland flow generation. Overland flow during early periods of the precipitation events was likely generated through infiltration excess, which occurs when precipitation rates exceed the soil infiltration capacity. Partially saturated soils have a lower hydraulic conductivity than fully saturated soil, and thus precipitation falling on relatively dry soil is less likely to infiltrate than precipitation on wet soil. Overland flow during later stages of the precipitation event, when gage height and thus discharge is at a maximum, is likely generated by saturation excess, which occurs when the soil column becomes fully saturated and any additional precipitation becomes runoff. The decreases in specific conductance at the peak gage height is likely caused by saturation excess.

Overall, the specific conductance and gage height data collected at the Hoagland monitoring site indicated capture of overland runoff and contributions of shallow groundwater to streamflow. The relatively similar but more subdued gage height at Hoagland as compared to the Arnold streamgage (fig. 12A) indicated that discharge at the Hoagland monitoring site was slightly less dependent on recent precipitation. However, the Hoagland record is more similar to the Mills Valley monitoring site (fig. 10) than the Finchville monitoring site (fig. 9), which suggests relatively young groundwater captured at Hoagland, similar to the Quaternary-age springs complex (table 7).