Water-Level and Nitrate Monitoring

Water-level measurements were made using standard USGS protocols for discrete water-level measurements using steel or electric tapes (Cunningham and Schalk, 2011). Water-quality samples were collected using USGS protocols for sampling wells (U.S. Geological Survey, 2018, 2023a). The discrete water-level and water-quality data collected from the groundwater monitoring network in Carson Valley are published online through the NWIS (U.S. Geological Survey, 2022).

The active groundwater level and nitrate wells monitored by the USGS range in depth from 11 to 608 ft below land surface (table 1). Spatially, the wells are sparsely distributed throughout Carson Valley (fig. 1). The network of wells used for monitoring water levels and nitrate was established in the 1980s and has largely remained consistent since 2010 with added water-level wells in Fish Springs and Johnson Lane community areas. Ruhenstroth, Gardnerville Ranchos, Alpine County, and the Airport community areas have only one water level well. To supplement the trend assessment, selected inactive wells were included for Ruhenstroth, Johnson Lane, and Indian Hills/Jacks Valley because, although these well locations were removed from the network, the wells have long-term monitoring data (greater than 5 years) that are valuable for this analysis.

The total number of wells, water levels, and nitrate and arsenic measurements within each community area are shown in table 2. The listed water-level wells and measurements correspond to the monitoring network from 1970 to 2022. Additional water-level data are available from inactive wells throughout the valley. The highest number of groundwater samples collected for nitrate analysis are within the active monitoring network that include Johnson Lane, Indian Hills/Jacks Valley, Ruhenstroth, and Gardnerville Ranchos. These areas were also included in previous investigations to evaluate the extent, sources, trends, and transport of nitrate (Thodal, 1996; Thomas and others, 1999; Rosen, 2003; Shipley and Rosen, 2005; Naranjo and others, 2013).

During a subset period from 1980 to 2022, the extent (number and distribution of sites) of monitoring varied (fig. 2). From 1985 to 1987, Thodal (1996) analyzed data from monitoring wells representing different land uses and sampled at varying frequencies (bimonthly, quarterly, and annually) to determine baseline groundwater chemistry conditions for Douglas County (including Carson Valley). Since that time, variation in sampling frequency has occurred because of updates to the monitoring well network, other monitoring efforts occurring within the valley, and changing well owner participation. In support of an investigation into water planning and modeling, water-level measurements increased from 2005 to 2006 (Yager and others, 2012). The increased number of nitrate measurements occurring from 2008 to 2009 was associated with a study designed to identify the source and fate of nitrate within the valley (Naranjo and others, 2013). Since the evaluation by Thodal (1996), there have been only discrete periods of arsenic monitoring (fig. 2C).

Trends in Water Levels and Nitrate Concentration

Water-level measurement and nitrate concentration trend assessments used data from active and selected inactive wells of the Carson Valley monitoring well network, as available from the USGS NWIS database (U.S. Geological Survey, 2022). The period of trend analysis for water levels was from 1970 to 2022. Time-series (temporal) variations in water levels are presented graphically by computing water-level change from the first measurement at the onset of monitoring. For these analyses, the depth to water was used to evaluate trends. Therefore, if the trend in depth to water is decreasing, water levels are rising. If the trend in depth to water is increasing, water levels are declining. Water-level measurements made immediately after well construction were not used in calculations because water-level perturbations from the process of drilling or injection of drilling fluid often have not equilibrated. Trend analysis for nitrate concentrations used data from the monitoring network obtained from 1983 to 2022. Changes in the depth to water and nitrate concentrations over time were analyzed in the statistical programming language R (R Core Team, 2021) using the Mann-Kendall nonparametric statistical test (Helsel and others, 2020) and were consistent with data analyses used by others (Thodal, 1996; Rosen, 2003; Shipley and Rosen, 2005; Naranjo and others, 2013). With this test, we chose an alpha value of less than 0.05 to represent a statistically significant monotonic trend at the 95-percent confidence limit. P-values greater than 0.05 are not considered statistically significant. Positive tau values indicate increasing trends, and negative tau values indicate decreasing trends. Spatial patterns of arsenic concentrations were mapped across Carson Valley, but given the lack of data, statistical trend analysis for arsenic could not be done.

Assessment of the Carson Valley Groundwater Monitoring Well Network

According to Garcia (1989), the groundwater monitoring well network in Douglas County (including Carson Valley) was designed to (1) monitor areas where heavily pumped municipal wells could draw in contamination or sites in areas with high potential for contamination; (2) gather data for long-term trend assessment of water quality; and (3) supplement data gaps and help define new sources of contamination. The intent was to design a monitoring network with the flexibility to adjust to changing groundwater conditions to better define the causes for the changes. As information on the long-term status of nitrate concentrations in Carson Valley become available, it may be necessary to adjust the network to ensure monitoring data provides suitable information for water resource planning purposes.

In the Carson Valley aquifer system, the well screen interval will affect the water quality drawn from wells. The general conceptual model shown on figure 3 illustrates the findings of Naranjo and others (2013) and Paul and others (2017) regarding the distribution of nitrate and arsenic in the alluvial aquifer of Carson Valley. Arsenic data assembled by Paul and others (2017) for the Gardnerville Ranchos, Minden, Gardnerville, Ruhenstroth, and Pinenut areas were shown to increase in concentration with aquifer depth. Deeper aquifer conditions also reflect longer flow paths, older groundwater, and greater contact with sediments, increasing the solubility and mobility of arsenic (Anning and others, 2012). Samples collected from wells throughout the valley (Naranjo and others, 2013) have shown nitrate concentrations near the water table are higher where inputs from septic systems are focused and steeply decrease with aquifer depth. The sharp gradients in nitrate concentrations are affected by oxidation-reduction (redox) conditions of the aquifer and the denitrification process (Naranjo and others, 2013). The State of Nevada well inventory database indicates that 225 wells have been deepened in Carson Valley since 1980 (State of Nevada Division of Water Resources, 2022). As domestic wells are deepened into the alluvial aquifer, they may have different water-quality characteristics. Given the risks to domestic and municipal water supply, the monitoring well network was reviewed to provide management considerations for potential updates to the nitrate and arsenic network in Carson Valley. The number of measurements per community area and the variability in concentrations and trends are important considerations in identifying needs for water resource planning. The assessment included a review of the existing and historical monitoring well data, knowledge gained from published reports, and a review of the monitoring frequency.

Carson River Basin Hydro Mapper

A web-based data visualization tool was developed to improve the dissemination and presentation of USGS data and other hydrologic data relevant to the Carson River Basin. The hydrologic dashboard, called the Carson River Basin Hydro Mapper (CBH), provides near real-time climate (snow water equivalent, precipitation, soil moisture), streamflow, and surface-water level data (for example, Lahontan Reservoir storage and level). The CBH interfaces with the USGS NWIS and the USGS National Map Services for watershed extents, the Natural Resources Conservation Service for snow measurement locations, the National Weather Service for rainfall and snow water equivalent, the National Oceanic and Atmospheric Administration for weather radar, and Douglas County, Nevada, for Carson Valley community area boundaries (table 3). The tool includes an interactive map viewer showing near real-time information and available discrete groundwater-level and water-chemistry data for select constituents. Assembling the integrated data into a single repository will allow for easier retrieval, analysis, and interpretation of data as they relate to important aspects of water resource management within the Carson River Basin.

Temporal Trends in Groundwater Levels

The change in water levels measured in the active well network in each community area are shown on figure 4. Select monitoring wells within the community areas of Johnson Lane, East Valley, Fish Springs, and Ruhenstroth have had more than 20 ft of water-level decline since the 1980s, with the onset of decline starting in the 1980s (figs. 4G, I–K). Water levels measured at monitoring well 22 in the Airport community area steadily declined by 14 ft at a rate of −2.9 feet per year (ft/yr) over the 41-year record (1981–2022; fig. 4A). Select wells within the community areas of Central Agriculture, Minden, Foothill, Alpine County, Gardnerville Ranchos, and South Agriculture fluctuate with rising and falling water levels likely because of recharge from the Carson River and proximity to nearby groundwater pumping wells (figs. 4B–F, H).

At Johnson Lane, monitoring wells in proximity to agricultural well 24 (well depth=218 ft) show similar patterns of decline (fig. 4G). The slope of water-level decline was −0.2 ft/yr at monitoring wells 23 and 24 over the 32-year (1990–2022) and 41-year record (1981–2022), respectively. For both wells, the most recent observation in 2022 reflects an average 7 ft of decline over the period of record. Monitoring well 25 underwent 20 ft of decline at a rate of −0.6 ft/yr over the 32-year record (1990–2022; fig. 4G). There is a substantial difference in the magnitude of decline and rate between two wells relatively close together, wells 23 and 24, and the more distal well 25. Using the average magnitude of decline of 7 ft and a rate of 0.22 ft/yr, well 25 declined an additional 13 ft and at nearly twice the rate at 0.41 ft/yr. At well 25, a long-term decline and annual variations likely were caused by a decrease in aquifer storage because of seasonal groundwater pumping. Well 23 and 24 water-level declines corresponded over the period of monitoring, but well 25 seems to have higher seasonal variations.

The change in water levels at monitoring wells 5 and 11 in South Agricultural were not apparent (fig. 4H). Variations in water levels between these wells and well 12 are depth dependent and are influenced by recharge from the Carson River, agricultural irrigation, (ditches), and groundwater pumping. Water-level change widely varied between 6.5 and 24 ft at monitoring well 12, a deeper (well depth=430 ft) agricultural well that is subjected to seasonal groundwater pumping. The well depths at monitoring wells 5 and 11 are relatively shallow (well depths=15 and 20.5 ft, respectively) and likely receiving sufficient replenishment from recharge from the land surface. Well depth information for all wells in the monitoring network used in this study is provided in table 1.

At East Valley, water levels measured at most monitoring wells are in decline. However, monitoring well 20 increased during an 8-year period between 1992 and 2001(fig. 4I). The water level at monitoring well 20 reached a maximum increase of 20 ft in 2008 and slowly declined 7 ft at a rate of −0.5 ft/yr. Coincidently, nearby monitoring well 18 had a water-level decline of 34 ft at a rate of −0.8 ft/yr during the 41-year record (1981–2022). The rate of water-level decline was also −0.8 ft/yr at monitoring well 16 during the 31-year record (1991–2022; fig. 4I). The rate of water-level decline was 60 percent greater at monitoring well 13 (−1.2 ft/yr) than the average rate of nearby wells 20, 16, and 18 (average=−0.7 ft/yr) during the more recent 22-year record (2000–22).

At Fish Springs, observed water levels vary widely at select wells with less than 10 years of record, from increasing to decreasing (fig. 4J). However, water-level declines are apparent at monitoring wells 7, 15, and 17. Although there is a 17-year gap in data at monitoring well 7, water levels have declined by 15 ft over the 32-year period (1990–2022). Declines were also observed at monitoring wells 15 and 17, which decreased by 27 and 26 ft over the 32- (1989–2021) and 41-year (1981–2022) record, respectively (fig. 4J). Although well 15 is considerably deeper (well depth=608 ft) than well 17 (well depth=95 ft) and is an agricultural well, the change in water levels closely compare over the period of monitoring.

The Ruhenstroth area has historically shown declining water levels (Maurer and Berger, 2007; Naranjo and others, 2013). However, the well network has been modified with the removal of several well locations where levels were monitored consistently since the 1980s, and the introduction of new monitoring locations which has disrupted our tracking of long-term water levels and began a new period of measuring water-level change. For example, monitoring well 4 was only recently added to the network in 2019 (fig. 4K). Over the 3-year period, water levels in well 4 have varied 8 ft, with the last measurement in 2022 showing a decline of 3 ft or −0.9 ft/yr. Inactive monitoring wells 37, 38, 39, and 40 provide a longer period of record of water-level change that has occurred since the 1980s. Monitoring well 37 had 11 ft of water-level decline over the 7-year record (1990–97) at a rate of −1.6 ft/yr. Monitoring well 38 had a water-level decline of 12 ft during the 32-year record (1981–2013) at a rate of −0.4 ft/yr. Monitoring well 39 had 7 ft of water-level decline over the 30-year record (1987–2017) at a rate of −0.2 ft/yr. Monitoring well 40, with a longer period of continuous measurement, measured 30 ft of water-level decline over the 38-year record (1981–2019) at a rate of −0.7 ft/yr.

The changes in water levels near monitoring well 20 in East Valley were examined more closely with time-lapse imagery and data from monitoring well 20 (fig. 5). The imagery indicates that the reservoirs were constructed between 1990 and 1994 and in use until 2006 just north of monitoring well 20. During that period, the reservoirs were unlined and stored reclaimed water temporarily on private property (Ed James, Carson Water Sub-Conservancy District, written commun., 2022). The period water was stored in the reservoirs coincides with a 20-ft increase in water levels at monitoring well 20 and a subsequent decline from reduction of recharge.

Spatial Distributions in Nitrate and Arsenic Concentrations

The spatial variability in the most current measurements of nitrate and arsenic concentrations in groundwater within Carson Valley are shown on figure 6. Figure 6A shows that most wells have nitrate concentrations of less than 5 mg/L. However, there are notable groups of wells within Indian Hills/Jacks Valley, Gardnerville Ranchos, Johnson Lane, and Ruhenstroth community areas with nitrate concentrations between 5 and 10 mg/L. There are also individual wells in Foothill and North Agricultural areas with nitrate concentrations greater than 10 mg/L. Spatially, arsenic concentrations are greater on the eastern side of Carson Valley and farther from the forks and mainstem of the Carson River (fig. 6B). In many select wells in Johnson Lane, East Valley, Ruhenstroth, Gardnerville Ranchos, South Agricultural, Minden, Central Agricultural, Airport, North Agricultural, and Indian Hills/Jacks Valley community areas, arsenic concentrations were greater than the EPA MCL criterion of 10 μg/L (U.S. Environmental Protection Agency, 2024).

Historical nitrate (1983–2022) and arsenic (1960–2019) concentrations measured from wells within each community area were plotted to show the range, distribution, and presence of MCL exceedances (fig. 7). For reference, the total number of measurements and wells in each community area are provided in table 2. Overall, the median nitrate concentrations observed from all wells within each community were less than 5 mg/L. However, wells within the North Agricultural, Indian Hills/Jacks Valley, Gardnerville Ranchos, Ruhenstroth, and Johnson Lane community areas have samples collected over the 37-year period with nitrate concentrations measured that exceed the EPA MCL of 10 mg/L (fig. 7A). These community areas are known to have had elevated nitrate concentrations in groundwater historically. From 2008 to 2009, 200 samples were collected from wells across Carson Valley that are influenced by different land-use categories. The highest nitrate concentrations existed in residential areas with high septic system density, and the lowest concentrations were in areas of vacant land and agricultural areas (Naranjo and others, 2013). The age of the septic system and depth to water were also important factors for increases in nitrate concentration in the aquifer.

In Central Agricultural, Johnson Lane, and Airport community areas, almost every well sampled had arsenic concentrations that exceed the MCL of 10 μg/L (fig. 7B). Median arsenic concentrations in all wells sampled in the North Agricultural, Central Agricultural, Johnson Lane, and Airport community areas are greater than the MCL of 10 μg/L. In many community areas, there are few groundwater samples analyzed for nitrate and arsenic. There is uncertainty on the spatial extent and temporal variability of nitrate and arsenic given that the monitoring network measures only a subset of areas and is with limited measurements collected over time. Further, the distributions for nitrate and arsenic (fig. 7) do not provide a complete representation of the aquifer within each community area; rather, they are measurements from a few wells sampled over the period of record. Elevated concentrations may be dependent on the screen interval depth in the well and proximity of the well to sources of the constituents.

Temporal Trends in Nitrate Concentrations

Discrete measurements of nitrate concentrations in the 11 monitoring wells in the active monitoring well network for each community area are shown on figure 8 (see table 1 wells 26–36). In Johnson Lane (monitoring well 34; fig. 8A) and Garnerville Ranchos (monitoring well 27; fig. 8F), nitrate has exceeded the EPA MCL of 10 mg/L (U.S. Environmental Protection Agency, 2024). Nitrate concentration is increasing steadily at monitoring well 33 (Johnson Lane) with the last measurement of 7.3 mg/L occurring in 2022 (fig. 8B). Nitrate concentrations observed in monitoring wells 29 (East Valley), 30 (Genoa), 31 and 32 (Johnson Lane), 35 and 36 (Indian Hills/Jacks Valley) are less than 2 mg/L and have not substantially varied over the period of record. Concentrations at monitoring well 31 (Johnson Lane) have been below detection since 2005 when the domestic well was deepened from a well depth of 175 to 350 ft (see table 3 in Naranjo and others, 2013). Before being deepened, nitrate concentrations steadily increased from 0.6 to 1.2 mg/L between 1993 and 2004 (USGS well 390055119421901; U.S. Geological Survey, 2022). Nitrate concentrations at monitoring well 26 (Ruhenstroth) increased over the period of record but have remained relatively constant at 5.6 mg/L since 2010. Nitrate concentrations at monitoring well 28 (East Valley) increased from 2001 to 2007 with a maximum of 9.1 mg/L then increased again from 2019 to 2021 to a second peak of 5.1 mg/L (fig. 8E).

Statistical Analysis of Water Levels and Nitrate Concentrations

Mann-Kendall trend analysis of depth to water level and nitrate concentrations in active and selected inactive monitoring wells show statistically significant increasing trends in most of the wells (table 4). In this analysis, the increasing depth to water trend indicates declining water levels, and the decreasing depth to water trend indicates a rising water table. The depth to water level from 17 of 26 monitoring wells (65 percent) analyzed is declining (increasing trend in depth to water; p<0.05). Analysis from 3 monitoring wells (12 percent) indicated depth to water was decreasing (p<0.05), and 6 wells indicated stable water levels with no apparent trend. The trend analysis for nitrate concentrations indicates 9 out of 14 monitoring wells (64 percent) are increasing (p<0.05). Only one monitoring well (well 36) indicated a decreasing trend, and three monitoring wells (wells 32, 39, and 41) indicated no trend in conditions. Analysis was not done on one well (well 31) because concentrations were below the level of detection (less than 0.01 mg/L).

Assessment of Monitoring Well Network

Whether domestic or municipal, groundwater pumping has been shown to contribute to water-level declines throughout the valley and declines of 5 to 40 ft have been forecasted (Yager and others, 2012). As water levels decline, domestic well owners have been deepening their wells. Within the monitoring well network, water-level measurements are made in 9 of the 16 community areas (excluding well 1 in Alpine County) that collectively represent about 24 percent of the Carson Valley area (table 2). There are no water-level measurements being made in Gardnerville, Genoa, Indian Hills/Jack Valley, North Agricultural, or Pinenut communities, which collectively comprise about 76 percent of the Carson Valley area.

In some areas of Carson Valley, nitrate in the groundwater continues to pose risks to domestic wells. Evidence indicates that nitrate contamination has increased rather than decreased in the well network at more locations over time. The extent to which water table variations affect aquifer nitrate concentrations is unknown. In controlled laboratory experiments, fluctuating water-level conditions may promote nitrification, contributing to highly variable nitrate concentrations in the aquifer (Zuo and others, 2023). Groundwater pumping for municipal, agricultural, and domestic uses may also contribute to nitrate removal from the aquifer (Naranjo and others, 2013). Thus, changes in land use or groundwater pumping rates play a role in mobilizing nitrate and arsenic. The existing monitoring well network may not be optimal in assessing the fate and transport of nitrate or arsenic to municipal or domestic wells. Periodic assessment of the monitoring well network can help to determine whether the existing well locations and frequency of sampling are sufficient to establish trends in water quality, especially in areas where there is high existing or planned municipal and domestic use. The monitoring network well density for community areas shown in table 2 can provide insight to areas where data gaps exist. Future expansion of data collection may be needed for tracking changes in water levels, nitrate, and arsenic concentrations with development in these community areas.

Nitrate transport simulations by Naranjo and others (2013) indicate that concentrations would continue to increase in areas of higher density of septic system use and in areas with older septic systems and longer septic-loading periods. The results of the trend analysis herein, a decade later, have shown that observed concentrations are continuing to increase in the alluvial aquifer. The existing network of wells could be improved to measure the change in nitrate concentrations in the community areas with high septic system density, as identified in Naranjo and others (2013), particularly in Indian Hills/Jacks Valley, Gardnerville Ranchos, Ruhenstroth, and Johnson Lane.

Stable trends in nitrate concentrations provide assurance that increased loading and transport from high septic system density areas (Naranjo and others, 2013) are not degrading water quality. For example, well locations 29–32, 35, and 36 (fig. 8) in the East Valley, Johnson Lane, Indian Hills/Jacks Valley, and Genoa have consistently had nitrate concentrations below 3 mg/L. Well location 31 has been consistently below the detection limit since 2002. At these wells, it might be acceptable to reduce the frequency of samples to every 5 years to ensure consistency in the overall trends. New monitoring wells along groundwater flow paths downgradient of high septic density areas, as identified in Naranjo and others (2013), can assist in determining transport away from source areas to other parts of the alluvial aquifer.

Historical data are too scant to allow for a clear understanding of the transport behavior of arsenic in the alluvial aquifer. Spatially, sufficient data exist to demonstrate that western Carson Valley generally has lower arsenic in groundwater than the eastern side. An improved understanding of the spatial distribution could be obtained if arsenic samples were included in the existing nitrate monitoring well network. However, the transport of arsenic to municipal wells would necessitate additional targeted monitoring. For example, an improved understanding of arsenic transport may warrant samples collected from monitoring wells from different redox zones (depths) within the aquifer.

Carson River Basin Hydro Mapper

The CBH tool was developed to provide easy, online access to groundwater, streamflow, and other spatially distributed hydrological data for the Carson River Basin (fig. 9; U.S. Geological Survey, 2023b). The tool provides access to near real-time information on climate, streamflow, and reservoir level as well as nitrate and arsenic data. Discrete water-level data measured and reported by the USGS, and the NDWR provide users with contextual hydrologic information from a dense monitoring network. The USGS-collected nitrate and arsenic concentrations are also shown through the CBH for groundwater and surface water sites in the Carson River Basin (fig. 10). Data from a single well can be compared to all wells within the Carson River Basin as a timeseries, allowing for efficient evaluation of conditions over time.