Naturally Occurring Metals in Unregulated Domestic Wells in Nevada, USA

Abstract

The dominant source of drinking water in rural Nevada, United States, is privately-owned domestic wells. Because the water from these wells is unregulated with respect to government guidelines, it is the owner’s responsibility to test their groundwater for heavy metals and other contaminants. Arsenic, lead, cadmium, and uranium have been previously measured at concentrations above Environmental Protection Agency (EPA) guidelines in Nevada groundwater. This is a public health concern because elevated levels of these metals are known to have negative health effects.

We recruited individuals through a population health study, the Healthy Nevada Project, to submit drinking water samples from domestic wells for testing. Water samples were returned from 174 households with private wells. We found 22% had arsenic concentrations exceeding the EPA maximum contaminant level (MCL) of 10 μg/L. Additionally, federal, state, or health-based guidelines were exceeded for 8% of the households for uranium and iron, 6% for lithium and manganese, 4% for molybdenum, and 1% for lead. The maximum observed concentrations of arsenic, uranium, and lead were ~80, ~5, and ~1.5 times the EPA guideline values, respectively. 41% of households had a treatment system and submitted both pre- and post-treatment water samples from their well. The household treatments were shown to reduce metal concentrations, but concentrations above guideline values were still observed. Many treatment systems cannot reduce the concentration below guideline values because of water chemistry, treatment failure, or improper treatment techniques. These results show the pressing need for continued education and outreach on regular testing of domestic well waters, proper treatment types, and health effects of metal contamination. These findings are potentially applicable to other arid areas where groundwater contamination of naturally occurring heavy metals occurs.

1. Introduction

For many drinking water contaminants, the United States Environmental Protection Agency (US EPA) has established maximum contaminant levels (MCLs). The MCL is developed by considering the maximum level of a contaminant in drinking water at which no known or anticipated adverse health effects would occur, as well as other factors such as cost, treatment technologies, and measurement techniques (US EPA, 2015a). Municipal water supplies are required to meet these levels, but privately-owned domestic drinking water wells are unregulated with respect to government guidelines. Therefore, it is the homeowner’s responsibility to test and mitigate contamination. In the US, it is estimated that 13.7 million households use a private well for their primary source of water (Bureau, 2019). In Nevada, private wells are the primary drinking water source in rural and peri-urban areas, with an estimated statewide private well population of 182,000 people (Kenny et al., 2009). To avoid exposure, many private well users install treatment devices and regularly maintain and monitor their performance or find alternative water supplies (e.g., a nearby well, bottled water) (Walker et al., 2008). Testing is the only way to ensure water is safe to drink and many states provide guidance on the regularity of testing (Zheng and Ayotte, 2015). However, studies have shown that many barriers exist for private well water testing in many US communities (e.g., socioeconomic, lack of a perceived problem, inconvenience, procrastination, complacency, lack of knowledge) (Chappells et al., 2015; Flanagan et al., 2016; Imgrund et al., 2011). This is a particularly relevant public health concern in the state of Nevada because of geologic, climatic, and population factors that could potentially expose private well owners to elevated concentrations of metals in domestic wells.

Arsenic (As) is an inorganic contaminant that is present above the EPA MCL of 10 μg/L in ~4.5% to 11% of US groundwater wells, generally because of geological (or geogenic) processes (Ayotte et al., 2011a; DeSimone, 2009; Focazio et al., 2006). Because of its elevated concentration in US groundwater and the potential health effects of exposure - such as skin, pulmonary and cardiovascular diseases, and cancers (Minatel et al., 2018) - As in drinking water has been the focus of many previous studies (e.g., Lombard et al., 2021; Podgorski and Berg, 2020; Shaw et al., 2005; Smedley and Kinniburgh, 2002; Welch et al., 1988; Zheng and Ayotte, 2015). Other heavy metals commonly found in US groundwater above EPA MCLs include uranium (U), lead (Pb), cadmium, and mercury (Ayotte et al., 2011a; DeSimone, 2009; Focazio et al., 2006; Hoover et al., 2017; Jurgens et al., 2010; Nolan and Weber, 2015). Exposure to these metals at elevated concentrations have been linked to kidney damage and increased cancer risk (US EPA, 2015b). Lead exposure at elevated concentrations has been linked to developmental issues in children (Gibson et al., 2022; US EPA, 2015b). Some metals do not have an EPA MCL, and therefore other health-based guidelines may be recommended. The variability of geogenic metal concentration in US groundwater is influenced by the geologic composition of aquifers, the geochemistry of aquifers (e.g., pH, oxidation-reduction conditions), and the hydroclimate of the region (e.g., infiltration and evaporation) (Ayotte et al., 2011a, 2011b).

Nevada has diverse geologic terrain, a complex tectonic history, and an arid climate – this results in groundwater that contains elevated levels of heavy metals, with As being the most commonly studied (Anning et al., 2012; Bevans et al., 1998; Bexfield et al., 2011; Lico and Seiler, 1994; Maurer et al., 2008; Paul et al., 2010; Pavelko and Orozco, 2015; Steinmaus et al., 2005; Walker et al., 2008, 2005; Walker and Fosbury, 2009; Welch et al., 1997, 1988; Whitney, 1994). National groundwater As modeling studies estimated that 14% of the domestic well population in Nevada (Ayotte et al., 2017) and 16% in the entire Great Basin hydrologic province (DeSimone et al., 2015) have As levels that exceed the MCL of 10 μg/L. Previous studies testing private groundwater wells in Nevada demonstrated As exceedances in northern Nevada, with the greatest As concentrations observed in the Carson Basin near the city of Fallon (Walker et al., 2008, 2005; Walker and Fosbury, 2009). In addition to As, these studies found elevated concentrations of iron (Fe), manganese (Mn), molybdenum (Mo), and U (Anning et al., 2012; Lico and Seiler, 1994).

In this study, we (1) surveyed 609 Nevadans who rely on private wells to assess their well water testing history, treatment types, and water consumption. For a subset of those surveyed, in 174 households (2) we measured a suite of metals in household water to assess the risk of exposure to heavy metals in Nevada private wells. We also (3) measured both pre- and post-treatment water samples from the same private well to determine the effectiveness of water treatments. We found there is a pressing need to educate Nevadans about regularly testing their private well, the potential hazard of metal contamination in private wells, and proper water treatment for the metals of concern.

2. Methods

2.1. Recruitment and survey of participants

Participants of the Healthy Nevada Project (HNP) were recruited for this research. The HNP is a large (>50,000 participants) all-comers population health study in Nevada that includes cross-referenced electronic health records, socio-demographic data, whole-exome sequences, and social health determinant data. Details about the HNP were previously published (Grzymski et al., 2020; Read et al., 2021; Schlauch et al., 2022, 2020). Specifically, HNP participants who consented to be contacted for future studies were invited via email to complete the HNP private well survey. The HNP private well survey contained 11 questions regarding their well, water treatment, and drinking water habits (Supplementary Material). The private well survey was created and disseminated on the Survey Monkey platform (www.SurveyMonkey.com).

2.2. Sampling

Survey respondents who had a private well and wanted to submit a well water sample were emailed an informed consent document for water sampling. Consented individuals were mailed an at-home sampling kit. The kit included sampling instructions, a data collection sheet, pre-cleaned (1% nitric acid washed) sample bottle(s), and a return-shipment box. Sampling protocols (Supplementary Material) requested participants to purge their well casing of stagnant water by allowing the water to run for 2 to 4 hours prior to sampling, which is similar to state well water sampling guidelines (Donaldson et al., 2012). After purging, participants were asked to sample from the tap they used for drinking, cooking, and other household activities (referred to as “household water” herein). Households without a water treatment system were instructed to sample from the tap they most commonly used for household activities (referred to as “households without treatment”). Homeowners with a water treatment system in their home were instructed to sample after the treatment system from the tap they most commonly used for household activities (referred to as “households with treatment”). For the purpose of this study, any method, media, etc. that is used to treat, purify, or filter well water is considered a “treatment.” To study the effectiveness of water treatment, households with treatment were asked to submit an additional pre-treatment sample. The pre-treatment sample represents the groundwater geochemistry and the households do not use the pre-treatment water for drinking, cooking, and other household activities. The post-treatment sample represents the water after undergoing water treatment.

Additional metadata requested of all participants were sample location, date, time, treatment method prior to sampling, color and smell of the well water, duration of well purging, and any other relevant information pertaining to the participant’s water and sampling experience.

2.3. Geochemical analysis

Participant sampling kits were mailed back to the Desert Research Institute (DRI) in Reno, Nevada. Samples were accessioned and de-identified. Related data were manually entered in an electronic database. Using ultra-high purity nitric acid (Aristar Ultra, VWR Chemicals BDH), the water samples were then preserved in 1% nitric acid and analyzed following EPA Method 200.8. All but one sample were received within the sample hold times outlined in EPA Method 200.8. The one sample received outside of the sample hold time was discarded. Sample geochemical analysis was conducted at the University of Nevada Core Analytical Laboratory using a Shimadzu 2030 Inductively Coupled Plasma Mass Spectrometry (Shimadzu Corporation, Columbia, Maryland, US) instrument. Settings for the ICP-MS are provided in Table S1. The targeted elements include As, copper (Cu), Fe, lithium (Li), Mn, Mo, Pb, and U. Calibration standards (Inorganic Ventures, Christiansburg, Virginia, US) for the ICP-MS instrument captured the concentration range of the samples.

Quality assurance (QA) and quality control (QC) steps were taken at DRI and during ICP-MS analysis. DRI laboratory blanks and internal standards were submitted with the samples for analysis. DRI internal standards were made using a standard of known elemental concentration (Inorganic Ventures, Christiansburg, Virginia, US) with the elemental concentration targeting expected concentrations. QC included comparing the calculated internal standard concentrations to measured values as well as ensuring the DRI laboratory blank elemental concentrations were below sample concentrations or below detection limits. The average offset of a ~10 μg/L internal DRI standard (n=6) was <5% for As and Pb and <11% for Cu, Mn, Mo, and U. The average offset of a ~50 μg/L internal DRI standard (n=4) was <5% for Fe, Mo, and Pb and <16% for As, Cu and Mn. Additionally, during ICP-MS analysis precision recovery check standards (Agilent, Santa Clara, California, US) were measured every 10 to 15 samples (depending on sample analysis duration) to ensure the ICP-MS instrument response did not drift > 5% during analysis. Also during ICP-MS analysis, internal standards (Inorganic Ventures, Christiansburg, Virginia, US) were added to further monitor ICP-MS instrument drift.

The detection limit for all analyzed metals was 0.05 μg/L. The detection limits were determined by a signal-to-noise ratio of >3, where the noise is the instrument response to the matrix. In some cases, the measured detection limit was below 0.05 μg/L, but 0.05 μg/L was used in this study as it is the reported detection limit for the element by the instrument manufacturer. The results were then reported to the homeowners including remediation options, as necessary.

2.4. Statistical analysis

Non-detects complicate subsequent data analysis. Discarding values <0.05 μg/L (i.e., below detection limit) or replacing them with zero may introduce a bias (Palarea-Albaladejo and Martín-Fernández, 2015) when computing summary statistics (i.e., mean median, and standard deviation), testing differences among groups, and correlation coefficients and regression equations (Helsel, 2011). Therefore, we chose to impute values <0.05 μg/L using R package (R Core Team, 2021) zCompositions (Palarea-Albaladejo and Martín-Fernández, 2015). We used the robust regression on order statistics (ROS) multiplicative lognormal replacement approach via the R package NADA for the imputation. This approach uses the measured values and assumes a distribution for the imputation of the censored portion (Helsel, 2005; Lee, 2020). To assess differences in metal concentrations in household water between homes without treatment and those with treatment as well as differences between pre- and post-treatment concentrations, we used the cendiff function in R, which is part of the NADA package. This function tests if there is a difference between two empirical cumulative distribution functions. The cendiff function is the Peto and Peto modification of the Gehan-Wilcoxon test (Helsel, 2005). This approach is appropriate for left-censored log-normal data, as is commonly observed in geochemical studies (Helsel, 2005), and was observed in this study.

3. Results

3.1. HNP private well survey

Since August 2020, a total of 41,632 invitations for the HNP private well survey were sent to consenting participants (Figure 1). We received a total of 2,263 (5.4%) complete answers. Of these, 609 (1.5%) individuals have a well and were interested in submitting a well water sample, of which 257 (0.6%) individuals consented to submit a sample, and 174 respondents returned water samples for analysis. Participants spent 2 minutes answering the survey questions.

Figure 1:

Numerical breakdown of participants in this study. Blue numbers are number of participants.

The findings from the 609 respondents to the HNP private well survey are summarized in Figures 2 and S1. Of the 609 respondents, 52.7% (n=321) never had their well water quality analyzed and 47.1% (n=287) had their well tested. Of the individuals who had their well water quality analyzed, 60.6% had tested their well in the last 10 years (Figure 2B) and 39.4% (n=113) either had their well tested more than 10 years ago or were unsure. Figure S1 summarizes the results with respect to home water treatment type. 44.7% (n= 272) of the respondents do not have a water treatment or are unsure if they have a water treatment system installed in their home (Figure S1A). Of those who do have a water treatment (n=337), the most common type of treatment is a filter (52%), followed by softener (50%), and 44.5% of respondents had more than one type of treatment (Figure S1C).

Figure 2:

Bar graphs showing results of the Healthy Nevada Project well water survey. A) Responses to the question if homeowners previously had their well water quality tested. “No” is in green and “Yes” is in black. B) The year of the well water quality test by percent of those that have tested their well.

3.2. HNP household water metal concentrations

174 HNP well water survey respondents returned water samples for analysis. Sampling began in November 2020 and was conducted through July 2021. Samples were returned from throughout Nevada with the majority of samples (n=160) from northern Nevada, 5 from northeastern California, and 9 from the Las Vegas, Nevada area (Figure 3). This spatial bias to northern Nevada was consistent with HNP recruitment focus. The majority of samples were returned from the Truckee Basin (64 samples), Carson Basin (44 samples), and North Lahontan Basin (42 samples) (Figure 3). Of the 174 HNP household water samples tested, 88 had no water treatment and 86 had water treatment (Table 1). We compared the concentration results to EPA guidelines for As, Cu, Pb and U (US EPA, 2015a); state secondary guideline for Fe and Mn (State of Nevada Legislative Counsel, 2018); the Health Based Screening Level (HBSL) value (Toccalino and Norman, 2006) for Mo; and the drinking water threshold from Lindsey et al. (2021) for Li (Table 1).

Figure 3:

Map of the study area. (A) Map of the United States showing the study area (red box). (B) Locations of homes that submitted a household water sample (red dots). Blue boundaries are the six-digit hydrologic unit classifications (HUC 6) for this region. Relevant HUC 6 basins are labeled in italics. Major cities are also shown.

Table 1:

Results from the household water samples for homes without water treatment and with treatment (results shown are post-water treatment). Detection limit for all elements was 0.05 μg/L.

Element	As	Cu	Fe	Li	Mn	Mo	Pb	U
Guideline (μg/L)	10^	1300^	600*	60#	100*	40&	15^	30^
Without Treatment
N	88	88	88	88	88	88	88	88
Minimum (μg/L)	0.10	0.05	35.70	0.23	0.10	0.05	0.05	0.05
Maximum (μg/L)	795	1202	1156	180	616	59	23	147
Mean (μg/L)	24.2	58.3	251.0	19.8	28.1	7.2	1.2	10.8
Median (μg/L)	2.5	12.6	142.2	6.9	1.9	2.8	0.3	2.9
Exceedances number (percent %)	18 (20)	0 (0)	9 (10.2)	6 (6.8)	4 (4.5)	3 (3.4)	1 (1.1)	7 (8.0)
With Treatment
N	86	86	86	86	86	86	86	86
Minimum (μg/L)	0.05	0.05	0.05	0.05	0.05	0.05	0.05	0.05
Maximum (μg/L)	175	421	4583	1855	8577	211	25	168
Mean (μg/L)	11.1	45.3	206.2	37.9	134.1	8.4	1.0	9.1
Median (μg/L)	2.4	9.3	55.7	4.2	0.9	2.7	0.1	0.7
Exceedances number (percent %)	21 (24)	0 (0)	5 (5.8)	5 (5.8)	6 (7)	4 (4.7)	1 (1.2)	8 (9)
p value 1	n.s.	n.s.	p<0.001	n.s.	p<0.01	n.s.	n.s.	p<0.001
Total exceedances number (percent %)	39 (22)	0 (0)	14 (8)	11 (6)	10 (5.7)	7 (4)	2 (1)	15 (8.6)

^{^ -}EPA guideline

^{* -}State guideline

^{# -}Drinking water only threshold, Lindsey et al. (2021)

^{& -}Health Based Screening Level

^{1 -}p values calculated using the cendiff function, n.s. = not significant

3.2.1. Households without water treatment

Households without a water treatment submitted a water sample from the tap most commonly used for household activities. The greatest concentration range of measured metals from HNP households without water treatments (n=88) was observed for Cu. The maximum concentrations of As, Mn, U, and Pb all exceeded guideline values by ~80, 6, 5, and 1.5 times respectively (Table 1, Figure 4). No exceedances above the EPA guideline were observed for Cu. 20% of the households exceeded guideline values for As, followed by Fe, U, Li, Mn, Mo, and Pb.

Figure 4:

Histogram of household water elemental concentration. Households without water treatment (A, grey) and with water treatment (B, blue). Results are plotted as Log₁₀ of the concentration, with the red line indicating the guideline value from Table 1.

3.2.2. Households with water treatment

Households with a water treatment submitted a water sample post-treatment from the tap most commonly used for household activities (n=86). The results show the greatest concentration range from HNP households was Mn followed by Fe (Table 1). The maximum concentrations of Mn, Li, and As all exceeded guideline values by ~86, 31, and 18 times, respectively (Table 1, Figure 4). No exceedances above the EPA guideline were observed for Cu. The greatest number of exceedances were observed for As, with 24% of the households with treatment, followed by U, Mn, Fe, Li, Mo, and Pb (Table 1).

3.2.3. Comparison between homes without and with water treatment

Median concentrations were lower for all metals in HNP households with water treatment compared to households without water treatment (Table 1). Significant (cendiff function; p<0.01) differences were observed for Fe, Mn, and U (Table 1). Although median concentrations were lower in households with water treatment, the number of and percent exceedances for As, Mn, Mo, and U were greater in homes with treated water compared to homes without treated water (Table 1). Conversely, the number and percent exceedances for Fe and Li were fewer in homes with treated water.

For the 174 households studied, 41% (n=72) of participants in the well water study had one or more elements exceeding guideline values (Table S2). In 11% (n=10) of homes without treatment and 15% (n=13) of homes post-water treatment, two or more exceedances were observed, with one household exceeding As, Li, Mn, Mo, and U guideline values (Table S2). The most observed co-exceedances were As and Li (n=5), As and U (n=5), and Fe and Mn (n=3).

3.3. Pre- and post-treatment results

Of the 86 HNP households that submitted a water sample post-treatment, 72 households also submitted a pre-treatment sample from the same well water system (Figure 1). Therefore, the post-treatment results shown in Table 2 comprise 72 of the 86 results shown in Table 1. Overall, median concentrations were reduced after treatment for all metals except for Cu (Figure 5), and differences between pre-and post-treatment concentrations were significant (cendiff function; p<0.01) for As, Fe, Li, Mn, Mo, Pb and U (Table 2).

Table 2:

Results from pre-water treatment and post-water treatment samples from the same household. Guideline values are found in Table 1.

Variable	As	Cu	Fe	Li	Mn	Mo	Pb	U
Pre-Treatment
N	72	72	72	72	72	72	72	72
Mean (μg/L)	22.4	49.4	731.3	26.4	54.1	14.0	25.2	15.3
Median (μg/L)	8.0	6.7	130.3	7.8	4.7	4.3	0.2	2.1
Exceedances number and (percent %)	34 (47)	0 (0)	14 (19)	9 (12.5)	10 (14)	5(6.9)	5 (7)	10 (14)
Post-Treatment
N	72	72	72	72	72	72	72	72
Mean (μg/L)	11.2	40.3	213.5	14.1	158.1	9.4	0.9	9.1
Median (μg/L)	1.4	8.1	44.3	3.5	0.8	2.5	0.1	0.4
Exceedances number and (percent %)	14 (19)	0 (0)	5 (7)	3 (4.2)	6 (8)	4(5.6)	1 (1)	6 (8)
p value ^	p<0.001	n.s.	p<0.001	p<0.01	p<0.001	p<0.01	p<0.001	p<0.01

^{^}p values calculated using the cendiff function, n.s. = not significant

Figure 5:

Box and Whisker plots showing elemental concentration pre-water treatment (green) and post-water treatment (blue) from the same household. Whiskers represent 5/95 percentile. Red lines denote guideline value.

Pre-treatment As concentrations were above the guideline value for 47% (n=34) households. For those households, approximately 61% of the As was removed post-treatment (Figure 6). Water treatments removed ~68% of the Fe when pre-treatment concentrations were above the guideline value (n=14). Variable treatment effectiveness was observed for Li, but ~47% of the Li was removed when pre-treatment concentrations were above the guideline value (n=9). Variable treatment effectiveness was also observed for Mn, with four households having an addition of Mn to the household water after treatment. Similarly, one treatment system added Mo to the drinking water. All systems were effective at reducing Pb, with 99% of Pb removed when groundwater concentrations were above guideline values (n=5). Approximately 37% of the U was removed when groundwater concentrations were above the guideline (n=10) value.

Figure 6:

Concentration of metals pre-treatment and post-treatment from the same well water system. Pre-treatment concentrations above guideline values are shown. Black line indicates no reduction in concentration, green dotted line a 50% reduction, and light green line a 90% reduction. Black dots are households with one treatment type (One), red dots are households with multiple treatment types (Multi), orange dots have multiple treatments including a Reverse Osmosis (RO) system (Multi-RO), and yellow dots are households with RO systems only. Copper is not shown as all pre-treatment concentrations were below the guideline value.

4. Discussion

4.1. Groundwater and drinking water metals

The majority of HNP households analyzed for metals in household water were located in northern Nevada within the Truckee, Carson and North Lahontan Basins. Hydrologically, these basins drain the Sierra Nevada and are part of the Great Basin hydrologic province, which include a network of internally drained valleys extending across northeastern California, Nevada, and western Utah. The study area is in the Basin and Range physiographic province that includes tectonically active areas of fault-bounded mountain ranges and valleys in the western US. The hydroclimate of northern Nevada is generally characterized by a wet season from November to April and dry season from May to October (Null et al., 2010). These geologic, hydrologic, and climatic factors are important for the resulting geochemistry of the groundwater of the basins.

Prior groundwater metal concentration studies in Nevada (Bevans et al., 1998; Lico and Seiler, 1994, 1994; State of Nevada Legislative Counsel, 2018; Steinmaus et al., 2005; Walker and Fosbury, 2009; Welch et al., 1997; Welch and Lico, 1998; Whitney, 1994) and national compilations (Ayotte et al., 2011a; DeSimone, 2009; Focazio et al., 2006) are available for comparison. We compare the exceedances observed from HNP households to these prior studies, as appropriate, to provide a hydrologic context of our observations.

4.1.1. Arsenic

For As, 20% of HNP homes without a water treatment system had concentrations above the EPA guideline. For HNP households with treatment, 24% had As concentrations post-water treatment above the EPA guideline. These results are greater than the statistically based model-predicted exceedances for As in private wells in Nevada (14%) (Ayotte et al., 2017) and the Great Basin (16%) (DeSimone et al., 2015). Our observations are in close agreement to the statistical modeling approach in the Southwest (including the Basin and Range aquifers tested for this study) from Anning et al. (2012) which showed 24.9% of groundwater wells in principal aquifers exhibited As concentrations greater than the MCL. Regional groundwater monitoring results in the Carson Basin Fallon area showed particularly high concentrations of As (>2000 μg/L) (Anning et al., 2012; Bevans et al., 1998; Lico and Seiler, 1994; Shaw et al., 2005; Walker et al., 2008, 2005; Walker and Fosbury, 2009; Welch, 1994; Welch and Lico, 1998). The greatest As concentrations observed in our study (794 and 534 μg/L) were also collected in the Carson Basin near Fallon, Nevada.

Our results are similar to prior As drinking water studies from the western US. A study on the Navajo Nation showed 15% of the 497 tested water sources had As concentrations above the guideline value (Hoover et al., 2017). Studies from North and South Dakota found 26% of 317 private wells (Powers et al., 2019) and 29% of 441 private wells (Sobel et al., 2021) exceeded the As guideline. In Arizona, the study of 16 untreated well water samples showed 37.5% exceeded As guideline (Lothrop et al., 2015).

Previous statistically-based modeling across the Great Basin and southwestern US demonstrated links between elevated As and internally drained valleys with high rates of evaporation (Anning et al., 2012; Ayotte et al., 2011b). In addition to elevated As, a greater pH, and Fe, Mn, Mo, and chloride concentrations were observed across the Great Basin and southwestern US (Anning et al., 2012; Ayotte et al., 2011b). More locally, similar correlations were observed in the Carson Basin, with greater As concentration corresponding to greater U, Fe, Mn and carbon concentrations attributed to evaporation, redox reactions and possibly adsorption (Welch and Lico, 1998).

4.1.2. Other metals

Unlike As, the state guideline for Fe is not a health-based threshold. Rather, it is a secondary guideline based on a concentration of Fe where aesthetic and taste issues occur. Prior work in the Carson Basin showed between 9% (7 of 79) and 27% (3 of 11) wells (Maurer et al., 2008; Welch, 1994) were above the guideline value for Fe. Our results were slightly lower, with 8% of HNP households exceeding the state guideline. Co-exceedances of Fe and Mn were observed potentially because these elements are more soluble under reducing hydrologic conditions (Ayotte et al., 2011b). The Fe and Mn correlation was observed by Welch and Lico (1998) in the Carson Basin. Regional monitoring results have shown elevated concentration of Mn in the Carson River Basin (Garcia, 1989; Lico and Seiler, 1994; Welch, 1994; Welch and Lico, 1998), including one study that showed 27% of 11 unfiltered groundwater supply wells studied exceeding 100 μg/L (Maurer et al., 2008). Another study showed 53% of 15 groundwater wells studied (Lico and Seiler 1994) and 37% of 76 groundwater wells studied (Welch, 1994) exceeded the Mn state guideline value. Approximately 6% of HNP homes had Mn concentrations above the state guideline value, less than the results of these previous regional studies. Similar to Fe, Mn is a secondary contaminant and is not widely studied in drinking water assessments (Ramachandran et al., 2021).

Few studies have focused on naturally occurring Li in groundwater and drinking water (Lindsey et al., 2021; Schlesinger et al., 2021) because the EPA does not regulate Li. However, Li concentrations from 1,464 public and 1,676 domestic national water supplies were recently compiled by Lindsey et al. (2021) and 9% and 6% of the samples, respectively, had concentrations greater than their threshold of 60 μg/L (Lindsey et al., 2021). Li concentration was found to increase in arid regions and 24% of wells studied in the western Basin and Range region had concentrations above the guideline value (Lindsey et al., 2021). Previous work in the Carson Basin showed 4% (3 of 69) (Welch, 1994) and 53% (9 of 17) (Lico and Seiler, 1994) of groundwater wells exceeded this value. We observed exceedances of Li in 7% of HNP wells, which was similar to the national observations of Lindsey et al. (2021).

Mo has also not been widely studied in US groundwater or drinking water (Pichler and Koopmann, 2020; Smedley and Kinniburgh, 2017). Previous studies showed Mo concentrations in the Basin and Range were higher in groundwaters with pH >7 (Ayotte et al., 2011a; Smedley and Kinniburgh, 2017) and relatively high Mo concentrations (up to 520 μg/L) were previously observed in northern Nevada groundwater wells (Lico and Seiler, 1994). Prior work in the Carson Basin showed 23% of the 13 groundwater wells studied had Mo concentrations above the HBSL (Lico and Seiler, 1994). We observed Mo concentrations above the HBSL in 4% of homes, greater than observations from Ayotte et al. (2011a) which showed 1.5% of wells across the US above the Mo guideline.

U has been more extensively studied in groundwater and drinking water because of its known health effects (Kurttio et al., 2002). In the Carson Basin, previous studies found elevated U was present in approximately 10% to 78% of wells (Bevans et al., 1998; Seiler, 2011; Welch, 1994; Welch and Lico, 1998), higher than our study (8% of households without treatment above guidelines). In the eastern San Joaquin Valley in California, 25% of the 122 domestic wells studied exhibited U concentrations above the EPA guideline (Jurgens et al., 2010), which has been linked to heavy groundwater pumping in the region (Ayotte et al., 2011b; Jurgens et al., 2010). In the Navajo Nation, 463 water sources were tested and U exceedances were observed in 12.8 % of the samples and U concentrations were greater in the vicinity of abandoned U mines (Hoover et al., 2017). Another study analyzed 185 well water samples on the Navajo Nation and concentrations above the U guideline value were observed in 5% of the drinking water samples (Corlin et al., 2016) and co-exceedances of U and As were observed in 4% of water sources (Corlin et al., 2016; Hoover et al., 2017). Similarly, As and U co-exceedances were observed in our study.

Pb and Cu elements are both more soluble under similar hydrologic conditions (Ayotte et al., 2011b). Elevated Pb concentrations have been previously observed in the Truckee Basin (Bevans et al., 1998) and few exceedances have been previously observed in the Carson Basin (Lico and Seiler, 1994; Welch, 1994), similar to our HNP results. No Cu exceedances were observed in our HNP study, and very few (0.1%) exceedances are observed throughout the US (Ayotte et al., 2011a).

In summary, the percent of HNP households with As concentrations above the EPA guideline were in general agreement with the groundwater statistical modeling study of Anning et al. (2012). Observations of Fe, Li, Pb and Cu in our study were similar to previous studies in northern Nevada, while fewer Mn, Mo, and U exceedances were observed in our study compared to prior work in Nevada or other areas. Overall, elevated concentrations of some elements in our study can be attributed to internally drained valleys with high rates of evaporation, greater pH, and redox reactions (Anning et al., 2012; Ayotte et al., 2011b; Welch and Lico, 1998). Continued monitoring of metal concentrations in groundwater — particularly for understudied metals such as Li, Mo, and Mn — is important for understanding the hydrogeologic factors that contribute potential hazardous drinking water conditions and informing health-based guidelines and public health measures.

4.2. Frequency of well water testing

Hydrogeologic factors of Nevada result in groundwaters containing elevated metal concentration (Anning et al., 2012). Well water testing is the best approach for assuring water is safe to drink and the recommendation in Nevada is to test every 10 years (Donaldson et al., 2012). Our study shows that 52.7% of HNP survey respondents never had their well water analyzed and 18.5% of HNP survey respondents either had their well tested more than 10 years ago or were unsure. Therefore, 70% of participants in the HNP private well water survey were not following the recommended guideline to conduct water quality testing every 10 years. We also show that 41% of participants in the well water study had one or more elements exceeding guideline values. Therefore, additional education and outreach is necessary to educate homeowners about the importance of regularly testing their well water.

Regular testing of domestic well water is particularly important because groundwater chemistry changes over time. One study showed an increase in As concentration over a one-year period from 39 to 110 μg/L in one groundwater well from the Truckee Basin (Sorg et al., 2014). Other studies, however, have shown stability in As concentration in some northern Nevada wells (Steinmaus et al., 2005; Thundiyil et al., 2007). Although previous work on temporal As concentration trends is inconclusive, predicting future changes to groundwater metal concentrations is also difficult. In a recent study conducted by Lombard et al. (2021), modeled As concentration increased because of climate change for many US groundwater resources. The probability of high As concentrations increased during drought conditions because of lower groundwater levels caused by reduced precipitation and increased evaporation that concentrated As in groundwater (Lombard et al., 2021). Although future precipitation and drought projections for Nevada are highly uncertain, the climate will likely be characterized by extreme climate variability in the form of extended periods of drought punctuated by short periods of enhanced precipitation (Dettinger, 2011). Regardless, guidelines for well water testing in Nevada (and other arid states in the western US) should be updated to encourage more frequent domestic well water testing, to account for potential hydroclimate variability. For example, the Dartmouth Superfund Research Program encourages individuals to test their well water every three years (Barnaby et al., 2017). Another study of private wells in New Jersey recommended testing guidelines based on As concentration, with testing every year for concentrations >5 μg/L and every 5 years for concentrations <5 μg/L (Mailloux et al., 2021). Such recommendations should be considered for Nevada.

4.3. Water treatment

When comparing pre- and post-water treatment metal concentrations, treatments were shown to significantly reduce the concentration of all metals except Cu. Results from the HNP well water survey show 44.7% of the respondents do not or are unsure if they have a treatment system installed. Additionally, 20% (n=18) of HNP homeowners without a treatment system have As concentrations >10 μg/L (Table 1) and 43% (n=38) of households without a water treatment system have one or more exceedances (Table S2). Therefore, metal exposure in many HNP households may be reduced by installing a treatment system.

Although treatments have been shown to be effective for reducing metal concentrations, here we show some treatments were unable to reduce concentrations below guideline values and some even added metals into the household water. Of the households with water treatments, exceedances were still observed (Table 1 and 2). For example, Walker et al. (2008) showed that 30% of the 59 households studied in the Carson Basin with RO systems had As concentrations above the MCL. A previous study in the same area showed similar failure rates (Walker et al., 2005). A study of 94 households in New Jersey and 156 households in Maine showed 16% and 19% of treatments, respectively, failed to reduce As (Yang et al., 2020). Another study of 386 households from Central Maine showed 15% of treatments failed for As (Flanagan et al., 2015). There are several possible explanations for treatment failures. Homeowners are required to maintain their treatment system regularly and barriers to maintenance include lack of information (e.g., not knowing how to maintain or who to call), financial constraints, or time. A previous study in Maine and New Jersey showed increased failure rates when treatment systems were installed or maintained by the homeowner versus by the vendor (Yang et al., 2020). Other explanations for water treatment failures include improper water treatment type or water chemistry factors.

Improper treatment type may be a contributing factor for water treatment failures observed here. For example, one study in Arizona showed As concentrations were reduced by 81% when RO systems were used, whereas activated carbon only reduced As concentrations by 24% (Lothrop et al., 2015). Studies have shown that RO is the most efficient system for As removal (Kartinen and Martin, 1995; Walker et al., 2008) and only 34% of the homes surveyed through the HNP private well survey with a water treatment have RO systems installed (Figure S1). Of the households with treatment systems and with As concentrations above the EPA guideline value post-water treatment (n=21), 16 households do not use a RO system for water treatment. This supports the hypothesis that improper treatment type is a contributing factor to the water treatment system failure in our study. A robust comparison of treatment types and removal efficiencies for the metals discussed herein is beyond the scope of our well water study. Previous work in the Carson Basin has shown that any well water treatment may encourage consumption even though some treatments may not be appropriate for reducing As exposure (Walker et al., 2005). Therefore, additional education about proper treatment types is necessary.

Although improper treatment is a contributing factor of the As exceedances observed in households with treatments, 5 homeowners with As concentrations above the EPA guideline used RO systems. Previous work in the Carson Basin tested RO systems for the removal of As in drinking water and showed that even though RO systems removed 95% of the As, 66% of those RO systems still did not reduce As to <10 μg/L (Walker et al., 2008). This is partly because of the high As concentration in the untreated well water and the presence of trivalent As³⁺, which reduces arsenic removal efficiency (Walker et al., 2008). Another study of 102 homes in the Carson Basin showed a 79% As concentration reduction after RO filtration, but 52% of the households RO systems failed to reduce As concentrations to below the guideline value (George et al., 2006). In a well water study from New Jersey and Maine, the higher groundwater As concentration and the presence of As³⁺ also resulted in ineffective As removal by RO systems (Yang et al., 2020). Previous studies have shown that RO efficiency depends on a variety of factors, such as maintenance (Yang et al., 2020), the age of the treatment system, and the type of RO membrane used (George et al., 2006). We hypothesize that a combination of issues may factor into RO treatment failure for the wells tested in our study.

Beyond treatment of As, adsorption and RO are the most commonly recommended treatment types to broadly reduce metal concentrations in drinking water (Chowdhury et al., 2016; EPA, 2005; Fu et al., 2013). A previous study in Arizona showed a reduction in Pb and antimony concentration in drinking water when using RO (Lothrop et al., 2015). Other treatment options for removing metals in drinking waters include coagulation, ion exchange, oxidation, and activated carbon, which vary in effectiveness depending on the water being treated (Chowdhury et al., 2016; Fu et al., 2013; Lothrop et al., 2015; Slotnick et al., 2006; Zheng, 2017). Here we show that treatments effectively reduce metal concentrations, but for some households post-treatment concentrations remained above guideline values. Future efforts should focus on assessing treatment types and removal efficiencies particularly for the less commonly studied metals. The treatment data and survey information highlight the importance of improving homeowner education about the importance of regularly testing their drinking water.

5. Conclusion

Northern Nevada lies at the nexus of population, geologic, and climatic factors that result in increased geogenic metal contamination of groundwater. Leveraging the HNP, we surveyed 609 Nevadans about their private well water treatment systems, and water use. The HNP private well survey revealed that many homeowners have not regularly tested their home water supply, which is concerning because testing is the best way to ensure water is safe to drink. More frequent testing of domestic well water for metals near guideline values is important as Nevada experiences population growth and climate change which can affect the geochemistry of groundwater.

Household water samples were submitted by 174 HNP participants for metal analysis. Exceedances were observed for all the metals studied (As, Fe, Li, Mn, Mo, Pb, and U) except Cu, with many homes exhibiting more than one exceedance. Although a water treatment system did not guarantee that water was safe to drink, home treatment systems did reduce the concentration of metals. Unfortunately, the risk of exposure to concentrations above guidelines was still present. This is partly because of water chemistry, improper treatment for the removal of the metal of interest, or because of water treatment system failure.

The primary metal of concern in Nevada groundwaters is As; however, U, Mn, Li, Mo, Pb, and Fe have also been observed at levels above guideline values. Concentrations of U in private wells are understudied in Nevada and Mn, Mo, and Li concentrations in drinking water are understudied nationally, which highlights the need for additional studies. The findings presented herein are potentially applicable to municipal drinking water systems and may provide public health officials, city planners, and other stakeholders with valuable insights on groundwater metal contamination. Additionally, our results may also be useful for the development of regional hydrogeologic models. Lastly, our findings are potentially applicable to other arid regions with private well users and potential geogenic metal contamination of groundwater, for example, Arizona, New Mexico, Utah, Idaho, Oregon, and Colorado.