Inorganic Constituents

The only inorganic constituent with a health-based benchmark detected at high concentrations was arsenic, which was detected at concentrations exceeding its MCL (10 micrograms per liter [µg/L]) in 8 percent of domestic groundwater resources (fig. 2). Arsenic also was detected at moderate concentrations in 18 percent of domestic groundwater resources (fig. 2). Wells with high concentrations of arsenic were in the eastern Sacramento Valley study area and were near the center of the Sacramento Valley basin (fig. 3A).

Additional inorganic constituents that have regulatory and non-regulatory benchmarks were detected at moderate concentrations with less prevalence. The trace element uranium and the nutrient nitrate were detected at moderate concentrations below their MCLs in 3 and 10 percent of domestic groundwater resources of the study unit, respectively (fig. 2). Hexavalent chromium, a carcinogenic trace element, does not presently (2024) have an MCL in California; therefore, it is regulated under the total chromium MCL of 50 µg/L. There were no detections of hexavalent chromium that exceeded 50 µg/L. The USGS health-based screening level for hexavalent chromium is 20 µg/L (Norman and others, 2018; fig. 2). One well had a moderate relative concentration when compared to the USGS health-based screening level because it had a concentration of 12 µg/L.

Constituents with non-regulatory benchmarks or SMCLs are not included in the water-quality overview charts on figure 1 and are considered separately to highlight aesthetic properties of groundwater, such as taste, color, or odor. Manganese is a trace metal that can affect the taste and color of groundwater but also can precipitate in pipes causing clogging of household plumbing systems. Manganese was present above its SMCL of 50 µg/L in 26 percent of domestic groundwater resources in the study unit (fig. 2). Manganese also has a SWRCB defined notification level of 500 µg/L (California State Water Resources Control Board, 2022) and was present at concentrations greater than the notification level in 10 percent of domestic groundwater resources. Total dissolved solids (TDS) are a measure of groundwater salinity and have both recommended and upper SMCLs of 500 and 1,000 milligrams per liter (mg/L), respectively, which were used as moderate and high thresholds for this study. Total dissolved solids were measured at moderate and high levels in 8 and 5 percent of domestic groundwater resources in the study unit, respectively (fig. 2). Chloride and sulfate are major ionic components of TDS with SMCL thresholds, but only chloride exceeded the recommended SMCLs in 3 percent of domestic groundwater resources in the study unit.

Organic Constituents

The only organic constituent exceeding regulatory benchmarks was the fumigant 1,2,3-trichloropropane (1,2,3-TCP), which was detected at a concentration above its California SWRCB defined MCL of 0.005 µg/L in 1 well (3 percent of domestic groundwater resources in the study unit; California State Water Resources Control Board, 2023; fig. 4). Fumigants are VOC pesticides that historically have been applied to agricultural crops, such as vineyards or orchards, for nematode control in the Central Valley.

Additional VOCs with regulatory benchmarks detected frequently (greater than or equal to 10 percent) in the study area included a gasoline byproduct, a solvent, and a disinfection byproduct. The gasoline byproduct methyl tert-butyl ether (MTBE) was detected at low concentrations in 32 percent of the domestic groundwater resources; the solvent tetrachloroethene (PCE) was detected at low concentrations in 10 percent of domestic groundwater resources; and chloroform, a disinfection byproduct, was detected at low concentrations in 34 percent of domestic groundwater resources (Lor and Dupuy, 2022).

Microbial Indicators

Microbial indicator constituents can be used to evaluate the potential for bacterial contamination of groundwater from wastewater sources. In this study, groundwater samples were tested for the presence or absence of total coliform bacteria, E. coli, and enterococci. Total coliform bacteria and enterococci were detected in 13 and 8 percent of domestic groundwater resources, respectively. E. coli bacteria were not detected. Total coliform bacteria were detected in both study areas, whereas enterococci was only detected in the foothills study area. Coliform bacteria are common and naturally present in soil and surface water and are generally harmless to humans. E. coli and enterococci are more specific indicators of fecal contamination because they are found in human and animal wastes (California State Water Resources Control Board, 2019). Because microbial MCL violations under the Revised Total Coliform Rule are based on repeat sampling, and survey results described in this study are from a single sampling event, they are insufficient to indicate MCL exceedances for these constituents (California State Water Resources Control Board, 2019).

Per- and Polyfluoroalkyl Substances

Per- and polyfluoroalkyl substances are a family of human-made chemicals that are commonly used as surfactants in a variety of industrial processes and are persistent in the environment (Kent, 2021). In April 2024, the U.S. Environmental Protection Agency (EPA) announced the final National Primary Drinking Water Regulation establishing MCLs for five individual PFAS compounds: perfluorooctanoate (PFOA), perfluorooctanesulfonate (PFOS), perfluorohexanesulfonate (PFHxS), perfluorononanoic acid (PFNA), and hexafluoropropylene oxide dimer acid (HFPO-DA; U.S. Environmental Protection Agency, 2024). The MCLs for PFOA and PFOS are 4 nanograms per liter (ng/L), and for PFHxS, PFNA, and HFPO-DA, the MCL is 10 ng/L. The PFAS compounds PFHxS, PFNA, HFPO-DA, and perfluorobutanesulfonate (PFBS) also are regulated as a PFAS mixture (U.S. Environmental Protection Agency, 2024). Before the establishment of the EPA MCLs, the State of California issued health-based advisory notification and response levels for the following PFASs: PFBS, PFHxS, PFOA, and PFOS (California State Water Resources Control Board, 2024b). The EPA MCLs supersede the California notification and response levels for PFOA, PFOS, and PFHxS. The California notification and response levels for PFBS are 500 and 5,000 ng/L, respectively.

In this study, 8 out of 28 measured PFAS were detected. At least one PFAS was detected in 26 percent of domestic groundwater resources (fig. 5). Two of the PFAS compounds with EPA MCLs, PFOA and PFOS, were detected above the MCL in 5 and 3 percent of domestic groundwater resources, respectively (fig. 5). The overall detection frequency of PFOA, PFOS, PFHxS, and PFBS, the PFASs with EPA MCLs or SWRCB notification or response levels, were 16, 16, 18, and 11 percent, respectively (fig. 5). The PFASs detected that do not have EPA MCLs or SWRCB notification or response levels were perfluoropentanoate (PFPeA), perfluorohexanoate (PFHxA), perfluoroheptanoate (PFHpA), and perfluorobutanoate (PFBA), which had detection frequencies of 16, 16, 8, and 3 percent, respectively (fig. 5).

Redox-Sensitive Constituents

Concentrations of some constituents in groundwater can be affected by the oxidation-reduction (redox) state of the aquifer from which the samples were drawn. Redox sensitive constituents sampled in this study that were detected at high concentrations were arsenic, iron, and manganese. Nitrate concentrations also are redox sensitive and were detected at moderate concentrations in some wells. The amount of dissolved oxygen (DO) in groundwater is an important indicator of redox condition. Before entering the aquifer, DO in water is typically in equilibrium with the atmosphere. However, it can be depleted once it enters the aquifer by reactions with organic carbon or inorganic reductants (McMahon and Chapelle, 2008). Generally, the redox state of groundwater is categorized as oxic (DO concentration greater than 0.5 mg/L) or anoxic (DO less than or equal to 0.5 mg/L). There are additional geochemical indicators of redox state in groundwater beyond DO concentration, and the relative concentrations of redox sensitive constituents like nitrate, manganese, iron, and sulfate can be used to determine if the redox state is mixed, meaning both oxic and anoxic conditions are present (McMahon and Chapelle, 2008; Jurgens and others, 2009).

Wells sampled in the study unit were categorized as oxic, anoxic, or mixed using the methodology described by Jurgens and others (2009) (one sample identified as suboxic was placed in the anoxic category). Most of the samples in the study unit were categorized as oxic (66 percent of wells) with the remainder classified as anoxic or mixed. Most of the anoxic wells were in the eastern Sacramento Valley study area (fig. 6). Generally, groundwater from wells in fine-grained alluvial sediments, especially those at the far end of groundwater flow paths, have a higher likelihood of being anoxic because of the geochemical evolution of groundwater as it moves through the system (McMahon and Chapelle, 2008).

About 26 percent of the wells in the study unit had anoxic groundwater. Groundwater with anoxic conditions allows for the mobilization of arsenic, iron, and manganese from dissolution of or desorption from minerals or mineral coatings containing these trace elements (Smedley and Kinniburgh, 2002). All but one of the wells with a high or moderate relative concentration of arsenic was in the eastern Sacramento Valley study area, and the three wells with high concentrations were classified as anoxic and were located near the center of the Sacramento Valley (fig. 3A). Previous studies of groundwater quality in the Sacramento Valley have shown that reducing conditions in these deposits are a major influencing factor on the groundwater chemistry, affecting concentrations of arsenic, DO, nitrate, iron, and manganese (Hull, 1984; Dawson, 2001; Bennett and others, 2011; Bennett, 2022). Nitrate is stable in oxic groundwater; however, in anoxic groundwater, nitrate is reduced, meaning it is consumed by redox processes that ultimately lower its concentrations (Burow and others, 2010). The four sites with moderate concentrations of nitrate were in wells classified as oxic and located closer to the east edge of the eastern Sacramento Valley study area where sediments are generally coarser than those located closer to the center of the Sacramento Valley (Hull, 1984; fig. 3B). The absence of moderate or high concentrations of nitrate in anoxic areas could either indicate that it was not in the groundwater to begin with or indicate that nitrate is being consumed by redox processes (Landon and others, 2011). All the wells with high concentrations of iron or manganese in the eastern Sacramento Valley study area also were in wells classified as anoxic or mixed, and with one exception, were located near the center of the Sacramento Valley in areas of generally fine-grained sediments (figs. 3C, 3D).

Anthropogenic Constituents

Detections of VOCs and PFASs are commonly related to factors associated with human activities on the landscape, such as degree of urbanization or agricultural practices. Additionally, there is a tendency for some of these constituents to become incorporated into water vapor, allowing them to be distributed into areas outside established urban or agricultural zones where they may have originated, through precipitation and runoff that eventually recharges the domestic groundwater resource. The following section discusses select VOCs and PFASs detected in the eastern Sacramento Valley and Foothill study areas within the context of factors that could affect their presence in the domestic groundwater resource.

The disinfection by-product chloroform (a trihalomethane) was the most frequently detected VOC (34 percent) in the study unit. This finding matches recent and past nationwide studies of VOC occurrence in which chloroform was the most frequently detected VOC in groundwater (Moran and others, 2002; Bexfield and others, 2022). Previous studies have examined chloroform and other trihalomethanes in drinking water because of their widespread presence in the hydrologic system and deleterious effects on human health at high concentrations (Ivahnenko and Barbash, 2004; Ivahnenko and Zogorski, 2006). Sources of chloroform to the groundwater system are numerous, which leads to its detection in urban and suburban areas as well as beneath agricultural, forested, and other undeveloped areas (Ivahnenko and Barbash, 2004). Detections of chloroform in the study unit were nearly equally divided between both study areas, with detections in the eastern Sacramento Valley generally occurring in the north or east-central side of the study area. In the foothills study area, detections of chloroform were uniformly distributed among the sampled wells (fig. 7A). Chloroform was detected in each of the three general land-use settings (urban, natural, and agricultural), but most occurred in or near areas identified as urban (fig. 7A). Treated drinking water has been cited as a potentially high source of chloroform either through landscape irrigation or leaking distribution pipes (Ivahnenko and Barbash, 2004). Another source of chloroform in groundwater could be related to shock chlorination of domestic wells. Shock chlorination is a treatment often prescribed to owners of domestic wells that need microbial contamination abatement and involves pouring chlorine directly into the well, allowing time for it to disinfect the well casing, then purging the well to remove the chlorine (Centers for Disease Control and Prevention and U.S. Department of Housing and Urban Development, 2006). This practice can result in residual chloroform forming in the aquifer and may be a source of chloroform in areas where there are no known urban sources of chlorinated water available to recharge the aquifer.

The occurrence of the solvent PCE in aquifers has been linked to urban settings. Tetrachloroethene was detected in 10 percent of the wells sampled at low relative concentrations. Tetrachloroethene is among the most frequently detected VOCs in groundwater in California and the Nation (DeSimone and others, 2015; Belitz and others, 2015; Bexfield and others, 2022). It was detected in both the eastern Sacramento Valley and foothills study areas in primarily urban areas (fig. 7B). Tetrachloroethene is a chlorinated solvent with a long history of use and many know applications in commercial and industrial settings (DeSimone and others, 2015). Its relatively frequent detection in groundwater is assumed to be related to its widespread use, which increases the potential for its disposal, potential spills, and leaks (Bexfield and others, 2022).

In this study unit, the only VOC detected at a high relative concentration was 1,2,3-TCP, an additive to soil fumigant mixtures. It was only detected in one well that was located near an orchard. Although not frequently detected in the Sacramento Valley, 1,2,3-TCP is detected statewide and has been linked to legacy agricultural land-use practices (Burow and others, 2019). In this study unit, its introduction to the aquifer is likely the result of past applications of fumigant mixtures containing 1,2,3-TCP beneath orchards and vineyards near the well after precipitation and irrigation transported it into the domestic aquifer system.

The gasoline oxygenate MTBE and PFASs were frequently detected in the study unit (32 and 29 percent, respectively). In the eastern Sacramento Valley study area, MTBE and PFASs were detected in or near the urbanized areas at the north and south end of the study area, and detections in the central part of the eastern Sacramento Valley were near the Feather River (figs. 7C, 7D). In the foothills study area, MTBE detections were evenly distributed throughout the study area (fig. 7C) with two of the three detections occurring near urban areas and one detection observed in natural land use (fig. 7C). The spatial distribution of MTBE and PFASs detections indicate a connection between the constituents that could be related to sources or mode of transport that allow these constituents to enter the domestic groundwater resource.

Methyl tert-butyl ether was a fuel additive used to raise octane levels to reduce harmful emissions, thereby improving air quality. As part of the Clean Air Act, implemented in 1990, oxygenated fuel use was mandated in areas out of compliance for ozone or carbon monoxide air quality standards. However, its use was banned in California after MTBE was discovered to be a groundwater contaminant; many studies have since detailed sources and occurrences of MTBE in groundwater throughout the Nation (Pankow and others, 1997; Baehr and others, 1999; Ayotte and others, 2005; Zogorski and others, 2006). California established an MCL for MTBE in 2000 (13 µg/L) and banned its use in 2004 (California State Water Resources Control Board, 2017). The maximum concentration of MTBE detected in the study unit was 0.79 µg/L (Lor and Dupuy, 2022).

Per- and polyfluoroalkyl substances, collectively, are a large group of chemicals (more than 12,000) used worldwide in hundreds of products that have been used and produced for decades in the United States (Kent, 2021; California State Water Resources Control Board, 2024b). Environmental sources are numerous because there is a very broad range of applications for this group of chemicals, which ranges from consumer goods, industrial applications, and emergency fire response (California State Water Resources Control Board, 2024b). Increased attention on PFASs in the environment is a result of an increasing awareness of their widespread occurrence and persistence in groundwater, surface water, soil, and precipitation that results from their unique chemical properties that make them highly resistant to processes that might otherwise break them down (California State Water Resources Control Board, 2024b). They are often referred to as “forever chemicals.” Adding to the increased attention are recently enacted regulations on specific PFAS constituents (California State Water Resources Control Board, 2024b; U.S. Environmental Protection Agency, 2024). Comprehensive monitoring efforts will be important to understand if mitigation efforts can effectively reduce these chemicals in the environment (Kent, 2021; California State Water Resources Control Board, 2024b; Dong and others, 2024).

In this study unit, detections of MTBE and PFASs generally occurred in areas of urban land use. Results of Spearman’s rank correlation testing, with a significance level of 0.05, showed both were significantly positively correlated with percentage of urban land use in 2002 (p<0.05), and total PFASs were significantly positively correlated with population density in 2000 within a 500-m buffer surrounding the sampled wells (Falcone, 2015, 2016). This finding is expected because their use, production, and emission into the environment primarily occurred or continues to occur there; however, they also were detected in areas of agricultural and natural land use (figs. 7C, 7D). Using geospatial data compiled by Tokranov and others (2024), which included distances to known sources of PFASs determined by the EPA, additional significant correlations were observed for total PFASs concentrations in wells in this study unit. Distances to fire stations, fire training sites, and machine metal manufacturing facilities had significant inverse correlations with total PFASs concentrations (p<0.05, Spearman’s rank correlation). It is unknown if these sources account for all the PFASs detections observed in the study unit, and additional analysis addressing specific sources is beyond the scope of this report. However, MTBE and PFASs are relatively frequently detected in this study unit and are seen in wells outside of urban areas (fig. 7D).

The atmosphere is a mechanism for the distribution of these constituents into areas outside of the urban environment. Atmospheric deposition of MTBE to groundwater has been demonstrated in numerous studies (Squillace and others, 1996, 1997; Baehr and others, 1999; Belitz and others, 2004; Fram and Belitz, 2014). In their study of groundwater quality throughout the Sierra Nevada for the GAMA-PBP, Fram and Belitz (2014) observed that concentrations of MTBE detected in public-supply wells were consistent with MTBE concentrations in precipitation that was a result of the partitioning of MTBE into water vapor from the atmosphere. Likewise, studies have shown that PFASs can be distributed far from their initial sources of production by way of the atmosphere, and they have been measured in precipitation (Scott and others, 2006; Kim and Kannan, 2007; Kwok and others, 2010; Gewurtz and others, 2019; Pike and others, 2021; Cousins and others, 2022; Pfotenhauer and others, 2022). Studies of PFAS concentrations in rainfall (primarily in the eastern United States) show PFAS concentrations ranging from less than 1 ng/L to more than 60 ng/L; however, concentrations vary by site, constituent, and proximity to local sources (Gewurtz and others, 2019; Pike and others, 2021; Pfotenhauer and others, 2022). Atmospheric contributions of PFASs to the environment are occurring globally, either through dry, or more likely, wet deposition (Thackray and others, 2020; Cousins and others, 2022). In this study, the concentration of PFAS compounds in groundwater ranged from not detected to 16.2 ng/L, with most compounds having maximum concentrations less than 10 ng/L (Lor and Dupuy, 2022).

Interestingly, higher concentrations of MTBE and PFASs were observed in wells in the foothills compared to wells in the eastern Sacramento Valley (Lor and Dupuy, 2022; figs. 8, 9). Wells that have detectable concentrations of MTBE are receiving recharge water affected by MTBE presence in the atmosphere between the early 1990s and the early 2000s. The same is true for PFASs because they have been manufactured for use in commonly available consumer products or industrial processes since the 1950s (Interstate Technology Regulatory Council, 2020) and, therefore, are an indication of modern groundwater. It is suspected that the differences could be related to differing aquifer dynamics between the study areas because they are very different aquifer types (alluvial versus fractured hard rock). The distribution of groundwater ages in wells completed in an alluvial aquifer system might yield lower relative concentrations of a constituent in modern groundwater if that alluvial aquifer well also is tapping into an older groundwater source. Conversely, a well completed in a fractured hard rock setting might only be receiving modern groundwater containing tracers of recent anthropogenic activity, which is not diluted with older groundwater that lacks the same tracer. Although a detailed analysis of the groundwater age of samples collected in this study using the complete set of tracers collected at all wells (tritium, sulfur hexafluoride, and carbon-14) was not done for this report, tritium concentrations were used to classify the samples into three different groundwater-age categories: modern, mixed, or premodern (Lindsey and others, 2019). Of the 12 wells with detections of MTBE, 9 were classified as modern, 2 mixed, and 1 premodern. Similarly, of the 11 wells with PFASs, 9 were classified as modern, 1 mixed, and 1 premodern.