Drinking water source and exposure to regulated water contaminants in the California Teachers Study cohort

Journal of Exposure Science & Environmental Epidemiology volume 35, pages454–465 (2025)Cite this article

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Abstract
Background
Pollutants including metals/metalloids, nitrate, disinfection byproducts, and volatile organic compounds contaminate federally regulated community water systems (CWS) and unregulated domestic wells across the United States. Exposures and associated health effects, particularly at levels below regulatory limits, are understudied.

Objective
We described drinking water sources and exposures for the California Teachers Study (CTS), a prospective cohort of female California teachers and administrators.

Methods
Participants’ geocoded addresses at enrollment (1995–1996) were linked to CWS service area boundaries and monitoring data (N = 115,206, 92%); we computed average (1990–2015) concentrations of arsenic, uranium, nitrate, gross alpha (GA), five haloacetic acids (HAA5), total trihalomethanes (TTHM), trichloroethylene (TCE), and tetrachloroethylene (PCE). We used generalized linear regression to estimate geometric mean ratios of CWS exposures across demographic subgroups and neighborhood characteristics. Self-reported drinking water source and consumption at follow-up (2017–2019) were also described.

Results
Medians (interquartile ranges) of average concentrations of all contaminants were below regulatory limits: arsenic: 1.03 (0.54,1.71) µg/L, uranium: 3.48 (1.01,6.18) µg/L, GA: 2.21 (1.32,3.67) pCi/L, nitrate: 0.54 (0.20,1.97) mg/L, HAA5: 8.67 (2.98,14.70) µg/L, and TTHM: 12.86 (4.58,21.95) µg/L. Among those who lived within a CWS boundary and self-reported drinking water information (2017–2019), approximately 74% self-reported their water source as municipal, 15% bottled, 2% private well, 4% other, and 5% did not know/missing. Spatially linked water source was largely consistent with self-reported source at follow-up (2017–2019). Relative to non-Hispanic white participants, average arsenic, uranium, GA, and nitrate concentrations were higher for Black, Hispanic and Native American participants. Relative to participants living in census block groups in the lowest socioeconomic status (SES) quartile, participants in higher SES quartiles had lower arsenic/uranium/GA/nitrate, and higher HAA5/TTHM. Non-metropolitan participants had higher arsenic/uranium/nitrate, and metropolitan participants had higher HAA5/TTHM.

Impact
Though average water contaminant levels were mostly below regulatory limits in this large cohort of California women, we observed heterogeneity in exposures across sociodemographic subgroups and neighborhood characteristics. These data will be used to support future assessments of drinking water exposures and disease risk.

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Introduction
Drinking water represents an important source of exposure to inorganics (e.g., arsenic and nitrate), radionuclides (uranium, alpha particles), disinfection byproducts (DBPs), and volatile organic compounds (VOCs) for populations in the United States (U.S.) and worldwide [1]. Approximately 90% of the U.S. population is served by public water systems, and 10% by private wells [2]. In California, approximately 95% of the population is served by public water systems [3]. Public water systems include at least 15 service connections or serve at least 25 people; community water systems (CWS) are a type of public water system that serve the same population year-round [2]. Public water systems are regulated by the U.S. Environmental Protection Agency (EPA) under the Safe Drinking Water Act [4]. The contaminants we evaluated are regulated through federally enforceable maximum contaminant levels (MCLs), which were determined based on economic and technical feasibility, treatment technologies, cost-benefit analysis, and public health benefit for specific health endpoints [4]. States generally have primacy over enforcement of federal drinking water regulations. Notably, the MCL goal, a non-enforceable standard based solely on risk to health, is 0 µg/l for arsenic, uranium, alpha particles, trichloroethylene (TCE), tetrachloroethylene (PCE), bromodichloromethane, bromoform, and dichloroacetic acid, as there is no known safe level of exposure to these contaminants [4]. Private wells are not federally regulated or monitored.

Nitrate is a common contaminant of drinking water supplies in agricultural areas, due to use of nitrogen fertilizers and concentrated animal feeding operation waste [5, 6]. Atmospheric deposition, erosion of natural deposits, and septic tank or sewage leakage contribute to nitrate contamination in rural and urban areas [4]. Geogenic arsenic occurs in groundwater across the U.S., with regional differences due to climatic and geological factors; arid climates can cause evaporative concentration of arsenic in shallow groundwater supplies and lead to high levels, such as in the San Joaquin Valley of California [7,8,9,10]. Mining and historical arsenical pesticide use are anthropogenic sources of arsenic contamination in water supplies [8]. Uranium is present in different rock types and is leached from host mineral phases to surface and ground water supplies; uranium mining/milling and mobilization of uranium via nitrate fertilizer use are anthropogenic sources of uranium contamination [11,12,13,14,15]. Uranium and other radionuclides can decay and release alpha radiation, often quantified as total gross alpha for monitoring compliance purposes. DBPs are formed by the reaction of chlorine and bromine with natural organic compounds during the disinfection of water supplies to treat pathogens [16]. DBPs are commonly found in public water supplies across the U.S., with the highest concentrations observed in those reliant on surface water or shallow groundwater [16]. While over 700 DBPs have been identified, the most abundant classes are trihalomethanes (THMs, which include the chemicals chloroform, dibromochloromethane, bromodichloromethane, and bromoform, and regulated as the sum total, TTHM), and haloacetic acids (HAA5, regulated as the sum of dichloroacetic acid, trichloroacetic acid, monochloroacetic acid, bromoacetic acid, and dibromoacetic acid) [4, 17, 18]. The VOCs TCE and PCE are solvents used in dry cleaning, metal degreasing, textile, art, and industrial processes, and may be found in some consumer products [19]. Toxic waste disposal sites, sometimes recognized as Superfund sites under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), are anthropogenic sources of inorganic arsenic, uranium, TCE, and PCE in groundwater [12, 19,20,21,22,23,24].

Numerous studies implicate one or more of these drinking water contaminants in adverse health effects, including cancer, cardiovascular disease, reproductive and developmental toxicity, nephrotoxicity, and other adverse health conditions [1, 16, 20, 25,26,27,28,29,30,31,32,33,34,35,36,37,38,39]. Inorganic arsenic is classified by the International Agency for Research on Cancer (IARC) as a cause of cancers of the bladder, lung, and skin, and is associated with increased risk of cancers of the kidney, liver, and prostate [26]. Inorganic arsenic is also a potent toxicant associated with numerous adverse health outcomes, including cardiovascular disease, hypertension, and reproductive disorders [26, 31, 32]. Uranium exposure through drinking water is associated with renal damage and nephrotoxicity, and an increased risk of colorectal, breast, kidney, prostate, and total cancer [20, 27, 33]. Nitrate is classified by IARC as a probable human carcinogen when ingested under conditions that result in the endogenous formation of N-nitroso-compounds, most of which are animal carcinogens [28, 34]. Cancers of the stomach, colon, bladder, kidney, ovary, and thyroid, and thyroid disease are associated with elevated nitrate ingested from drinking water; however, the number of studies of most cancer sites is limited [29, 35]. Higher intake of DBPs through drinking water is associated with increased bladder cancer risk, and a limited number of studies suggest DBP exposures are potential risk factors for colon, rectum, and endometrial cancer [16, 36]. TCE is classified as carcinogenic to humans based on kidney cancer, and PCE (Group 2A) as probably carcinogenic to humans based on bladder cancer evidence [30]. Occupational studies also support adverse developmental, neurological, and hepatotoxic effects of TCE and PCE exposures [19]. Assessment of long-term drinking water contaminant exposures and associated health risks have traditionally been limited by the lack of water quality data that could be assigned to individuals in epidemiologic cohorts; understanding large-scale water quality data at the level of consumer intake is a critical research gap [40]. Additionally, there are relatively few cohort studies evaluating drinking water exposures at levels below the MCLs and World Health Organization guidelines that are commonly experienced by the general U.S. population [1]. Inequalities in CWS arsenic, uranium, and nitrate exposures by sociodemographic characteristics such as, race and ethnicity, income, education, region, and rurality/urbanicity have been documented [41, 42]. Few studies have evaluated sociodemographic inequalities in DBP and TCE/PCE exposures in the United States.

Our primary objective for this study was to describe exposure to regulated, frequently detected and measured contaminants in drinking water in the California Teachers Study (CTS), a large prospective cohort of women. We described the spatial linkage of participants’ residences to their drinking water source and corresponding estimates of contaminant concentrations. For a subset, we evaluated the agreement between address-assigned and self-reported drinking water source and described the daily intake of tap water and CWS contaminants. Additionally, we examined inequalities in CWS exposures across sociodemographic groups.

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