Social disparities in nitrate-contaminated drinking water in California's San Joaquin Valley

Abstract

Background: Research on drinking water in the United States has rarely examined disproportionate exposures to contaminants faced by low-income and minority communities. This study analyzes the relationship between nitrate concentrations in community water systems (CWSs) and the racial/ethnic and socioeconomic characteristics of customers.

Objectives: We hypothesized that CWSs in California's San Joaquin Valley that serve a higher proportion of minority or residents of lower socioeconomic status have higher nitrate levels and that these disparities are greater among smaller drinking water systems.

Methods: We used water quality monitoring data sets (1999-2001) to estimate nitrate levels in CWSs, and source location and census block group data to estimate customer demographics. Our linear regression model included 327 CWSs and reported robust standard errors clustered at the CWS level. Our adjusted model controlled for demographics and water system characteristics and stratified by CWS size.

Results: Percent Latino was associated with a 0.04-mg nitrate-ion (NO3)/L increase in a CWS's estimated NO3 concentration [95% confidence interval (CI), -0.08 to 0.16], and rate of home ownership was associated with a 0.16-mg NO3/L decrease (95% CI, -0.32 to 0.002). Among smaller systems, the percentage of Latinos and of homeownership was associated with an estimated increase of 0.44 mg NO3/L (95% CI, 0.03-0.84) and a decrease of 0.15 mg NO3/L (95% CI, -0.64 to 0.33), respectively.

Conclusions: Our findings suggest that in smaller water systems, CWSs serving larger percentages of Latinos and renters receive drinking water with higher nitrate levels. This suggests an environmental inequity in drinking water quality.

Figure 1

Schematic of a community water system (CWS) indicating the location of point-of-entry sources and the use of average nitrate concentration in the distribution system as a proxy for tap water quality. Water entering the distribution system may flow from a groundwater well (point a) or from a surface water source (i.e., stream; point b). Water may then be treated (point c; different treatment techniques may be used, depending on the contaminant of interest and original source). Water then enters the distribution system at points of entry (point d). In this example, nitrate samples would be used from point d, because points a and b flow into the same point of entry. Average nitrate level at point d is used to represent average water quality in the distribution system (point e). Nitrate levels in the distribution system are a proxy for tap water quality (point f). If points a and b are separate points of entry (i.e., do not flow into a shared point d), nitrate sample points would be used from each source separately. Constant and equal flows are assumed.

Figure 2

Average nitrate concentration of CWSs in California’s San Joaquin Valley, 1999–2001 (n = 327). Estimates are based on an average of each point-of-entry source’s average concentration. Data are from CDPH (2008) Water Quality Monitoring and PICME databases (CDPH 2008a, 2008b). Approximate locations of CWSs are depicted, but not true boundaries. Because of close proximity of some CWSs, some CWSs not fully visible.

Figure 3

Percentage of CWSs with low, medium, and high average nitrate concentrations, by quartiles of percent Latino and home ownership. Average system-level nitrate concentration is derived from the average of each source’s average nitrate concentration at point of entry. Low is < one half the MCL of 45 mg NO₃/L. Medium is one half the MCL up to the MCL (22.5 mg/L to 44.9 mg/L NO₃). High is ≥ to the MCL.