A new study has uncovered a disparity between the safety of drinking water in certain regions of California—serving majority Latino communities—and water from community water systems in other areas of the state. Led by University of California, Santa Barbara (UC Santa Barbara) researchers, the study uncovered that some drinking water systems have higher nitrate and arsenic contamination levels, and that the contamination is exacerbated by droughts, which may become more frequent with climate change.

Long-term exposure to contaminants such as arsenic and nitrate in water is linked to an increased risk of cancers, cardiovascular diseases, developmental disorders, and birth defects in infants. According to the study, in the U.S., there is an acknowledged disparity in exposure to contaminants in tap water provided by community water systems. Scientific literature demonstrates that, oftentimes, systems distributing water with higher contamination levels exist in areas that lack adequate public infrastructure or sociopolitical and financial resources.

In the present study, soon to be published in the American Journal of Public Health, researchers investigated the drinking water quality in California’s community water systems serving majority Latino communities. The project analyzed trends in nitrate and arsenic concentrations in drinking water sourced from both surface and groundwater, using a varied set of data, including water sampling data, historical drought records, sociodemographic characteristics of the populations, measures of agricultural intensity, and community water system characteristics from the period 2007–2020.

The findings revealed that systems serving majority Latino populations consistently exhibit higher and more variable levels of nitrate and arsenic compared to those serving non-majority Latino populations. Additionally, instances of drought were seen to increase the contamination in community water systems serving these communities. Specifically, drought increased nitrate concentrations in majority Latino communities, with the effect doubling for community water systems with more than 75 percent Latino populations served. Arsenic concentrations in surface sources also increased during drought for all groups.

According to the study, nitrate concentrations in groundwater-sourced drinking water increased from a baseline of 2.5 milligrams per liter (mg/L) in 1998 to a peak of 3.1 mg/L in 2018 for majority Latino community water systems. In contrast, nitrate levels in non-majority Latino community water systems decreased from 2.1 mg/L to 1.8 mg/L over the same period. The disparity in nitrate exposure was especially pronounced in surface-sourced water, where majority Latino community water systems showed a mean nitrate concentration of 2.2 mg/L, significantly higher than the 1.2 mg/L observed in non-majority Latino community water systems as of 2020.

Drought conditions worsened the disparities, with a notable impact on surface-sourced drinking water. For majority Latino community water systems, drought conditions lead to an increase in nitrate levels; a 2-unit increase in the normalized drought index resulted in a 0.04 mg/L rise in nitrate concentrations for community water systems serving more than 25 percent Latino populations. This increase is even greater in systems serving over 75 percent Latino communities, with a 0.16 mg/L rise.

The trend is worst in very small (less than 500 connections) and privately operated community water systems, where nitrate concentrations are more susceptible to drought conditions. Surface-sourced water showed a drought-related increase in nitrate levels of 0.17 mg/L—more than double the increase observed in groundwater sources (0.07 mg/L).

Although more community water systems (approximately 77 percent) are supplied by groundwater, more people (nearly 80 percent) are served by community water systems that use surface water as their primary source.

Moreover, arsenic concentrations in drinking water exhibited variability under drought conditions. Drought increased overall arsenic concentrations in surface-sourced drinking water for both majority and non-majority Latino community water systems. Interestingly, however, for majority Latino communities, drought lead to a statistically insignificant decrease in arsenic levels in groundwater-sourced drinking water. This trend contrasts with recent findings in the San Joaquin Valley, where drought-related intensified agricultural groundwater pumping has significantly increased nitrate prevalence by three to five times in public supply wells, highlighting a broader regional issue.

The project was led by Sandy Sum, M.A., a Ph.D. candidate in economics and environmental science at the Bren School at UC Santa Barbara.