ARSENIC AND NITRATE IN DRINKING WATER IN DISADVANTAGED COMMUNITIES IN CALIFORNIA’S CENTRAL VALLEY

Arsenic and nitrate can be elevated in groundwater in California’s Central Valley as a result of agricultural activity, such as groundwater overpumping and use of fertilizers. Arsenic is mobilized when overpumping pulls groundwater from arsenic-rich clay layers in the aquifers, rather than the more permeable sand and gravel layers with low arsenic. Nitrates can be leached into groundwater following fertilization of crops. This research focused on San Joaquin County, where groundwater is a major source of drinking water and more than half of the population is low income. We explored whether disadvantaged communities in the county had higher levels of arsenic and nitrate in their groundwater, relative to non-disadvantaged communities. To identify disadvantaged communities, we used CalEnviroScreen 3.0. This index uses pollutant burdens and population characteristics to classify census tracts (communities of 1,200 - 8,000 people with common characteristics) as disadvantaged or non-disadvantaged. We then used Geographic Information Systems (GIS) technology to overlay disadvantaged community locations with groundwater well data from July 2017 - July 2020. The well data were obtained from the state-run Groundwater Ambient Monitoring and Assessment Program (GAMA). Across both types of communities, about 23 census tracts received degraded water, with wells exceeding arsenic or nitrate drinking water standards (10 µg/L and 10 mg/L, respectively). Of the 343 wells sampled for arsenic, 57 exceeded drinking water standards. Of the 57 exceedances, 36 were located in disadvantaged communities. Arsenic concentrations were 1.4 µg/L higher in groundwater wells from disadvantaged communities relative to their non-disadvantaged counterparts. This difference was statistically significant (t-test, pooled variance, p<0.05). In contrast, nitrate concentrations were not statistically different between disadvantaged and non-disadvantaged communities (t-test, pooled variance, p=0.686). We also explored whether nitrate concentrations in groundwater were related to land use type (e.g., farmland, livestock, open space, urban, or water). In conclusion, the two pollutants do not behave similarly with regard to their distribution in disadvantaged communities.