FLUORIDE CONCENTRATION TRENDS IN GROUNDWATER USED FOR PUBLIC DRINKING WATER SUPPLY IN CALIFORNIA

Fluoride in groundwater serves as both an essential trace mineral and potential contaminant. At low concentrations around 1 mg/L fluoride has been linked to improved dental health, and in many parts of the U.S., municipal drinking water is artificially fluoridated to increase ambient concentrations. However, at levels above 2 mg/L (the California maximum contaminant level, MCL), adverse health effects such as dental and skeletal fluorosis can occur. In groundwater used for drinking water in the U.S., fluoride concentrations are typically low (<0.7 mg/L), although there are regions where fluoride concentrations can exceed 2 mg/L (McMahon et al., 2020). This study investigates temporal trends of fluoride concentrations in untreated groundwater from public supply wells in California and is part of the California Groundwater Ambient Monitoring and Assessment Program Priority Basin Project (GAMA-PBP). Approximately 15% of groundwater resources used for public supply have fluoride concentrations above 2 mg/L, and these high concentrations are most prevalent in the groundwater basins of the Mojave and Sonoran Deserts in southern California and in the southern end of the Sierra Nevada.

Of the public supply wells with sufficient data from 1974 to 2019 to test for temporal trends using the Mann-Kendall test (Jurgens et al., 2020), about 12% have a statistically significant long-term trend (1974–2014) of decreasing concentrations. The strongest negative trends are correlated with the high fluoride concentrations and found primarily in the southern California deserts and southern Sierra Nevada. A lesser proportion of wells has increasing fluoride concentrations and there is no geographic pattern. Decreasing fluoride trends are associated with increasing calcium concentrations but increasing fluoride concentrations are not related to calcium concentration trends, which suggests multiple mechanisms are responsible for the fluoride trends across the state. Geochemical and spatial analyses are used, along with geochemical modelling to identify potential drivers behind the observed trends, particularly as they relate to hydrologic and land use changes.