UC Merced

The San Joaquin Valley is a rural, agricultural area in California with high rates of poverty and food and water insecurity. Applications of soil fumigants and pesticides in agriculture are widely understood to be a leading cause in groundwater contamination. From the 1940s until the mid-1980s in California, agricultural production often involved the use of pesticides, particularly those containing 1,2,3-Trichloropropane (TCP). After numerous studies found that TCP caused cancer and infertility in mice, TCP was banned from agricultural use in the 1980s. TCP, however, remains in groundwater at concentrations exceeding state regulations, especially in the San Joaquin Valley. Previous toxicology studies suggest that TCP is unsafe even at its lowest detection limit, 0.005 μg/L. One of the most assumed but understudied determining factors of contamination level is precipitation. This study statistically analyzes Groundwater Ambient Monitoring and Assessment (GAMA) Groundwater Information Systems data from 2020-2024 to determine if TCP concentrations vary by season (wet vs. dry). Wet season is defined from October through April, and dry season is defined as May through September. Understanding potential fluctuations with seasonality may help domestic well users and municipal water system operators understand when water quality may be at its lowest or highest levels of contamination. Individual wells were averaged for each county by year and season. Results showed variation in sampling amounts by year, individual wells and well type. Three out of eight SJV counties had significant (p<0.05) differences between seasonal contamination levels, where each of those counties had higher contamination during the wet season. Further research is recommended with longer term data, comparing to soil type, and more sampling at varying well depths. Additional TCP remediation needs to address needs and factors for water treatment for Central Valley residents. This should be done with a One Water approach through engagement with the local public to meet long-term community and environmental needs.