Identification of the Source of H2s and Characterization of Vertical Groundwater-Chemistry Trends in the San Joaquin Valley’s Unconfined Aquifer

The San Joaquin Valley’s (Valley) unconfined aquifer (Aquifer) is saturated with hydrogen sulfide (H2S) contaminated groundwater that has no verified source. H2S in the Aquifer was initially observed by Mendenhall et al. in 1916, but no thorough investigations into the source of H2S have been conducted. As a result, the surficial and vertical extent of H2S has not been estimated. To estimate surficial extent this study compiled H2S data from the water-quality databases of the National Water Quality Monitoring Council and the California State Water Resources Control Board. Based on the estimated H2S extent and the Aquifer’s hydrogeologic characteristics this study hypothesized that H2S was generated as a byproduct of microbial mediated sulfate-reduction. Bacterial sulfate-reduction (BSR) is associated with the ecological succession of the terminal electron accepting processes (TEAP). This study classified the predominant TEAP in groundwater by measuring the concentration of redox related constituents and comparing results to the known evolution of microbial reduction. Results demonstrated the progressive depletion of dissolved oxygen, manganese, and iron from 50 to 200-feet (ft) below ground surface (bgs). From 200 to 240 ft bgs, these constituents, plus sulfate, were abruptly removed from the groundwater system due to the onset of BSR resulting in the initial detection of aqueous H2S. At 240-ft bgs, the depletion of sulfate, production of H2S, detection of sulfate-reducing bacteria (SRB), and the assigned sulfate-reducing TEAP were sufficient evidence to unambiguously conclude that the Aquifer’s source of H2S is H2S generated as a byproduct of BSR mediated by SRB. In the future, the defined vertical groundwater-chemistry trends of redox related constituents can predict whether H2S is actively being produced within a portion of the Aquifer.