GEOLOGIC INFLUENCES OF SECONDARY MINERAL DISSOLUTION TO THE HYDROGEOCHEMISTRY OF SHALLOW GROUNDWATER DISCHARGING TO URBAN CATCHMENTS WITHIN THE SANTA MONICA MOUNTAINS WATERSHED

Dissolution of secondary minerals associated with the regional geology of the Santa Monica Mountains watershed greatly influences the water quality of shallow groundwater that discharge into urban catchments. This study shows that shallow groundwater samples collected from fine textured aquifer material formed in sedimentary marine environments tend to have the highest total dissolved solid concentrations, relative to groundwater collected in geologic strata composed primarily of lower solubility igneous rocks and sandstones. In this study, 29 groundwater samples were collected throughout the study area from groundwater springs and seeps that contribute to local perennial streams. Field index parameters (conductivity, pH, salinity, dissolved oxygen, and temperature) were obtained during sample collection, with split samples submitted to a laboratory for analysis of major ions, selenium and arsenic. Groundwater samples were also tested for hydrogen and oxygen stable isotopes to help determine the origin of the shallow groundwaters in the area. Chemical analysis show that groundwater samples collected from fine textured aquifers of sedimentary marine origins have high TDS (1,500 to 3,500 mg/L), including high chloride (250 to 500 mg/L Cl^-), and very high sulfate (1,000 to 2,500 mg/L SO₄^2-) concentrations. Most notably, the more saline groundwater samples collected from the marine sedimentary formations were influenced by gypsum layers and calcite cements that had formed as secondary mineral precipitation products. Spatial mapping shows the highest concentrations of selenium from groundwater samples originate from one of these sedimentary marine rocks, the Miocene Monterey and Modelo Formations. Isotopic analysis of all of the samples reveals that groundwater contains the signature of mixed imported and local waters with local waters usually being more dominant (approximately >75%). Both local and imported waters have a slightly evaporative isotopic signature, synonymous with infiltrated water that had been subjected to some evaporation prior to recharge. Imported water is apparently recharging shallow aquifers in the study area, probably as a result of leakage from water pipes, sewer pipes, and urban storm drains.