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Description
A geochemical review, evaluation, and modeling simulation were completed for a 334.1 square kilometers (km2) rural study area, the Anza-Terwilliger study area. Water chemistry data were compiled from the years 1950 to 2013, and the data were separated by location and geology into seven sub-areas. Geochemical trends of the groundwater for each sub-area were assessed through evaluations of the groundwater chemistry at various locations within each sub-area. Specifically, TDS, chloride, sulfate, and nitrate-as-nitrogen concentrations at various locations in each sub-area were compared amongst themselves and to the concentrations of other sub-areas. TDS and sulfate concentrations were generally greatest in West Anza areas of groundwater pumping. The areas with the greatest nitrate-as-nitrogen concentrations were found to correlate with fractured bedrock aquifer geology and being located in more urban land use areas, which is consistent with the septic sewage system based study area. Through temporal site evaluations, it was determined the geochemistry of most groundwater sites remain unchanged during the period of record. In an effort to better understand the geochemical processes controlling water chemistry along flow paths, geochemical inverse modeling was completed with the freeware PHREEQC. Evaluations of aluminum concentrations over three orders of magnitude from 0.001 mg/L to 0.1 mg/L were completed and results indicate they have little impact on saturation indices of primary minerals nor a large impact on the inverse modeling results. Two different scenarios of groundwater flow were modeled using granodiorite as the aquifer material. The results from the modeling are non-unique, but the combination of the results provide insight into potential reactions that influence the geochemistry of the groundwater. Ultimately, the combination of models for these scenarios represent the phases necessary to affect groundwater chemistry along flow.
