Abstract

This work presents a volume model of groundwater total dissolved solids (TDS) in the vicinity of the Fruitvale and Rosedale Ranch oil fields near Bakersfield, California. California Senate Bill 4 (2013) and the 1974 Safe Drinking Water Act authorize the protection of underground sources of drinking water (USDW), defined as non-exempt aquifers containing water with <10,000 parts per million (ppm) TDS. TDS data from produced water samples indicate that TDS increases with depth, but the depth at which TDS reaches 10,000 ppm varies greatly. Currently, the spatial distribution of available TDS data do not provide adequate coverage to clearly define the distribution of USDW near the oil fields, causing uncertainties in strategies for monitoring protected groundwater and management of wastewater injection operations. Therefore, geophysical logs from 50 oil and gas wells were evaluated to fill spatial gaps. Resistivity, porosity, and temperature data from these logs were used to model TDS concentrations in three dimensions. The model uses Archie’s Equation and kriging to predict TDS values (with uncertainties) within the volume. Model parameterizations are found by mathematical optimization with the sum of the squared residuals as the objective function to be minimized. Modeling shows the 10,000 ppm boundary is reached at ~3,200 ft (975 m) below sea level in Rosedale Ranch and deepens to the southeast in Fruitvale to ~4,200 ft (1,280 m). Mapping groundwater TDS concentrations at this resolution reveals that concentration is primarily controlled by depth, recharge, and stratigraphy, as well as faulting and facies changes in some places.