Novel methods for groundwater monitoring within regions of oil and gas production

As California enters its fourth year of drought, protecting shallow groundwater quality for municipal, domestic and agricultural use becomes critically important. Many of California’s sedimentary basins host both potable groundwater aquifers and hydrocarbon-producing zones, and Senate Bill 4 (Pavely, 2013) requires groundwater monitoring within hydrocarbon zones that potentially impact shallow aquifers by certain types of oil production. One approach to understanding the relationship between deeply seated fluids associated with oil production and shallow drinking water aquifers is to establish current ambient groundwater quality conditions using available data (both current and historic), then develop a predictive mixing model between ambient groundwater and “produced” waters associated with oil production. In addition, in order to fill datagaps useful for the model, novel methods of analyzing certain chemical analytes in produced waters must be developed. Produced waters typically contain complex hydrogeochemical matrices (e.g., high salinity, TDS, etc.) but also contain effective isotopic tracers such as isotopes of radium. Utilizing produced water samples from three large oil fields in California, a novel method to analyze radium-226 in produced waters was developed. The analytical method utilizes liquid scintillation counting (LSC) to provide accurate and efficient results with a two-week turn around time. Radium analysis in produced water samples will further refine the mixing model and therefore groundwater monitoring programs in oil producing regions. Data from the Kern County groundwater basin demonstrates the novel groundwater monitoring methods. The San Joaquin Valley’s Kern County groundwater basin contains the majority of California’s oil production (~80% of the active wells in California). In addition, the county relies almost entirely on groundwater for its water supply, and is the most productive agricultural county in the US. Considering Kern County’s historically stressed groundwater system, the Department of Water Resources ranks the basin as a high priority in the statewide groundwater elevation monitoring (CASGEM) program. This research consists of a database investigation to define the current water quality conditions within Kern County’s groundwater system and to examine the relationship between shallow groundwater and produced waters. An end-member mixing model utilizing multivariate statistics compares the geochemical data between the two distinct waters. Two publically available data sets are used to define the end-members: the California State Water Resources Control Board’s Groundwater Ambient Monitoring and Assessment program data are used to define the shallow zone, and the USGS Produced Waters data are used to define the deep formation waters. The multivariate mixing model indicates that six common groundwater chemical analytes (Ca, Cl, Mg, Na, SO4 and TDS) distinctly segregate shallow groundwater from produced waters. The multivariate model will aid groundwater monitoring programs by providing a statistical test of whether deeply seated basin fluids are mixing with shallow groundwater.