Abstract

In order to characterize groundwater quality near oil fields within the Los Angeles Basin, California, the U.S. Geological Survey collected groundwater samples from 38 monitoring wells at 17 locations across the basin. This work was in collaboration with the California State Water Resources Control Board and the Water Replenishment District of Southern California. Shallow (~100 m), moderate, and deep (~600 m) monitoring wells within or near oil fields, along with three control sites upgradient from oil fields, were sampled. Samples were analyzed for a comprehensive suite of analytes including dissolved hydrocarbon gases (C1-C6), and their isotopes.

Results show concentrations of methane gas ranged from 0.002 to 150 mg/L up to five times saturation values, which suggest an external source for the hydrocarbon gases. Analyses of ≏13C and ≏D of the methane ranged from -80.8 to -45.5 per mil, and -249.8 to -134.9 per mil, respectively, and identified microbial methanogenesis as the primary source of the methane, and CO2 reduction as the main formation pathway. In addition, 14C was measured in the samples to provide an estimate of groundwater residence time (age); 14C values range from <1 to 130.1 percent modern carbon and inversely correlate with methane concentration, suggesting an increase in dissolved methane with groundwater residence time. The methane concentrations, isotopic analyses, and groundwater radiocarbon results indicate that the excess methane in Los Angeles groundwater originated from relatively shallow microbial production, rather than the migration of deeper thermogenic methane associated with oil fields in the basin.