Abstract

As part of the State Water Resources Control Boards Regional Monitoring Program of Water Quality in Areas of Oil and Gas Production, chemical and isotopic data from 40 water wells and 27 oil-field water samples were used to examine groundwater radium (Ra) activities in and near three mature oil fields (Fruitvale, Lost Hills, and South Belridge). In 18 percent of the wells, 226Ra+228Ra activities (range=0.010-0.51 becquerel/liter) are larger than the 0.185 becquerel/liter (5 picocuries/liter) drinking-water standard. Fourteen of the 40 sampled water wells are used to supply drinking water, but none of the samples from those 14 wells have 226Ra+228Ra activities above the drinking-water standard. Correlations of Ra activities with total dissolved solids (145-15,900 milligrams/liter), manganese+iron (Mn+Fe) (<0.005-18.5 milligrams/liter), and pH (6.2-9.2) indicate Ra mobility is controlled by salinity, oxidation-reduction, and pH conditions in the aquifers. Trace-element, noble-gas, and other data indicate high-Ra samples (>0.185 becquerel/liter) adjacent to the South Belridge field contain 40 to 45 percent oil-field water leaked from unlined disposal ponds. High-Ra samples overlying the Lost Hills field contain less than 10 percent oil-field water from subsurface sources, possibly due to natural mixing processes. Discordance between 228Ra/226Ra ratios in groundwater affected by pond leakage (median=3.1) and oil-field water (median=0.51) indicate the high groundwater Ra in these samples is not derived directly from oil-field water but from interactions of this water with aquifer sediments. Geochemical model (PHREEQC) calculations and spatial patterns in groundwater chemistry suggest Ra in pond (oil-field) water is removed onto aquifer sediments by co-precipitation with barite and adsorption on Mn-Fe precipitates in the near-pond environment. Subsequent infiltration of saline, organic-rich pond water into the aquifer mobilizes Ra from downgradient aquifer sediments through Ra-desorption and Mn/Fe-reduction processes. This study shows the effects of oil and gas production activities on Ra activities can be complex, requiring multiple tracers in both groundwater and oil-field water to understand them.