Occurrence and Sources of Radium in Groundwater Associated with Oil Fields in the Southern San Joaquin Valley, California

Geochemical data
from 40 water wells were used to examine the occurrence
and sources of radium (Ra) in groundwater associated with three oil
fields in California (Fruitvale, Lost Hills, South Belridge). 226Ra+228Ra activities (range = 0.010–0.51
Bq/L) exceeded the 0.185 Bq/L drinking-water standard in 18% of the
wells (not drinking-water wells). Radium activities were correlated
with TDS concentrations (p < 0.001, ρ =
0.90, range = 145–15,900 mg/L), Mn + Fe concentrations (p < 0.001, ρ = 0.82, range = <0.005–18.5
mg/L), and pH (p < 0.001, ρ = −0.67,
range = 6.2–9.2), indicating Ra in groundwater was influenced
by salinity, redox, and pH. Ra-rich groundwater was mixed with up
to 45% oil-field water at some locations, primarily infiltrating through
unlined disposal ponds, based on Cl, Li, noble-gas, and other data.
Yet 228Ra/226Ra ratios in pond-impacted groundwater
(median = 3.1) differed from those in oil-field water (median = 0.51).
PHREEQC mixing calculations and spatial geochemical variations suggest
that the Ra in the oil-field water was removed by coprecipitation
with secondary barite and adsorption on Mn–Fe precipitates
in the near-pond environment. The saline, organic-rich oil-field water
subsequently mobilized Ra from downgradient aquifer sediments via
Ra-desorption and Mn/Fe-reduction processes. This study demonstrates
that infiltration of oil-field water may leach Ra into groundwater
by changing salinity and redox conditions in the subsurface rather
than by mixing with a high-Ra source.