Radium isotopes and 222Rn in shallow brines, Kharaghoda (India)

Abstract

The concentrations of Ra isotopes (²²⁴Ra, ²²³Ra, ²²⁸Ra and ²²⁶Ra) and ²²²Rn have been determined in brines from Kharaghoda, western India. The Ra isotopes concentrations in these samples are orders of magnitude higher than that reported in potable groundwaters. The ²²⁴Ra activities lie between 28 to 277 dpm per kg; about 4% to 96% of that expected from its production, calculated using ²²²Rn as a recoil flux monitor. The low [²²⁴Ra/²²²Rn] activity ratios are typical of less saline brines. Retardation factors for Ra in these brines, derived from the ²²²Rn-²²⁴Ra-²²⁸Ra system and based on a reversible exchange model, are in the range of about 0.3–114. These values are 3–4 orders of magnitude lower than that in potable groundwaters. The residence time of Ra in Kharaghoda brines is approximately one day. Our data suggest that in briny aquifers Ra is considerably less ‘particle-reactive’ than in potable groundwaters. The retardation factor for Ra shows a strong negative correlation with salinity. This anticorrelation suggests that salinity would play a crucial role in determining the mobility of Ra and its geochemical homologues through groundwater systems.