Freshening and Salinization are Important Factors Controlling Concentrations of Boron and Uranium in NW Indian Groundwater

Abstract:

The heavily exploited alluvial aquifers of Northwestern India are plagued by multiple quality problems, including high concentrations of boron and uranium. The arid to semiarid environment has contributed to elevated groundwater salinity through high evapotranspiration rates, yet even in areas where irrigation with imported surface water has led to aquifer freshening, quality problems quality problems persist. Using geochemical and isotopic data, we explore how salinization and freshening affect the groundwater quality in the Indians states of Punjab, Rajasthan, and Gujarat. In Gujarat oxygen and hydrogen isotopes show clear evaporation trends with respect to the local meteoric water, Br/Cl ratios suggest halite dissolution, and very high δ¹¹B with low B/Cl ratios indicate adsorption related to salinization. These are aquifers with high TDS (median = 1940 mg/L) and boron concentrations (median = 565 µg/L), but relatively low uranium concentrations (median = 14.2 µg/L). Conversely, in Punjab, which has more extensive irrigation with imported surface water than does either Gujarat or Rajasthan, oxygen and hydrogen isotopes reveal mixing of groundwater with surface water, and low δ¹¹B with high B/Cl ratios that are indicative of boron desorption related to aquifer freshening. Groundwater from these aquifers have lower TDS (median = 827 mg/L), but high B (median = 507 µg/L) and U (median=40.8 µg/L) concentrations. Rajasthan’s groundwater shows a mixture of freshening and salinization processes, with low Br/Cl ratios typical of halite dissolution, but in many samples this is accompanied by very high Na/Cl ratios (molar ratio >1) that are typical of ion exchange from freshening. Beyond the adsorption/desorption and dilution processes most typically associated with freshening which affect boron concentration and TDS, freshening also affects uranium concentrations by promoting calcite dissolution. Soils throughout the study area are calcareous, and by decreasing calcite saturation by both ion exchange and dilution, freshening raises groundwater bicarbonate concentrations, which play a key role in uranium solubility. Thus, while alleviating some groundwater salinity issues though dilution with low TDS surface water, freshening, can promote other water quality issues, such as uranium.