Mineralogical sources of groundwater fluoride in Archaen bedrock/regolith aquifers: Mass balances from southern India and north-central Sri Lanka

The Maheshwaram and Waipally catchments of Andhra Pradesh, India, and the Plonnaruwa catchment of north-central Sri Lanka. The distribution of F across eight crystalline phases and between the bedrock and the regolith at eleven sites in three catchments is documented. Mineral contributions to F release during weathering and regolith development are quantified. An estimate of weathering duration for the in situ regolith in Andhra Pradesh, 250–380 Ka, is close to a previous estimate for southern India. Partial or total destruction of the primary F-bearing bedrock minerals and consistent depletion of F in the remnant minerals result in a much reduced total F content in the regolith. Leaching experiments and field relationships, however, indicate a greater potential for F mobilisation to groundwater from the regolith than the bedrock. Schemes for managed aquifer recharge should beware the risk of mobilising additional F to groundwater.