MINERALOGICAL DISTRIBUTION OF F AND SOURCES OF GROUNDWATER FLUORIDE IN THE GNEISSIC BEDROCK/REGOLITH AQUIFER OF ANDHRA PRADESH, INDIA

Fluoride in groundwater-based drinking water is a widespread concern over large parts of India, particularly in the granitic gneiss bedrock/regolith catchments of Andhra Pradesh, one of the most severely affected states. Development of conceptual models for the mobilisation of F to groundwater, to guide mitigation actions, requires knowledge of (1) the distribution of F mineralogical sources within and between bedrock and the derived regolith, and (2) the relative availability of F-bearing minerals in bedrock and regolith as sources of groundwater F^- in the present-day. Understanding the mechanisms and progress of chemical weathering of the granitic gneiss is key to these questions, ie how effectively is F removed from its primary source(s) as the bedrock weathers? And, to what extent is F^- flushed from the weathering profile and/or re-sequestered by secondary mineral phases as the regolith develops? To address these questions, we have applied optical petrography, XRD, scanning electron microprobe analysis, whole-rock chemical analysis and leaching experiments to samples of bedrock and regolith from two catchments in Andhra Pradesh. We have quantified the distribution of F between its principal mineralogical sources (biotite, apatite, titanite, hornblende, fluorite, calcite, and sorbed phases) and between bedrock and regolith. Leaching experiments show there is no straighforward relationship between whole-rock F content and leached [F^-]; in some instances regolith samples leach higher F^- concentrations than the fresh granitic gneiss. Results shed light on conflicting conceptual models of F release to groundwater in gneissic bedrock/regolith aquifers. Accounting for groundwater [F^-], simple estimates of groundwater flux in the catchments suggest that the extant regolith in Andhra Pradesh has developed over a period of c.350 ka, during which time >70% of the original bedrock F has been removed by chemical weathering.