WASTE-WATER IMPACTS ON GROUNDWATER: CL/BR RATIOS AND THEIR IMPLICATION FOR ARSENIC POLLUTION OF GROUNDWATER FOR 200 MILLION CONSUMERS IN THE BENGAL BASIN
Plotted in Cl/Br vCl space, compositions of groundwaters from wells in palaeo-channel settings scatter along mixing lines between waste-water and the most dilute groundwater, with many falling near the end-member value for septic-tank effluents of around 1560 for Cl/Br mass ratio at 125 mg/L Cl. End-member modelling shows that between 25% and 50 % of such wells yield water that comprises > 10% of waste-water. Compositions of groundwater from wells in palaeo-interfluvial settings are not affected by waste-water contamination unless sited at the margins of the palaeo-interfluves, where the palaeo-interfluvial aquifers are invaded by contaminated palaeo-channel groundwater. Settings are identifiable by well-colour survey, owner information, water composition (As, Fe, Mn, U, V, Mo), and drilling. These data show that shallow palaeo-channel groundwater of the Bengal Basin has been widely contaminated by waste-water derived from pit latrines, septic tanks, and other methods of sanitary disposal.
Groundwater in the Bengal Basin is also severely polluted by dissolved As. Because values of Cl/Br and faecal coliform counts are both inversely related to concentrations of pollutant As in groundwater, waste-water contributions to groundwater in the near-field of septic-tanks and pit-latrines (within 30 m?) may be suppressing the mechanism by which such pollution arises and may be lessening its prevalence and severity. In the far-field of such sources, organic matter in waste-water may increase As-pollution of groundwater by driving reduction of sedimentary FeOOH and release of As to groundwater.