WASTE-WATER IMPACTS ON GROUNDWATER: CL/BR RATIOS AND THEIR IMPLICATION FOR ARSENIC POLLUTION OF GROUNDWATER FOR 200 MILLION CONSUMERS IN THE BENGAL BASIN

Some 200 million, mostly rural, consumers living in the Bengal Basin (West Bengal and Bangladesh) rely on groundwater from shallow tubewells for their domestic water supply, including their drinking water. The groundwaters occur in two principal settings viz.palaeo-channel and palaeo-interfluve. Palaeo-channel settings comprising palaeo-valleys infilled by grey sands during and after sea-level rise since the end of the last glacial maximum (LGM); palaeo-interfluvial settings comprise brown sand aquifers capped by a palaeosol that formed between river channels prior to the LGM and are now buried by more recent sediments.

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.