Abstract:
Demand for groundwater in urban centres across Asia continues to rise with ever deeper wells being
drilled to avoid shallow contamination. The vulnerability of deep alluvial aquifers to contaminant
migration is assessed in the ancient city of Varanasi, India, using a novel combination of emerging
organic contaminants (EOCs) and groundwater residence time tracers (CFC and SF6). Both shallow and
intermediate depth private sources (<100m) and deep (>100m) municipal groundwater supplies were
found to be contaminated with a range of EOCs including pharmaceuticals (e.g. sulfamethoxazole, 77%
detection frequency, range <0.0001e0.034 mg L 1), perfluoroalkyl substances (e.g. PFOS, range <0.0001
e0.033 mg L 1) as well as a number of pesticides (e.g. phenoxyacetic acid, range <0.02e0.21 mg L 1). The
profile of EOCs found in groundwater mirror those found in surface waters, albeit at lower concentrations,
and reflect common waste water sources with attenuation in the subsurface. Mean groundwater
residence times were found to be comparable between some deep groundwater and shallow groundwater
sources with residence times ranging from >70 to 30 years. Local variations in aquifer geology
influence the extent of modern recharge at depth. Both tracers provide compelling evidence of significant
inputs of younger groundwater to depth >100m within the aquifer system.
