• India;
  • Murshidabad;
  • West Bengal;
  • arsenic;
  • groundwater;
  • stable isotopes

[1] Arsenic (As) concentrations and stable hydrogen (δ2H) and oxygen (δ18O) isotope ratios of surface and groundwaters from a representative site in West Bengal, India, are reported. Shallow groundwaters (≤35 m) from the study site have among the highest As concentrations measured in the entire Bengal Basin, reaching values in excess of 4600 μg kg−1. Stable isotope ratios of waters from constructed, perennial ponds indicate the ponds are chiefly recharged during the summer monsoon, and subsequently undergo extensive evaporation during the dry (winter) season. In contrast, groundwaters with high As concentrations plot along the local meteoric water line (LMWL) near where the annual, volume-weighted mean precipitation values for δ2H and δ18O would plot. The stable isotope data demonstrate that groundwaters are directly recharged by local precipitation without significant evaporation, and thus are not recharged by, nor mixed with, the pond waters. Furthermore, reactive transport modeling indicates that dissolved organic matter (DOM) derived from pond waters does not fuel microbial respiration and As mobilization at depth in the underlying aquifer because travel times for pond-derived DOM exceed groundwater ages by thousands of years. Instead, organic matter within the aquifer sediments must drive dissimilatory iron reduction and As release to groundwaters.