A study on coliforms in arsenic-contaminated aquifer sediments of Chakdah, West Bengal, India

Groundwater contamination by geogenic arsenic (As) is globally a significant health risk. The availability of As in groundwater as well as its toxicity depends on its oxidation state. Metal reducing bacteria in groundwater-associated aquifer sediments can reduce sediment boundless toxic arsenate (AsV) more toxic arsenite (AsIII) and mobilize them into groundwater. Under anaerobic conditions as prevalent in deep surface sediments, dissimilatory arsenate-respiring prokaryotes are the predominant arsenate-reducing microbial members. However, in anaerobic environments of subsurface sediments of lesser depth, microbial As cycling, and their mobilization are rarely studied. In rural parts of India and Bangladesh, open defecation, use of pit latrines, and soak pits are standard practices, and seepage of fecal matter, as well as coliforms into adjoining sediments, could be of high possibility. Herein, distribution of aquifer sediment-associated arsenate tolerant microbial population in general and coliforms (including fecal coliforms) in particular from shallower depths were studied. The difference in the recovery of total arsenate-resistant microbial count under aerobic and anaerobic conditions was evident. Prevalence of fecal coliforms, specifically Escherichia coli (revealed by molecular characterization), was observed at greater depths due to their facultative anaerobic nature.