ROLE OF DISSOLVED ORGANIC MATTER IN CONTROLLING GROWTH OF COLIFORM BACTERIA IN ARSENIC CONTAMINATED GROUNDWATER OF SOUTHEAST BANGLADESH

Contamination of surface water resources with pathogens promoted use of shallow groundwater as a primary drinking water source in Bangladesh several decades ago. However, the shallow groundwater containing elevated levels of arsenic (As) caused a severe health threat to millions. Recently it was observed that As-contaminated groundwater also contained detectable concentrations of fecal (coliform) bacteria. In this study, we investigate interrelationships between As, coliform bacteria and dissolved organic matter (DOM) which appears to be a key factor in As mobilization in shallow aquifer of Bangladesh. Nineteen groundwater samples (13-236 m depth) and eight surface water samples from Matlab (southeast Bangladesh) were collected and analyzed for their inorganic, organic and microbial composition. Our results show that shallow groundwaters with elevated As (0.2-407 µg/L), also contain detectable (unsafe) levels of total coliform bacteria (12-238 CFU), whereas surface waters contained minimal As, and several orders of magnitude higher total coliforms (308-241,957 CFU). Dissolved As concentrations decreased with the depth, whereas, that of total coliforms did not show any depth-dependent variations. Stable isotopic composition (δ¹⁸O and δ²H) of groundwaters with elevated total coliforms showed signatures similar to local precipitation, indicating faster recharge and lesser degree of natural filtration. Groundwater with elevated total coliform counts also showed the youngest C-14 age (65 years) among all samples, DOM with lower freshness index (0.48) calculated from fluorescence spectroscopy indicating fresh DOM supply, and a lower Cl:Br mass ratio consistent with organic matter degradation. Molar C:N was found to be inversely correlated with As consistent with prior studies, and positively correlated with total coliforms indicating C-based heterotrophic metabolism. We hypothesize that locations of rapid recharge may draw coliforms from the surface due to insufficient natural filtration. Sedimentary organic carbon that supports As mobilization under reducing condition, may also support growth of coliform bacteria. While As remains to be an important health concern in groundwaters of Bangladesh, the co-presence of coliform bacteria even at >200 m depths is alarming.