Identification of microbiogeochemical factors responsible for arsenic release and mobilization, and isolation of heavy metal hyper-tolerant bacterium from irrigation well water: a case study in Rural Bengal

In Bengal Delta Plain (West Bengal and Bangladesh), shallow aquifer (< 50 m) groundwater is often used in irrigation for paddy cultivation. The present study highlights the role of anthropogenic activities on natural process and focuses on arsenic mobilization in the shallow irrigation well water of rural Bengal. The major focus is to examine the role of geochemistry, arsenic mobilization and their association with local microbial community in irrigation well water. The results suggest that the groundwater of monitored wells is usually devoid of dissolved oxygen and consists of high concentration of dissolved redox elements like arsenic (As-T) (mean value 58.7 mu gL(-1)) and iron (Fe-T) (mean value 2.6 mgL(-1)), with low amount of oxyanions like sulphate (SO42-) and nitrate (NO3-) (mean value 8.3 mgL(-1) and 1.8 mgL(-1), respectively). High concentration of alkaline earth metals like calcium (Ca) (mean value 252.0 mgL(-1)) and magnesium (Mg) (mean value 96.9 mgL(-1)) with high alkalinity (mean value 400.3 mgL(-1)) suggests that carbonate dissolution (calcite and dolomite) is the key process in these monitored wells. The factor analysis reveals a positive and strong correlation (r(2) = 0.716) between arsenic (As) and iron (Fe). Some of the wells are contaminated with high concentration of chloride (up to 128.5 mgL(-1)), total coliforms (no. of wells contaminated with coliforms = 7) and faecal coliforms (no. of wells contaminated with faecal coliforms = 5), which point towards the local anthropogenic influence (leakage of sewage). Some irrigation wells with non-permissible arsenic concentration also harbour arsenic (As3+) and iron (Fe2+) hyper-tolerant bacteria. The microenvironment of the irrigation wells consisting of suitable local reducing condition together advocates the dominance of existing microbial community, particularly arsenic and iron hyper-tolerant bacteria in them. In this study, an arsenic and iron hyper-tolerant bacterium has been isolated from irrigation well water. Scanning electron microscopy and 16S rDNA sequencing further established the identity of this bacterium as Enterobacter sp., which is a facultative anaerobe. The addition of fresh organic matter through local anthropogenic activities could enhance the microbial activity in these monitored wells. The silver nitrate test reveals the biotransformation potential of this bacterium from arsenite (As3+) to arsenate (As5+). Altogether, these results point towards the dependence of arsenic mobilization process on multiple microbial and geochemical factors, where bacterium like Enterobacter sp. might also play an important role. Such biogeochemical processes are not only responsible for the unsafe nature of irrigation well water (for both domestic and irrigation purposes) in rural Bengal, but also convert it into a major potential source for soil arsenic accumulation, thereby contaminating the food chain.

Automated summary

This study emphasizes the impact of anthropogenic activities on natural processes, particularly the mobilization of arsenic in irrigation well water in the Bengal Delta Plain. The results reveal the dominance of dissolved redox elements like arsenic and iron, as well as the presence of high alkaline earth metals in these wells. The contamination of chloride and coliform bacteria suggests human influence, while the presence of arsenic and iron hyper-tolerant bacteria indicates the important role of microbial communities in arsenic mobilization and bio-transformation.