ARSENIC ENRICHED GROUNDWATER DISCHARGE FROM A MICRO-TIDAL SUBTERRANEAN ESTUARY, AT THE BAY OF BENGAL, INDIA

Coastal aquifers are important links between terrestrial and marine biogeochemical cycles. Hence, solutes discharging via the process of submarine groundwater discharge (SGD) may impact coastal and marine ecosystems. This influence of SGD on marine ecosystems is pronounced at the freshwater-seawater mixing zone of a coastal aquifer. This mixing zone, by analogy to surface estuary, is called as ‘subterranean estuaries’. Several studies showed that SGD affects coastal trace metal budgets. The focus of this study is to investigate the biogeochemical processes that control arsenic (As) discharge and cycling in a subterranean estuary located at the Bay of Bengal, India. Porewater samples were collected for hydrogeochemical parameters from a series of multi-depth observational wells from February 2012 to September 2013. Samples were collected along a 100 m transect from the inter-tidal zone. Dissolved As concentrations ranged from 1 µg/L to 71 µg/L, with many of the samples having concentrations higher than the World Health Organization (WHO) guideline value of 10 µg/L. Depth profiles of As concentrations show elevated concentrations at a depth of about 45cm. Total dissolved Fe and Mn concentrations vary from 0.09 mg/L to 45.2 mg/L and 0.03 mg/L to 5.9 mg/L, respectively. Elevated arsenic zones were found to be associated with elevated dissolved Fe and Mn zones, indicating that Fe and Mn cycling probably controls the Arsenic budget. Offshore distribution of As shows elevated concentrations from shoreline to 40 m offshore site. Dissolved As is low again at 60 m to 80 m offshore. Freshwater-seawater transition occurs around 40 m offshore site, indicating hydrologeochemical cycle of SGD, is the reason behind offshore variations in As concentrations. The total chemical flux of As by SGD to Bay of Bengal is yet to be determined.