PRE AND POST-MONSOON QUALITY OF DISSOLVED ORGANIC MATTER IN HIGH ARSENIC SITES IN WEST BENGAL, INDIA

The role of dissolved organic matter (DOM) in the biogeochemical interactions resulting in iron (Fe) and arsenic (As) mobilization in reducing aquifers of West Bengal (India) has been documented recently. Monsoonal recharge in this region is also an important hydrologic control influencing the redox conditions, groundwater flow and quality. Studies have reported the effect of monsoonal recharge on levels of As, Fe and HCO₃^-, however its effect on the characteristics of DOM has not been studied thus far. In this study, pre (PRM) and post-monsoon (PSTM) spectroscopic properties of DOM in the groundwater samples from the shallow (50 – 150 ft) Holocene aquifer (Nadia) and deep (200 – 360 ft) Pleistocene aquifer (Hooghly) were examined using absorbance and 3D fluorescence spectroscopy followed by a three component parallel factor analysis. Total As, anions, dissolved organic carbon (DOC) and bulk proteins were measured. Total As in shallow PRM and PSTM samples ranged from 50 to 315 μg/L and 42 to 300 μg/L respectively, while for deep PRM and PSTM samples from 0.44 to 10.62 μg/L and 0.55 to 9.62 μg/L. At shallow depths, chloride (Cl^-) concentrations were found to have mixing effects from monsoonal recharge while at greater depths (>200 ft) this effect did not exist. All other chemical and DOM properties showed variations (increased/decreased) at all depths after monsoonal recharge. Shallow samples showed decrease in total As with a decrease in DOC, slight increase (~20%) in fluorescence index (FI), slight decrease (~15%) in freshness index (β:α), decrease in specific ultraviolet absorbance (SUVA₂₅₄) and decrease in bulk proteins. Deep samples showed increase in total As with a decrease in DOC, increase in FI, slight increase in β:α, decrease in SUVA₂₅₄ and decrease in bulk proteins. The Cl^- concentrations varied at shallow depths but remained unchanged at greater depths after monsoonal recharge. In addition to small change (~20% decrease at shallow and increase at greater depths) in As concentrations, the results indicate the significant change in DOM quality after monsoon. This change at shallow depths may be linked directly to vertical mixing, while at greater depth it may be attributed to recharge but via complex biogeochemical reactive transport processes induced by increased water table depth and complex flow of groundwaters.