Monsoonal rainfall initiates autochthonous alteration of dissolved organic matter composition in Indian groundwaters

Groundwater contamination is a considerable threat to public health in many regions of the world. Strong seasonal variation in monsoon-affected regions can have significant effects on groundwater quality, yet these variations are not fully understood. Using excitation-emission matrix (EEM) fluorescent spectroscopy, we explore the seasonal dynamics of dissolved organic matter (DOM) composition along a transect in West Bengal, India. Groundwater, river water and ponds were sampled (n = 59) on a weekly/fortnightly basis, from the onset of monsoon to late-July, to gain an understanding of DOM temporal dynamics in aquifers at a critical point in seasonal hydrological conditions. Several fluorescent DOM (fDOM) components and indices were used to infer the source and nature of DOM. Although dissolved organic carbon (DOC) remained consistent throughout the sample timeframe, precipitation-associated spikes in fluorescence index (FI), specific ultraviolet absorbance (SUVA₂₅₄) and redox potential (Eh) likely point towards rainfall-induced increase of autochthonous DOM and the increase of microbial metabolic activity in response to oxygenated recharge. We suggest that observed fluctuations in organics were associated with concomitant changes in redox- and solubility-controlled elements (e.g. Fe and Mg), thus having wider implications on groundwater geochemistry and particularly the mobility of redox- and organic-sensitive solutes.