ABSTRACT
Understanding the interactions among Dissolved Organic Carbon (DOC), arsenic and fecal contamination in the groundwater of southeast Bangladesh is crucial as groundwater is the most important drinking water sources in Bangladesh. Groundwater samples (13 – 236 m depths) from four piezometer nests and nearby surface water samples from Matlab (SE Bangladesh) were collected and analyzed for inorganic, organic and microbial constituents. The groundwaters contained total dissolved As (AsT) ranging from 0.2 to 407 µg/L, dissolved organic carbon (DOC) between 0.7 and 12 mg/L, molar C:N between 0.2 and 8, and total coliform (MPN) between 12 to 238 CFU. Absorbance and fluorescence spectroscopic analyses of dissolved organic matter (DOM) revealed that the groundwater contained aromatic humic-like fluorescence signature, characterized by humification index (HIX) of 10±7, and specific ultraviolet absorbance (SUVA) ranging from 0.3 to 4 L/mg.m. Surface water samples from the study area contained low AsT (2-11 µg/L) and very high total coliform (308 – 241,957 CFU). These results indicate that almost all the groundwater samples analyzed in this study contained unsafe levels of AsT (>50 µg/L) as well as presence of total coliform bacteria in detectable levels suggesting potential pathogens. Higher DOC, lower C:N and high humic-like fluorescence signature of groundwater DOM are consistent with arsenic mobilization by reductive dissolution of iron bearing aquifer sediments by microbes under reducing conditions and fueled by DOM. The presence of coliform bacteria in groundwater, even at >200 m depths, is alarming. The source of coliform bacteria in groundwater may be via infiltration of surface water that is contaminated by anthropogenic wastes, and further supported by higher DOC concentrations in the groundwater.
