EFFECT ON GROUNDWATER QUALITY FROM PROXIMAL SURFACE WATER BODIES AND EFFECT ON ARSENIC DISTRIBUTION IN BANGLADESH: GEOCHEMICAL CONTROLS

Abstract:

Matlab is one of the areas in SE Bangladesh highly affected with elevated concentrations of dissolved As in drinking waters. Matlab is stratigraphically composed of thick floodplain deposits of Holocene age overlying Plio-Pleistocene grey fine to coarse sands with considerable clay (Dupi Tila). The dissolved As concentrations in the studied area ranged from <1 to 3644 µg/l and more than 70% of functioning tubewells exceeded the WHO guideline of 10 µg/l. To test the hypothesis that young labile organic carbon (LOC) accelerates As release in reducing aquifer systems as long as As-bearing iron oxides/hydroxides are available, we collected 19 groundwater (depths from 14 to 240 m) and 9 surface water (8 pond and 1 canal in proximity to the piezeometer nests) samples from 4 nests in north and south Matlab, respectively, during monsoon months (Jun-Jul, 2014). Dissolved organic carbon (DOC), fluorescent properties of dissolved organic matter (DOM) and fecal indicator bacteria (FIB) (e.g., E. coli) counts using field Colilert^TM test kits were undertaken to understand the chemical character and potential sources of dissolved organic matter (DOM) in groundwater and surface waters. DOC concentrations were high (1.5-12.2 mg/l) in shallow aquifers (14-33 m) where dissolved As ranged from 100-500 µg/l. DOC concentrations were lower (0.7-4.2 mg/l) in intermediate to deeper depths (52-240 m) with lower As concentrations of 0-50 µg/l. In surface water bodies, the range of DOC was 2.9-8.1 mg/l. MPN based analyses for both ground and surface waters indicate the frequency of E. coli detection in shallow well waters (MPN= 3.6-74.1) was high as well as in ponds and canals (MPN= 8.5-433.4). Microbial activity in groundwater was lower than in unprotected surface waters. Freshness index (β:α), humification index (HIX), fluorescence source index (FI) values showed that DOM in shallow and surface water bodies was distinct from deep groundwater. Concurrent with the lower DOC in deeper wells, the overall fluorescence intensities decreased with depth. The results thus far point to more humic DOM in shallow groundwaters, which is not expected to be a labile carbon source for microorganisms, but which may be involved in complexation or other biogeochemical reactions that mobilize arsenic.