The Hydrology of Shallow Aquifers in Araihazar, Bangladesh: A Potential Link to Temporal Variability in Dissolved Arsenic Concentrations

To shed light on the complex interactions between dissolved As levels in shallow aquifers and recharge, nests of three very shallow (6-9 m deep) wells were monitored bi-weekly over 2 years. At Site X, a strong vertical gradient in redox conditions and groundwater composition between the shallowest and deepest well is indicated by Eh (+134±64 mV vs. -4±43 mV), dissolved As (3±1 vs. 230±24 ug/L), and Cl (267±54 vs. 60±14 mg/L). Whereas the composition of groundwater at the shallowest and deepest well remained relatively constant over two years, considerably larger seasonal fluctuations were observed at the intermediate well (7.6 m): 44±52 mV in Eh, 196±93 mg/L in Cl and 15±11 ug/L in As. Hydraulic head measurements indicate that these changes reflect a combination of changes in the direction of groundwater flow and mixing of groundwater from above and below 7.6 m depth. At two other sites (Y and Z), no sharp vertical gradient in the same aquifer properties were observed and neither were there any significant seasonal fluctuations over two years. However, the monitoring data from three shallow wells at site Y show a gradual decline of ~5 ug/L per year in average As. Relative hydraulic heads between sites Y and Z, and a broad trend of increasing downward hydraulic gradients at site Y that parallels the declining average As suggest an enhancement of vertical recharge by irrigation pumping. In contrast, no gradual changes in groundwater chemistry or hydrology were recorded at site Z. These observations provide further evidence that recharge can occasionally drive seasonal and perhaps longer-term variations in groundwater composition including As in shallow aquifers of Bangladesh.