Hydrological control of As concentrations in Bangladesh groundwater

Abstract

[1] The elevated arsenic (As) content of groundwater from wells across Bangladesh and several other South Asian countries is estimated to slowly poison at least 100 million people. The heterogeneous distribution of dissolved arsenic in the subsurface complicates understanding of its release from the sediment matrix into the groundwater, as well as the design of mitigation strategies. Using the tritium-helium ( 3 H/ 3 He) groundwater dating technique, we show that there is a linear correlation between groundwater age at depths <20 m and dissolved As concentration, with an average slope of 19 mg L À1 yr À1 (monitoring wells only). We propose that either the kinetics of As mobilization or the removal of As by groundwater flushing is the mechanism underlying this relationship. In either case, the spatial variability of As concentrations in the top 20 m of the shallow aquifers can to a large extent be attributed to groundwater age controlled by the hydrogeological heterogeneity in the local groundwater flow system.

Temporal variability of groundwater chemistry in shallow and deep aquifers of Araihazar, Bangladesh

“…The reasons for much of this remain unclear, but sites with net loss of As located on older meanders is consistent with the notion that flushing over time may gradually decrease the inventory of As in the Holocene aquifer but that process will take thousands of years if not longer (McArthur et al, 2001;Stute et al, 2007;van Geen et al, in press). The increasing trends observed in Sites E and G suggest that near surface mobilization and subsequent transport (Polizzotto et al 2005) may have provided a fresh source of As to the aquifer in very young flood plain.…”

“…Groundwater ages in deep aquifers at sites A and B are 10-1000 times higher than in the corresponding shallow aquifers of Araihazar (Zheng et al, 2005;Stute et al, 2007). Radiocarbon ages of dissolved inorganic carbon from wells C-53 m, F-58 m, and G-52 m were 10700 ± 55, 6240 ± 30, and 3620 ± 35 years, respectively (Dhar, 2006).…”

“…Finally, strong temporal decoupling between As and Fe, Mn and S in the shallowest wells (5-7 m) from our sites imply that the chemical compositions of recharge water could be highly heterogeneous both spatially and highly variable temporally. Stute et al (2007) have pointed out on the basis of paired measurement in groundwater from the same set of shallow monitoring wells (< 20 m) that there is a surprisingly linear relationship between As concentrations and groundwater age across a wide range of settings (Table 1). It is noted that the deepest wells of most sites including A-15 m, B-19 m, E-14 m, F-19 m, G-14 m are affected by mixing, therefore were not included in regression analysis ).…”

“…There is growing evidence that at least part of this heterogeneity can be attributed to variations in local geology and its effect on shallow groundwater flow Stute et al, 2007;Aziz et al, in revision;Weinman et al, in press.). Such spatial variability naturally leads to the concern that shallow groundwater As concentration may also change over time, especially because subsurface flow is likely to be affected by large water withdrawals for irrigation in certain areas of Bangladesh (Harvey et al, 2002;Klump et al, 2006).…”

Advection of surface-derived organic carbon fuels microbial reduction in Bangladesh groundwater

“…These observations combined indicate that the advection of DOC is delayed relative to that of groundwater. We estimate the lower limit of the retardation factor for DOC transport of 5.5 by dividing the ratio of the vertical groundwater velocity at this site (29) to the fastest potential vertical penetration of bomb-labeled DOC (11 m in 30 y). The 14 C content of the DNA extracted at 11m (410 ± 90 14 C y or Δ 14 C −56 ± 10‰) indicates that little bomb-spiked DOC has been metabolized by the resident microbes.…”