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

Abstract

Samples were collected every 2-4 weeks from a set of 37 monitoring wells over a period of 2-3 years in Araihazar, Bangladesh, to evaluate the temporal variability of groundwater composition for As and other constituents. The monitoring wells are grouped in 6 nests and span the 5-91 m depth range. Concentrations of As, Ca, Fe, K, Mg, Mn, Na, P, and S were measured by high-resolution ICPMS with a precision of 5% or better; concentrations of Cl were measured by ion chromatography. In shallow wells <30 m deep, As and P concentrations generally varied by <30%, whereas concentrations of the major ions (Na, K, Mg, Ca and Cl) and the redox-sensitive elements (Fe, Mn, and S) varied over time by up to ± 90%. In wells tapping the deeper aquifers > 30 m often below clay layers concentrations of groundwater As were much lower and varied by <10%. The concentrations of major cations also varied by <10% in these deep aquifers. In contrast, the concentration of redox-sensitive constituents Fe, S, and Mn in deep aquifers varied by up to 97% over time. Thus, strong decoupling between variations in As and Fe concentrations is evident in groundwaters from shallow and deep aquifers.Comparison of the time series data with groundwater ages determined by 3 H/ 3 He and 14 C dating shows that large seasonal or inter-annual variations in major cation and chloride concentrations are restricted to shallow aquifers and groundwater recharged < 5 years ago. There is no corresponding change in As concentrations despite having significant variations of redox sensitive constituents in these very young waters. This is attributed to chemical buffering due to rapid equilibrium between solute and solid As. At two sites where the As content of groundwater in existing shallow wells averages 102 µg/L (range: < 5 to 648 µg/L; n=118) and 272 µg/L (range: 10 to 485 µg/L; n=65), respectively, a systematic long-term decline in As concentrations lends support to the notion that flushing may slowly deplete an aquifer of As. Shallow aquifer water with > 5 yr 3 H/ 3 He age show a constant As:P molar ratio of 9.6 over time, suggesting common mechanism of mobilization.Corresponding author, Y. Zheng,

A Review of Groundwater Arsenic in the Bengal Basin, Bangladesh and India: from Source to Sink

“…One consensus that has been achieved in all these years is that, in general, temporal variations in arsenic concentration of different wells are limited, and that the variations are the highest in the shallowest depths and the concentrations seem to be quite stable in deeper parts, owing to the fact that the groundwater flow is much more vigorous along with larger inputs in the shallower depths of the aquifers. Dhar et al [40] reported a variation of <30 % in the arsenic concentrations of the wells shallower than 30 m, whereas for wells deeper than 30 m, often capped by clay layers, the variance was as low as <10 % in their study site in Bangladesh.…”

“…Some wells are reported to show statistically significant trends, while others show irregular and random changes in concentrations [13,29,40,[42][43][44]. Such variations in the concentration of arsenic in groundwater have been attributed to changes in pumping rate during the course of a day, seasonal variations in recharge and precipitation, seasonal oscillations in redox processes affecting the Fe and S cycling, irrigational abstractions, head gradients, aquifer flushing, long-term changes in climate and groundwater levels, changes in pumping regimes and depth of abstraction in relation to the gradual reduction of water level due to pumping and entrainment of high As shallow groundwater into a deeper low As well due to construction defects [20,44].…”

“…CGWB [21], on the other hand, reported a seasonal variation in groundwater arsenic concentrations in West Bengal, with minima during the post-monsoon period which is thought to be due to dilution of groundwater arsenic by monsoonal recharge. Cases of an overall decrease in arsenic concentration in wells over time or during wet seasons can be attributed to the concept of dilution and/or aquifer flushing [28,40,44,46,47]. Mandal et al [48] claimed that the deeper tube wells which were once arsenic free, may get contaminated over the years in areas where the shallower tube wells are arsenic contaminated.…”

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

“…1 and 2) (23). The five shallow wells ranging between 6 and 25 m in depth below ground tap a Holocene (<10,000-y-old) aquifer that has developed the characteristic vertical As gradient with concentrations increasing from 0.007 to 2.7 μM (<1-203 μg/L) (24,25). Within the same depth interval, concentrations of Fe, methane (CH 4 ), and DIC increase with depth while redox potential (Eh) and sulfate concentrations decrease, consistent with microbial respiration of OC coupled to reductive dissolution of Fe oxides, sulfate reduction, and methanogenesis ( Fig.…”

“…The study site (site F) has been the focus of extensive investigations and is located in Lashkardi village of Araihazar upazila, ∼20 km east of the capital Dhaka (24,25,29) (Fig. 1).…”