Comparison of arsenic concentrations in simultaneously-collected groundwater and aquifer particles from Bangladesh, India, Vietnam, and Nepal

Abstract

One of the reasons the processes resulting in As release to groundwater in southern Asia remain poorly understood is the high degree of spatial variability of physical and chemical properties in shallow aquifers. In an attempt to overcome this difficulty, a simple device that collects groundwater and sediment as a slurry from precisely the same interval was developed in Bangladesh. Recently published results from Bangladesh and India relying on the needle-sampler are augmented here with new data from 37 intervals of grey aquifer material of likely Holocene age in Vietnam and Nepal. A total of 145 samples of filtered groundwater ranging in depth from 3 to 36 m that were analyzed for As (1–1000 μg/L), Fe (0.01–40 mg/L), Mn (0.2–4 mg/L) and S (0.04–14 mg/L) are compared. The P-extractable (0.01–36 mg/kg) and HCl-extractable As (0.04–36 mg/kg) content of the particulate phase was determined in the same suite of samples, in addition to Fe(II)/Fe ratios (0.2–1.0) in the acid-leachable fraction of the particulate phase. Needle-sampler data from Bangladesh indicated a relationship between dissolved As in groundwater and P-extractable As in the particulate phase that was interpreted as an indication of adsorptive equilibrium, under sufficiently reducing conditions, across 3 orders of magnitude in concentrations according to a distribution coefficient of 4 mL/g. The more recent observations from India, Vietnam and Nepal show groundwater As concentrations that are often an order of magnitude lower at a given level of P-extractable As compared to Bangladesh, even if only the subset of particularly reducing intervals characterized by leachable Fe(II)/Fe >0.5 and dissolved Fe >0.2 mg/L are considered. Without attempting to explain why As appears to be particularly mobile in reducing aquifers of Bangladesh compared to the other regions, the consequences of increasing the distribution coefficient for As between the particulate and dissolved phase to 40 mL/g for the flushing of shallow aquifers of their initial As content are explored.

Retardation of arsenic transport through a Pleistocene aquifer

“…In 2006, drilling was briefly interrupted at 7 sites to increase the vertical resolution of both sediment and groundwater data using the needle-sampler (31). Groundwater was pressure filtered under nitrogen directly from the sample tubes.…”

“…With the exception of needle-sample data and the nest of 10 wells in the Holocene portion of the aquifer, which had to yield to construction, groundwater As, Fe, and Mn concentrations reported here represent the average for samples collected without filtration in April and May 2012. Groundwater data from 2006 were previously reported in (6) and (31). …”

Solute chemistry and arsenic fate in aquifers between the Himalayan foothills and Indian craton (including central Gangetic plain): Influence of geology and geomorphology

“…Bhattacharya et al, 1997;Harvey et al, 2002;Stü ben et al, 2003;van Geen et al, 2004;Mukherjee et al, 2008b) by dissimilatory microbial reduction (e.g. Islam et al, 2004) and/or compet- itive anion exchange (Acharyya et al, 1999;Appelo et al, 2002;van Geen et al, 2008). Zheng et al (2004) proposed that As is released from metal (oxyhydr)oxides or other sediment phases by Fe and S cycling, which was advanced by Mukherjee et al (2008b) by hypothesis of As retention in solution by partial redox equilibrium, leading to re-oxidation of previously reduced phases.…”

Arsenic in groundwater in the southern lowlands of Nepal and its mitigation options: a review

“…These units span a wide range of various rocks being metamorphic, sedimentary, and igneous in origin, making it possible for their differential erosion to account for some of the groundwater arsenic heterogeneity we see in the foreland and delta (i.e. Gurung et al 2005;Shah 2008;van Geen et al 2008;Guillot et al, 2015). The Terai Plain is an active foreland basin consisting of Quaternary sediments that include molasse units along with gravel, sand, silt, and clay.…”