Natural organic matter in sedimentary basins and its relation to arsenic in anoxic ground water: the example of West Bengal and its worldwide implications

Abstract

In order to investigate the mechanism of arsenic release to anoxic ground water in alluvial aquifers, we sampled ground waters from three piezometer nests, 79 shallow (< 45m) wells, and 6 deep (> 80m) wells, in an area 750 m by 450 m, just north of Barasat, near Kolkata (Calcutta), in southern West Bengal. High concentrations of arsenic (200 -1180 µg l -1 ) are accompanied by high concentrations of iron (3 -13.7 mg l -1 ) and phosphate (1 -6.5 mg l -1 ). Ground water that is rich in manganese (1 -5.3 mg l -1 ) contains < 50 µg l -1 of arsenic. The composition of shallow ground water varies at the 100m-scale laterally and the metre-scale vertically, with vertical gradients in arsenic concentration reaching 200 µg l -1 m -1 . The arsenic is supplied by reductive dissolution of FeOOH and release of the sorbed arsenic to solution. The process is driven by natural organic matter in peaty strata both within the aquifer sands and in the overlying confining unit. In well waters, thermo-tolerant coliforms, a proxy for faecal contamination, are not present in high numbers (< 10 cfu/100 ml in 85% of wells) showing that faecally-derived organic matter does not enter the aquifer, does not drive reduction of FeOOH, and so does not release arsenic to ground water.Arsenic concentrations are high (>> 50 µg l -1 ) where reduction of FeOOH oxide is complete and its entire load of sorbed arsenic is released to solution, at which point the aquifer sediments become grey in colour as FeOOH vanishes. Where reduction incomplete, the sediments are brown in colour and resorption of arsenic to residual FeOOH keeps arsenic concentrations below 10 µg l -1 in the presence of dissolved iron. Sorbed arsenic released by reduction of manganese oxides does not release arsenic to ground water because the arsenic resorbs to FeOOH. Arsenic pollution of ground water is common in the alluvial aquifers of the Bengal Basin because Himalayan erosion supplies immature sediments, with low surface-loadings of FeOOH on mineral grains, to a depositional environment that is rich in organic mater so that complete reduction of FeOOH is common.

A Gel Probe Equilibrium Sampler for Measuring Arsenic Porewater Profiles and Sorption Gradients in Sediments: II. Field Application to Haiwee Reservoir Sediment

“…Reductive dissolution of Fe(III) oxides and the resulting secondary mineral transformations can promote As mobilization under certain conditions and may be important in many types of reducing sediments and aquifers (19)(20)(21)(22)(23). When sorption sites remain available on the solid, As can readsorb onto the solid phase even as reductive dissolution progresses (24). Readsorption is affected strongly by the porewater composition, and the presence of competitively adsorbing ions can increase the amount of As in the porewater.…”

Correlação entre uso da terra e qualidade da água subterrânea

“…Dentre as fontes mais prováveis, podem ser citados os aportes antropogênicos e a degradação da matéria orgânica (BHATTACHARYA et al, 2006;McARTHUR et al, 2001McARTHUR et al, , 2004. O aporte antropogênico reside, fundamentalmente, nos efluentes domésticos, fossas sépticas, currais e na aplicação de fertilizantes.…”

“…A liberação de P na água subterrânea decorre da ação microbiana na dissolução redutiva do Fe +3 ligado ao P. Em condições anaeróbicas, o Fe +3 é reduzido a Fe +2 e, com isso, tem-se a liberação do P presente na matéria orgânica (BHATTACHARYA et al, 2006;McARTHUR et al, 2001McARTHUR et al, , 2004 …”

The Source of Arsenic and Nitrate in Borrego Valley Groundwater Aquifer

“…Nevertheless, the processes leading to the release of arsenic in groundwater aquifer are not yet fully understood. The literature synthesis indicates various hypothesis concerning the release of arsenic 1) by oxidation of arsenic-bearing pyrite [9]- [11], 2) by competitive exchange of phosphate from fertilizers [12] and 3) by reduction of iron hydroxides under anoxic conditions [13] [14]. Our study, however, provides a baseline of groundwater quality in BV using the current ongoing research results and literature synthesis addressing the source and origin, trend, and pattern of contamination in BV aquifer.…”