Mobilization of Arsenic in Groundwater of Holocene Delta, Indian Segment

Recent incidences of high arsenic exposure from drinking water and human suffering in SE Asia notably in Bengal Delta Plain (BDP) appear to be quite different from the global scenario. The source of arsenic in groundwater of BDP is considered to be geogenic and mostly restricted to Holocene sediments. Several models (oxidation of As-rich pyrite, reductive dissolution of Fe(III)-Fe(II) systems, anion exchange of sorbed As mostly phosphate) have been put forward. However, the models have their own weakness and the paper deals with such discrepancies to focus the arsenic sourcing (primary, secondary and tertiary), release mechanism and mobilization in Holocene deltas. The paper also deals with groundwater quality and (hydro) chemistry vis-a-vis alluvial Holocene sediment. The stratigraphic profiling indicates that the shallow aquifers (< 40m) are significant contributors (~91 percent, n = 497) of high arsenic in groundwater. The major hydrochemical features are low Eh (range: - 149-37 mv; mean: - 53mv) and nitrate (range 0.001- 1.72; mean 0.13 mg/L) followed by high alkalinity (range 100 - 630 mg/L; mean 280 mg/L), iron (1.23- 38.4, mean 7.48 mg/L), phosphate (range 0.002 - 3. 99 mg/L; mean 0.56 mg/L), hardness (range 46-490 mg/L; mean 235) and sulphate (0.28 - 48.62 mg/L; mean 6.43 mg/L). Groundwater (hydro) chemistry indicates that the affected aquifers are reducing in nature. The four different local situations (sanitation, surface water bodies, land use and sanitation coupled with eco-system) are identified and most important is sanitation where groundwater arsenic concentration is highest (up to 1180 mg/L) among the different local situation. Arsenic hotspots are identified as a multi-level phenomena and unveiled greater complexities at a deeper level. Morphology and sediment profiling of BDP deltas are influencing arsenic sourcing where sandy useable aquifers are identified with overbank deposits rich in arsenic host environment. Shallow aquifers ( 5-70 m) are often enriched with DOC ( range- 1.2- 7.0 mg/L) where as deep aquifers ( 100- 304 m ) are relatively containing low amount of DOC ( 0.4- 1.9 mg/L). This reflects the influence of local processes at shallow depths where oxidative as well as microbial degeneration of carbon sink is playing crucial role in arsenic mobilization. This also suggests that the break down of organic matter is the principal process in the shallow reducing aquifers with high/low arsenic along with high / low - coupled redox sensitive species and DOC. The presence of DOC in shallow aquifers also indicates that organic matter is relatively young and more reactive. High PCO2 values, relatively high redox sensitive elements, low Eh and absence of DO are the hydrogeochemical fingerprints of the BDP shallow aquifers. Delta building processes are also important to explain arsenic sourcing and mobilization (deeper arsenics- bearing middle delta aquifers with low concentration of chloride as well as deeper saline tidal deposits aquifers of lower delta with / without low arsenic). Both the deeper aquifers are also containing high bicarbonate. This indicates that mineral carbonates (both simple and complex) are also playing important role in arsenic mobilization at least in deeper aquifers. The study also deals with the role of secondary minerals (mica/clay) in arsenic mobilization. The XPS studies on mica further strengthen that the surface chemistry and role of Fe (II) are also important issue to understand the difference of high/low arsenic in between shallow and deep aquifers in BDP.