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Abstract: In order to test the hypothesis that the ambient iron concentrations could regulate sulfate reducing activity (SRA) in mangrove areas, 10 cm cores were examined from test and reference sites. The test site at Diwar mangrove ecosystem is highly influenced by iron released by the movement of barges carrying iron ore during the non-monsoon seasons and the reference site at Tuvem is relatively pristine. The average iron concentrations were 17.9% (±8.06) at Diwar and 6.3% (±1.5) at Tuvem. Sulfate reducing rates (SRR) ranged from 50.21 to 698.66 nM cm−3 d−1 at Tuvem, and from 23.32 to 294.49 nM cm−3d−1 in Diwar. Pearson’s correlation coefficients between SRR and environmental parameters showed that at Tuvem, the SRR was controlled by SO4 −2 (r = 0.498, p < 0.001, n = 60) more than organic carbon (r = 0.316 p < 0.05, n = 60). At Diwar, the SRR was governed by the iron concentrations at an r-value of −0.761 (p < 0.001, n = 60), suggesting that ca.58% of the variation in SRR was influenced negatively by variations in ambient iron concentrations. This influence was more than the positive influence of TOC (r = 0.615, p < 0.001, n = 60). Laboratory experiments to check the influence of iron on SRR also supported our field observations. At an experimental manipulation of 50 ppm Fe3+ there was an increase in SRR but at 100 ppm an inhibitory effect was observed. At 1000 ppm Fe3+ there was a decrease in the SRR up to 93% of the control. Thus, our study showed that ambient iron concentrations influence SRR negatively at Diwar and counters the positive influence of organic carbon. Consequently, the influence could cascade to other biogeochemical processes in these mangrove swamps, especially the mineralization of organic matter to carbon dioxide by sulfate respiration. [Copyright &y& Elsevier]

Abstract: Arsenic-enriched groundwater has been a pressing human health issue for more than a decade, with tens of millions of people worldwide being at risk of chronic As poisoning through the consumption of As-burdened groundwater. To elucidate the importance of dissolved S on the scale of As concentrations, the composition of groundwater samples from 926 locations spanning over the floodplains of three severely arsenic affected regions in Asia (Bengal-, Mekong-, Red River deltas), were assessed. A binary mixing model based on Cl− or B as conservative tracers implies that two types of water may be regarded as end-members with respect to groundwater composition in these deltas, namely surface derived water (approximated by river water) and saline water identical to residual sea water. Six redox zones were distinguished by comparing the model-calculated concentrations with the measured values. Only one zone (denoted methanogenic) had very high average As concentrations and they were significantly higher than in the other zones – for all three regions, regardless of applying Cl− or B as a tracer in the model. Average As concentrations±standard error in the methanogenic zone were 182±23μgL−1 (n =50%), 41±6μgL−1 (n =43%), and 61±20μgL−1 (n =24%) in the Mekong, Red River and Bengal delta, respectively. Arsenic levels were significantly lower in the SO4-reducing and the Fe-reducing zones, where averages were 23±7μgL−1 (n =27%, zone I), 14±3μgL−1 (n =48%, zone S) and 26±9μgL−1 (n =64%, zone S). These results suggest that a sufficient supply of inhibits the release of As to groundwater and that reduction may be as important as Fe reduction in controlling the enrichment of As in groundwater. [Copyright &y& Elsevier]