| 1. |
- Neidhardt, H., et al.
(författare)
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Influences of groundwater extraction on the distribution of dissolved As in shallow aquifers of West Bengal, India
- 2013
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Ingår i: Journal of Hazardous Materials. - : Elsevier BV. - 0304-3894 .- 1873-3336. ; 262, s. 941-950
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Tidskriftsartikel (refereegranskat)abstract
- Here we report temporal changes of As concentrations in shallow groundwater of the Bengal Delta Plain (BDP). Observed fluctuations are primarily induced by seasonally occurring groundwater movement, but can also be connected to anthropogenic groundwater extraction. Between December 2009 and July 2010, pronounced variations in the groundwater hydrochemistry were recorded in groundwater samples of a shallow monitoring well tapping the aquifer in 22-25m depth, where Astot concentrations increased within weeks from 100 to 315μgL-1. These trends are attributed to a vertically shift of the hydrochemically stratified water column at the beginning of the monsoon season. This naturally occurring effect can be additionally superimposed by groundwater extraction, as demonstrated on a local scale by an in situ experiment simulating extensive groundwater withdrawal during the dry post-monsoon season. Results of this experiment suggest that groundwater extraction promoted an enduring change within the distribution of dissolved As in the local aquifer. Presented outcomes contribute to the discussion of anthropogenic pumping influences that endanger the limited and yet arsenic-free groundwater resources of the BDP.
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| 2. |
- Neidhardt, H., et al.
(författare)
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Organic carbon induced mobilization of iron and manganese in a West Bengal aquifer and the muted response of groundwater arsenic concentrations
- 2014
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Ingår i: Chemical Geology. - : Elsevier BV. - 0009-2541 .- 1872-6836. ; 367, s. 51-62
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Tidskriftsartikel (refereegranskat)abstract
- The exact circumstances that cause the widespread enrichment of Mn and As in groundwater of the Bengal Delta Plain (BDP) and many other Asian delta areas still remain a matter of debate in the scientific community. We conducted an in situ field experiment in the central BDP region to investigate the influence of organic matter on the mobility of Fe, Mn and As in shallow aquifers. The groundwater at our study site was initially characterized by a circum-neutral pH, low concentrations of O2, NO3 - and SO4 2-, and increased Fe, Mn and As concentrations, reflecting reducing conditions in the aquifer. Since organic matter controls microbially mediated redox processes which are believed to result in the mobilization of Fe, Mn and As from Holocene aquifer sediments, an easily degradable carbon source (sucrose) was introduced into a shallow aquifer via four nested monitoring wells and distributed by circular pumping. Initial sucrose concentrations reached up to 2.55mM in the local groundwater and induced a strong increase in the activity of indigenous microbes that decomposed the sucrose within the following 14days stepwise into intermediate catabolic products (e.g., acetic acid), and finally to CO2/HCO3 -. The formation of organic acids was accompanied by a temporary decline in the pH and the redox potential, as well as an increase in the concentration of most major and trace elements in the groundwater by several times. While Mn concentrations rose up to 81.3μM (representing a 7.5 fold increase), Fe (on average 96.7% Fe(II)) concentrations reached a considerable transient maximum of 1390μM, which was 36 times higher than the initial baseline value. The most significant observation of this experiment is that the relative increments of dissolved As (on average 95.8% As(III)) reached between 19 and 49% only, which is in clear contrast to the pronounced mobilization of Fe, Mn and other trace elements. Changes in the groundwater composition during the experiment imply that the mobilization of Fe and Mn was primarily caused by a reductive dissolution of Mn-oxides and Fe-(oxyhydr)oxides, resulting from the stimulation of indigenous bacteria by the addition sucrose. In this context, the release of As can be attributed to the dissolution of Fe-(oxyhydr)oxides, which constitute the principal source of As in the aquifer sediments according to mineralogical and geochemical analyses. In contrast to the pronounced mobilization of Fe, the response of groundwater arsenic concentrations appeared to be muted, as indicated by subsequently declining As to Fe mol ratios that dropped one order in magnitude. The remarkable decoupling of As from Fe mobilization indicates that the aquifer sediments were apparently capable of compensating for the additional release of As. We attribute this As buffer potential to remaining Fe-minerals and potentially newly formed Fe(II)- and mixed Fe(II/III)-mineral phases, which were able to readily immobilize dissolved As. Sequential extraction results of the initial aquifer sediments further support this interpretation, revealing that up to 85% of the total As in the sediments was already present in adsorbed form, with Fe-minerals as principal hosts. Hence, the experimental data implies that a biogeochemically controlled environment of competing As release and retention arose after the addition of sucrose, where Fe-mineral phases played a key role in buffering the release of As. We further conclude that organic carbon limited aquifer systems in the BDP with increased As concentrations in groundwater may exhibit an unexpected buffer potential towards an additional As release, even when vast amounts of easily degradable organic carbon are introduced into the system.
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| 3. |
- Neidhardt, H., et al.
(författare)
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Reconstructing the sedimentation history of the Bengal Delta Plain by means of geochemical and stable isotopic data
- 2013
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Ingår i: Applied Geochemistry. - : Elsevier BV. - 0883-2927 .- 1872-9134. ; 36, s. 70-82
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Tidskriftsartikel (refereegranskat)abstract
- The purpose of this study is to examine the sedimentation history of the central floodplain area of the Bengal Delta Plain in West Bengal, India. Sediments from two boreholes were analyzed regarding lithology, geochemistry and the stable isotopic composition of embedded organic matter. Different lithofacies were distinguished that reflect frequent changes in the prevailing sedimentary depositional environment of the study area. The lowest facies comprises poorly sorted fluvial sediments composed of fine gravel to clay pointing at high transport energy and intense relocation processes. This facies is considered to belong to an early Holocene lowstand systems tract that followed the last glacial maximum. Fine to medium sands above it mark a gradual change towards a transgressive systems tract. Upwards increasing proportions of silt and the stable isotopic composition of embedded organic matter both indicate a gradual change from fluvial channel infill sediments towards more estuarine and marine influenced deposits. Youngest sediments are composed of clayey and silty overbank deposits of the Hooghly River that have formed a vast low-relief delta-floodplain. Close to the surface, small concretions of secondary Mn-oxides and Fe-(oxyhydr) oxides occur and mark the fluctuation range of the unsaturated zone. These concretions are accompanied by relatively high contents of trace elements such as Zn, Ni, Cu, and As. To sum up, the outcomes of this study provide new insights into the complex sedimentation history of the barely investigated central floodplain area of West Bengal.
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