Geological Society of America

T233 Arsenic in Geologic Systems I

Wednesday, 3 November 2010
8:00 AM - 12:00 PM
Room 612

DEPTH DEPENDENT GEOCHEMICAL PROCESSES AND OCCURRENCE OF HIGH DISSOLVED ARSENIC IN GROUNDWATER, WEST BENGAL, INDIA

Paper number: T233-11
Program: Technical Sessions
Time: 10:20 AM – 10:35 AM

Authors

Ashis Biswas Presenting Author
University of Kalyani, Kalyani, N/A
Dipti Halder
University of Kalyani, Kalyani, N/A
Herald Neidhardt
Karlsruhe Institute of Technology (KIT), Karlsruhe, N/A
Subhamoy Bhowmick
University of Kalyani, Kalyani, N/A
Santanu Majumder
University of Kalyani, Kalyani, N/A
Debasree Saha
University of Kalyani, Kalyani, N/A
Prosun Bhattacharya
KTH Royal Institute of Technology, Stockholm
Zsolt Berner
Karlsruhe Institute of Technology (KIT), Karlsruhe, N/A
Debashis Chatterjee
University of Kalyani, Kalyani, N/A

Abstract

The problem with elevated arsenic (As) in groundwater is most severe in Bengal Delta Plain (BDP) where high As groundwater is often encountered in shallow aquifer (<50 m) with heterogenic distribution in the aquifer. It is important to trace the various (bio)geochemical processes occurring in the aquifer to explain this feature of the problem and it might alsconsiderable heterogeneity. The present study presents the data on periodical monitoring of groundwater samples from five nested piezometers with an aim to highlight temporal changes in groundwater composition and the underlying redox processes responsible for the high dissolved As in groundwater. Equilibrium geochemical modeling was carried out to advocate how different geochemical processes like precipitation, dissolution of some key mineral phases regulate groundwater chemistry and hence influence ongoing redox processes in the aquifer.

The groundwater is generally Ca–HCO₃ type. The concentration of major constituents in groundwater is relatively high in shallow aquifer and gradually decreases with increasing depth in most cases. This suggests that surface processes (local recharge, pollution from sanitary source) are possibly influencing the groundwater chemistry of shallow aquifer. The speciation of As and Fe indicate persisting reducing environment in the aquifer [As (III): 87–97% of As_T and Fe (II): 76–96% of Fe_T]. The SI calculation reveals that groundwater is usually saturated to supersaturated with carbonate mineral phases like calcite (CaCO₃), dolomite [(CaMg(CO₃)₂], siderite (FeCO₃) and rhodochrosite (MnCO₃) and Fe mineral phases like ferihydrite [Fe(OH)₃], hematite (Fe₂O₃) and goethite (FeOOH). The dissolution/precipitation of these mineral phases may be important for the mobilization of As together with Fe and Mn. This study also suggests that the geochemical process via Fe and / or Mn oxyhydroxide reduction is the principal process to release As in groundwater. However several other mechanisms like exchange process and infiltration (dilution from the surface recharge) are also working locally in the upper aquifer zone. The positive correlations of U and V with As, Fe and Mn indicate redox processes responsible for mobilization of As in the deeper screened piezometers are mainly microbially controlled.