Paper No. 47-0
DISTRIBUTION, HYDROCHEMICAL CONTEXT AND MOBILITY OF ARSENIC IN ALLUVIAL AQUIFERS OF THE BENGAL BASIN
BURGESS, William1, AHMED, K. Matin2, BURREN, Melanie3, CARRUTHERS, Alison4, COBBING, Jude5, CUTHBERT, Mark O.6, MATHER, Stephen7, MCCARTHY, Eileen1, PERRIN, Jérôme8, and CHATTERJEE, Debashis9, (1) Geological Sciences, Univ College London, Gower Street, London, WC1E 6BT, United Kingdom, william.burgess@ucl.ac.uk, (2) Geology, Dhaka Univ, Dhaka, 1000, Bangladesh, (3) Geological Sciences, Univesrity College London, Gower Street, London, WC1E 6BT, United Kingdom, (4) Chemistry, Univ College London, Gower Street, London, WC1E 6BT, United Kingdom, (5) Hydrogeology, British Geol Survey, Maclean Building, Crowmarsg Gifford, Wallingford, Oxon, OX10 8BB, Uruguay, (6) Hydrogeology, ENTEC UK, 160-162 Abbey Foregate, Shrewsbury, SY2 6BZ, United Kingdom, (7) Groundwater, Mott MacDonald Ltd, St. Anne House, Wellesley Road, Croydon, CR9 2UL, United Kingdom, (8) Centre of Hydrogeology, Neuchâtel Univ, Neuchâtel, Switzerland, (9) Chemistry, Kilyani Univ, West Bengal, India

In a transect of the Holocene alluvial aquifers of the Bengal Basin, we have established the distribution and hydrochemical context of arsenic in groundwater at five 15 km2-study areas, with a sampling density of 5 - 10 tubewells per km2 and depth range 10 – 250 m. A conceptual model of arsenic in the aquifer is proposed which supports numerical models of arsenic in tubewell discharge.

Dissolved arsenic is associated with abundant iron and bicarbonate, and absence of oxygen and nitrate. Arsenic is released from iron oxyhydroxides by reductive dissolution, linked to oxidation of organic matter in the aquifer sediments. Favourable hydrochemical conditions are widespread, but the arsenic source is discontinuous and depth-specific.

Highest arsenic concentrations and the greatest range (0 - 1000 mg/l) occur in groundwater shallower than 75 m. At greater depths, arsenic has a lower concentration and more limited range. Sediment cores from western Bangladesh provide a more precise, site-specific depth-distribution of arsenic in the aquifer to 60 m. At the single source-horizon between 18 and 21 m, porewater arsenic exceeds 300 mg/l against a background less than 50 mg/l. Distribution patterns of arsenic in groundwater suggest sedimentological controls on the extent of the source. However, arsenic concentrations may increase with tubewell age. Oxygen and hydrogen isotope profiles confirm the downward leakage of arsenic-rich groundwater.

Tubewell catchments in the semi-confined, multi-layered alluvial aquifer are limited by vertical leakage. Arsenic concentration at tubewells is governed by the degree of overlap between the catchment and the discontinuous arsenic source, the path-length between source and tubewell screen, and the duration and rate of pumping. Numerical models reproduce the observed variability and indicate arsenic concentration increasing over decades or longer. Most tubewells in Bangladesh have been installed since 1985. There are important implications for treatment, tubewell location and design, and monitoring.

GSA Annual Meeting, November 5-8, 2001
General Information for this Meeting
Session No. 47
Natural Arsenic in Groundwater: Science, Regulation, and Health Implications
Hynes Convention Center: 210
1:30 PM-5:30 PM, Monday, November 5, 2001
 
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