Arsenic Distribution in Groundwater of Tamil Nadu

The Arsenic concentration in the groundwater of Chennai city. The As concentration in the five groundwater samples is exceedingly higher than 50µgl^-1. The highest concentration of 146µg l^-1 has been observed in the groundwater collected near Adyar creek. The high concentration spots are the two places corresponding to the flow channel of the Adyar River and the Buckingham canal, which are the discharge points for industries to dump their effluents and for domestic sewage. The present reconnaissance survey indicates that thee is an urgent need to carry out a detailed systematic study to estimate the affected by As contamination. The occurrence of As in natural waters is usually associated with sedimentary rocks of marine origin, weathering of volcanic rocks, fossil fuels, mineral deposits, mining wastes, agricultural use, and irrigation practices. As occurs naturally in the earth’s crust at about 1.8mg kg^-1. Numerous minerals, rocks, sediments and soils contain As partly as a constituent of sulfide minerals. As is also adsorbed onto clay colloids bound to organic matter to form water-soluble complexes with Al, Fe, Ca and Mg. Over-extraction of groundwater, especially during summer, enhances the liberation of As into the water column, which eventually surfaces by forming volatile complexes.

There is sufficient evidence to suggest that there is a geological control in the distribution of As with alluvium sediments. Anthropogenic sources of As are numerous, both in the form of organic metal complexes and inorganic ions. As compounds are used in paint industries as paint pigments, and in textile and tanning industries. Household detergents may also contain about 10-70ppm of As, as is exemplified by the highest concentration along a section of the Adyar river. Prior to 1968, inorganic forms of As was used extensively in agriculture as insecticides, fungicides, herbicides and desiccants. Although inorganic arsenicals are no longer widely used in agriculture, there is still concern about the residual effects of its high application in the past. Further, the organic effluents discharged by the industries can complex with As to form non-degradable metal complexes and enter the groundwater from the river systems. Arsenic accumulates in the biota and other inorganic materials because of its low mobilization factor of 3.3 and soon becomes poisonous. In the following section, an attempt is made to review some of the processes involved in As mobilization in the environment. Several processes have been proposed to explain the release of sedimentary As into groundwater and here we will discuss some of the recent observations by researchers.

1. Oxidation of iron pyrite by air: Das et al. 1996 studied As concentration in groundwater of West Bengal and confirmed the presence of As in iron pyrite sediments. They suggested that the high volume extraction of groundwater in the region has exposed the deltaic sediments to air, which through oxidation reaction causes the decomposition of iron pyrites to ferrous sulfate (FeSO₄), ferric sulfate Fe₂(SO₄)₃ and sulfuric acid. The process fires up As which is then oxidized into arsenite and arsenate both of which are soluble in groundwater.

2. Reduction reaction of arseniferrous iron-hydroxides: The oxidation theory however fails to explain the increase in As level in deeper wells and anoxic condition. They observed that As-rich groundwater is mostly restricted to the alluvial aquifers of the Ganges Delta and concluded that the source of As must lie in the upstream source region of the Ganges in Bangladesh.

3. Microorganisms: Analysis of surface water samples taken from around Montpellier, France a region with large heavy metal deposits, indicated that the pH in the samples varied from 2.5 to 3.5 and As content was 100 to 300 mg l^-1. The presence of a group of microorganism, the Stromatolytique was considered the main cause of high As levels in drinking water.

The aquifers of Chennai region are recharged by precipitation, irrigation return, flow from river beds and surface water bodies. Hydrogeochemical studies performed here are mainly based on a single time sampling and hence a detailed study is required to understand the processes mentioned above.

The As concentration in groundwater of Chennai appears to result from a combination of three factors:

i) A source of As (present in the aquifer sediments)

ii) Its mobilization (i.e., As is released from the sediments to the groundwater particularly during summer when there is over extraction of groundwater) and

iii) Transport (i.e. As is flushed away in the natural groundwater circulation).

Thus, the presence of As content in groundwater in many parts of the world is a major threat to the local community and the environment. With the view to understand the source of the As problem in groundwater around the world it is proposed to undertake an International Geological Correlation Programme (IGCP) that will link across various continents, the source of As bearing horizons, the corresponding As levels in the aquifers and any relationship in their respective geological age and stratigraphic position in the regional geographic set up. Since the source of As is also likely to be common for Cd and Hg as well, the above study has to be addressed to understand the sources of these toxic metals in the groundwaters that pose serious health hazards in many parts of the world, particularly in densely populated regions of South east Asia.