Geology Reference
In-Depth Information
Studies in the hard rock terrains covered with black cotton soil in basaltic
terrain and red lateritic soil in granitic areas indicate that a minimal rainfall
of about 246 mm and 412 mm are required for initiation of deep percolation
in red and black cotton soils respectively. However all of this will not be
available for utilization as part of it will be lost to base flow and effluent
seepage to surface drainage system. This also does not include other sources
of recharge such as return flow from irrigation.
WATER QUALITY
In addition to the problem of inland salinity, overexploitation of groundwater
has resulted in seawater intrusion in coastal areas. This problem is more
severe in the coastal parts of Gujarat, Orissa, Tamilnadu and Kerala. Further,
higher concentration of fluoride, iron and arsenic is reported from some
areas. Higher concentration of fluoride (more than the permissible limit of
1.5 mg/lit) is reported from parts of Andhra Pradesh, Tamilnadu, Rajasthan
and Uttar Pradesh. The cause of high fluoride in these areas is geogenic i.e.
due to the dissolution of fluoride bearing minerals. At some places,
deflouridation plants are installed to remove high concentration of fluoride.
The occurrence of arsenic in groundwater, reported in recent years from
parts of West Bengal, has caused great concern. In West Bengal, life of more
than five million people is at risk due to high As in groundwater and already
about half a million people suffer from various arsenic related diseases.
Similar problem is reported from the neighbouring country of Bangladesh
where high concentration of arsenic (0.3 to 1.1 mg/lit) is reported from
shallow alluvial and deltaic aquifers in the depth range of 15 to 75 m below
the ground surface. The permissible limit of arsenic in drinking water in
India is 0.05 mg/lit, while the WHO has put a limit of 0.01 mg/lit.
Studies in West Bengal by the scientists of the Bhaba Atomic Research
Center (BARC) and CGWB show that groundwater from shallow unconfined
aquifers (depth 20 to 80 m) has low dissolved oxygen, negligible SO
4
and
higher concentration of As (0.5 to 1.0 mg/lit or more), and bicarbonate; pH
being above 7. In most of the areas the arsenic concentration is localized.
Higher concentration of As is in areas where clay pockets predominate.
Isotope data indicate modern recharge to the shallow aquifer. Groundwater
in deeper semi-confined to confined aquifers (>100 m) contains negligible
amount of As and is much older in age (5000 to 13,000 years) indicating that
these are palaeowaters. Surface water in these areas does not have any
arsenic.
The cause of high arsenic in groundwater of both India and Bangladesh
is somewhat controversial. Some workers attribute it to the presence of
arsenic bearing pyrite in the clay, silt and peat formations interbedded with
alluvial aquifers. Lowering of water table, due to excessive withdrawal of
groundwater, has resulted in the oxidation and leaching of As from the
sediments.
