Nature of Coastal Aquifers in Parts of East Coast,

Coastal aquifers of Andhra Pradesh form a part of a discontinuous strip of alluvium along the East Coast of India. These aquifers comprise ridge like features generated by tidal action and the deltaic fluvial deposits. They show many of the characteristic morphological features of stream and deltaic sediments, like channel bars, levees, etc., Their maximum thickness is reported to be around 400 metres. Deltaic sediments are derived from 3 major river systems: Godavari, Krishna and Pennar and many smaller but, locally important streams. They range widely in thickness, grain size distribution and by implication in aquifer parameters and well yields which can be attributed to their depositional environment. Seasonal variation in yields of shallow wells is related to changes in the thickness of the zone of saturation. These aquifers are sustained by average normal annual rainfall is more than 1000 mm with an effective infiltration rate of 10 to 15%, and an average specific yield of around 10%. Use of groundwater continues to increase rapidly in these areas. This has resulted in up-coning of Salt water fresh water interface, but as yet there is no reversal in regional flow direction. Aquifer chemistry shows evolution towards higher chlorides at the expense of bicarbonates and sulfates after initial rise in calcium content. Revelle’s ratio progressively decreases away from the coast. However, complexities in chemical characteristics of the aquifers are introduced on four counts: 1. saline water ingress in locally over-exploited areas, 2. cyclonic activity that transports large quantities of sea water directly over land, 3. over-exploitation and 4. industrial contamination. Further, deviations in expected trends are introduced by geomorphic characteristics of the terrain, aquifer geometry, basement characteristics, etc. Thus northern part of the coast being devoid of wide alluvium is hardly effected by saline ingress. But, vast tracts of Aqua-culture along alluvial coast in the southern part of the area has rapidly effected the shallow unconfined aquifers. Protection measures are controlled abstraction rates, proper designing of wells, artificial recharge and drainage, remedial pumping, etc.