Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

P. N. Ballukraya ¹, R. Ravi ¹

Affiliations
1 Department of Applied Geology, University of Madras, Madras - 600 025, IN

Abstract

Subsurface lithological mapping has shown the presence of a crystalline rock ridge along the east coast of Madras City, and the aquifer above this ridge shows some characteristic features distinct from those on the western side. The presence of a fresh water ridge along the coast has checked the intrusion of sea water even though water levels are as low as 3m below mean sea level at certian places. Several pockets of brackish ground water have been identified which have been established as due to the presence of tanks and depressions therein. Identification of marine sands below the alluvium in some places gives an idea of the marine transgression that had occurred during Tertiary periods. Studies indicate that most of the alluvium is of estuarine origin, and that the crystalline ridge along with part of the east coast may be updergoing slow uplift.

Keywords

Hydrogeology, Madras, Tamil Nadu.

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P. N. Ballukraya ¹, R. Ravi ¹

Affiliations
1 Department of Applied Geology, University of Madras, Guindy Campus, Chennai, 600 025, IN

Abstract

Hydrogeochemical characterisation is a very important aspect of groundwater management. The unconfined aquifer of Chennai city is heavily exploited for meeting the water requirements of the city. Hydrogeochemical investigations carried out showed the existence of diverse hydrogeochemical facies of groundwater. Analysis of the data indicates that the quality of groundwater is related to hydrogeological environment, both in the present and the geological past. Relationship between water level fluctuations and groundwater quality in the area has been established and this is controlled by the lithology of the host rock and overlying material. The enrichment of various ionic constituents in groundwater is controlled by recharge-discharge processes as well as the local environment. Increase of TDS in groundwater after the recharge is observed in predominantly clayey areas while in sandy aquifers, it is seen to decline with recharge. The hydrogeochemistry of the groundwater shows that most of Chennai area can be classified as recharge area pointing to the dominance of vertical recharge processes.

Keywords

Groundwater Management, Hydrogeochemistry, Unconfined Aquifer, Chennai.

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P. N. Ballukraya ¹, R. Ravi ¹

Affiliations
1 Department of Applied Geology, University of Madras, Chennai 600 025, IN

Abstract

Hydrogeological studies of the groundwater in Chennai area have shown that the alluvial aquifers have several geochemical facies with distinct characteristics. Factor analysis of the data has been helpful in characterising these facies, since it is noticed that the factor 1 measures salinity, hardness and alkalinity, while factor 2 measures pollution. It is observed that lithology, land use and hydraulic characteristics exercise significant control on the geochemistry of groundwater. The behaviour of groundwater quality in coastal areas, river banks and surface reservoirs is also well explained by factor scores and many of these form distinct hydrogeochemical facies. The study further shows that the geochemistry of the groundwater in these categories gets much more distinct in the pre-recharge periods indicating the effectiveness of the rock-water interaction in the aquifers.

Keywords

Groundwater, Geochemistry, Factor Analysis, Chennai.

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P. N. Ballukraya ¹, R. Ravi ¹

Affiliations
1 Department of Applied Geology, University of Madras, Chennai - 600 025, IN

Abstract

Hydrogeological investigations along the Chennai (Madras) coast showed that groundwater levels are lower than mean sea level, often as much as -3 to -6m. However, the quality of groundwater in the near-surface unconfined aquifer, tapped by shallow wells, is reasonably fresh, though deeper wells show distinct signs of sea-water intrusion. Presence of fresh water below mean sea level in the absence of positive fresh water head is not possible in a coast line having hydraulic continuity with the marine environment. Detailed studies show that a narrow fresh water ridge, created by the beach dune complex, exists along the coast. Along most of the beach ridge groundwater abstraction is relatively less and also there is regular recharge into the aquifers from monsoon rains, and thus the static water level remains slightly above mean sea level along a narrow zone for most of the coastline. This fresh water ridge prevents contamination of the shallow aquifer to the west of the ridge, even though the water levels fall below mean sea level in the aquifer. Where the beach area has been urbanized or where there is large-scale abstraction from beach zone, groundwater has been contaminated with saline water as there is no fresh water barrier in such locations. Thus, the beach area has, to a large extent, safeguarded the shallow aquifers from serious saline water intrusion in Chennai city. This is a fortuitous situation which also selVes as a warning not to urbanise beach zones in coastal cities.

Keywords

Hydrogeology, Freshwater Ridge, Electrical Conductivity, Coastal Aquifer, Chennai, Tamil Nadu.

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