HYDROGEOLOGICAL CONDITIONS OF BENGALURU- A REVIEW

G.V. Hegde Former Senior Geologist Department of Mines and Geology, # Geokuteer, Adarshanagar, 3rd Cross, College Road, Sirsi-581 402 Email: [email protected]

(Sri Suryaparakash Rao Endowment lecturedelivered by G.V. Hegde, Former Senior Geologist, Department of Mines and Geology, on June 22, 2015)

With the formulation of Bruhat Bengaluru Mahanagara Palike (BBMP),Bengaluru has an extent of 800 sq.km and witnessed anunimaginable spurt in population from51million (2001) to 84.23million(2011).It is reported that Bengaluru has 862 slums and nearly ten percent of Bengaluru population are slum dwellers (Subhash Chandra and Hegde,2015). The built up area of 600 sq. kmis a mosaic of concrete structures and asphalt roads. Rest of the 200 sq.km,forms by and large an open area.Because of unplanned and haphazard growth of the city and encroachments by the builders and developers, many of the rivulets and streams in most part of the city are now distorted and/or diverted and have shrunk both in width and depth. They have become the flow paths for sewage and industrial effluents of the city. As a result, low lying areas often get inundated even when there is rain for even a short duration (Hegde and Subhash Chandra,2012). The normal annual rainfall of Bengaluru is 830mm. The spread of normal rainfall during pre-monsoon is152mm, SW monsoon is 454 mm and NE monsoon is224 mm.The evapotranspiration values of Bengaluru urban extent is estimated at 71.14 % of the normal rainfall (Hegde and Subhash Chandra, 2012).

Geology and Geomorphology:The Bengaluru city is composed mainly of gneisses, granodiorites and granites. The general N-S stretch of the conspicuous high land is the granite belt forming a divide between the catchment of the Cauvery basin in the west and the catchment in the east. The field traverses by Subhash Chandra and Hegde (2015) has revealed the granite body extending beyond 13°5′ in the north (Fig.1) and not terminating at 13°parallel as is shown in the map publishedby the Geological Survey of in the year 2000.The gneissic terrainto the west of granite belt, forming a part of the Vrishabhavathi River system of Arkavathi River catchment, dotted with hills and mounds is highly undulated with deep rock cutvalleys and rapid topographic fall towards SW. The drainage pattern of the Vrishabhavati is to an extent controlled by fairly well evidenced shears and faults, the evidences of which are now masked mainly by the effects of urbanisation. The area to the east of granite belt forming the Ponnaiyar River Catchment is aplateau, with gradual fall in topography of70mover a length of 15km towards east and southeast. The flow path in the Hebbal valley at the north and Belandur valley at the south dominate the Ponnaiyar drainage system in the city. North of 12°55′, to the east of granite belt up to the eastern and northern periphery,30to 60m thick in-situ saprolite spread(chemically weathered product of gneissic rock) is at around 880m above the mean sea level. Isolated exposure of gneissic rock in the saprolite part of the terrain is not uncommon.

Figure 1Geological map of Bengaluru(afterSubhash Chandra and Hegde)

Status of lakes: Bengaluru lakes were once interconnected by intricate network of canals to capture monsoon precipitation of the city.Series of such interconnected cascading tanks were created over centuries along the drainages in the valley systems of Hebbalin the north, Koramangala-Challaghatta, Belandur-Varthur in the south and Vrishabhavati in the West to almost accommodate the storm water flow of the city. But, the tragedy is that most of these water bodies,in the name of urban developmentare now encroached upon for building construction and many of the lakes have now become the repositories of the city sewage load. The urbanisation and loss of tanks has resulted in the loss of catchment yield, water storage capacity and depletion of groundwater table. It is reported that out of 127 lakes in the conurban area, 60 of the surviving lakes are contaminated fully with sewage and rest of the 67 under different stages of deterioration.

Aquifer system of Bengaluru:Thegeological, geomorphological and geohydrological characteristics ofthe Arkavati River catchment in the west differ from that of the Ponnaiyar catchment in the east.In the Arkavati catchment part, the gneissic rocks are weathered to a depth of 15 to 20m and grade into unaltered fresh rock with joints becoming sparse and compact at depths. In general theydisplay a ‘composite aquifer system’. Presently unconfined, semi-unconfined, and part of the semi-confined aquifers have gone dry. In most of the cases, the water is now being struck at depths around 220-250m, but with a meagre yield of 1 to 1.5 lps. Most of the bore-wells drilled beyond 280 m have not yielded water. The joints/fractures are the only groundwater conduits and not the rock matrix as it is fresh, hard and compact. The extraction of groundwater resources is much beyond the annual recharge resulting in mining of groundwater resources. There is an indication of the entire composite aquifer system of the Arkavati catchment going dry sooner than later. In the Ponnaiyar catchment, thesaprolite and lateritic soil both being clayey, togetherforms an impermeable layer and functions as ‘aquiclude’. The built up area with concrete structures and asphalt roads also does not promote infiltration / percolation of water and this is resulting in heavy over land flow. The unaltered gneissic formation below the saprolite, similar to the condition explained in Arkavati catchment has remained hard and fresh with joints being compact and sparse at depths. Thefresh unaltered, sparsely jointed compact rock body with impermeable saprolite above and massive rock horizon below forms a ‘confined aquifer zone’ and not the semi-confined aquifer as earlier opined by many (Subhash Chandra and Hegde,2014). If there can be groundwater recharge into this zone, it can possibly be through the lineaments of the area or from the composite aquifer zone of the neighbourhood locality that may have some extended connectivity through joints and fractures. Given the condition of poor to negligible annual replenishmentand that the water stored since geological past in the confined aquifer is on the verge of becoming barren. Many bore wells drilled even beyond 300m depth have remained dry andnot sustainable.

Scope for groundwaterrecharge:There are suggestions from certain corners to directly inject the roof top rain water onto the deep dry bore wells as a measure for groundwater recharge. But the characteristics of the composite aquifer system display a varied role. While the unconfined aquifer can accept and release the water in a moderate way, the semi-confined aquifer comprised of hard and massive rock matrix with joints being compact and sparse at depth, the rate of intake of water from the unconfined media into that zone gets hindered.That means, the load of rain water injected as a measure for groundwater recharge cannot be accepted by the rock body at depth. Similarly, the saprolite and lateritic soil together forming an impervious layer and the rock matrix in the confined aquifer zone below is massive, fresh and the joints are sparse, the water intake will be minimal.Thus the groundwater recharge from rain water harvesting measures through bore wells will not be a viable approach in these areas. Hence, conservation of rain water through roof top harvesting and direct utilisation is more appropriate.

References:

Hegde, G.V and Subhash Chandra, K.C (2012), Resource availability for water supply to city, ; Current Science, Vol.102, No.8 pp 1102-1104

Subhash Chandra, K.C and Hegde, G.V (2014), Occurrence of groundwater in the Ponnaiyar river catchment covering eastern parts of Bangalore city; Current Science, Vol.106, No.10, pp 1353-1356

Subhash Chandra, K.C and Hegde,G.V (2015), Bengaluru Water Resource Management – Challenges and Remedies; Published by Institute for Natural Resources Conservation, Education, Research and Training, Bengaluru. ISBN: 81-89650-51-3