Wednesday, August 10, 2011

Ground water is just 150 ft deep in Hyderabad; further down is hard rock which does not yield any ground water

By Syed Akbar

Hyderabad: How deep one should dig a bore well in Hyderabad and Secunderabad to extract 
ground water? As builders and house owners vie with one another to go deeper into the 
soil for “perennial source” of ground water, geologists clarify that the water table in 
twin cities is just 150 ft deep. They warn that any drilling into the earth beyond this 
point will only harm the ground water table and affect its prospects as time passes by. 
It’s waste of money and labour.

The geology of Hyderabad and its surrounding areas is peculiar in the sense that the 
depth up to which it can withhold water is about 150 ft or a little more in certain 
pockets. Beyond 150 ft one encounters the bedrock, which does not hold any ground water, 
unless it has some fractures. Going deeper and deeper may give temporary benefits, but in 
the long run spoils the water table.

According to Dr VVS Gurunatha Rao, deputy director, National Geophysical Research 
Institute, hard rock is encountered in twin cities up to depths up about 150 ft. This is 
the lower limit for drawl of ground water. “Our studies have shown that hard rock is 
encountered in many places in and around Hyderabad at 30 meters deep or a little below. 
The upper loose soil and the area below it are the potential sources of tapping ground 
water. Below it the ground water potential is relatively less. And lower further down is 
the bedrock, which is hard and impermeable. The rain water that percolates the ground 
will not seep further down the hard rock,” he clarifies.

Explaining the reason for drying up of many bore wells in Hyderabad despite the increase 
in ground water table in the last three years, Dr Gurunatha Rao blames the culture of 
constructing cellars and double cellars. “The first few metres of top soil have the 
greatest potential of withholding water. But by constructing cellars and double cellars 
we are robbing of a large portion of this potential ground water zone,” he adds. A cellar 
occupies about six metres of this important water holding area. Double cellar means 
occupation of 12 to 15 metres of the top soil.

The NGRI scientist suggests that one should avoid constructing cellars to ensure more 
recharge of ground water. He also points out that going deeper into the earth will not 
yield ground water perennially at least in twin cities.

According to the city-based International Water Management Institute and Jawahar Lal 
Nehru Technological University, the Musi sub-basin is mainly
covered by Archaean granites with Deccan Traps. As in a typical hard rock
aquifer region, the yield of the bores decreases with depth due to the  reduction of the 
fracture density. The risk of water scarcity in case of
a drought year is thus exacerbated.

A joint study by teams from IWMI and JNTU has revealed that the ground water resource in 
Hyderabad and its surrounding areas is mostly represented by typical unconfined shallow 
aquifers in hard rock which generally occupy the upper 20 metres of the subsurface 
profile. “These composite aquifers derive primarily from the geomorphologic processes of 
deep weathering and
erosion. They can therefore be considered as a multi-layered system,” the study points 

The upper layer is unconsolidated weathered mantle (saprolite or regolith), which is from 
negligible to several tens of metres thickness. When saturated, this layer constitutes 
the reservoir of the aquifer (water source). Instead of tapping this zone, builders are 
going deeper to the second layer, which is fractured-weathered layer, characterised by a 
fracture density that decreases with depth. This in other words means as one goes deeper 
the potentiality of ground water decreases. The lower layer is fresh basement, which is 
permeable only locally where deep tectonic fractures are present.

The study found that in the last two decades, natural recharge from rainfall in Musi 
sub-basin accounted for 9.4 per cent of the total annual rainfall. But the exploitation 
was much higher leaving a wide deficit. In certain places, however, there are deeper 
aquifer systems thanks to major joints, fractures or crevices. At these places ground 
water can be harnessed up to 700 feet. But only a fortunate few will get such deeper 
aquifer systems. But as Dr Gurunatha Rao argues the average depth of bore wells in twin 
cities should not cross 150 ft, and to ensure perennial source of ground water, 
scientifically-test rain water harvesting methods should be adopted.
AP virtually sits on well explosion
Andhra Pradesh virtually sits on a well explosion. In the last three decades 
the number of wells in the State has gone up by four times, up from 8 lakhs to 24 lakhs. 
And hold your breath. The well population in Andhra Pradesh is increasing at the rate of 
50,000 a year. But what’s worrying is that scientific steps are not taken to ensure that 
the quantity of water drawn from ground water is properly replenished by rain water.

According to official figures, area irrigated through groundwater increased from 10 lakh 
hectares to 28 lakhs hectares since 1990. Ground water constitutes about 45 per cent of 
the total area irrigated. Interestingly, almost 80 per cent of the drinking water needs 
are met through ground water and just 20 per cent from rivers and lakes.

With increasing concretization, recharge of ground water during monsoon has been hit in 
the last 10 years. Adding to the problem, the number of rainy days has come down. 
Geologists point out that the more the number of rainy days the more the recharge of 
ground water. Heavy rains only lead to run off. For ground water to be recharged fully, 
rain should be moderate and spread over a number of days.

Predicting the “future scenario”, the State ground water department forecasts that by 
2020 the number of wells will be touch 36 lakhs, mostly deep bore/tube wells, irrigating 
about 36 lakh hectares.

While bore wells may meet the water needs of people in Telangana and Rayalaseema, they 
will harm the delicate ecological balance in the coastal regions. “If ground water is 
over exploited in coastal regions, there’s the threat of sea water intrusion into the 
ground water. This will make the soil salty and unfit for agriculture,” warns Dr VVS 
Gurunatha Rao, deputy director of NGRI.

In case of Hyderabad and Rangareddy district, of the 40 ground water micro basins 
present, only 10 can be declared as safe. The remaining fall under over-exploited and 
critical categories, meaning there’s danger to the water table if immediate steps to 
recharge it are not taken. There’s the problem of ground water pollution too thanks to 
the use of fertilizers and pesticides and unscientific disposal of waste by chemical 

No comments: