Thursday, December 4, 2008

Groundwater Flow in Basalts, Caught on Video

The volcanic rock basalt underlies the city of Pune and much of the state of Maharashtra. Its rock mass is made up of an interlocking fabric of crystals and it doesn't have interconnected pore spaces through which water could be transmitted. The permeability of the rock mass is negligible. But basalts can be prolific aquifers. The reason is the various kinds of fractures and joints that serve to store and transmit water.

I came across a construction site near my house the other day and saw that they had dug a pretty deep hole for a foundation. I could see a thick rock profile consisting of two basalt flows. The upper flow was an amygdaloidal vesicular basalt. The lower one was a compact basalt. These are field terms used to categorize flow units with different physical characteristics. At the junction of the two flows was a fractured zone. The fractures were horizontal giving the basalt a sheeted appearance. And water was flowing out of these fractures at a fair pace. Take a look.

Groundwater flow in Basalt

This narrow flow zone which is a few feet thick is really the aquifer in this case. The compact basalt below forms the base of the system. Above this water seep you can see that the amygdaloidal basalt is bone dry. I wrote before about the enormous influence basalt hydrology has on the lives of Maharashtra farmers. You can get an idea why that is so. Finding water in basalts can be tough work because of such narrow permeability zones. Farmers often have the misfortune of farming tiny plots of land over basalts with no or very deep permeability zones. Digging wells into basalts is expensive and risky. Without proper geological guidance many poor farmers remain without access to groundwater.

So far this shallow groundwater system is not being exploited as a resource within the city of Pune. But that might change. Looking twenty years ahead, an expanding population and more water intensive life styles as people become prosperous might strain the surface water supply, which currently is sufficient to give citizens of Pune one of urban India's highest per day per capita allowance of around 200 liters.

Add to that are the vagaries of the monsoons. Even without global warming induced perturbations to rainfall, Indian monsoons are characterized by a decadal scale natural variability. Pune over the last 3-4 years has experienced higher than average rains but extended periods of lower than average rainfall is also a likely scenario in the future. A combination of lower rainfall and higher population might mean that the surface water reservoirs fall short of supplying enough water. The aquifer underneath Pune might gain importance in this context.

Unfortunately hydrologists don't really know how much water is present in the aquifers underneath Pune. The state ground water board periodically issues completely useless statements about the level of groundwater either going up or going down by said amounts. These are based on a few observation wells in areas where the aquifer is not being exploited. But there is practically no quantitative assessment of what will happen to the groundwater system if people suddenly start sucking water out of it. The system has not been studied under stress.

There is an opportunity to do that. On the outskirts of the city private water suppliers have sunk dug wells and bore wells. Year after year they are pumping water out of the shallow and deep aquifers. The water balance is not understood. How much should be taken out so that natural recharge will balance extraction? But I don't see government scientists rushing with their measuring tools to take advantage of these potential data points. Scientific progress relies as much on opportunism as on any other attribute. But state hydrologists lack the flexibility to deviate from their 2 year or 3 year of 5 year plans and schemes they are directed to follow.

Pune must have a science backed plan if and when the time comes to start exploiting the underlying aquifers. The time to start a serious research program is now.

No comments:

Post a Comment