Wednesday, August 28, 2013

The Aquifer Underneath My House

... is very prolific.



The photo above taken in March 2012 is of an excavation for a building about half a kilometer away from my home in Pune, India. The developer struck water at around 30 feet below the surface. Water began gushing out of sheet cracks in the basalt rock. Within a couple of days the water level had risen to just a few feet below the surface and then stabilized.

The water level you see in the picture is not the water table but the potentiometric surface. The developer had punctured a confined aquifer. Water in this type of aquifer is under hydrostatic pressure. The puncture or hole is this case creates a pressure gradient and water flowed from the aquifer (high pressure) into the hole (low pressure). It rose until the water pressure at the bottom of the hole equaled the water pressure in the aquifer at which point water stopped flowing out of the aquifer and hence stopped rising in the well /excavation.

In the picture below the red arrows point to the sheet cracks from which groundwater is seeping out.


This picture was taken in May 2012 after pumping had reduced water pressure in the aquifer and water seepage was a mere trickle.



I did a rough calculation back in March 2012 and figured that the hole contained about 15-20 million liters of water. Since then the developer has been periodically pumping water out of the hole. Between March 2012 and May 2012 I figured more than 100 million liters of water was likely pumped out.  But recharge during the monsoons in June and July 2012 stymied his efforts and he let the site be flooded. Resuming after the monsoons, he continued pumping water and tried to build a foundation to the site. During this 2013 monsoon, rains in June and July recharged the aquifer and the site is flooded again.

Pune has in the past been well supplied by water from several surface reservoirs. In the last few years though, rapid city growth, loss through leakages and a tussle for water allocation between the city and agriculture has led to shortages in the city beginning around March. The response has been to drill private tube wells to supplement dwindling city water supply. While this may provide citizens with an alternative in the short term the danger remains that excessive number of wells and pumping might deplete this underground resource too.

But what is excessive and unsustainable? That is a fundamental question to which the municipal corporation has no answer to because basic data on the hydrogeology of these aquifers has not been collected.  For example in my neighborhood, private tube wells are striking water at depths between 25 feet and 40 feet. We can assume that there is a significant aquifer at that depth (there may be aquifers at deeper depths too). Judging from my rough calculations at the construction site more than one hundred million liters of water were pumped out in a span of a few weeks between March 2012 and May 2012 before there was a notable decrease in water pressure. This was an unusual situation of sustained pumping at just one location. The more relevant situation would be scores of private wells pumping water for short time spans from this aquifer. What would be the sustainable water extraction budget for this aquifer in this area?  These are the kind of questions the city government must try to answer.

Last year the water shortage became so acute that the city government considered sequestering private tube wells and converting them into a public water source.

If such times are coming upon us we need to come to a quantitative understanding of these aquifers.

But how does one map these deeper aquifers in the first place?  The agency in charge of state groundwater the Groundwater Survey and Development Agency does have a few observation wells scattered through the city. The location and number follow watershed boundaries. Each watershed or micro-watershed has a fixed number of wells. One could look at the log - geological description with depth - and try to get some idea of the rock properties at depth. But these wells are too few and far between to get a finer resolution idea. So, one way to improve understanding of these aquifers is to drill more observations wells. While this will give us a better coverage there is a limit to how many wells the agency can drill and monitor. The location of these wells too is crucial, a point I will come back to later. Besides, the very narrow diameter of a tube/bore well means that one doesn't get a good idea about the nature of the network of cracks that control the flow of groundwater.

I have two more suggestions. One is to use construction sites like the one near my house. Unlike trying to  interpret geology from a few rock chips that come out of a bore well, such large excavations offer a clear view of the rock section. The relationship of fracture networks and lava flows can be clearly seen. Such an opportunity to understand aquifer properties must not be wasted. The city government should mandate that a state geologist must be notified whenever a large excavation exposes rock layers. The geologists will then describe the site and the aquifer properties exposed along the walls of the excavation and in case of a situation like the one I have depicted, carry out pumping tests to ascertain flow rates of groundwater. Not all excavations will go down to the same depth, but Pune is growing rapidly, construction sites are aplenty and collating data over the next 10 years or so will give the city government a good idea of the subsurface geology up to depths of 30 to 40 feet.

Construction sites will be separated from each other. A technology like ground penetrating radar will help in filling up the gaps. Radar imaging is a useful method to map out subsurface fracture patterns in rocks. Groundwater in hard crystalline rocks like granite and basalts is stored in and moves along cracks and fractures in the rock. Recent papers in the Journal of Geological Society of India and Current Science demonstrate its usefulness in mapping subsurface structure up to depths of tens of meters in granitic terrains around Hyderabad. A similar approach will help Pune too map subsurface network of cracks and fractures in the basalt rock.

Instead of expanding the network of observation wells by drilling willy nilly or by some predetermined formula such as "number of wells per micro-watershed", a more useful approach is to follow the data. As more and more data about subsurface aquifers and fracture systems is collected, an optimal network of observation wells can then be drilled to monitor the system.

India's new National Water Policy and well as the Maharashtra Groundwater Act will require that aquifers are mapped and that a groundwater use plan be prepared by the authorized agency based on the properties of the aquifer. This is a welcome move. Far too long groundwater management ideas followed the contours metaphorically and literally of surficial watersheds. However, aquifer boundaries need not coincide with those of watersheds. Subsurface geology is complicated and may change over few hundred meters. Moreover, this basalt terrain being made up of layered lava flows, there may be several aquifers at different depths. A sensible groundwater use plan will be based not on watershed as a management unit but on an aquifer as a management unit. For that, the dimensions and spatial variability of each aquifer needs to be well understood and a groundwater management plan to be of any practical use needs to be scaled to that variability in aquifer properties.

Finally, the water in these aquifers ultimately is rain water falling on the ground and seeping through the column of rock. Which means unpaved ground like parks and playgrounds that can act as recharge zones for aquifers must be protected.  Geologists I have talked to identify the toe of the slopes of the various hills in Pune as important recharge areas. Even as the city pours more concrete and grows, it is imperative we protect these zones from being paved over along with other efforts to augment recharge such as constructing recharge pits within large housing complexes.

4 comments:

  1. Very interesting and indepth. As I too have observed, indiscriminate paving around trees is uprooting them. When I looked down at a gaping hole left by a huge fallen tree, even a layperson like me could understand that it was due to stoppage of seeping rain water, due to concretisation which caused this.
    I agree that more research should be conducted to tap our natural water reserves.

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  2. Has anyone (in India) looked at stable isotope ratios in such trapped aquifers?

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  3. yes.. not sure about Pune.. but stable isotopes are used for a variety of env., provenance and climatic studies.. do you have a specific application in mind?

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  4. In perhaps a first among biological applications in India, they were only recently used to determine the origins of dragonflies over the Maldives - http://dx.doi.org/10.1371/journal.pone.0052594 - I was wondering if geologists could use them to provide some idea of groundwater recharge based on differences between surface waters that undergo some heavy isotope enrichment through evaporation versus underground and trapped waters. In any case I found that this is indeed in use - http://dx.doi.org/10.1016/j.jhydrol.2006.10.004

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