With widespread
extraction of groundwater in Bangalore, the search for a sustainable solution
for the usage of groundwater is the call of the hour. What has been a repeated
cause of worry in terms of supply has been the drying up of borewells that are
an integral source of water to a majority of the population. In a scientific
point of view, the borewells are our access points to what are called the ‘intermediate
aquifer’ and the deep aquifer’. Intermediate aquifers can be found at a depth
of 30 m to 40 m below ground level and the deep aquifers found at a depth of
150 m or lower. What has been the case for the past few years has been the
vanishing of the intermediate aquifers, which has forced the digging of
borewells further to try and extract from the deeper aquifers.
Particularly in the South
Eastern part of Bangalore, the Sarjapur Road area and its surroundings, most
borewells are preposterously deep, going as low as 1000 feet in search of
water, and sometimes still failing to identify a sustainable source of water. Therefore
we are now approaching a reality where having extraction of groundwater from
where we live is as close to not being possible, for the resource has been nearly exhausted.
The BWSSB supply of the Cauvery River water is non-existent in this
part of the city in terms of infrastructural connection. In terms of the
quantum of supply of BWSSB water, a bleak picture is being painted on that front
as well. Currently, the Cauvery Water Supply Scheme (CWSS) Stage I, II, II & Stage IV Phase I and II
amounts to the total allocated water from the Cauvery River provisional for
Bangalore city. The total installed capacity as of right now is 1310 MLD. Below given is the water supply requirements based on the future anticipated populations. Notice the amount of shorftfall in demand, which is already being experienced.
Source: https://bwssb.gov.in/content/about-bwssb-2
As it can be seen above, to put it bluntly, we just do not have enough supply of water to meet our basic needs, let alone our over consumption! Therefore, alternative sustainable options must be administered in order to solve such a crisis. A good example of such an occurrence is in Adarsh Palm Retreat who are fortunate to have an alternative resource in place that can aid in reducing their current dependency on unsustainable sources.
Shallow Aquifers: Back to the Good Old Times!
Above, it was mentioned that our borewells largely tap onto the intermediate and deep aquifers. There is in fact a third form of aquifer that has been somewhat forgotten, the shallow aquifer. To say the least, this groundwater resource very close to ground level is not unheard of. We were in fact supporting our basic needs through this resource for thousands of years, while having access through open wells! The case of Adarsh Palm Retreat (APR) is a symbolic attempt to kind of back track our preexisting technologically advance solutions and revert to a simple solution that will provide far more sustainable results.
Through a study done by Biome, a solution was identified to reform the water management strategy that is in place for the community. It was established that right under the 128 acres of APR, there lies a humongous resource in the form of a shallow aquifer, or to simply put, groundwater found at a depth of 10 m below the ground. This resource was disguised in the form of a problem for the residents, as during the rainy months, there was sever flooding that occurred in certain basements of houses. This provided Biome a suspicion that there seems to be a hidden resource in plain sight. After some deliberation, the size of the aquifer was identified with respect to APR's boundaries. Below given is information on the aquifer.
Weather Zone
|
10
|
m
|
Specific Yield of Aquifer
|
0.060
|
|
Storage
|
310976
|
KL
|
In terms of rain
|
600
|
mm of rain
|
The weathered zone, in effect the depth of the shallow aquifer has been taken as 10 m which is a conservative assumption made that needs to be ascertained. However, with the consultation of leading hydrologists with vast experience, it is certainly a good working assumption for now. To simplify the above data, the capacity of the aquifer has been determined to be 600 mm of rain. Now with an overall objective to harvest this aquifer twice in a year, which would amount to 1200 mm of water in terms of rainfall provisional for usage. With comparison to 2015's rainfall of 1188 mm this will ensure the utilization of the entire rainfall received in a year. With a systematic approach, this somewhat giant storage unit can be fed rainwater and extracted reasonably all throughout the year. This in turn can aid in reduction of dependency on other not so reliable resources such as tanker water.
In the given below link is a quick presentation summarizing the story of APR and how this solution can be of benefit to the community-
Great! How do I tap this Shallow Aquifer?
Finally, a solution would need to be developed in order for the extraction of the water from this shallow aquifer. However, it must also be bared in mind that this solution can only be considered sustainable if adequate amount of water is also recharged into the aquifer. Therefore, there needs to be a system in place that will ensure a balance in withdrawing as well as in recharging. This can be conducted by having a network of withdrawal wells, that will be the extraction point for the water to be transported to the water treatment plant. Alongside, there will be an adequate number of recharge wells, whose purpose would be to recharge the well and that can be done by harvesting the rainwater runoff on APR's boundaries.
The first withdrawal well was constructed on Phase 2, Lane 8 of APR. The well has a diameter of 5 ft and a depth of 25 ft. The volume of the well is around 12,000 L. Here are some photos of the withdrawal well.
Here is a quick video explaining the use of the withdrawal well and the challenges the community members faced in the execution of such a solution.
Specifications of the Withdrawal Well
The effective volume of the well is around 12,000 L. Recently, the performance of the withdrawal well was analyzed. This test was conducted by pumping out a majority of the water with the help of a 2 HP pump. It took roughly 20 minutes to pump out the water. Due to practical difficulties, unfortunately the volume of water could not be estimated in this test. Once the water was pumped to a dept of 20 feet, an automated data logging sensor was installed. This setup can then be utilized for understanding the recovery rate of the well. The meaning of the recovery rate of a well is the rate at which water rushes back into the well from the ground once space is made for it by pumping out the existing water. The sensor's function was to log the water level of the well every 5 minutes. The sensor was kept for 2.5 days and no pumping was conducted during the time frame, Below is the results that were found in the duration.
To understand the graph, picture 0 being ground level and the y axis variables signifying the depth at which water was found below the ground level. Therefore, more the water in the well, closer is the surface to the ground. As it can be inferred, around 4 m of water was recovered during the span of the sensor being installed. That amounts to a volume of around 6,600 L! It can also be noticed that the majority of the recovery occurs in the first 24 hours. Zooming in on that portion of the data for analysis.
With this data, the initial recovery rate of the well can be calculated. In a span of 24 hours, the water level increases a column of about 3 m which gives a rate of 5833 L/day.
Water Quality
A water quality test was conducted previously, by sampling the water that was accessible from a private dewatering well which was constructed in order to cease the basement seepage problem that the resident was facing. In the below picture, the results of the test are given.
Through logical understanding, the water found in that well is from the same shallow aquifer. Therefore, we suspect the quality to be similar as well. One of the subsequent activities would be to conduct another water quality test for the water from the withdrawal well.
Pump Test
Another important activity that will be conducted is something known as a 'Pump Test'. This test would involve the monitoring of water levels while the pumping of the water occurs as well and data is collated for multiple days. This will allow us to further make more conclusions about the aquifer that will help fine tune the strategy and translate it into physical infrastructure to sustainably and effectively use the shallow aquifer.
