Groundwater bankingMarch 22, 2008
With depleting ground water table, water banks are a dire need
|Artificial storage and recovery is what every city needs, says S.Vishwanath|
Precious: Storing water also recharges aquifers
Groundwater in India is a crucial resource and indeed the very lifeline of our water needs. In a city like Bangalore anywhere between 30 to 50 per cent of the water needs may be coming from groundwater.On the one hand, groundwater tables are falling and borewells are being sunk to depths of 1,250 feet and more. On the other hand, stormwater floods city streets and wastewater flows in storm drains. Groundwater is not being adequately replenished nor is the potential that the ground offers to store water being utilised.
Artificial storage and recovery (ASR) or managed aquifer recharge is a method by which water banks are created using rainwater, stormwater and treated wastewater and this water is drawn in times of need. This essentially makes use of the potential of the ground to store and to a certain extent treat water, preventing evaporation losses.
Hydro-geology Developing a good understanding of the hydro-geology of a place and understanding the nature of the soil and rock is crucial. We must know, for example, the porosity of the soil, the transmissivity, hydraulic conductivity and storage. The thickness, depth and geographical extant of aquifers need to be understood.
The lithology of aquifers also need to be determined. In simple words, the capacity of the ground to absorb, retain and return water needs to be determined.
Rooftop rainwater is ideal because it tends to be the cleanest. If a simple first rain separation is done and the rainwater filtered, it is excellent for artificial recharge. Stormwater running in properly managed catchments without pollution can also be used.
Filters These are usually held in retention ponds and allowed to seep into the ground. If stormwater is to be cleaned, sand filters and wetlands can be designed to remove physical turbidity. The traditional tanks were great recharge structures when they functioned as percolation tanks.
Sewage water, if treated to the adequate levels, can also be used for artificial storage and recovery. In Cubbon Park and Lalbagh in Bangalore, tertiary treatment plants have been set up which are using membranes to treat sewage to high levels.
If the salts, nitrates and phosphates can be removed the water will be ideal for storage. In fact the presence of large open spaces and aquifers nearby provides an excellent opportunity for such controlled experiments.
A limited confined aquifer is ideal. However, the process of ASR can be done in property which have their own sewage treatment plants and a recharge zone.
Failed borewells and open wells provide opportunities for artificial recharge but aquifer characteristics may prevent controlled access to the water so recharged. It is to be understood that the unit of ASR is determined by hydro-geological boundaries rather than surface boundaries.
Potential zones City-level attempts or even neighbourhood-level attempts centered around a surface water body is possible. Each surface water body is topographically located at the lowest point. Sewage flows and stormwater flows are also in this direction.
With a treatment plant located close-by, it should be possible to pick both stormwater and treated sewage water for ASR. In a city all surface water bodies are potential zones for ASR.
Conventionally, treated wastewater has been immediately used for non-potable purpose such as toilet flushing or watering landscapes. Especially during the rainy season the zero discharge policy enforced by the State Pollution Control Boards leads to excess treated wastewater which is difficult to manage.
In such circumstances it is better for the treated wastewater to be of such high quality that it is possible to use the aquifer as a bank to store it and then to use it later during times of need.
A holistic policy of understanding rainwater, surface water and groundwater along with robust knowledge of treated wastewater will enable better sustainable practices to be put in place for water use.
New systems such as high level treatment of sewage and ASR will be the future of better water management and in pursuing the science behind this lies water wisdom for our cities.