Deep Aquifer Recharge

India relies heavily on groundwater usage, and it is the world's largest extractor of groundwater. Groundwater is a depleting resource and many cases of dug wells, bore wells and aquifers going dry, issues of high fluoride content and increasing salinity are being noted. Niti Aayog report mentions depleting groundwater resources for major Indian cities in the years to come. In this scenario of water stress, issues around access and distribution, rainwater harvesting offers a huge potential solution.

There are many borewells in India that have either gone dry or could not discharge water because of local aquifer conditions. In many of these defunct borewells, groundwater is depleted or they have reached hard rock and it no longer serves the purpose of withdrawing water. It takes decades and centuries to recharge the deep confined aquifers to recharge naturally and so, the existing defunct borewells are great locations for groundwater recharge because the drilling and well structure already exists. So with certain pipe modifications and filter mechanism, we can divert rooftop rainwater and surface runoff to the borewell which will very well serve the function of a recharge structure.

Through this recharge method:

(i) Groundwater level increases

(ii) Salinity levels decrease due to dilution in areas with high salinity

(iii) Pumping costs and drilling costs are saved. (Groundwater levels decrease with more withdrawal and no recharge, so motor with higher power are installed and eventually the borewell is dug deeper to access water - These costs can be saved with deep aquifer recharge)

For deep confined aquifer recharge , it is important to understand:

Aquifer characteristics:

It is important to understand aquifer characteristics before implementing the recharge structure. There are several strata of different aquifer media such as sand, clay, silt, gravel, rocks and each have different permeable properties. For deep aquifer recharge it is important to know at which permeable strata should the filtered rainwater be released. These conditions vary locally and a local geology survey is needed or local borewell diggers can be contacted for the purpose. Also water which will be recharged into deep aquifers, it has to be cleaned and there should not be any contaminants in the water.

Sources and types:

Rainwater from roof, surface runoff, watershed landscapes, roads or water bodies can be used for recharging deep confined aquifers through borewells. These borewells can be existing functional ones, or defunct borewells which have gone dry over time or newly constructed recharge borewells. According to local site conditions, rainfall availability, a combination of these types of recharge borewells and sources of rainwater catchment can take place. Moreover, site specific conditions will need customised design of borewell recharge.

Components of borewell recharge:

• Catchment: Catchment is an area within which the rainwater that falls is collected, and its cleanliness must be ensured. It can be rooftop or roads, fields or barren land or any landscape. It is important that this water is filtered and removed of impurities and sediments before the water is diverted to the borewell for recharge.

• Pipes and drains: Pipes from roof and storm water drains from roads channel rainwater from their respective catchment to the harvesting structure.

• Filter: For surface runoff or storm water collection, a desilting chamber is the first part where water reaches with silt and soil. This chamber allows the silt to settle and the overflow reaches filter media of sand, gravel and boulders; from which it enters borewell casing through perforations.

• Borewell: Through an existing defunct or newly constructed borewell, the filtered rainwater artificially recharges the deep aquifer. Perforations in the borewell pipe are made at the level of aquifer strata having high permeability like sand or fractured rock, for water to be released in the aquifer.

• Air vent: Air vent is provided to release trapped air in the recharge borewell system.

Maintenance:

• It is important to ensure that the rainwater diverted for recharge does not have impurities. Especially for surface runoff, care must be taken that there are no chemical spillages or open defecation in the catchment.

• For recharging in a functional well, the rooftop as a catchment should be preferred. This way there is control over catchment and its cleanliness can be ensured and supervised. With experts’ consultation one must understand protocols for taking care of the submersible motor, while recharging functional borewell.

• The filter media used should be cleaned well at least once before monsoon season. Similarly pipes and recharge pit should be cleaned to avoid any clogging.

Recharge borewells can play a crucial role in decentralised water conservation and management. They help in increasing groundwater tables for deep aquifers and improving the water quality. For agriculture practices that are heavily dependent on groundwater as their source of irrigation, it should be a must to identify defunct and abandoned borewells and implement recharge solutions.