Many rural properties and industrial operations make use of boreholes to extract water from the water table to be used as a source of potable or industrial water. In many instances the extraction rate exceeds the rate of replenishment. Ultimately the water table will continue to drop and the borehole will dry up.
Rainwater harvesting can be used as an effective means to augment the water supply in ground water.
There are a number of steps that can be used in combination to augment ground water.
The first step is the retain water on the ground for as long as possible in or around the property, before it can flow away. Water that makes direct contact with the soil must be channelled to areas where the water can effectively be retained and provided with an opportunity to permeate into the soil. Physical structures in the natural waterflow can be constructed to temporarily “dam up the water”.
Physical plugs like small check-dams, bunds and gabions walls can be constructed in the dry natural waterflow contours to increase retention time. Bigger structure like ground water dams can also be built, but designers must ensure effective overflows and make the water surface area as big as possible to ensure maximum water permeation into the soil.
The second step is to ensure that all water that came into contact with a roof surface can be recovered before contact with the ground or soil. This water will need some form of pre-filtration before storage. If there is sufficient space and the water can be stored in tanks and re-used it will alleviate pressure on the ground water resource.
Alternatively, if the recovery surface area is too great, it is recommended that the water is stored in an effectively sized attenuation pond. The pond will be used as a temporary storage while the water is sufficiently purified and redirected to the underground water table.
Critical steps in treating this water are to remove all organic material and silt before transferring it to a specially designed recharge pit or well that is indirectly connected to the ground water table.
Removing sizeable organic material can only be achieved with physical screening or filtration. After screening, the water must be filtered by natural means like slow sand filtration before it is discharged back into the ground water table. This will minimise ground water contamination and ensure effective and sustainable recharging of the underground water table.
