Water Portal / Rainwater Harvesting / Groundwater recharge / Infiltration ponds
Infiltration ponds are large open water ponds that are either excavated or in an area of land surrounded by a bank, and normally will not exceed 15,000 m3. They store rainwater but with the main aim of infiltrating the water to aquifers where it can be extracted using wells or from springs. They are constructed in areas where the base of the pond is permeable and where the aquifer to be recharged is at or near the surface. Examples include dune infiltration ponds in South Africa, Tajamar ponds in Paraguay, and infiltration basins in Niger. Large dams can also be used to artificially recharge aquifers – in Jordan, one dam was constructed to recharge a well field 8km from the dam site, and experience from the past 6 years shows that groundwater levels have increased by 25-40 metres. In Nepal, small ponds traditionally helped to recharge spring water.
Suitable conditions
The aquifer to be recharged needs to be at or near the surface. The base of the pond needs to be permeable. The typical amount of water going into the ground is 30 m/year for fine texture soils (e.g. sandy loams), 100 m/year for loamy soils and 300 m/year for coarse clean sands. A field method to determine seepage rates in the bottom of reservoirs has been developed which can be used to assist in design. Ideally infiltration rates should exceed evaporation rates.
Pond size should be decided according to catchment area and number of fillings possible per year. In order to efficiently capture runoff in a catchment, similar design techniques to contour trenches could be employed for infiltration ponds.
Construction, operations and maintenance
The main issue is to minimize silting, as this will reduce infiltration capacity through the base and sides. There are several techniques to minimize this:
- Any diversion and intake structures should be made so as to minimize input of silt to the ponds. Sedimentation basins can reduce silt load before water enters infiltration pond. What might work better is to keep a good cover of indigenous grasses in the run-off area. Kambiti Farm in Kitui District provides a good example of previously degraded land being managed and where open dams did not silt up due to pasture management. Contour lines with trees or grasses in the runoff area also work. If the inflow channel is defined, silt traps can be tried out to reduce silt load as is done with Charco dams in Tanzania. In this case, stones laid across the channel form mini dams and perennial vegetation can be grown between these mini dams to reduce the flow velocity of water, thereby encouraging silt deposits.
- Where aquifer material is fine, clogging may occur rapidly but can be delayed by covering the base and sides of the pond with a 0.5m thick layer of medium sand.
- A rotational system of ponds can allow some to dry while others are used – the ones that dry up can be scraped out to restore infiltration rates, while the drying process is also good for killing algae. In this case, the pond should be shallow enough to allow rapid draining when scraping is needed.
- Constructing ridges on the floor of the basin and controlling water level can allow fine silt to deposit in troughs, allowing most infiltration to take place on the sides of the
ridges.
- Mechanical ploughing of the floor of the basin can also increase permeability.
De-silting will most probably need to be carried out at some stage. There may be more sustainable ways of doing this compared to the usual approach used in the recovery stage of DCM, where this process is often paid for by NGOs and where there is a lack of community will to contribute. Experience from infiltration ponds in India shows that securing participation is very difficult to achieve when users/farmers do not see any direct benefit from the ponds. An institutionally-resilient way to de-silt (or even construct) ponds may be to promote ponds on private land, where one landowner has a vested interest to maintain and de-silt the pond, thus reducing the need for NGO intervention in the longer run. Experience in India seems to support this where the farmer providing the land for the johad (pond) would be the prime beneficiary, of the recharged water on adjacent land, but where the community also benefited.
Pond depth should be deep enough to prevent excessive algae and water plant growth, but not too deep so that anaerobic conditions are avoided – this means a depth of between 1-4 metres.
