Subsurface Groundwater Recharge

Artificial groundwater recharge is the planned infiltration of effluents from sanitation systems (e.g. waste stabilisation ponds, surface, horizontal flow or vertical flow constructed wetlands), stormwater or surface runoff into the aquifer in order to increase the natural replenishment of groundwater resources. Artificial subsurface groundwater recharge refers to recharge techniques (generally recharging wells) that release the water below the ground (as opposed to surface groundwater recharge where water percolates through the soil before entering the groundwater aquifer). Groundwater recharge is increasing in popularity as groundwater resources are being depleted and as saltwater intrusion is becoming a greater threat to coastal communities (TILLEY et al. 2008).

Although the primary objective of this technology is to preserve or restock groundwater resources, artificial recharge has been used for many other beneficial purposes. These include conservation or disposal of floodwaters, control of saltwater intrusion, storage of water to reduce pumping and piping costs as well as temporary regulation of groundwater abstraction. Furthermore water quality can be improved through the removal of suspended solids via soil filtration (see also surface groundwater recharge) or through the dilution with naturally occurring groundwater (ASANO 1985). Moreover artificial recharge is used in wastewater disposal, waste treatment, secondary oil recovery, prevention of land subsidence, storage of freshwater within saline aquifers, crop development and streamflow augmentation (OAKSFOARD 1985; UNEP 1998).

Subsurface groundwater recharge, as opposed to surface water recharge does not provide any additional treatment through soil filtration. Therefore polluted wastewater should be treated before subsurface groundwater recharge. The effective and efficient operation of an artificial groundwater recharge system depends on the method of recharge, the characteristics of the aquifer, the residence time (hydrological study), the amount of blending with other waters and the history of the system. Careful analysis of these factors should precede any recharge project (TILLEY et al. 2008). For subsurface recharge, particularly the blending conditions must be studied carefully.

To avoid chemical reactions that would reduce aquifer porosity and recharge capacity, the recharge water must be chemically compatible with the naturally occurring groundwater and the aquifer material that it flows through. During operation, the quality (turbidity from sediments but also algae and bacteria, temperature, suspended solids, BOD, nitrogen and phosphorus, other chemicals, etc.) and quantity of the water to be recharged must be carefully controlled and monitored.

A variety of methods have been developed and applied to artificially recharge groundwater reservoirs in various parts of the world. Direct subsurface recharge methods can reach aquifers located at greater depth and require less land than direct surface recharge methods, but are more expensive to construct and maintain. Recharge wells, commonly called injection wells, are generally used to replenish groundwater resources when aquifers are located at greater depth and confined by materials of low permeability. All subsurface methods are prone to clogging because of suspended solids, biological activity or chemical impurities.

Recharge wells have been used to dispose of treated industrial wastewater, to add freshwater to coastal aquifers experiencing saltwater intrusion, and to force water under pressure into permeable bedrock aquifers to arrest land subsidence resulting from extensive withdrawals of groundwater, although with variable success (CGWB 1994). In many places, including the United States, Japan and Thailand, the use of injection wells is still considered experimental (UNEP 1998).

To ensure the effective and efficient operation of an artificial recharge system, a thorough and detailed hydro geological study must be conducted before selecting the site and recharge method. In particular, the following basic factors should be considered (UNEP 1998):

• Location of geologic and hydraulic boundaries
• Transmissivity of the overlying material
• Depth of the aquifer and lithology
• Storage capacity
• Porosity
• Hydraulic conductivity
• Natural in- and outflow of water to/from the aquifer
• Availability of land
• Surrounding land use and topography
• Quality and quantity of water to be recharged
• Economic and legal aspects concerning the recharge
• Degree of public acceptance

Recharge or injection wells are subsurface groundwater recharge techniques used to directly discharge water into deep water-bearing zones. Recharge wells can be cased with the material covering the aquifer. If this material is unconsolidated, a screen can be placed in the well in the zone of injection. Recharge wells are suitable only in areas where a thick impervious layer exists between the surface of the soil and the aquifer that is to be replenished. They are also advantageous in areas where land is scarce (as surface groundwater recharge requires large areas for infiltration). A relatively high rate of recharge can be attained with this method (O'HARE et al. 1986).