Surface Groundwater Recharge

Artificial groundwater recharge is the planned infiltration of effluents from sanitation systems (e.g. wastewater stabilisation ponds, surface, horizontal flow or vertical flow constructed wetlands), storm water or surface runoff into the aquifer in order to increase the natural replenishment of groundwater resources. Artificial surface groundwater recharge refers to different groundwater recharge techniques that release effluent from above the ground into the groundwater aquifer via soil percolation. Groundwater recharge is increasing in popularity as groundwater resources are being depleted and 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 increase groundwater resources, artificial recharge has been used for many other beneficial purposes. These include additional treatment (through soil filtration) and conservation or disposal of treated wastewater or 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 soil aquifer treatment) or through the dilution with naturally occurring groundwater (ASANO 1985). Moreover, artificial recharge is used in waste treatment, secondary oil recovery, prevention of land subsidence, storage of freshwater within saline aquifers, crop development and stream flow augmentation (OAKSFORD 1985; UNEP 1998).

A contaminated aquifer (e.g. by toxic chemicals) is next to impossible to recover. Therefore heavily polluted wastewater should be treated before surface groundwater recharge, even though the method provides some soil filtration treatment during percolation.

For surface groundwater recharge the assimilation capacity of the receiving soil body and the hydro geological conditions must be studied carefully. The following basic factors should be considered in particular (UNEP 1998):

• Location of geologic and hydraulic boundaries
• Depth of the aquifer and transmissivity of the overlying material
• Lithology
• Storage capacity
• Porosity
• Hydraulic conductivity and natural in- and outflow of water to/from the aquifer
• Availability of land, surrounding land use and topography
• Economic and legal aspects concerning the recharge
• Degree of public acceptance

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. Generally these methods are classified as surface and subsurface groundwater recharge.

Direct surface recharge techniques are among the simplest and most widely applied methods. Subsurface groundwater recharge conveys water directly into an aquifer without the filtration or oxidation that occurs when water percolates naturally through an unsaturated zone (UNEP 1998).

Direct Surface Groundwater Recharge

With direct groundwater recharge water moves from storage above ground to the aquifer via soil percolation. Most of the existing large-scale artificial recharge schemes in Western countries make use of this technique,which typically employs infiltration basins (spreading basins) to enhance the natural percolation of water into the ground. Field studies of spreading techniques have shown that, of the many factors governing the amount of water that will enter the aquifer, the area of recharge and length of time that water is in contact with soil are the most important (TODD 1980). In general direct surface recharge systems based on percolation have relatively low construction costs and are easy to operate and maintain.

Spreading Basins

This method involves surface spreading of water in basins that are excavated in the existing terrain. For effective artificial recharge, highly permeable soils are required and maintenance of the water layer above the soil surface is necessary. Recharge in spreading basins is most effective where there are no impending layers between the land surface and the aquifer and where clear water is available for recharge. In addition this method tolerates more turbid water than any well recharge methods does (O'HARE et al. 1986).

Recharge Pits and Shafts

Shafts can have circular, rectangular, or square cross sections and may be backfilled with porous material enhancing the percolation process and preventing the stagnation of the water (which can lead to insect breading). The shaft may end above the water table or reach below the water table and serve as hydraulic connector (O'HARE et al. 1986).

Ditches

Ditches are similar to spreading basins but they have a different shape. A ditch can be described as a long narrow trench, its bottom width being less than its depth. A ditch system can be designed to suit the topographic and geologic conditions that exist at a given site (O'HARE et al. 1986). Water fills up in the ditches and percolates naturally through the soil.

Other Recharge Techniques

Another method of artificially recharging groundwater is to use permeable pavements,which allows water surface runoff (e.g. storm water) to trickle underground, rather than allowing it to pool on the surface and evaporate (SMITH 2003).

Flood irrigation of surface water applied to surrounding farmlands is also a vital source of groundwater recharge. As agricultural land is converted to urban use, identifying sites for additional groundwater recharge is essential to keep water supplies balanced (water consumption of urban areas are generally lower, but impact of deep percolation from flood irrigation is lost; FRESNO 2005).