These two options are grouped together because the technology for achieving them is the same. Also known as ``aquifer recharge,'' the two main technologies involved are reservoir infiltration and groundwater injection. Both methods of aquifer recharge aim to replace groundwater supplies with treated wastewater. In a region that experiences groundwater depletion through pumping for irrigation or municipal use, this option is especially desirable.
Prior to recharge, wastewater is treated to a higher degree than is required for surface water discharge, but not to finished drinking water standards. The water is then piped to large basins where it is allowed to infiltrate into the ground. In regions with aquifers that have barriers to surface recharge, or where space for reservoirs is too expensive, the water is pumped down into the aquifer through injection wells. While they are more energetically expensive, injection wells take up less space and can sometimes recharge greater quantities of water than surface basins (WRA, 2001).
Aquifer recharge protects against saltwater intrusion in coastal regions, and helps to prevent subsidence wherever it is employed. There is typically a residence time of 5 years or more between injection and re-withdrawal for potable or non-potable use, during which time natural physical and biological processes help clean the water of pathogens and chemicals (NRC, 1998).