Protecting the areas in a groundwater basin where water infiltrates most easily preserves the critical function of recharging the aquifer, as well as protecting the aquifer from potential future contamination
Groundwater recharge or infiltration is the process whereby water moves downward from surface water to groundwater; it is the primary method through which water enters an aquifer. Recharging groundwater is an essential component of conjunctive use projects and critical for sustainable groundwater management.
Alluvial groundwater basins are complex systems with sediments and soils that vary across the area of the basin; in some areas, prevailing geologic conditions allow for greater infiltration of water into the groundwater basin than in others. Protecting those areas where conditions for groundwater infiltration are the most optimal ensures that those areas continue to be functional and available for use, rather than being covered by urban infrastructure.
HOW RECHARGE OCCURS
Groundwater recharge can occur through instream or offstream infiltration, or through injection wells. Infiltration utilizes areas in the groundwater basin that are geologically suitable for recharge; these can either be instream (meaning within the natural channels of a waterway) or off-stream in spreading basins located over suitable areas in the basin. Injection wells can be used in areas where purchasing large tracts of land for spreading basins is cost prohibitive.
However, each method of recharge has its pros and cons. Regular maintenance is needed for spreading basins; the basins can get clogged by suspended sediments in the recharge water, reducing the rate of recharge considerably. Injection wells can get clogged unless the water is treated and turbidity is minimal.
Besides infiltration basins or injection wells, there are other methods of recharging aquifers. Flood detention facilities provide flood control benefits, and can recharge aquifers as it is held behind dams and reduced at a slower rate. Other flood detention basins serve as grassy parks most of the year, but can fill with runoff during storms. Large spaces in the urban landscape, such as playgrounds, school grounds, and parking lots can be retrofitted to collect, treat, and funnel stormwater to dry wells or other small scale infiltration facilities. Low-impact development and other best management practices can be used to enhance groundwater recharge.
AVOIDING CONTAMINATION OF THE AQUIFER
The natural characteristics that make recharge areas effective in recharging aquifers also make them a concern for groundwater quality because if contamination occurs in these areas, substances can easily enter the ground and migrate to the aquifer. Thus by protecting recharge areas and preventing contamination, expensive treatment is avoided that would have been needed in the future to make the water suitable for use.
Many potentially contaminating activities such as farms, dairies, and industrial complexes have been allowed in recharge areas, resulting in contamination, and in some places, these practices continue today. Remediation of contaminated groundwater can take decades or longer; cost millions, billions, or potentially even more, as well as increase greenhouse gas emissions from remediation systems. In virtually all cases, contamination can never be fully removed from the aquifer and will instead require costly wellhead treatment before it is suitable for use.
Urban stormwater runoff that is used for groundwater could potentially affect groundwater quality. To avoid this, best management practices are recommended, such as construction of low-flow basins for runoff from areas that could be a source of chemical contamination, pretreatment of stormwater runoff, water quality monitoring, and periodic evaluation of project data. (Click here for more on stormwater capture and management.)
STATEWIDE MAP OF HYDROLOGICALLY VULNERABLE AREAS
In response to Executive Order D-5-99, in 2000 the State Water Resources Control Board staff compiled a map showing locations statewide where published information suggests conditions may be more vulnerable to groundwater contamination due to activities taking place at the ground surface. The map also identifies areas where geologic conditions allow recharge to underlying water supply aquifers at rates substantially higher than in other areas of the groundwater basin.
Groundwater recharge is a primary component of sustainable groundwater management and is critical to providing a sustainable, reliable, and high quality groundwater supply. Protecting recharge areas ensures that groundwater aquifers will continue to be able to be replenished; protecting these areas from high-risk uses helps to preserve groundwater quality and avoids costly wellhead treatment or other remediation costs in the future. In some cases, diverting floodwaters to recharge facilities may have flood control benefits.
Costs can be incurred to purchase or lease the land that will be used as a recharge area, and depending on land prices, these costs could be significant or even prohibitive. The land used as a recharge area oftentimes cannot be used for other purposes; there could be impacts to local tax revenues as a result. However, it should be noted that protection of recharge areas may also help prevent future costs by reducing the need for expensive groundwater treatment as preventing contamination today will reduce future costs of drinking water.
Map of Hydrologically Vulnerable Areas: In 2000, the State Water Resources Control Board, Division of Water Quality, created a California map identifying soil or rock conditions that may be more vulnerable to groundwater contamination. Click here for the map and supporting documentation.