An aquifer is an underground layer of water-bearing permeable rock, rock fractures or unconsolidated materials such as sand, silt or gravel from which groundwater can be extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology. An aquitard is a bed of low permeability along an aquifer and aquiclude is a solid, impermeable area underlying or overlying an aquifer. If the impermeable area overlies the aquifer, pressure could cause it to become a confined aquifer.

Groundwater Basins and Storage
In Southern California, groundwater basins have about 3.1 million acre feet of available storage, or storage that can be pumped and used (Atwater, 2011).  Significant capture and storage of stormwater is dependent upon greater and more efficient use of groundwater basins for storage. Because of the relationship between surface water and groundwater recharge, in particular within the Los Angeles River watershed, it is useful to review and understand the local groundwater basins and their storage potential. 

The Water Replenishment District of Southern California (WRD) was formed in 1959 to protect and sustain the groundwater resources of the Central and West Coast basins. Prior to the formation of WRD in 1959, over-pumping caused many water wells to go dry and allowed sea water intrusion to contaminate coastal groundwater. WRD protects the Central and West Coast basins through a centralized management scheme where annual pumping allocations are closely monitored. Additionally, WRD leads regional groundwater replenishment, deterrence of sea water intrusion, and groundwater quality monitoring of contamination through assessments on water pumped from the WRD service area.

The San Fernando Basin, Central Basin, and West Coast Basin are key to the region’s efforts to capture and store stormwater for later use. The San Fernando Basin is generally considered unconfined, where water can readily infiltrate through the soil matrix down into the water table. The Central and West Coast Basins are generally considered confined, where recharge occurs through underflow from upgradient regions (such as the Forebay) or through mechanical forcing (injection). Please refer to the confined/unconfined image, groundwater basins map, and forebay map below. Descriptions of these basins, and the associated challenges to greater use of these basins, are presented herein as these basins will likely play a significant role in projects to capture and use stormwater regionally.

Other regionally relevant groundwater basins not discussed here include the Main San Gabriel, Hollywood, and Santa Monica groundwater basins.