The purpose of Water Factory 21 is to produce a water supply for the Talbert Barrier to prevent seawater intrusion. Final plant effluent, meeting all California Department of Health Services primary and secondary drinking water standards, is pumped into the groundwater basin via a series of 23 multi-point injection wells with a total of 81 individual injection points. Injection wells are placed approximately every 600 feet along Ellis Avenue from the Santa Ana River to the bluffs at Beach Boulevard. This injected water forms a freshwater mound between the ocean and the groundwater basin, providing a hydraulic barrier to seawater intrusion and permitting the groundwater basin to be safely drawn below sea level.

Once underground, some of the injected water flows toward the ocean, forming the seawater barrier. The majority of the water, however, flows into the groundwater basin to augment Orange County's domestic groundwater supply.