The Upper Santa Ana Watershed study unit covers approximately 1,000 square miles. Within this study unit, the San Jacinto and Elsinore groundwater basins each constituted a study area, and the nine subbasins of the Upper Santa Ana Valley groundwater basin were grouped into four additional study areas based on geography: Bunker Hill/Cajon/Rialto- Colton; Cucamonga/Chino; Riverside-Arlington/Temescal; and Yucaipa/San Timoteo (Kent and Belitz, 2009).

The study unit has hot, dry summers and cool, moist winters. Average annual rainfall ranges from 10 to 24 inches in the valleys and from 24 to 48 inches in the mountains within and surrounding the study unit.

The Santa Ana River, with its headwaters in the San Bernardino Mountains, is the longest river in Southern California and flows southwestward through the Upper Santa Ana Valley. Streamflow in the Santa Ana River varies seasonally; the highest flows are generally in the winter and spring. In the dry season, most streamflow in the Santa Ana River consists of tertiary treated wastewater (Belitz and others, 2004).

Most aquifers of the study unit are unconfined, with some confined groundwater in the Bunker Hill and Chino Subbasins and the eastern part of the San Jacinto Basin. Groundwater flow in most of the study unit is affected by numerous faults cutting the alluvial sediments. The primary aquifers of the Upper Santa Ana Watershed study unit are defined as those parts of the aquifers underlying the study unit and corresponding to the perforated intervals of the wells listed in the California Department of Public Health (CDPH) database. Public-supply wells typically are drilled to depths of 200 to 650 feet, consist of solid casing from the land surface to depths of about 175 to 500 feet, and then are perforated below the solid casing. Water quality in the primary aquifers may differ from that in the shallower and deeper parts of the aquifer system.

Land use in the study unit is approximately 44 percent (%) natural, 21% agricultural, and 35% urban. Most of the land use adjacent to the study unit is natural and consists of steep mountains or hills generally covered by forest or chaparral.

Recharge to the groundwater system occurs through direct infiltration, return irrigation flow, and stream-channel infiltration from the Santa Ana and San Jacinto Rivers and their tributaries, and from numerous engineered recharge facilities operating in the study unit. The primary sources of discharge are water pumped for municipal supply, evaporation from areas with a shallow depth to water, and discharge to streams.