Prior to the development of the Solano Project, groundwater was extensively used in Solano County both for municipal supplies and for agriculture. One of the main reasons for the development of the Solano Project was to rectify groundwater overdraft in some of the agricultural areas. Once the Solano Project started making agricultural water deliveries, groundwater levels rebounded.
The Cities of Rio Vista and Dixon are served exclusively with groundwater from basins underlying the cities. Vacaville gets approximately one third of its municipal water supply from groundwater underlying the city. Most of the growers within Solano Irrigation District (SID) use surface water supplied by SID, but SID has its own wells to supplement their surface water supply from the Solano Project. Maine Prairie Water District and Reclamation District No. 2068 provide surface water to their growers, and do not currently use groundwater underlying their districts. Growers outside of districts that provide surface water rely entirely on groundwater unless they have an individual right to a surface water supply. The amount of this groundwater use has not been accurately quantified.
Most rural residential landowners have individual shallow groundwater wells that serve their domestic needs. There are also some small rural residential water systems that distribute groundwater to their customers.
The largest groundwater basin underlies the northeastern part of Solano County. This groundwater basin starts from the foothills above Vacaville and goes to the Sacramento River. The groundwater basin goes from Putah Creek to the north to the boundaries of Fairfield to the south. There are two basic levels to the groundwater basin. The shallower aquifer provides agricultural water and local domestic supplies.The shallower aquifer is underlain by the Tehama Formation aquifer. This aquifer is quite deep (over 1,000 feet) under Vacaville, but surfaces in the English Hills area north and west of Vacaville. Vacaville's wells draw from the Tehama Formation for its groundwater supply.
Public agencies that overlie this groundwater basin have developed groundwater management plans as specified in AB 3030,the state law that authorizes local agencies to prepare groundwater management plans. SCWA prepares biannual reports on groundwater levels for the groundwater basin. groundwater level data comes from DWR and local public agencies utilize the groundwater basin. Groundwater level data comes from the California Department of Water Resources and local public agencies utilize the groundwater basin. These reports show no trend of overdrafting with current levels of groundwater use. Groundwater levels drop in dry years, but rebound in wet years.
There may be a potential to more aggressively utilize the groundwater basin. Areas that have a surface water supply that are underlain by a groundwater basin are good candidates for conjunctive use projects. A typical conjunctive use project includes the installation of groundwater wells that are used in drier years instead of surface water that can be sold or exchanged. In wet years, the groundwater basin is recharged and the use returns to surface water.
Rio Vista has done studies on its groundwater basin.
Groundwater basins outside of the Tehama Formation area and Rio Vista have not been studied much.