Vallecito Valley is located in the arid region of eastern San Diego County, California, where hydrologic studies are virtually nonexistent. The ground water supply in this 13 sq mi (32 sq km) valley is a concern of the 30 residents, who rely on the aquifer as their water resource. This study has determined that present pumping conditions of 2 acre-ft/yr (2,500 cu m/yr) will not overdraft the aquifer when compared to estimated annual recharge of 2,400 acre-ft/yr (2,900,000 cu m/yr). The water table decline in the past can be associated with irrigation pumping and abnormally dry seasons. Underlying the alluvial deposits are low yield Cretateous plutonic and late Mesozoic metasedimentary rocks. Alluvial material reaches over 1,000 ft (330 m) in thickness with aquifer tests showing no significant variation in hydraulic conductivity throughout the valley. Ground water movement is west to east and is retarded across the central part of the valley by the transecting south branch of the Elsinore fault. Ponding on the southwest side of the fault has brought the water table near land surface, which allows phreatophyte growth and mineral precipitation. This has created a weathering resistant fault line scarp. Ground water nears land surface at the eastern end of the valley as a result of shallowing bedrock. The soil tension gradient in the unsaturated zone is in the upward direction throughout the year, with a minimal effect from precipitation. Relatively high soil moisture tensions reflect an environment where soil moisture primarily moves in the vapor phase. Moisture movement is uncertain in other areas of the valley, such as Vallecito creek, where ephemeral surface runoff from Mason Valley enters the valley. Significant recharge most likely occurs in the upper reaches of Potrero and Storm Canyons. Water chemistry is influenced by high versus low TDS ground water entering the aquifer via Campbell Grade and Storm Canyon, respectively. Differences in these water types include a higher calcium to sodium ratio from the Storm Canyon area. Ultimately, both recharge sources began in the intermountain region of the Laguna Mountains. However, ground water from the Campbell Grade underflow is of greater age than Storm Canyon recharge, having travelled through Mason Valley aquifer before entering the Vallecito basin. Hydrologic data indicates that there is currently no problem concerning depletion of the local aquifer due to residential pumping. The problem may instead by water quality degradation of the alluvial aquifer. Historical data from the residential area records a gradual increase in the TDS over the past 3 decades, which may be associated with localized residential pumping.