Hydrogeology of the Tierra Rejada Groundwater Basin, Ventura County, California

The Tierra Rejada Hydrologic Basin is located in an unincorporated area of Ventura County, California. The basin is approximately 1,900 acres in area and is contained within a 4,500 acre watershed. The basin is relatively small in size and the aquifer is unconfined. Nevertheless, it is an important water source for both agricultural irrigation and domestic water supply. Currently, groundwater extraction from the basin is un-managed. Prior to the completion of this study, there were no detailed estimates of the aquifer characteristics or storage. The purpose of this study was to evaluate existing data and obtain additional data concerning the basin climate, geology, aquifer depth and boundaries, historic water levels, water quality, aquifer characteristics, and basin water budget. Groundwater is stored primarily in sandstones and conglomerates with a matrix predominantly composed of volcanic rock of the Topanga Formation, and in fractured basalts and basalt breccias of the Conejo Volcanics. The aquifer is contained in an east-west trending syncline that parallels the Simi fault and plunges towards the west. Based on the analysis of geologic and water well data, the aquifer thickness in the central and western portion of the basin is more than 600 feet. The saturated zone iospach map was used to estimate the volume of the aquifer and the volume of groundwater available in storage. A groundwater pumping test performed in the western portion of the basin near the outlet of the Arroyo Santa Rosa using the Camrosa Water District domestic supply well 2N/19W-15N3 that is screened within the upper section of the Conejo Volcanics. 2N/19W-15N3 that is screened within the upper section of the Conejo Volcanics. The test results indicate that the transmissivity of the aquifer in this area is approximately 3.0 x 103 feet squared per day (ft2/d) and the specific yield is 0.11. Although the specific yield may vary spatially, by assuming a specific yield of 0.11 for the entire basin, the total available groundwater in storage is about 80,000 acre feet (ac-ft). The average yearly inflow and outflow to the basin are both estimated to be approximately 6,200 ac-ft per year (ac-ft/yr). The major source of inflow is precipitation (5,250 ac-ft/yr). Other sources of inflow include stream flow (50 ac-ft/ yr), subsurface inflow (480 ac-ft/yr), treated waste water (280 ac-ft/yr), and imported water (150 ac-ft/yr). The majority of the outflow goes to evapotranspiration or consumptive use (5,075 ac-ft/yr). An estimated 770 ac-ft/yr of water flows from the Tierra Rejada Basin to the Santa Rosa Basin both as surface outflow (540 ac-ft/yr) and subsurface outflow (225 ac-ft/yr). An additional 370 ac-ft/yr of water is exported for domestic supply. Between the 1940s and 1970s, the water levels in the basin decreased by more than 150 feet in depth. Between 1970s and 1990s, water levels steadily increased. In 1997, the water table elevations are equivalent to the levels reported in 1950. Water quality in the basin is generally good. The primary water type is magnesium bicarbonate with low levels of chloride, sodium, and nitrate. Total dissolved solids (TDS) in water wells have increased over time from below 500 milligrams per liter (mg/L) in the 1950s to over 800 mg/L in the 1990s. In recent years, groundwater use in the basin has increased both from increased farming activities and water export for domestic supply. Historic fluctuation in groundwater levels have shown that the water supply is limited. Drought conditions or increased water well pumping would likely lead to significant decreases in groundwater levels. In order to maintain current water levels and ensure that the quality of the water is maintained, and before any increased long term groundwater pumping is conducted, it is suggested that a groundwater basin management plan be considered by the major water users.