The Las Flores basin is 1ocated in northwest San Diego County, California, on the Camp Pendleton Marine Corps base. The Las Plores basin is one of four groundwater basins which supply drinking water to Camp Pendleton. The purpose of this study was to develop an understanding of the hydrogeology of the basin, determine a water budget, and calculate a safe yield value in order to better manage the basins groundwater resources. Groundwater in the Las Flores basin is extracted from the San Mateo aquifer located in the coastal basin. Other hydrogeologic units within the Las Flores basin are not viable groundwater resources due to their inaccessibility or low yields. The San Mateo aquifer is overlain by unconsolidated alluvial deposits and underlain by the San Onofre Breccia. Groundwater in the San Mateo aquifer is unconfined except for a locally occurring confining condition located in the vicinity of Interstate 5. The direction of groundwater flow is southwest along the axis of Las Pulgas Canyon. Well log data and eleven electrical resistivity surveys conducted using the Schlumberger sounding technique were evaluated to estimate the thickness of the San Mateo aquifer. The minimum thickness of the San Mateo aquifer ranges from 33 to 1400 feet. Aquifer tests were conducted at five locations within the study area. The estimated transmissivity of the San Mateo aquifer ranges from 1.2 to 22.4 feet2/ minute, the specific yield is 0.12 and hydraulic conductivity ranges from 0.03 to 0.19 feet/minute based on the aquifer test data. Groundwater from this basin is an acceptable drinking water supply with TDS concentrations less than 900 mg/l and nitrate concentrations less than 7 mg/l. Degradation of groundwater quality due to recharging treated sewage effluent was examined. Flow velocity calculations indicate that nitrate from sewage effluent recharge has not reached wells in the coastal basin. However long-term nitrate degradation may occur if sewage effluent recharge is increased. Nitrate degradation may occur in the basin if sewage recharge exceeds 1430 acre-feet/year. A water budget was calculated using inflow components of precipitation infiltration, stream recharge infiltration, and sewage effluent infiltration and outflow components of domestic pumpage, stream baseflow, subsurface outflow and phreatophyte evapotranspiration. A long term safe yield value was calculated from the water budget using a predictive polynomial regression equation. The criteria used to determine the maximum safe yield was preventing salt water intrusion and long-term decline of the water table. The maximum safe yield of the Las Flores basin is an additional net pumpage of 100 acre-feet/year beyond the present net pumpage of 125 acre-feet/year. Net pumpage is the amount of water pumped from the basin that is not recharged by way of sewage effluent. The tota1 pumpage for the Las Flores basin can be approximately 1655 acre-feet/year as long as net pumpage does not exceed 225 acre-feet/year and 1430 acre-feet/year of treated sewage effluent is recharged to the basin.