The Los Osos Valley groundwater basin is located on the coast of central California. It extends about 3 miles inland and an unknown distance offshore, where it is in hydraulic connection with the Pacific Ocean. Its maximum onshore depth is about 1,000 feet. Groundwater flow in the basin was investigated using existing data, geologic mapping, drilling, and measurements of potentiometric head, water quality, and streamflow. The basin is underlain largely by relatively impermeable rocks of the Franciscan Complex. Basin fill consists of unconsolidated, complexly layered sediments of Miocene age or younger. Individual beds are generally thin and discontinuous. A three-dimensional digital model of the basin was developed and calibrated to match potentiometric heads and headdependent flows during water years 1970-77 and 1986. The model simulated poorly the steep vertical potentiometric heads, and it could not simulate perched water tables. In other respects, model results were good and not highly sensitive to likely errors in input data. Infiltrated rainfall contributes about 80 percent of annual recharge to the groundwater basin, and groundwater inflow accounts for almost all the remainder. Net recharge from Los Osos Creek is small. In water year 1986, outflow from the basin consisted of net municipal pumpage (24 percent), net agricultural pumpage (25 percent), net outflow across the ocean boundary (26 percent), perched-water runoff (16 percent), and phreatophyte transpiration (9 percent). During water years 1970-77 net municipal pumpage and net agricultural pumpage accounted for only 10 and 22 percent of total outflow, respectively. The population of Los Osos Valley is expected to increase by about 20,000 people by the year 2010, and municipal water use is expected to increase by about 1,700 acre-feet per year. The model was used to simulate the effects of various future water-supply and wastewater-disposal alternatives on the groundwater system. Results indicated that if wastewater is centrally treated and recharged to the groundwater basin, the entire projected municipal water demand can be met with locally pumped ground water without inducing seawater intrusion, even during droughts lasting 1 to 3 years. If wastewater is exported from the basin, however, large amounts of seawater intrusion are likely to occur even if nearly half of the municipal water demand is met with imported water. Consulting geologist registered with State of California.
Introduction 1 Purpose and scope 2 Description of study area 2 Previous studies 4 Acknowledgments 4 Geohydrology 4 Geology 4 Geometry of groundwater basins 5 Hydrologic characteristics 6 Water levels 6 General trends 6 Seasonal variation 7 Comparison of shallow and deep groundwater levels Aquifer properties 9 Water quality 11 Sample collection and analysis 11 Factors affecting water quality 11 Variation of water-quality characteristics 12 General conditions 12 Comparison of shallow and deep ground water 14 Characterization of nitrate sources 15 Fertilizers 15 Sewage 16 Distribution of nitrate and tritium near Lake Merced 18 Seawater contamination 18 Estimation of groundwater recharge 21 Climate 22 Temperature 22 Rainfall 23 Land and water use 24 Land use 24 Water use 25 Sources of recharge 26 Infiltration of rainfall 26 Infiltration of irrigation water 29 Leakage from water and sewer pipes 29 Recharge interception 30 Rainfall runoff 30 Evapotranspiration 30 Calibration and sensitivity of hydrologic routing model 32 Sewer flow 33 Monthly calibration 35 Simulation of storms 36 Sensitivity analysis and limitations of the model 37 Estimated groundwater recharge 39 Summary and conclusions 40 References cited 41 Appendix 1. Description of variables used in hydrologic routing model 65 Appendix 2. Equations used in hydrologic routing model 67 Contents
Hydrographs showing simulated effects of a single winter without recharge on groundwater levels for 1988-89 for the Santa Rosa and the San Simeon groundwater basins, San Luis Obispo County, assuming normal pumping rates, and effects of selected decreases in agricultural pumping for the San Simeon
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.