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Hydrogeology and Water Resources of the Los Osos Valley Ground-Water Basin, San Luis Obispo County, California

U.S. Geological Survey (USGS) | July 18th, 1988

Summary

The Los Osos Valley ground-water basin is located on the coast of central California. It extends about 3 miles inland and an unknown distance offshore, where it is in h

The Los Osos Valley ground-water 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. Ground-water 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 head-dependent 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 ground-water basin, and ground-water 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 ground-water system. Results indicated that if wastewater is centrally treated and recharged to the ground-water 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.

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Hydrogeology and Water Supply of Salinas Valley

Monterey County Water Resources Agency (MCWRA) | June 15th, 1995

Summary

The Monterey County Water Resources Agency (MCWRA) convened a panel of 10 geologists, hydrogeologists, and engineers familiar with Salinas Valley ground water basin to at

The Monterey County Water Resources Agency (MCWRA) convened a panel of 10 geologists, hydrogeologists, and engineers familiar with Salinas Valley ground water basin to attempt to reach agreement on the basic physical characteristics of the basin, and the surface and ground water flow within the basin. Agreement on the completeness and accuracy of existing data and previous hydrogeological studies was seen as an important first step in identifying and implementing a technically sound solution acceptable to the public that would stop seawater intrusion that began some 60 years ago. Mike Armstrong, General Manager of MCWRA, instructed the panel to review and, if possible, reach consensus on the hydrogeological characteristics of the basin, define clearly the water resources problems in the basin, and determine surface water and ground water flow within the basin. We were not requested to discuss specific local projects or political and institutional aspects of the problems. The panel met in a closed-door session in Monterey on May 24 and 25, 1995. The session was closed to the public and the press to enable the panelists to discuss and explore ideas and opinions freely without worrying about statements, questions, and hypotheses being repeated out of context. Members of the panel believe the process worked very well. This report presents our findings, conclusions, and recommendations. We were able to achieve more than our original scope of work. There was remarkable unanimity of opinion on our understanding of the physical characteristics of the basin, the hydrologic system, the interaction between surface water and ground water, and definition of the specific ground water problems in the basin. In summary, the facts we agreed upon point so compellingly toward an already identified regional solution to the Valley's ground water resources problems that the panel has included a potential solution. We have included a strong recommendation in this White Paper for implementing that regional solution.

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Hydrologic framework of the Santa Clara Valley, California

Geological Society of America (GSA) | June 1st, 2015

Summary

The hydrologic framework of the Santa Clara Valley in northern California was redefined on the basis of new data and a new hydrologic model. The regional groundwater flow

The hydrologic framework of the Santa Clara Valley in northern California was redefined on the basis of new data and a new hydrologic model. The regional groundwater flow systems can be subdivided into upper-aquifer and lower-aquifer systems that form a convergent flow system within a basin bounded by mountains and hills on three sides and discharge to pumping wells and the southern San Francisco Bay. Faults also control the flow of groundwater within the Santa Clara Valley and subdivide the aquifer system into three subregions. After decades of development and groundwater depletion that resulted in substantial land subsidence, Santa Clara Valley Water District (SCVWD) and the local water purveyors have refilled the basin through conservation and importation of water for direct use and artificial recharge. The natural flow system has been altered by extensive development with flow paths toward major well fields. Climate has not only affected the cycles of sedimentation during the glacial periods over the past million years, but interannual to interdecadal climate cycles also have affected the supply and demand components of the natural and anthropogenic inflows and outflows of water in the valley. Streamflow has been affected by development of the aquifer system and regulated flow from reservoirs, as well as conjunctive use of groundwater and surface water. Interaquifer flow through water- supply wells screened across multiple aquifers is an important component to the flow of groundwater and recapture of artificial recharge in the Santa Clara Valley. Wellbore flow and depth-dependent chemical and isotopic data indicate that flow into wells from multiple aquifers, as well as capture of artificial recharge by pumping of water- supply wells, predominantly is occurring in the upper 500 ft (152 m) of the aquifer system. Artificial recharge represents about one-half of the inflow of water into the valley for the period 1970–1999. Most subsidence is occurring below 250 ft (76 m), and most pumpage occurs within the upper-aquifer system between 300 and 650 ft (between 91 and 198 m) below land surface. Overall, the natural quality of most groundwater in the Santa Clara Valley is good. Isotopic data indicate that artificial recharge is occurring throughout the shallower parts of the upper-aquifer system and that recent recharge (less than 50 yr old) occurs throughout most of the basin in the upper-aquifer system, but many of the wells in the center of the basin with deeper well screens do not contain tritium and recent recharge. Age dates indicate that the groundwater in the upper-aquifer system generally is less than 2000 yr old, and groundwater in the lower-aquifer system generally ranges from 16,700 to 39,900 yr old. Depth-dependent sampling indicates that wellbores are the main path for vertical flow between aquifer layers. Isotopic data indicate as much as 60% of water pumped from production wells originated as artificial recharge. Shallow aquifers not only contain more recent recharge but may be more susceptible to anthropogenic and natural contamination, as evidenced by trace occurrences of iron, nitrate, and volatile organic compounds (VOCs) in selected water-supply wells. Water-resource management issues are centered on sustaining a reliable and good-quality source of water to the residents and industries of the valley. While the basin has been refilled, increased demand owing to growth and droughts could result in renewed storage depletion and the related potential adverse effects of land subsidence and seawater intrusion. The new hydrologic model demonstrates the importance of the aquifer layering, faults, and stream channels in relation to groundwater flow and infiltration of recharge. This model provides a means to analyze water resource issues because it separates the supply and demand components of the inflows and outflows.

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Increasing threat of coastal groundwater hazards from sea-level rise in California

Nature Portfolio (Springer Nature) | August 17th, 2020

Summary

Projected sea-level rise will raise coastal water tables, resulting in groundwater hazards that threaten shallow infrastructure and coastal ecosystem resilience. Here we

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Inorganic, isotopic, and organic composition of high-chloride water from wells in a coastal southern California aquifer

Applied Geochemistry (Elsevier) | August 1st, 2005

Summary

Chloride concentrations were as high as 230 mg/L in water from the surface discharge of long-screened production wells in Pleasant Valley, Calif., about 100 km NW of Lo

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Integrated Coastal Groundwater Monitoring Program and Plan

Monterey County Water Resources Agency (MCWRA) | May 19th, 2019

Summary

California American Water Company (CalAm) is working to increase sustainability of their water supply portfolio to meet the long-term needs of customers in their

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Integrated Hydrologic Model of Pajaro Valley, Santa Cruz and Monterey Counties, California

U.S. Geological Survey (USGS) | September 5th, 2014

Summary

Increasing population, agricultural development (including shifts to more water-intensive crops), and climate variability are placing increasingly larger demands on avail

Increasing population, agricultural development (including shifts to more water-intensive crops), and climate variability are placing increasingly larger demands on available groundwater resources in the Pajaro Valley, one of the most productive agricultural regions in the world. This study provided a refined conceptual model, geohydrologic framework, and integrated hydrologic model of the Pajaro Valley. The goal of this study was to produce a model capable of being accurate at scales relevant to water management decisions that are being considered in the revision and updates to the Basin Management Plan (BMP). The Pajaro Valley Hydrologic Model (PVHM) was designed to reproduce the most important natural and human components of the hydrologic system and related climatic factors, permitting an accurate assessment of groundwater conditions and processes that can inform the new BMP and help to improve planning for long-term sustainability of water resources. Model development included a revision of the conceptual model of the flow system, reevaluation of the previous model transformed into MODFLOW, implementation of the new geohydrologic model and conceptual model, and calibration of the transient hydrologic model. The conceptual model identified inflows and outflows that include the movement and use of water from natural and human components. The groundwater flow system is characterized as a layered geologic sedimentary system through which downward flow is driven by the combined effects of the application of irrigation water at the land surface and the pumping of groundwater from deeper in the system. Overall, groundwater meets most of the agricultural demand in the initial part of the growing season, augmented by precipitation during wet winter and spring seasons. In addition, the amount of groundwater used for irrigation varies from year to year in response to climate variation and can increase dramatically in dry years. Data on agricultural pumpage is a major component of simulated outflow that was previously unavailable; therefore, a coupled farm-process model was used to estimate historical pumpage for Pajaro Valley by subregion (water-balance subregions) as well as the delivery of surface water to and from the Harkins Slough Aquifer-Storage-and-Recovery System (HS-ASR) and related Coastal Distribution System (CDS) since 2002. The new, integrated hydrologic model includes new water-balance subregions; delineation of natural, municipal, and agricultural land use; streamflow networks; regions of tile drains; and, the groundwater flow system. The redefinition of the geohydrologic framework and incorporation into the simulation revealed the importance of the confining units at the base of the alluvial deposits and between the upper and lower Aromas Sand for regional groundwater flow.

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Interpretation of Hydrostratigraphy and Water Quality from AEM Data Collected in the Northern Salinas Valley, CA

Marina Coast Water District | March 15th, 2018

Summary

Groundwater management, in the Monterey area of California, requires an accurate understanding of the hydrostratigraphy of the coastal aquifer system, as well as an unde

Groundwater management, in the Monterey area of California, requires an accurate understanding of the hydrostratigraphy of the coastal aquifer system, as well as an understanding of the groundwater quality variability within that system. The hydraulic properties of, and connectivity between, the individual units in the aquifer system determine potential routes for the movement of groundwater within and between aquifers. An understanding of the distribution of water quality within the aquifer system and an understanding of the hydrostratigraphy of the system are needed to evaluate the current state of groundwater resources, and to assess the potential impact of any proposed activity on the groundwater resources in the area. The focus of this study is the Northern Salinas Valley. Figure 1 shows the specific region of interest outlined in pink. The orange contour shows the extent of saltwater intrusion into the uppermost aquifers in this area, as determined from well data by the Monterey County Water Resources Agency as the regions where groundwater contains concentrations of chloride greater than 500 mg/L. But there is some question as to whether this contour accurately captures the extent of saltwater intrusion into these aquifers. The detection of water with anomalously low concentrations of total dissolved solids in five newly constructed monitoring well clusters suggests the presence of isolated lenses of fresher groundwater in the Dune Sand Aquifer and Perched Dune Sand Aquifer (outlined in light blue) and in the 180-Foot Aquifer (outlined in dark blue) (Hopkins Groundwater Consultants, 2016). The locations of seven wells from the Monterey Peninsula Water Supply Project (MPWSP) are also shown in Figure 1 as red diamonds, and wells owned by the Marina Coast Water District (MCWD) are shown as green circles. The two groundwater subbasins in the region of interest—the Monterey Subbasin and the 180/400 Aquifer Subbasin—are outlined in purple and blue, respectively. The objective of this study was to use the geophysical method, airborne electromagnetics (AEM), to evaluate the current understanding of the hydrostratigraphy in this area and to interpret the distribution of groundwater quality indicated by available well data. To evaluate the understanding of the hydrostratigraphy, a hydrostratigraphic model of the region of interest was first built using existing data, and was then updated with the information supplied by the AEM data. This approach was followed for two separate initial models. The first initial model was based on the North Marina Groundwater Model (NMGWM), developed by Geoscience Support Services as part of the MPWSP. The second initial model was built in-house using lithology data and hydrostratigraphic interpretations from a variety of previously published reports. Our interpretation of the distribution of groundwater quality in the region is based on statistical relationships built between water quality samples and borehole geophysical data collected in the wells shown in Figure 1.

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Mapping saltwater intrusion with an airborne electromagnetic method in the offshore coastal environment, Monterey Bay, California

Journal of Hydrology (Elsevier) | April 1st, 2019

Summary

Study Region: The northern coast of the Monterey Bay, California, USA, extending from the coastline to 3.5 km offshore. Study Focus: Three-hundred and twenty line-kil

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Midcoast Groundwater Study Phase III, San Mateo County, California

San Mateo County | June 4th, 2010

Summary

San Mateo County ‘Midcoast’ extends from northern Half Moon Bay to Devils Slide along Highway 1. It encompasses the communities of Montara, Moss Beach, Seal Cove, Pri

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Hydrogeology and Water Resources of the Los Osos Valley Ground-Water Basin, San Luis Obispo County, California

Hydrogeology and Water Supply of Salinas Valley

Hydrologic framework of the Santa Clara Valley, California

Increasing threat of coastal groundwater hazards from sea-level rise in California

Inorganic, isotopic, and organic composition of high-chloride water from wells in a coastal southern California aquifer

Integrated Coastal Groundwater Monitoring Program and Plan

Integrated Hydrologic Model of Pajaro Valley, Santa Cruz and Monterey Counties, California

Interpretation of Hydrostratigraphy and Water Quality from AEM Data Collected in the Northern Salinas Valley, CA

Mapping saltwater intrusion with an airborne electromagnetic method in the offshore coastal environment, Monterey Bay, California

Midcoast Groundwater Study Phase III, San Mateo County, California

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