Can Los Osos Valley Groundwater Basin Provide a Sustainable Water Supply?

California State University, Monterey Bay (CSUMB) | September 16th, 2010

The aquifers underlying the community of Los Osos in California are impaired by nitrate pollution from septic systems (shallow aquifers) and saltwater intrusion f

The aquifers underlying the community of Los Osos in California are impaired by nitrate pollution from septic systems (shallow aquifers) and saltwater intrusion from overdraft (lower aquifers). The development of a centralized waste water system, now in the final pre- construction stages, is proceeding concurrently with an adjudicated basin planning process involving local water purveyors in an effort to develop a management plan for a sustainable basin and water supply. Development of the wastewater system aimed at protecting the shallow aquifer has highlighted the need for a comprehensive groundwater basin management plan that focuses the County, the local water purveyors, the residents, and resource agencies on a common set of actions to stop seawater intrusion and protect the entire basin. The final basin management plan will be scrutinized and critiqued as a potential model for myriad other hydrologically independent communities that are, or will soon be, undergoing the similar quest for reliable water. Stakeholders in the Los Osos Valley Water Basin have the opportunity to lead coastal California on the difficult path toward sustainable water supply through creative local solutions that do not presume the long-term viability of imported water or desalination. While there are a number of detailed analyses and suggested actions in this report, the overarching conclusions/recommendations include: The saltwater intrusion models used in the draft EIR and technical memoranda (TMs) recently developed for the purveyor basin planning process attempt to balance saltwater intrusion against inputs and outputs to and from the aquifers. The net groundwater extraction amounts simulated in the models (corresponding to the purported safe yields) are considered aggressive (too much extraction) because they do not stabilize, or result in seaward migration, of the saltwater/freshwater interface (toward the northwest). In fact, the TM safe yield may allow the saltwater to move inland, especially if California climate becomes drier, as projected by State Government. Modeling results presented in our report indicate that if rainfall patterns persist as they have in the past, there is a reasonable chance that saltwater intrusion will progress farther, rather than remain stable, under anticipated pumping conditions. Further investigation is recommended to resolve uncertainties associated with the TM safe yields, including permeability of the regional (AT2) aquitard, how the AT2 affects lower aquifer recharge, and whether sufficient recharge will occur to prevent seawater intrusion in the upper aquifer. These uncertainties are under- appreciated in the EIR and TMs. We recommend the development of a “contingency plan” that allows flexible adaptive management to address unintended consequences that may occur following the implementation of the Los Osos Valley Wastewater Plan. Contingency plans identify mission-critical sections of the plan that have uncertainties, recommend monitoring to ascertain progress of the plan, and define remedial actions that will mitigate various unintended consequences, including divergence from, or failure of the plan. Several mission-critical arenas are discussed in the report. Rooftop rainwater harvesting and low impact development (LID) options could produce a substantial amount of water for local irrigation, reducing potable water use, while also augmenting groundwater recharge. Some LID alternatives, e.g., rain gardens and vegetated swales, can also provide attractive low-water using landscape features that may also help Los Osos residents adapt to the potential changes in groundwater levels and soil moisture content resulting from project implementation that may affect landscaping. Both on-site and community LID features will also reduce pollution of surface waters in the area. While the method used to calculate potential basin recharge from rainwater and LID features in this report is inconclusive, it suggests that the benefits of these measures may be considerable. Rainwater harvesting and LID are proven technologies that should be considered on a broad basis in the Los Osos Valley to help balance the basin and adapt to potential impacts from the project. Treatment wetlands can provide economical, efficient waste water treatment systems and/or the means to provide additional treatment, storage, and recharge of the groundwater basin. They also provide habitat and quality of life benefits (open space and passive recreational opportunities, such as hiking trails). Nitrate removal in wetlands has been studied in small scale experiments in Monterey County, has been implemented in large scale applications with positive success. Wetlands to treat surface water and wastewater have been used successfully in many communities around the world. We recommend an emphasis on agricultural exchange to maximize its benefits on seawater intrusion and help balance the basin. Tertiary-treated waste water is safe for irrigation and can reduce groundwater pumping, as well as energy and fertilizer use for farmers, with related costs savings. When well water from farms is exchanged for recycled water, potable water can be used to offset pumping causing seawater intrusion. While it is best to balance the hydrologic budget using resources within the basin, there is also an option to trade agricultural-grade treated water to regional farms in exchange for drinking grade water from wells outside the basin if ample water reserves are developed in the future. The draft Los Osos Valley sustainable water basin management plan, “Achieving a sustainable Los Osos Valley Water Basin: Framework for a 21st century basin management plan” (Wimer 2009) presents realistic, well-supported solutions for addressing the water needs of the Los Osos Community using sustainable, water saving and LID methods while preserving the basin for future use. The plan integrates methods reviewed in this report (rainwater harvesting, LID, and agriculture exchange), with urban reuse and intensive indoor and outdoor conservation to balance the basin and provide flows to sensitive ecosystems, with margins of safety that also address current and future uncertainties. We recommend that the plan is further developed, possibly with the help of water use efficiency expert, and considered for implementation. To answer the question posed by the title--yes, Los Osos can become a model of sustainable water use for coastal California if stakeholders implement reasonable conservation measures to complement an appropriate waste water treatment strategy and an adaptive management plan. The great number of uncertainties in the system makes any plan “experimental.” Given the uncertainties, due diligence will include erring on the side of caution (being conservative, not aggressive, in planned groundwater production), and devising a realistic contingency plan that includes adaptive management strategies. The Advanced Watershed Science and Policy (ENVS 660) graduate course at California State University Monterey Bay produced this report. It presents the results of extensive literature review, synthesis, and new analyses. The breadth of the report spans many of the key water supply issues facing the people of the Los Osos Valley. It provides information that can help this region move toward a sustainable water supply through innovative waste water management and sound conservation measures.

Case History No. 9.14. Santa Clara Valley, California, U.S.A.

United Nations Educational, Scientific and Cultural Organization (UNESCO) | June 30th, 1984

CASGEM Alternative Monitoring Plan Fact Sheet

California Department of Water Resources (DWR) | August 15th, 2023

Castac Lake Valley Groundwater Sustainability Plan

Castac Basin Groundwater Sustainability Agency | September 17th, 2020

The Castac Lake Valley Groundwater Basin (also referred to herein as “the Basin”), California Department of Water Resources (DWR) Basin No. 5-029, is located at the

The Castac Lake Valley Groundwater Basin (also referred to herein as “the Basin”), California Department of Water Resources (DWR) Basin No. 5-029, is located at the southern end of Kern County. The Basin is identified by DWR as being a very low priority basin, and as such, this GSP and associated SGMA compliance efforts have been conducted on a voluntary basis. Due to size, the file has been divided into two parts. Part 1, Part 2

Central Coast Groundwater: Seawater Intrusion and Other Issues

California Water Foundation | August 4th, 2014

The Central Coast area relies on groundwater for more than 80 percent of its water supplies, a greater percentage than any other region in the state. Over 40 percent of t

The Central Coast area relies on groundwater for more than 80 percent of its water supplies, a greater percentage than any other region in the state. Over 40 percent of the groundwater basins in the Central Coast region were ranked as either “high” or “medium” priority as part of the California Statewide Groundwater Elevation Monitoring (CASGEM) prioritization released in June 2014, indicating that they are among the most threated by present or future increases in groundwater demand. Central Coast groundwater supplies are seriously threatened by over-pumping that causes seawater intrusion, a challenge unique to coastal communities. Seawater intrusion results in saline groundwater that is unsuitable for irrigation or domestic use, directly threatening the Central Coast’s economy, where many high-value, salt-sensitive crops, such as strawberries, are grown. Groundwater basins in the Monterey and Salinas Valley area are in a state of long-term overdraft, resulting in falling groundwater levels, regional seawater intrusion, and reduced groundwater storage. Seawater intrusion reaches more than five miles inland in the Salinas Valley and three miles inland in the Pajaro Valley. In addition to salinity issues, several groundwater contaminants are present in these aquifers, including elevated levels of total dissolved solids, iron, manganese, nitrates, and other trace elements. The Paso Robles groundwater subbasin supplies water for 29 percent of the population and an estimated 40 percent of the agricultural production of San Luis Obispo County. Groundwater levels in the basin have been dropping rapidly in recent years, with many areas experiencing groundwater level declines of more than 70 feet. As a result, many wells have gone dry and groundwater users have been forced to deepen their wells or drill new wells. In July 2013, over 100 rural property owners complained to the San Luis Obispo County Board of Supervisors that their wells were running dry and in August 2013 the Board adopted the “Paso Robles Groundwater Basin Urgency Ordinance” as a temporary action to prevent new pumping from worsening overdraft conditions. Additionally, land surface subsidence has been noted in the Paso Robles area, and the recent rapid groundwater level declines may have triggered additional subsidence in this area. Although groundwater levels in the Oxnard Plain area have been relatively stable or have shown an increasing trend, in the coastal regions this stability is largely due to seawater intrusion and results in water of unusable quality replacing high quality groundwater. Rising groundwater levels in the Las Posas Valley are a result of active management to increase groundwater recharge beneath the Arroyo Las Posas. Seawater intrusion began in the Oxnard Plain area by 1930s and was widespread as early as the 1940s. Changes in groundwater management, including pumping reductions, shifting of pumping locations, construction of the Freeman Diversion, and the operation of the Pumping Trough and Pleasant Valley pipeline systems have significantly reduced seawater intrusion, but seawater intrusion conditions persist. Other contaminants present in the region include nitrates, iron, manganese and sulfate. In addition to causing seawater intrusion, over-pumping also caused irreversible land subsidence ranging from 2 .5 to 5 feet in several areas of the basin. The Central Coast is critically dependent upon groundwater, but this resource is under serious threat in a number of basins. In light of the limitations of existing state policies, the following measures would improve the effectiveness of groundwater planning and support sustainable management of California’s Central Coast’s groundwater resources . Establish a statewide policy to manage groundwater sustainably. Organize and empower local groundwater agencies to manage groundwater subbasins. Require the development and implementation of groundwater sustainability plans by local groundwater agencies. Provide local agencies with technical guidance and financial support from the state. Empower the State to oversee and ensure the implementation of groundwater sustainability plans.

Central Coast Hydrologic Region Groundwater Update

California Department of Water Resources (DWR) | April 1st, 2015

The primary goal of the Central Coast Hydrologic Region (Central Coast region) groundwater update is to expand information about region-specific groundwater conditions fo

The primary goal of the Central Coast Hydrologic Region (Central Coast region) groundwater update is to expand information about region-specific groundwater conditions for California Water Plan Update 2013, and to guide more informed groundwater management actions and policies. A second goal is to steadily improve the quality of groundwater information in future California Water Plan (CWP) updates to a level that will enable regional water management groups (RWMGs) to accurately evaluate their groundwater resources and implement management strategies that can meet local and regional water resource objectives within the context of broader statewide objectives. The final goal is to identify data gaps and groundwater management challenges meant to serve as a guidepost for prioritizing future data collection and funding opportunities relevant to the region. This regional groundwater update is not intended to provide a comprehensive and detailed examination of local groundwater conditions, or be a substitute for local studies and analysis.  Consequently, where information is readily available, the update does report some aspects of the regional groundwater conditions in greater detail.

Central Kings Groundwater Sustainability Agency Groundwater Sustainability Plan

Central Kings Groundwater Sustainability Agency (CKGSA) | December 11th, 2019

Central Kings Groundwater Sustainability Agency Groundwater Sustainability Plan (Revised)

Central Kings Groundwater Sustainability Agency (CKGSA) | July 13th, 2022

Central Valley Region Salt and Nitrate Management Plan

Central Valley Salinity Alternatives for Long-Term Sustainability (CV-SALTS) | January 19th, 2017

Elevated salinity and nitrates in surface water and groundwater are increasing problems affecting much of California, other western states, and arid regions throughout th

Elevated salinity and nitrates in surface water and groundwater are increasing problems affecting much of California, other western states, and arid regions throughout the world. In California, as surface and groundwater supplies become scarcer, and as wastewater streams become more concentrated, salinity and nitrate impairments are occurring with greater frequency and magnitude. In 2006, the Central Valley Water Board, the State Water Board, and stakeholders began a joint effort to address salinity and nitrate problems in California’s Central Valley and adopt long-term solutions that will lead to enhanced water quality and economic sustainability. Central Valley Salinity Alternatives for Long-Term Sustainability (CV-SALTS) is a collaborative basin planning effort aimed at developing and implementing a comprehensive salinity and nitrate management program. The goal of CV-SALTS is to maintain a healthy environment and a good quality of life for all Californians by protecting our most essential and vulnerable resource: WATER. In July 2008, the Central Valley Salinity Coalition (CVSC) was formed.  CVSC represents stakeholder groups working with the Board in the CV-SALTS effort.  Its purpose is to organize, facilitate and fund efforts needed to fulfill the goals of CV-SALTS.  CVSC coordinates the meetings of the CV-SALTS committees, maintains an independent web site, and manages the projects originating from this effort.  Information and materials regarding the stakeholder committees and other activity, including the meeting schedule, are posted on their website: www.cvsalinity.org. The Final SNMP for Central Valley Water Board Consideration was completed in December 2016. The Central Valley Water Board staff plans to release this version of the SNMP and all of its supporting attachments in January 2017 for a comment period so that the public has the opportunity to review the SNMP documentation before a general information workshop is held with the Central Valley Water Board, currently planned for March 2017.

Chapter A of Natural and Anthropogenic (Human-Made) Hexavalent Chromium, Cr(VI), in Groundwater near a Mapped Plume, Hinkley, California: Introduction to Study Area Hydrogeology, Chromium Sources, Site History, and Purpose of Study

U.S. Geological Survey (USGS) | April 25th, 2023

Documents in this series: Chapter A: Introduction to Study Area Hydrogeology, Chromium Sources, Site History, and Purpose of Study Chapter B: Survey of Chromium and

Documents in this series: Chapter A: Introduction to Study Area Hydrogeology, Chromium Sources, Site History, and Purpose of Study Chapter B: Survey of Chromium and Selected Element Concentrations in Rock, Alluvium, and Core Material Chapter C: Chromium in Minerals and Selected Aquifer Materials Chapter D: Analyses of Regulatory Water-Quality Data Chapter E: Groundwater Chemistry and Hexavalent Chromium Chapter F: Environmental Tracers of Groundwater Source, Age, and Geochemical Evolution Chapter G: Evaluation of Natural and Anthropogenic (Human-Made) Hexavalent Chromium Chapter H: Predevelopment Water Levels, Groundwater Recharge, and Selected Hydrologic Properties of Aquifer Materials, Hinkley and Water Valleys, California Chapter I: Sequestration and Reoxidation of Chromium in Experimental Microcosms Chapter J: Summary and Conclusions Between 1952 and 1964, hexavalent chromium, Cr(VI), was released into groundwater from the Pacific Gas and Electric Company (PG&E) Hinkley compressor station in the Mojave Desert 80 miles (mi) northeast of Los Angeles, California. Remediation began in 1992, and in 2010, site cleanup was projected to require between 10 and 95 years and was expected to cost between $36 and $176 million. A 2007 PG&E study estimated the natural Cr(VI) background in groundwater in Hinkley Valley to be 3.1 micrograms per liter (μg/L). This concentration was used for interim regulatory purposes by the Lahontan Regional Water Quality Control Board (RWQCB). In the fourth quarter (October–December) 2015, the regulatory Cr(VI) plume extended about 3.0 mi downgradient from the release location within the Hinkley compressor station, while groundwater having Cr(VI) concentrations greater than 3.1 μg/L was present more than 8 mi downgradient. Although rocks and minerals in the area are naturally low in chromium, alluvium eroded from the San Gabriel Mountains and transported to Hinkley Valley by the Mojave River, and locally small exposures of mafic rock, including hornblende diorite and basalt, may contribute Cr(VI) to groundwater. In response to limitations of the PG&E 2007 Cr(VI) background study’s methodology, uncertainty in the natural Cr(VI) background concentration, and an increase in the mapped extent of groundwater having Cr(VI) concentrations greater than the interim regulatory background of 3.1 μg/L, the Lahontan RWQCB concluded that the 2007 PG&E background Cr(VI) study should be updated. The purpose of the updated study is to estimate background Cr(VI) concentrations in groundwater within the upper aquifer upgradient, downgradient, near the margins, and within the footprint of the PG&E Cr(VI) plume in Hinkley, California. The scope of the study included eight tasks; results from those tasks are presented in the chapters within this professional paper.