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

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.