Pismo Beach Regional Groundwater Sustainability Project
The City of Pismo Beach is a member of the Northern Cities Management Area (NCMA) of the Santa Maria Groundwater Basin in San Luis Obispo County, California. There are three major sources of water that supply the NCMA—Lopez Lake (surface water), the State Water Project Coastal Branch (imported water), and groundwater pumping. Each source has a defined delivery volume that varies annually.
In coastal regions, freshwater meets saltwater in a transition zone that includes the mouth of the river (surface water) and the aquifer (groundwater). Excessive pumping from a coastal aquifer can reduce the seaward flow of freshwater to the extent that saltwater intrudes into the aquifer, rendering it useless for agriculture or consumption. As new inland wells are constructed, regional groundwater levels can be lowered, allowing for further saltwater migration.
A 2009 NCMA monitoring study revealed that coastal groundwater elevations were below mean sea level (MSL), creating conditions conducive to saltwater intrusion. The study also detected water quality measurements that indicated the potential of saltwater intrusion affecting the City of Pismo Beach, Grover Beach, San Luis Obispo, Arroyo Grande, and Oceano.
The five cities implemented water conservation methods and reduced groundwater pumping, which restored groundwater elevations to above MSL within a year. In subsequent extended drought conditions, sentry wells detected groundwater elevations similar to those of the 2009 study, increasing the potential for saltwater intrusion.
Clearly, the five cities needed to reduce the risk of saltwater intrusion and improve sustainability for the region’s water supply. Because the basin is near the ocean, the project required a numerical model that could account for groundwater salinity and density, which affects flow.
Because Geoscience has expertise and experience in density-dependent groundwater flow modeling, the City of Pismo Beach turned to Geoscience to model alternatives and develop a sustainable plan to assure a reliable source of potable water going forward.
Because overpumping created an imbalance that threatened saltwater intrusion, Geoscience developed a highly-advanced model to assess the efficacy of using artificial recharge in the groundwater basin to create a salinity barrier. Using an average of the previous five years as a baseline, the model compared two types of recharge—wastewater infiltration and stormwater infiltration. The model could also determine whether the wells would pump freshwater or saltwater, and how effective the mitigation would be.
Since the flow of wastewater is typically reliable even during dry years, the model anticipated that wastewater could be treated and used to significantly enhance groundwater supplies and protect groundwater resources during drought conditions.
The model enabled NCMA to design a solution and advance the project to the permitting phase. The target solution includes injecting advanced-treated recycled water into the groundwater basin to establish a seawater barrier and improve groundwater supply reliability, especially during drought conditions. When implemented, the solution is projected to increase groundwater supply by 30 percent while reducing costs. It will also promote water independence by reducing reliance on the State Water Project.
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