Pismo Beach Regional Groundwater Sustainability Project

Client Profile

The City of Pismo Beach, a member of the Northern Cities Management Area (NCMA) of the Santa Maria Groundwater Basin in San Luis Obispo County, California, needed groundwater sustainability solutions.

The NCMA gets water from three major sources—Lopez Lake (surface water), the State Water Project Coastal Branch (imported water), and groundwater pumping. Each source has a defined delivery volume that varies, sometimes significantly, every year.

Challenge

In coastal regions, freshwater meets saltwater in transition zones that includes river mouths (surface water) and aquifers (groundwater). Excessive pumping from a coastal aquifer can critically reduce the flow of freshwater seaward allowing saltwater from the submarine aquifer to intrude into the inland freshwater aquifer. This saltwater intrusion would make the freshwater aquifer useless for agriculture or consumption—a serious issue for farmers, water utilities, and municipalities.

As wells are constructed and begin pumping freshwater out of the groundwater basin, groundwater levels can be further reduced along with the force of freshwater flow toward the ocean, allowing for more saltwater migration farther inland.

A 2009 NCMA monitoring study revealed that groundwater elevations (levels) along the coast were below mean sea level (MSL), creating conditions conducive to saltwater intrusion—the saltwater aquifer was higher and had more pressure than the inland freshwater aquifer, which would allow the saltwater to push inland and contaminate the freshwater aquifer.

The study also detected water quality measurements indicating the potential for saltwater intrusion would affect the cities of Pismo Beach, Grover Beach, San Luis Obispo, Arroyo Grande, and Oceano. 

The five cities worked to implement and enforce water conservation to reduce groundwater pumping, which helped restore groundwater elevations to above MSL within a year reducing the risk of saltwater intrusion. But extended drought conditions had a significant impact on groundwater recharge and flow, and sentry wells (used to monitor groundwater levels) detected groundwater elevations similar to those of the 2009 study, once again increasing the potential for saltwater intrusion.

Clearly, the five cities needed to reduce the risk of saltwater intrusion and improve the region’s water supply sustainability before irreversible saltwater contamination occurred.

Because the basin is near the ocean, the project required a numerical model, which incorporates groundwater salinity and density, to accurately model groundwater flow.

Geoscience’s expertise with numerical models and density-dependent groundwater flow modeling, empowered the City of Pismo Beach to explore options and develop a sustainable plan to produce reliable potable water.

Geoscience Solution

Because over pumping created an imbalance that threatened saltwater intrusion, Geoscience developed a highly-advanced model to assess an artificial recharge solution which would inject more freshwater in the groundwater basin to create a salinity barrier. Using an average of the previous five years as a baseline, our model compared two recharge types—wastewater infiltration and stormwater infiltration.

The model also provided insight to determine whether the wells would pump freshwater or saltwater and the mitigation efficacy of each potential solutions.

Since wastewater volumes are typically reliable even during dry years, the model predicted wastewater could be treated and injected into the basin as a supplemental freshwater source to significantly increase groundwater flow and protect the freshwater aquifer from saltwater intrusion during drought conditions.

Customer Results

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.

Additionally, the groundwater sustainability project will promote water independence by reducing NCMA’s reliance on the State Water Project.

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