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Facts about the Project's Slant Wells

The MPWSP will rely on “slant wells” to maximize capture of seawater. This technology prevents damage to ocean life, and is the technology preferred by the State of California for coastal desalination plants. The slant wells will also minimize impacts to inland groundwater resources, and will assist local groundwater management agencies to meet the requirements of the Sustainable Groundwater Management Act.

The wellheads for the slant wells will be located behind the dunes on the disturbed Cemex property, a former sand mining operation, in the City of Marina.

California American Water constructed a test slant well in 2015 to assess the feasibility of the technology and to monitor impacts to ocean life and inland groundwater supplies. The test slant well is approximately 700 feet in length and reaches a depth of 203 feet below sea level.

Long-term pumping tests were conducted with the test well between April 2015 and February 2018, totaling over 817 days of pumping.

  • Results from the long-term pumping tests showed that the test well drew 95 percent sea water and the remaining five percent was degraded brackish water from sources in the vicinity of the slant well.

California American Water has committed to provide the desalinated brackish water to Castroville to reduce their inland pumping to further alleviate ongoing seawater intrusion in the Salinas Valley Groundwater Basin.

The test well design is similar to the additional slant wells that will be constructed to supply the MPWSP, except that the additional slant wells are expected to be about 1,000 feet or longer in order to extend the wells further under the seafloor. Each slant well is expected to produce up to 3,000 gallons per minute.

The MPWSP slant wells will withdraw seawater and brackish groundwater from a small portion of the Dune Sand and 180-foot Aquifers, part of the larger Salinas Valley Groundwater Basin. Operation of the slant wells will not adversely impact groundwater conditions and will not impact other groundwater pumpers. Marina Coast Water District’s (MCWD) production wells withdraw from the 400-foot and Deeper Aquifers. The nearest MCWD (MCWD Well #12) is approximately 1.75 miles inland from the MPWSP and is inactive. The nearest active MCWD well (MCWD Well #11) is approximately 2.75 miles from the site and extracts from the 900-foot Deep Aquifer.

Monitoring is an important component of the project to confirm that operation of the slant wells is not causing negative impacts to Marina’s and other’s groundwater supplies, and to trigger mitigation if the slant wells are causing impacts.

The monitoring program will be independently overseen by the Monterey County Water Resources Agency.

  • Sixteen “sentinel” monitoring wells around the ocean test well have already been constructed and their data was collected as part of the long-term slant well pumping test. MCWRA may require additional wells to be installed prior to pumping in areas that lack coverage and that warrant monitoring in order to evaluate potential effects from slant well pumping. MCWRA will determine the number of new monitoring wells and their location.

  • Water level and electrical conductivity (to record salinity) are recorded by downhole data loggers every 15 minutes.

  • If impacts are detected, California American Water and an independent hydrogeologist directed by MCWRA will coordinate with the well owner to develop a course of action needed to alleviate any harm.

2022-10-24 14_02_36-CA-Mtry_WSP-OceanSlantWell-factSheet_FINAL95.pdf - Adobe Acrobat Pro (
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