Basin Invasion

The county’s most recent seawater intrusion map for the 400-foot aquifer, using 2015 data, reflects the biggest advancement of intrusion since 1985, and shows the appearance of three “islands” to the east.

Monterey County’s seawater intrusion maps can tell many stories, but when they show seawater advancing further into the Salinas Valley Groundwater Basin, the stories share a theme: The basin is being overpumped, putting agricultural and municipal water supplies at risk.

In a July 11 meeting, hydrologists from the Monterey County Water Resources Agency presented the 2015 seawater intrusion maps to their board and the County Board of Supervisors. They found the results alarming.

The maps show contour lines of the intrusion, year by year, dating back to 1944. The 2015 data reveals significant advancement of seawater intrusion – which fouls wells, making them unusable – into the 400-foot aquifer (named for its depth), a primary source of irrigation for many growers. That aquifer also provides roughly half the drinking water for the residents of Salinas, Marina and the former Fort Ord; they get the other half of their water from a deeper aquifer, an ancient water supply with little recharge.

For residents of Marina and the former Fort Ord, who are supplied water by the Marina Coast Water District, the new maps show what might become an existential threat to their water supply: Marina Coast has four wells right along the southeastern edge of the contour line in the 400-foot aquifer map.

There is some good news revealed by the maps: Seawater intrusion in the 180-foot aquifer showed almost no advancement, which MCWRA senior hydrologist Howard Franklin attributes to projects like the rubber dam, which traps water in the Salinas River so that it can diverted, treated and used for irrigation.

But seawater is moving in the shallower aquifer – it’s just gone vertical. The most interesting revelation of the maps is the discovery that in some areas, seawater has migrated downward, creating what MCWRA hydrologist Tamara Voss calls “discrete islands” of intrusion in the 400-foot aquifer. On the map they look like detached purple blobs, unconnected to the the coast.

Voss, who analyzes the data used to create the maps, says how the water is migrating vertically remains unknown, but it is suspected to be occurring through abandoned wells.

The agency expects to release the 2017 intrusion maps next summer, meaning that the next set of maps are likely to look worse before they look better: Franklin, at the July 11 meeting, said it takes one to two years for rainfall to percolate into groundwater. That means the 2017 maps will reflect groundwater levels before impact by the past winter’s deluge, and will instead show the tail end of a five-year drought.

This year’s map was released a year late, which agency officials attribute to understaffing. When Voss first came on in 2007, there were six people in her department. It is now just her.

Outside of MCWRA, concern runs high. Brenda Granillo, district manager for Cal Water, which supplies Salinas, says some of Cal Water’s wells are showing elevated levels of chloride – a harbinger of seawater intrusion.

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“I think what the maps show is that we have a big problem in the Salinas Valley,” says County Supervisor Jane Parker, who represents District 4, where much of the intrusion is occurring.

Franklin says he’ll return to the MCWRA board Aug. 21 with recommendations to address the issue.


Clarification (7/21): The original version of this article stated the rubber dam is beneficial due to trapping water in the Salinas River and allowing it to percolate. While that may be true to some extent, the key benefit is trapping it to be diverted, treated and piped to growers, who in turn pump less groundwater.

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