Seawater Intrusion and the Drought


Salinas River Mouth

There’s a giant hole under the city of Salinas; a hole that stretches all the way across the valley from the mouth of Toro Creek to the base of Fremont Peak. There, on the East Side, it reaches its greatest depth; more than 100 feet below sea level. It’s a hole in the head elevation of the groundwater table and it’s the reason why the Salinas Valley has a seawater intrusion problem.

The hole, which the hydrologists prefer to call a “head trough,” is created and maintained by pumping. Not just pumping from the hole itself, but also upstream pumping, which greatly reduces the flow of groundwater reaching the hole.

Before heavy pumping, groundwater in the Salinas basin moved gradually down the Valley and out to sea. Now, what’s left of the groundwater flow from upriver, simply falls into the hole. But the hole doesn’t fill. The pumping rate, 523,000 acre feet per year on average over the past 54 years, is simply too high.

Between the hole and the beach, groundwater, which once flowed toward the ocean, now flows backwards toward the hole, bringing salt water from the ocean along behind it. Stopping seawater intrusion means allowing the hole to fill and getting the flow moving in the right direction by maintaining a groundwater level higher than sea level. The exact “protective elevations” needed to accomplish this may be found in this November 2013 report.

Salinas Valley Hole

All water heads toward the bottom of the hole. This graphic, from the December 2014 Salinas River Groundwater Basin Report, better known as the State of the Basin Report, prepared for the Monterey County Water Resources Agency, is intended to show conditions as of Fall 2013. The situation has only grown worse since. The arrows depict direction of flow and the contour lines depict groundwater head elevations. They range from, minus 70, near the bottom of the hole, to zero, near the beach, and a positive 70, near Gonzales. Click image to enlarge.

According to the 2014 State of the Basin Report, fresh water storage in the Salinas basin has been declining by an average of about 6,000 acre feet per year, while another 11,000 to 18,000 acre feet have been lost on average each year to seawater intrusion. This adds up to a total average annual loss of 17,000 to 24,000 acre feet. This is the amount by which the basin is considered to be out of hydrologic balance.

But it isn’t a steady decline. The real damage occurs during times of drought.

Pumping is heaviest during the summer and early fall, and the basin relies on substantial (over 1,000 acre feet per day) releases from lakes Nacimiento and San Antonio during these critical months to keep groundwater levels from plummeting. When there isn’t enough water in the lakes to allow those releases to be made, as there wasn’t last year and won’t be this year, groundwater storage can decline by as much as 215,000 acre feet in a single year.

These declines are reflected in the Water Agency’s Quarterly Salinas Valley Water Conditions Reports. The latest report, for the second quarter (January – March) of the 2015 water year, was released on April 27.

The report reveals that the water level in the Pressure 180-Foot Aquifer averaged 7 feet less in March 2015 than it did in March 2014, 21 feet below the level considered normal for March, and 5 feet below the previous record low for March, set in 1991 (the final year of the last multi-year drought).

In the Pressure 400-Foot Aquifer, average water levels were down 5 feet over last March, 14 feet below normal, and 4 feet below the previous (1991) record.

In the Eastside Subarea, the numbers are 3 feet less than last year (the previous record low for March), 28 feet below normal and 9 feet below 1991 levels.

In the Forebay Subarea, it’s a 6 foot loss since last year, 12 feet below normal for March, and a foot above the 1991 record low.

In the Upper Valley the loss is 8 feet since last year, 10 feet below normal and less than a foot above the 1991 record low.

As water levels in the areas immediately adjacent to the seawater intrusion front are now lower than they have ever been before, it’s worth asking what consequences followed the 1991 lows.

The State of the Basin Report notes that seawater intrusion began to accelerate during the 1984-1991 drought, but that the rate of intrusion does not appear to have peaked until over 5 years after the drought ended, between 1997 and 1999. “If this latent response to an extended drought is repeated in the Basin,” the Report continues, “water quality impacts stemming from the current drought may not manifest for several years.”

Historic Intrusion Map

The huge advances in seawater intrusion following the drought in the late-70’s and the 1984-1991 drought are clear from this Water Agency graphic. Another advance of this magnitude could easily render a huge number of wells, serving the city of Salinas, as well as some of the world’s most productive cropland, unusable. It may be 5 to 10 years before we know the full extent of the damage being wrought by today’s low water levels.

This is why it is foolish to conclude, as many with a stake in Salinas Valley development have, that slowing rates of seawater intrusion over the past 15, mainly wet, years show the problem is nearly, if not completely, solved. Such claims should definitely be taken with, not just a grain, but a very large dose of 500mg per liter salt.

As the State of the Basin Report puts it, “(t)he consequences of no action under continued drought conditions will be the imminent advancement of seawater intrusion within the next few years…”

By “action” they mean reducing pumping in the vicinity of the hole by piping water in from areas further up valley and with closer hydrologic connections to the river, something reports have been suggesting since at least 1946, or drilling deeper wells to tap the Deep Aquifer. Unfortunately, importing water from more easily recharged areas upstream has been a political nonstarter (“not currently consistent with County Policy,” as the Report puts it), as it involves “stealing” water from one group of growers in order to give it to another; and tapping the Deep Aquifer is expensive and speculative, in that no one knows whether it has sufficient quantity and quality of water, whether it too will be subject to seawater intrusion, or whether it is sufficiently separated from the higher aquifers to make any difference at all.

In any case, nothing is likely to happen in time to mitigate the next great leap forward in seawater intrusion. That’s just the way it is.


3 Responses to Seawater Intrusion and the Drought

  1. Don Fischer says:

    Great report. Thank you for this effort and for its clarity.

  2. Gordon Miller says:

    Thanks for the brilliant update from east of Eden. I am always striving to find the best view of Salinas.

  3. Thanks for a great read. Water in the Moneterey county area is a nightmare. We need to change the way we deal with water because survival is at stake, not just the needs of the mega rich.

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