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Seawater Intrusion Threatens Some of California’s Richest Farmland

The lush farming region has faced seawater intrusion from Monterey Bay for decades. But it has become an emergency due to unregulated well drilling, explains Gary Petersen, general manager of the Salinas Valley Basin Groundwater Sustainability Agency.

Written by Matt Weiser Published on Read time Approx. 5 minutes
A farm worker pulls weeds from a field of lettuce in California’s Salinas Valley. The region is famous for its agricultural production, but groundwater – lifeblood for both farms and cities – now faces an emergency due to expanding seawater intrusion.George Rose/Getty Images

California’s Salinas Valley, one of the world’s most productive farming areas, faces a groundwater emergency. The problem is seawater intrusion into freshwater aquifers, which are the region’s lifeblood.

The issue has been understood for a long time. It’s a result of groundwater pumping so intense that it has drawn in seawater from Monterey Bay from some 30 miles away. Twenty years ago, Monterey County spent about $70 million to build the Castroville Seawater Intrusion Project, which injects recycled wastewater underground to hold back seawater from the Bay. It worked, but could not keep pace with another threat: Unregulated well drilling.

Dozens and maybe hundreds of new wells have been drilled into the aquifer, primarily to serve agriculture. In addition, about 140 old farm wells were not properly sealed and destroyed, creating Swiss cheese-like pathways for seawater to penetrate ever deeper areas of the aquifer.

The extent of the seawater intrusion became known last year, when new results from groundwater monitoring emerged. The county created a working group of groundwater users and experts to find solutions, which were presented in April.

Water Deeply spoke with Gary Petersen, general manager of the Salinas Valley Basin Groundwater Sustainability Agency and coordinator of the working group, to explain the situation further.

Water Deeply: What is the nature of the groundwater in this area?

 Gary Petersen: Well, there are basically three aquifers. There is the 180ft aquifer, which has been heavily intruded for a very long time. Underneath that is a 400ft aquifer, which people have gone to more and more once they hit seawater intrusion in the 180ft aquifer. Then below that is what is called the deep aquifer. That starts about 800ft and maybe goes to 2,000ft.

There are currently 41 wells that have been permitted into that deep aquifer. But not a lot of time has been spent in characterizing that aquifer and understanding what is there.

Water Deeply: How extensive is the seawater intrusion?

Petersen: The salinity movement that raised a concern, and what the emergency is about, was the growth of seawater intrusion in the 400ft aquifer. It suddenly started showing up in areas that were disconnected from the continuous movement of the seawater front. Out in front of it, there were spots of salinity intrusion that are starting to form. That is thought to be a process of vertical migration through well casings from the 180ft aquifer to the 400ft aquifer.

As people have gone deeper, the concern is that seawater intrusion has happened in the 400ft aquifer because of bad well casings or poorly destroyed old wells. There is a handful of those that are known: About 140 wells were identified decades ago as needing to be destroyed but they were not.

Water Deeply: Why weren’t those old wells destroyed?

Petersen: There are a lot of reasons why that didn’t happen. I think part of it is funding. Part of it is that pumping restrictions that were called for initially, when the Castroville Seawater Intrusion Project was built, were not implemented.

Which leads then to the greatest fear: If the 400ft aquifer is being contaminated by the 180ft, the only known recharge for the deep aquifer is thought to be from the 400ft aquifer above it. So it sets up a nightmare scenario of vertical migration into the deep aquifer. Drinking water has been coming from there; more and more agriculture water has been coming from there. So if we don’t get a handle on this – certainly at the 400ft level – plus if we don’t start dealing with those wells that we know are causing intrusion, and if we don’t stop putting more wells in the ground, it’s going to get worse until we lose everything.

Water Deeply: What solutions did your working group propose?

Petersen: First we’ve got to protect the drinking water. There’s a great Will Rogers quote that I love: “The first thing you do when you find yourself in a hole is stop digging.” So though we didn’t place limits on well pumping, we recommended a moratorium on any new wells in the 180ft and 400ft aquifers until we get the groundwater sustainability plan in place so that we can better characterize that.

Agriculture backs up to a tract of suburban homes near the town of Gonzales in California’s Salinas Valley. The region, in close proximity to Monterey Bay, is famous for its lettuce, wine and other agricultural products but the groundwater that serves farms and cities is threatened by seawater intrusion. (George Rose/Getty Images)

There were two more proposals that sprung out of our work. One is that the Water Resources Agency should immediately go to work to characterize the deep aquifer and understand that, so that when decisions are made about our wells that the consequences are also understood.

There are eight wells known to be contributing to vertical migration. We recommended those be immediately destroyed – within 90 to 120 days. We thought that money should come from the county as an emergency action. We do not know if that will happen or not.

The other thing that I think was tough for folks was that we drew a line. We know there’s an area that is heavily impacted by seawater intrusion. We know that front is moving. So we proposed enlarging the area where new wells are banned. We’re not trying to stop progress here. We’re not trying to halt agriculture in that area. But we had to put a line somewhere, so we expanded the area.

Another recommendation has to do with replacement wells. We recommended that if you are a farmer and you’re in the 400ft aquifer and you lose a well to seawater intrusion, that you be authorized to replace that well with a well into the deep aquifer to provide ongoing use of the land.

The average cost for a deep well right now is about a million and a half [dollars]. So nobody is rushing out to do these, because you have to be able to justify the cost. We’re just trying to get to some short-term solutions that will prevent things from getting worse.

Water Deeply: It sounds like you had to make some tough choices.

Petersen: One of our overarching principles was to protect public health and safety via protection of drinking water. If it comes to people losing drinking water versus agriculture having irrigation water, people should get drinking water.

At the same time, this area in question is completely farmed. Agricultural interests are all over that area. So we needed to make decisions that took into account retaining the viability of the agricultural industry – and keep that in mind in our decision-making in hopes we could protect that as much as possible.

Our third piece was to not do anything that would, in two years, cripple us when we release our groundwater sustainability plan. It can’t get in the way of getting what we need to head toward sustainability.

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