Toxic chemical detected in SLO groundwater 40 years ago. Now city wants to clean it up

The city of San Luis Obispo wants to pump groundwater again — but first it must clean up decades-old chemical pollution detected within the basin.

In the 1980s, city wells pumping under the city detected tetrachloroethylene (also known as PCE), a chemical associated with dry cleaning and industrial operations, in the San Luis Obispo Valley groundwater basin.

Early on, the contamination didn’t exceed any regulatory standards, so San Luis Obispo was still able to deliver drinking water to customers, according to Mychal Boerman, the city’s utilities deputy director of water.

The city stopped using groundwater in 2015, to solely use surface water and recycled water from its wastewater treatment plant, he added.

By 2025, SLO hopes to start pumping groundwater again to serve as drinking water for residents. This will help diversify its water sources and provide an extra buffer against drought, Boerman said.

“The goal for the city is ultimately to be able to use groundwater again,” Boerman said. “We just want to make sure that we’re making the issue better and not making it worse.”

SLO wants to tap into groundwater basin

The San Luis Obispo Valley groundwater basin sits under the city, stretching southeast into the Edna Valley. It currently primarily feeds agricultural operations in the Edna Valley.

The basin has historically been overpumped largely due to agriculture operations in the Edna Valley, according to a report documenting the state of the basin in 2021.

That, coupled with drought conditions, has caused the basin’s storage levels to plummet.

Even given that, the city wishes to tap into the basin. First, it needs to clean the pollution within it.

How big is chemical pollution plume?

To begin its cleanup efforts, the city applied for and received a $2.2 million grant from a program created by Proposition 1, also known as the Water Quality, Supply, and Infrastructure Improvement Act, which voters passed in 2014. That money funded a plume characterization study to fully map the pollution within the basin.

“What we’re doing is delineating the plume, so we’re essentially figuring out exactly how big it is,” Boerman said. “That can be used to compare to prior studies to see if there’s an active source that’s continually allowing PCE to be discharged into the groundwater basin., or if it’s just continuing to make its way through the basin and getting diluted overtime.”

That study is now nearly complete.

According to maps created by the city, the plume of PCE stretches from near Highway 101 and Los Osos Valley Road north to about Highway 101 and Higuera Street.

“The plume is over a small area of the basin. It’s not the entire basin,” Boerman said.

Measurements taken by the city show that levels of PCE in the groundwater basin are up to about 20 micrograms per liter, according to Greg Bishop, a senior engineering geologist with the Central Coast Regional Water Quality Control Board . That agency is managing the pollution cleanup.

To meet drinking water standards, the city must treat the groundwater so that the amount of PCE is about five micrograms per liter or less, Bishop said.

PCE can be toxic to humans if consumed at levels higher than that, he added.

“We don’t think there’s any active discharge meaning the PCE isn’t still being spilled,” Bishop said. “But there are remnants of historic releases.”

Bishop noted that the regional water quality control board is working with the city to identify exactly where those sources of contamination are to fully determine whether there is any ongoing pollution in the basin.

What’s next for city?

Once the city completes the site characterization study of the contaminated plume, it will send it to the regional water quality control board for approval.

Next, the city will need figure out exactly how it will install groundwater pumping wells over the basin to extract the water while also ensuring the pollution does not make it into the taps of residents.

To do so, the city applied for a $6 million grant from the state to help fund the planning and installation of groundwater wells that would be specially fitted with decontamination heads.

Should the city receive the grant, Boerman said it will work with the regional water quality control board to get wells in the ground producing water for residents by 2025.

This will alleviate pressure on the city’s other water sources such as recycled water and water from Nacimiento Lake, Santa Margarita Lake and Whale Rock Reservoir.

Although city officials have said SLO has plenty of supply to serve residents for at least another five years even given current drought conditions, a state regulation requires the city to implement its water shortage contingency plan because of water shortages statewide.

“Having access to supply from groundwater as a backup both helps us in cases of loss of infrastructure and also helps us as we continue to deal with prolonged periods of drought,” Boerman said.

How to learn more about groundwater pollution

The city will hold a meeting on Wednesday at 5:30 to 8 p.m. at the Ludwick Community Center, 864 Santa Rosa St. in San Luis Obispo, to show residents what it knows about the plume.

For more information, visit the city’s website at www.tinyurl.com/slogroundwater.

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