SLO wins $6.6 million grant to clean up toxic chemical in groundwater. Here's how

Apr. 2—The San Luis Valley Groundwater Basin stretches from San Luis Obispo to Edna Valley — but a toxic chemical swirling in the water prevents the city from using the resource for drinking water.

That will soon change, however.

San Luis Obispo won a $6.6-million grant to install wells that remove tetrachloroethylene, a chemical also known as PCE, from the groundwater, according to city water resources program manager Nick Teague.

The wells should be operational by 2026 and will allow the city to fulfill about 12% of its drinking water needs, he said.

SLO groundwater contaminated with PCE

A section of the basin under San Luis Obispo contains about 57 gallons of PCE pollution, a toxic chemical formerly used by dry cleaners, according to Teague.

The largest amount of PCE is concentrated underneath Higuera and South streets. The plume extends southwest along Highway 101 to Los Osos Valley Road where a fault line blocks the pollution from spreading further, he said.

"If you think about the aquifer like a sandbox, that's the end of the sandbox," Teague said. "Water kind of hits that, moves to the surface and then usually discharges to the creek."

A "bedrock high," which looks like an underground hill, separates the SLO section of the basin from Edna Valley's section. Water flows slowly over the bedrock high and prevents PCE from spreading into the Edna Valley portion of the basin.

The city stopped using the basin for drinking water during the 1990s, but City Farm SLO and the Madonna Inn still pump groundwater for irrigation, Teague said.

The Edna Valley side of the basin experiences an annual overdraft of about 1,100 acre-feet per year due to the high demand of agriculture, while the SLO side has a surplus of about 700 acre-feet of water per year, Teague said.

Grant funds pollution cleanup

The California State Water Resources Board awarded the city a $6.6-million grant to clean up PCE pollution in the basin.

The grant will fund the construction of two wells that use a granular activated carbon system to remove PCE from the ground water — similar to how a Brita removes contaminants from tap water, Teague said.

After filtration, the water will meet standards set by the California Division of Drinking Water and the Environmental Protection Agency.

Each well will have two filters, one that removes the PCE from the water and another that serves as a backup unit. The city will test the water between each filter and replace them as needed, according to Teague.

"You've always got that redundant system so there's no chance of PCE sneaking through," he said.

Meanwhile, the city will install eight to 12 monitoring wells to evaluate water levels and the distribution of PCE in the basin.

Eventually, this process will remove almost all PCE from the groundwater.

"I think the coolest part about this is that it's a win-win," Teague said. "We're decreasing contamination and providing potable water to the city in a energy efficient, cost effective way."

Right now, the city sources about 5,000 acre-feet of drinking water per year from the Whale Rock Reservoir, Salinas Reservoir and Nacimiento Reservoir.

"It costs money to pump those from their source to the water treatment plant, and then there's a cost associated with treating that water," Teague said.

This new project, however, will allow the city to pump about 700 acre-feet of groundwater per year — meeting 12% of the city's drinking water needs.

Pumping and cleaning the groundwater will be cheaper and more energy efficient than cleaning surface water because groundwater experiences a natural filtration process, according to Teague.

It also provides the city a backup plan if something happens to the reservoirs.

"If for some reason we had to do repairs on the water treatment plant and shut it down for a short period of time, we could supply most of the community with this groundwater for a short period of time," Teague said. "We're always looking for ways to become more resilient."

The city will monitor groundwater levels during the project to determine if they should reduce pumping.

"We don't want to overpump, we want groundwater levels to stay consistent," Teague said.

This story was originally published April 2, 2024, 9:00 AM.

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