Seawater seeping into Los Osos water basin poses threat

Seawater continues to advance into the drinking water supply that serves Los Osos, a new monitoring report shows.  

And while the community is cutting back on its water use to counter the effects of the drought, a threat to the basin exists.

Those are some of the takeaways from a draft water monitoring report released July 2 by Cleath-Harris Geologists Inc. The San Luis Obispo firm produced the report for the basin’s governing agencies. 

The report also reveals a chloride level that’s within safe drinking limits but exceeds recommended targets in four of the basin’s western wells that are closer to the ocean where seawater poses an urgent threat.

There are 30 wells in the monitoring network of the lower aquifer, 20 of which are owned by agencies serving larger groups of customers. (Those agencies are the Los Osos Community Services District, Golden State Water Company, the county of San Luis Obispo and S&T Mutual Water Co.). Ten are owned privately in the lower aquifer area.

The assessment noted that chloride levels rose 8.5 percent between October 2014 and April, the last time monitoring was done, analyzing water quality results from four key agency-owned wells that are part of the monitoring network.

At 190 milligrams per liter, the chloride levels are nearly twice as much as the target of 100 milligrams per liter identified in the Updated Basin Plan.  

The basin plan is an effort between the agencies to manage the aquifers that supply the community’s water.

The U.S. Environmental Protection Agency recommends chloride levels of no higher than 250 milligrams per liter to avoid a salt taste in drinking water.

“It’s definitely a situation that’s serious,” said hydrogeologist Spencer Harris, who drafted the technical memo. “That’s what the metrics are telling us. The community has done a great job of conserving and cutting back. But it has coincided with an exceptional drought. That’s why we’re not seeing any relief in the metric.” 

Harris said an accurate reading on the rate of seawater intrusion couldn’t be calculated with the latest monitoring in April. That’s because of chloride influence at the Palisades Well site near South Bay Community Park. 

The dilution of chloride in the lower aquifer originated from water leaking down from the upper aquifer through the borehole of the Palisades Well when the well was out of service for a few months. 

Lower levels of chloride exist naturally in the basin’s upper aquifer water, but when they mix with the lower aquifer supply that’s impacted by seawater, it skews the reading used to gauge saltwater intrusion. 

The base of the upper aquifer is as far as 200 feet down from the ground while the base of the lower aquifer is as far as 700 feet down in the Los Osos basin, with the range of where that bottom sits depending on the area of the basin, Harris said.

The latest memo didn’t assess the rate of intrusion, but an average rate of between 200 and 250 feet per year in Zone D — the western area of the lower aquifer — has occurred between 2009 and 2014, which Harris inferred is continuing based on overall chloride and  water levels.

A rate of seawater intrusion in Zone E — in the central area of the lower aquifer — is 100 to 170 feet per year. 

Rates can be difficult to accurately assess and they’re considered an estimate, rather than an exact science, based on chloride and water level information, Harris said.

The Los Osos Community Services District, Golden State Water Co., San Luis Obispo County and S&T Water Co. are working cooperatively to manage the basin as part of an agreement in a lawsuit filed in 2004 over water rights and consumption.  

When groundwater levels are low, it reduces the pressure that keeps out the saltwater, which is denser.

Low water pressure allows seawater to infiltrate the freshwater aquifer, rendering that water useless for potable purposes without desalination. 

The option of shifting production from the western wells in the lower aquifer to wells in the upper aquifer, has been identified in the Updated Basin Plan as a strategy to allow pressures to recover and prevent seawater from moving inland.

The plan recommends shifting more production to the upper aquifer and treating that water, which has high nitrate levels. That would help increase the freshwater pressure in the lower aquifer. 

The county Board of Supervisors will hear a presentation on the Updated Basin Plan and related implementing agreements at its meeting Tuesday. 

As part of settlement talks in the lawsuit, the four agencies involved are finalizing a Joint Powers Agreement that will clear the way to establish methods of operating that are mutually beneficial.

The next San Luis Obispo Superior Court date in that case will be Aug. 14 with Judge Martin J. Tangeman.