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Saltwater is still seeping into Los Osos lower aquifer

Recent monitoring shows that saltwater continues to seep into the Los Osos groundwater basin.

Monitoring conducted in December and January shows fingers of saltwater are pushing deeper into the basin’s lower aquifer, which supplies drinking water for the communities of Los Osos and Baywood Park. County Public Works officials believe three years of drought have exacerbated the seawater intrusion problem.

“Overall, it’s gotten worse in the last couple of years,” said John Waddell, Public Works engineer. “Too much water is being taken from the lower aquifer and not enough from the upper aquifer.”

Water purveyors in Los Osos avoid the upper aquifer because it has high levels of nitrates, a pollutant that causes health problems. Septic leach fields have been identified as the source of the nitrates.

Hydrologists estimate that the rate of saltwater intrusion between 2005 and 2010 was 700 feet per year. The seawater plume has reached as far inland as a well near Los Osos Community Park and Los Osos Valley Road.

County officials believe current water demand is within the basin’s safe yield, the amount of water that can be withdrawn from the basin without negative effects. However, adjustments will have to be made to halt saltwater intrusion.

These include switching to wells farther inland and taking more water from the upper aquifer. The Los Osos sewer project is also expected to benefit the groundwater basin by removing the source of the nitrates and reusing water, Waddell said.

A basin management plan is being developed for Los Osos and is expected to be released by the end of the year. Safe yield for the Los Osos basin is estimated at 3,200 acre-feet per year, of which 2,100 acre-feet are available for domestic use.

Meanwhile, county Public Works officials hope to clear the final regulatory hurdle to the Los Osos sewer project as soon as June. The state Coastal Commission will hold a permit hearing on the sewer when it meets from June 9 to 11 in Marina del Rey.

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