Los Osos groundwater report shows ‘positive signs.’ Could that one day lead to growth?
The Los Osos groundwater basin appears to be inching closer to providing a sustainable drinking source for its current population, according to a new report recently approved by the basin’s management committee.
And that, in turn, could one day allow the town to grow after decades in limbo.
“There are positive signs, but there is still work to do,” said Dan Heimel, the executive director of the Los Osos Basin Management Committee.
The report, prepared by Cleath-Harris Geologists, documents the 2021 conditions of the basin. The Los Osos groundwater basin serves as the sole source of water for the unincorporated town along the Morro Bay Estuary.
A development boom in the 1970s and ‘80s caused the Los Osos groundwater basin to deplete faster than it was able to refill. Soon, the basin’s water quality deteriorated due to seawater intrusion and nitrates leaking from septic systems.
Fast forward to 2015, and a plan was drawn up to restore the water supply for Los Osos and ensure the town can eventually grow in population. At the time, the basin was adjudicated, meaning its oversight was placed in the hands of a basin management committee that must report annually to the San Luis Obispo Superior Court on its progress toward bringing the basin to sustainability.
As the 2021 annual report shows, the basin is still facing saltwater and nitrate contamination. It also saw a drop in freshwater storage from 2020 by about 100 acre-feet, or nearly 32.5 million gallons, according to the report.
Although those variables indicate that the basin is still not suited to produce water for more people than it currently serves, there is room for hope.
“It’s showing some progress on doing what we need to do: to have the basin provide a sustainable source of water for existing residents,” said San Luis Obispo County District 2 Supervisor Bruce Gibson, who sits on the Los Osos Basin Management Committee.
What the Los Osos water report shows
While seawater intrusion appeared to have moved inland in the lowest portion of the aquifer, it receded back out to the ocean in one of the upper zones, according to the report.
And nitrate pollution concentrations decreased from 2020 levels, although it still has not met its target value, the report says.
Additionally, less water was used from the basin in 2021 than in 2020, according to the report, indicating that conservation measures may be working.
The 2021 report used a sustainable yield benchmark of 2,760 acre-feet, while only 2,000 acre-feet of water was pumped from the basin that year. That means in 2021, only about 72% of the basin’s sustainable capacity was used, the report shows.
That would put it comfortably below the goal of using no more than 80% of the basin’s capacity.
However, the report makes note of a big caveat.
In October 2021, the basin management committee revised the sustainable yield of the basin using more up-to-date rainfall data and better analysis of seawater intrusion and nitrate contamination.
As a result, it found that only 2,380 acre-feet of water could be safely pumped from the basin annually, versus the previous target of 2,760 acre-feet, according to the annual report.
Using the updated sustainable yield value in 2021 would have put the basin above its target.
“Current calculations are that we’re at 84% of the sustainable yield,” Heimel said. “I s
till see that as a positive sign — that pumping in the Los Osos basin is less than the sustainable yield estimate.”
Goal is to be able to grow Los Osos’ population
Heimel further explained that “when they developed the basin plan, they said, ‘We don’t want to just stop seawater intrusion. We want to push it back to some historical extent, to try to reverse it.’ That provides an additional buffer for the existing community, and mitigation against potential future climate change impacts.”
Reversing the effects of the historical overpumping by using less than the sustainable yield would ensure the basin has a reliable source of freshwater for a growing population. The county has drafted a plan that, if approved by the California Coastal Commission, could allow the community to grow by nearly 30% by 2040.
The Coastal Commission has recently sent letters to the county noting that it doesn’t see that Los Osos has a sustainable water source for its current population, and asked the county to stop approving any new water-using development within the basin’s boundaries.
Gibson noted that the 2021 report shows that the conservation efforts and other activities taking place to bring the basin into balance may be working. That being said, he recognized that the basin may not be ready for a growing population quite yet, especially given how the ongoing and extreme drought could potentially cause water levels to dip.
“We do have to make good policy off of good science,” Gibson said. “My job is to lead on land-use policy for the county. And we have to do that consistent with resources available to support new growth, if we can do it. And I’m perfectly happy to take a careful look at these resources and to continue working with the purveyors on developing the resources that Los Osos needs to realize its future.”
