Famously the unincorporated town of Los Osos on the California Central Coast, was the subject of a 35-year building moratorium that was only lifted earlier this year. The community of 15,000 is overseen by the county Board of Supervisors who has now allowed a few residential projects that would hook up to the local groundwater supply to move forward. Currently Los Osos water supply is 100% sourced by groundwater.The supervisors use a formula that is dependent on the health of the groundwater aquifer as measured each year.
For years, there has been a debate within the community over whether we are pumping too much of that groundwater allowing seawater to intrude into the community’s aquifers.But now a new “more accurate” groundwater model released this month appears to show the town may not have the water to sustain growth without significant new initiatives. Without new hook ups – seawater is already moving in, says the model. What to do?
The Los Osos Basin Management Committee (BMC) is charged with monitoring the basin.The committee made up of the town’s three water purveyors and a public member (Supervisor Bruce Gibson), files an annual report to the court as a result of a 2015 Stipulated Judgment.
For years the BMC has used a “static” older model to estimate how much water the community can pump without causing seawater to move into the aquifers below the town.Overpumping can reduce the pressure to keep the saltwater at bay. For the past few years the BMC board has sought consultant expertise to put together a new updated “Transient” model to guide them.Now the model is in place. The consultant engineering firm that developed the model made big news this past week.
Some background
Seawater intrusion is the movement of saline water into freshwater sources, both groundwater and surface water. It can accelerate its pace not just from groundwater pumping but from rising sea levels says a November study in Geophysical Research Letters that evaluated more than 60,000 coastal watersheds around the world, mapping how diminished groundwater recharge and sea level rise will each contribute to saltwater intrusion while estimating what their net effect will be. The study’s authors found that by the year 2100 rising sea levels alone will tend to drive saltwater inland in 82% of coastal watersheds studied. So this is not just a Los Osos problem.
A JPL report adds “Spurred by planetary warming, sea level rise is causing coastlines to migrate inland and increasing the force pushing salt water landward. At the same time, slower groundwater recharge — due to less rainfall and warmer weather patterns — is weakening the force moving the underground fresh water in some areas.”
If there is less natural pushback against rising seas due to climate change, Los Osos and all coastal communities need to push back harder, increase the volume of recharge to battle seawater intrusion or face contamination.
Here are some key effects of seawater intrusion:
-Degradation of Water Quality
-Reduced Freshwater Availability
-Increased Water Treatment Costs.
This week at the June BMC meeting, a new model was introduced that suggests that the older groundwater flow model was flawed. The change of tune has major implications for the community who just recently, under the county jurisdiction, adopted a new community plan that allows limited growth in this community. Critics say the new information is a wake up call. But for what?
The new so-called “Transient” model released by Spencer Harris and Cleath-Harrris Geologists of San Luis Obispo, the basin committee’s consultant firm, says the new model accounts better for hydrology variation.The transient nature of the model allows for simulating variable hydrologic patterns that include wet and dry cycles over a 45-year modeling period. The current steady-state Model, due to its limitations, simulated constant hydrologic conditions occurring year over year and could not account for the impacts of variable hydrology (i.e. extend drought).”
“Current pumping not sustainable”
The new model indicates “that pumping at current average rates and spatial distribution would cause seawater to intrude further into the Basin which is an indication that current average pumping and distribution is not sustainable.”
The old model suggested the basin could pump 2380 acre feet per year without causing an influx of seawater. The headline here is that the five-year basin average of 1830 AFY of pumped groundwater is already causing seawater intrusion. Just what number is sustainable” has yet to be determined.
Seawater moving inland 24 feet a year
“The extent of the saltwater intrusion over the next 45-years at the current Basin pumping distribution is simulated to advance up to 1,100 feet inland (24 feet per year) in Lower Aquifer Zone D” says the model. “The greatest advance occurs towards downtown from Cuesta-by-the-Sea and would be detected at the new Skyline monitoring well cluster.”
Skyline is just north of Los Osos Valley Rd that divides the town north and south.The 1100 ft penetration is approaching a quarter of a mile inland movement over the next generation.
“In Zone E, the inland limit of the intrusion front stalls at the Community Park, with up to 400 feet of intrusion over 45 years (9 feet per year) continuing at LA11 near Pasadena Avenue. The intrusion front does not reach any active community supply wells, but is close to LA10.”
So now what?
If current groundwater pumping is not sustainable – what is the number? That will be the next task of BMC with more help from their consultant. The committee says it will use the Transient Model to develop updated estimates of sustainable yield for the Basin and to evaluate different recycled water and/or supplemental water supply alternatives to help improve Basin sustainability. This work is currently ongoing.
Initiatives under consideration
On the recycled water front, treated water supplied from the wastewater treatment plant will be replacing pumped groundwater now. The county has used ARPA federal funds to connect recycled water to community Middle School, Community Park and Monarch Grove Elementary School this month, enabling these entities to stop using water pumped from their own wells for their vast grounds. More use of recycled water is a key initiative that could help.
Key meeting on pipeline June 25
As to the quest for supplemental water for the basin, Community Services District (CSD) general manager Ron Munds has been negotiating with the US Army Corp of Engineers on releasing the $8 million funded by Congress and signed by President Biden last year for a proposed 2.5 mile pipeline that would connect to State Water near Morro Bay. But it needs to be approved by the Army Corp of Engineers to move forward with Munds reporting to the CSD board earlier this month that the Corps had put the project on their 2025 work plan, a positive move. So what’s next?
Munds now says that he has an important meeting with the Corps of Engineers June 25 and “we should know more after that.” The original estimate was that the pipeline could bring in 200 acre feet per year of supplemental water although perhaps a higher number could be realized. The water coming in would be drinkable as it is for Morro Bay.
Critics suggest that water would be released based on availability and can’t be counted on. But despite drought and surplus years, next door Morro Bay has never been short of their requested amount that is the majority of their supply.
Call to action – “dire state of the basin”
Beth Reineke (S&T Mutual Water Co Mgr) BMC board member offered her take at the June 18 BMC meeting. Fellow board member Bruce Gibson tried to convince the board that since consultant Spencer Harris was not present to discuss his memorandum and the new model, policy decisions should not be discussed prematurely. Mr. Harris was said to be under the weather.
But Reineke,who is a water quality analyst, said the consultant’s written report spoke for itself and added that the impressive peer reviewed information prompted her today to make a call to action to address the “dire state of our basin.”
“We are currently over-pumping by a large and yet-to-be-determined amount.”
Here are some of her bullet points from her presentation.
- Prioritize a reduction in pumping to stop and reverse seawater intrusion.
Connect to the State Water pipeline to help offset pumping from our basin in wet years. This could prevent the pumping of 200-600 acre feet a year (AFY) from our
basin.
Use the 2023 Maddaus Study as a guideline for where conservation is still needed in the community. This would save 118 AFY for our basin.
Seek out solutions to reduce the estimated pumping from private well owners (Maddaus reported an average of 390 gallons each house per day).
Insist on maximal use of recycled water in each possible area of Los Osos. - Connect all remaining non-sewered homes to the sewer collection system.
Cabrillo Estates is ready and willing to connect to the sewer; the BMC should support this project.
The other non-sewered areas of Los Osos should also be added to the sewer to maximize the flow of recycled water and reduce nitrate loading into the basin. - Implement a stormwater capture program.
Use the Transient Model to identify opportunity areas for capture measures.
Educate the community and incentivize household-scale catchment systems. - Add forever chemicals (PFAS) and chromium-6 to our Basin Management Plan.
The BMC must take a stance on the contamination in our basin. We need to work vigorously to stop the sources and mitigate the ongoing issues of PFAS and chromium-6 in our water supply.
Reineke believes the entire community needs to be connected to water meters and that this would cut water use in the basin.
By installing meters and increasing water rates, S&T Mutual halved our water use in the span of a decade from 55 AFY in 2013 to 27 AFY in 2023.
“It is imperative that all water users share equal responsibility in terms of both cost and conservation. Financial incentives and monitoring have been crucial in achieving water- saving goals in Los Osos. By installing meters and increasing water rates, S&T Mutual halved our water use in the span of a decade from 55 AFY in 2013 to 27 AFY in 2023. So, I believe it is time to extend this philosophy to the remaining users who also rely heavily on our shared water resources.
By ensuring that all users contribute fairly, we can promote sustainable water management as we strive to bring the basin back into balance.”
The town’s three water purveyors pump around 1000 acre-ft per year for the urban area residents and businesses of Los Osos and Baywood Park.These connections are all metered. What is not metered are the rural resident wells and ag wells that together are estimated to pump almost 700AF a year (see chart). Not in the report but much of the ag water goes to pasture and hay production with limited commodities for human consumption.
In the next few months the Los Osos BMC is revisiting this issue using this model to estimate various scenarios to fight off more seawater influx. Some of these initiatives are controversial and long term.
But if the sustainable yield is 1600AFY for the sake of argument, the adopted policy to do 80% of the sustainable yield would be 1280AFY pumped. That amount can be augmented with imported water from the state pipeline by whatever amount we can secure – 200 to 600 AFY, suggests Reineke. That is water we can use but don’t have to pump.
Ron Munds says connecting the two schools & park to recycled water can yield perhaps 50AFY.
Capturing stormwater might be done in a couple seasons with some engineering.
The county’s Growth Management Ordinance passed last year established a 0.4% growth rate for Los Osos in Calendar Year 2025 including an allocation of 25 new dwelling units in Los Osos.
Given this latest report,the Los Osos growth rate this coming year might arguably be zero.
