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By Chris Frost
Ventura—The conversation about increased pumping fees for groundwater in the Oxnard basin continues with seawater invasion and how it relates to the groundwater locally.
The Fox Canyon, Groundwater Management Agency, held its fourth workshop, Aug. 21, and discussed the reasons why the area will reduce pumping in the future to meet its sustainability goals as it moves toward 2040.
Cities can expect considerable pumping fee increases per acre-feet of water and can have far-reaching effects on the local economy.
Jill Weinberger made the presentation to the attendees and said because of groundwater elevations and the tie to seawater intrusion in Oxnard; the undesirable result is net seawater intrusion over climate cycles of wet and dry periods.
For groundwater in storage, it's a volume of groundwater produced that would exceed the amount of fresh-water recharge over cycles of drought recovery.
"Seawater intrusion, it's an intrusion that results in a net landward, so we're talking about the expansion of those areas and making them larger," she said. "Net landward migration of that saline water impact from beyond the currently impacted area. Groundwater quality is tied to that landward migration of the saline water impact front. Any subsidence is subsidence that interferes with surface land uses. These are undesirable results."
Finally, she said interconnected surface and groundwater, which you can sometimes see in streams, can be a loss of a water-dependent ecosystem habitat.
"For Pleasant Valley, there is a little bit of a change," Weinberger said. "Although those water levels interact with the water levels in Oxnard and influence each other, there is no direct seawater intrusion into the Pleasant Valley Basin. We've done modeling and particle tracks, and there is no expectation that under any scenario of modeling that seawater itself would extend inland enough to impact the Pleasant Valley Basin physically. But the Pleasant Valley Basin and the groundwater elevation in Pleasant Valley can impact the ability of Oxnard to eliminate that undesirable result."
Undesirable results in Pleasant Valley include groundwater levels that do not recover to pre-drought conditions during multi-year periods of above-average precipitation.
"If you're in a wet cycle, but your groundwater elevations are still down, then you haven't allowed that groundwater to recover with the climate," she said. "That's an undesirable result. Also, groundwater levels that would prevent the Oxnard sub-basin from stopping landward migration of that saline water impact is an undesirable result."
Any measurable increase in brine migration along faults in the Pleasant Valley Basin, she said, is an undesirable result related to the groundwater elevation.
"Groundwater and storage are the same," she said. "It's the volume of groundwater produced that exceeds the recharge cycles in drought recovery is not directly applicable to the Pleasant Valley Basin. It's applicable in the sense of the interaction between the two, but it's not directly to Pleasant Valley. The groundwater quality is a gradient that would cause expansion of currently impacted areas for water quality. Subsidence is the same and is any subsidence that would interfere with surface land uses. Interconnected surface and groundwater are the same. It's the loss of groundwater-dependent ecosystem habitat."
To truly examine the potential impacts on the sustainability indicators and understand undesirable results, Weinberger said you need a tool that looks at the basins as a whole, called the numerical groundwater model.
"We used the United Water Conservation District model for Oxnard and Pleasant Valley," she said. "We also used the West Los Posas Management area. The model is used to investigate production rates that would mitigate undesirable results, starting with the assumption that the 2015-2017 average production rate is continued into the future."
It was the baseline model run, and the model assumption to see if there would be any undesirable results under that scenario.
"There are a few required modeled scenarios, and there are additional scenarios that were run as part of this," she said. "The required ones assess the future baseline and continue production at the average 2015-2017 production rates for the next 50 years. We're also looking at what the sustainable yield might be if it determines there were undesirable results and if they did, what would the sustainable yield be if we didn't include projects and what would it be if we did incorporate projects?"
In this context, she said the sustainable yield is the production rate where they avoid undesirable results.
"Those were measured for the aquifer systems, the upper and lower aquifer system, and they were not measured for each aquifer individually," she said. "No individual model scenario was used to manage the basins. The model scenarios were analyzed as a group to understand what the sustainable yield would be. The sustainable yield comes from the group of model scenarios."
The 2015-2017 scenarios included production and a future baseline with projects.
"For the projects to be incorporated, they were submitted by stakeholders; and they had to meet the criterion for inclusion in the Groundwater Sustainability Plan (GSP) or approved by the Fox Canyon Board for analysis as part of the GSP process.
"That doesn't mean the projects were approved to occur in the future, it means it needs to be analyzed as part of the process," she said. "Some of these projects may be adopted, and some of them may not be. Maybe all of them are, and maybe none of them are. It was to see what the potential impacts of these projects would be."
From there, they did a scenario that reduced pumping and still had the projects.
"There where three scenarios run with reduced pumping and the projects weren't incorporated," she said. "The potential projects are fowling land in Pleasant Valley which would generate an additional 2,200 AF a year of reduced pumping, 500 AF a year in the Oxnard sub-basin and there is 4,600 AF a year of water from the great project in Oxnard basin and West Las Posas because this model covers the West Las Posas Management Area, as well."
In the upper aquifer system, she said the 2015-2017 average production rate was 39,000 AF per year, and the lower aquifer system was 29,000 AF per year in the Oxnard sub-basin.
"In Pleasant Valley, 2015-2017 average rate was 14,000 AF per year, and the same was true for the West Las Posas Management Area," she said. "The scenarios that did incorporate reduced production did it in a way that was a gradual linear ramp-down over the 20-year-period. Each year an addition reduction was taken to get from point A, the 2015-2017 rate to the reduced rate."
What you are going to see as part of the ongoing is a review of the projects to optimize management of the basin to examine ways the sustainable yield can be increased.
“If you look at where there is no net seawater flux, and you look at the production rate in the upper aquifer system, you come to about 32,000 AF,” she said. “In addition to that, there was significant work done to develop uncertainty because all models have uncertainty. When you look at quantifying the uncertainty in these estimates, you come up with plus or minus 4,000 to 6,000 AF a year.”
When you start with the upper aquifer system, Weinberger said the 2015-2017 rate was 39,000 AF each year. If they use the upper end of the uncertainty, the total is 38,000 AF each year.
“If you look at the lower aquifer system, the story is a little bit different,” she said. “From 2015-2017, the production was 29,000 AF a year. The upper end of the uncertainty was more like 10,000 or 11,000 AF a year.”
In Pleasant Valley, the 2015-2017 rate was 14,000 AF each year, and the upper end of the uncertainty was 12,600 AF each year.
“The difference is what was happening in 2015-2017 and what was predicted as the estimated sustainable yield with all the incorporated projects,” she said. “The cautions I gave about those varies with the aquifer system, and it varies with geographic location.”
They can’t manage the basin by reporting pumping and the sustainable yield, she said, and under SGMA, the way to report is by defining a threshold that unsustainable results occur.
“The minimum thresholds are static pumping levels, not water pumping levels,” she said. “When the water levels recovered, they were selected for key wells.”
Sustainable yield is based upon the relationship between the seawater flux and groundwater production from all the model scenarios.
“The one level where the minimum threshold is higher is in Pleasant Valley where the spring water level is 173 feet higher than the minimum threshold,” she said. “The historical water level has been below the minimum threshold in that well.”
If the area has more than one well in the basin, she said the minimum threshold would be the water level where undesirable results occur.
“We have more than one well,” she said. “We define basin-wide undesirable results by the number of wells that are below a minimum threshold.”
Oxnard resident Steve Nash asked if the number is meaningless in terms of goals.
“It is not,” Weinberger said. “The basin will be evaluated as to whether or not it has reached sustainability. We’re not reporting pumping; we’re reporting water levels in this case. By 2040, the water levels need to be above the minimum thresholds.”
When the basin reaches the desired elevation, she said there is still some time for it to decline during a drought, when groundwater tractions need to be increased before it hits the minimum thresholds and triggers an undesirable result.
“There are still static water levels, and we are still sticking with a set of key wells in Fox Canyon,” she said.
She pointed to one well where the minimum objective is 7 feet above sea level, and the measurable goal is 22 feet above the mean sea level.
“They are based on the water level at which there is no net seawater flux into or out of these aquifers,” she said. “During wet periods, you may have groundwater flowing out to the ocean, and during dry periods there may be seawater flowing into the basin. We need to stop the net landward migration of that saline groundwater impact front.”
She told the crowd that the information would be used in the future to set triggers for groundwater extraction reductions, but they are not there yet.
“The board will consider the specific groundwater reduction method after the GSP (Groundwater Sustainability Plan) adoption,” she said. “Because we have groundwater elevations that are below the minimum threshold right now, we have to set targets to get there in the next 20 years. There are a couple of different paths to get there. One is looking at dry climate conditions, one is looking at average climate conditions, and you can have wetter than average conditions.”
The agency has interim thresholds to communicate with the DWR (Department of Water Resources) about how well it’s doing to reach water sustainability by 2040.
“Every five years, an interim milestone is set,” Weinberger said. “If we have drier conditions, it’s going to be hard to make it to that measurable objective. You might only make it up to the minimum threshold. The GSP does talk about the methodology for evaluating whether or not the interim milestone has been met and we’ll be evaluating that every five years.”
The basin has multiple wells, not one well, and they use a more extensive set of criteria to assess whether the whole basin is experiencing undesirable results.
“It’s not triggered by one water level in one well below the minimum threshold,” she said. “What it would be evaluated by in the upper aquifer system is if six of the 15 key wells are below the minimum threshold. That’s sufficient to say the upper aquifer system in the Oxnard sub-basin has an undesirable result. If any of the key wells are below the historical low; that is also an undesirable result. The historical low in Oxnard is below the 2015 water levels.”
If any of the key wells fall below the minimum threshold for three consecutive monitoring periods over 18 months, she said, or for three of five successive monitoring periods.
“You can’t have a basin where one well is always below the minimum threshold,” she said.
The process is just starting and mandated in the regulations (at a minimum) is an evaluation of the GSP every five years.
“It’s not voluntary,” she said. “It’s part of what needs to happen to manage these basins. As data gaps are filled and projects are brought forward that are different than the ones proposed for this initial step of the process. The minimum thresholds and measurable objectives will be re-evaluated. Ultimately, basin management needs to be optimized.”
This story will conclude in the Sept. 27 edition.