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Groundwater survey could lead to new water storage approach

November 9, 2021 — This Thursday, residents around the Ukiah groundwater basin may see a helicopter flying low, hauling a large hoop. It’s part of a state-sponsored program designed to map the geological features of groundwater basins. 

Katherine Dlubac is an engineering geologist and the project manager for the Department of Water Resources’ stateside Aerial Electromagnetic (AEM) surveys. She laid out some of the ways that information from the surveys can be used, with the larger goal of implementing SGMA, the Sustainable Groundwater Management Act of 2014, which requires local jurisdictions to come up with plans to manage the resource.

“It tells us about the aquifer structures,” she said; “where we have thick layers in the subsurface of sands and gravels that allow for water to flow, but also for water to be stored. It also tells us where we have layers of clays and silts, so fine grained materials that inhibit water movement. And so while the AEM data still needs to be combined with other types of data...what it can do is it can provide you a better picture of what’s happening in the aquifer...so that you can make those management decisions of whether you want to try recharging water in the area from the surface, or if you want to try injecting water into the aquifer to store it there as another type of reservoir.”

Supervisor Glenn McGourty is part of the local Groundwater Sustainability Agency tasked with gathering as much information as possible to craft the plan. The Ukiah groundwater basin relies mostly on the Russian River, but is also fed by about 160 smaller tributaries.

“Any data we get is going to be useful,” he said; “because you can’t usually see groundwater. You have to have some way of measuring it. Often it’s delineated by wells, so you don’t know for sure what you’re looking at. And there are a couple of mysteries, because we don’t really have uniform geology here in the valley. So the two mysteries are, where is the groundwater? And the second mystery is, how does it get recharged? The surface groundwater interface, as we call it, is the thing that’s really hard to understand.”

Dlubac says the electromagnetic technology has been around since the middle of the last century, but it was used mostly in mining applications. After Prop 68, a Parks, Environment, and Water Bond passed in 2018, the Division of Water Resources carried out a pilot program in central and Southern California counties to gather data for their groundwater management plans. The survey taking place now will measure basins across the state for the next two years, taking what Dlubac calls “a snapshot” of their geological features. After the two-year survey of “coarse grid data,” she hopes to go back for a finer picture. “When we go in and start collecting fine grids of data, we can start to get more high resolution information about some of the space in between the coarse grid where we didn’t collect information,” she reported. “And that can support defining recharge areas, better understanding where we have clays in the subsurface, where we have subsidence, and other areas that can support the implementation of SGMA.”

McGourty is interested in experiments that are currently underway in the Central Valley, on how to store water in the ground, rather than in surface reservoirs. Knowing  what kinds of sediments are where could further that approach locally. “The idea would be to divert the river during really high flows and to inundate some parts of the basin where there’s fairly permeable alluvium, gravel, principally,” he explained, adding that the City of Ukiah already uses recycled wastewater to recharge the aquifer. “One of the things we’re not really sure about,” he reflected, adding to the mysteries still to be solved; “is what are the parameters of the river underflow and things like that. So any information about where water is coming from is of interest to us.”

Dlubac expects the information from the survey to be available to local groundwater management agencies in about eight months.


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