While the ground in Arbuckle and other parts of Colusa County isn't necessarily sinking, there are areas of subsidence raising concerns for some local experts.
Since 2008, the most impacted area in the county — in Arbuckle along Interstate 5 — has dropped 2.08 feet. The rest of Arbuckle has experienced anywhere from 4 to 8 inches of subsidence. Around the city of Colusa, there has been about 2 to 4 inches of change over the past eight years.
"Subsidence is not a good thing," said Roy Hull, engineering geologist for the Department of Water Resources. "It can cause damage to roads, to railroads, to buildings, buried utilities, to wells and to conveyance systems."
In 2008, DWR conducted a study of the Sacramento Valley to establish elevations at different points. DWR did this study so that it could return in the future and take additional measurements to determine whether the ground was moving.
Hull said DWR has received reports from people living within the county that could be related to subsidence. The reports included damaged wells, foundations and walls cracking, retaining walls cracking, the ground slumping, field levels being off, and conveyance lining cracking and buckling.
There are two types of subsidence: elastic and inelastic. Elastic subsidence is a normal occurrence. DWR was monitoring for inelastic subsidence.
"The one we are most concerned about is the inelastic, which is the stuff that compresses and never returns," Hull said.
Inelastic subsidence occurs when the structure of a clay is compromised during compaction, to the point where it is unable to expand to its original thickness even when groundwater levels rise. When that layer of clay, or silt, in the underground compresses, ground levels drop.
Subsidence is not uniform. There are certain areas that can drop due to subsidence while the surrounding area stays the same.
"You get a lumpy type of dropping going on," Hull said. "Slumping on roads, stuff like that."
Hull said this can occur in California from three different factors: tectonic movement, oil and gas extractions, and groundwater extractions.
While DWR has not confirmed the cause of subsidence in the county, some experts believe there is a good chance it is occurring due to groundwater extra tions, in addition to the impact of the drought over the past few years.
"It's when the groundwater levels decline, and we've seen some decline in the Arbuckle area since the drought," said Mary Fahey, water resources coordinator for Colusa County. "It's a number of factors. The drought played its part in it because there was a lack of surface water. The TC Canal serves that area and it had a zero allocation over those two years."
DWR has conducted small surveys since 2008. It wasn't until a NASA study came out last year that confirmed DWR's concerns. This prompted DWR to schedule a full-scale survey for spring 2017.
Subsidence could also come into play when the Sustainable Groundwater Management Act takes effect in 2022. In the act, the county must address different areas of concern, and subsidence is one of them.
"That really prompted DWR to come in to resurvey in the Arbuckle area," Fahey said. "It's something that we are going to have to address in our plan. It's definitely something to take note of because it's not something we've seen before."
As part of SGMA, the county must create a groundwater sustainability agency tasked with establishing a groundwater sustainability plan by 2022. The county is already underway with forming the agency. Once established, the agency will create a plan that must address concerns like subsidence to show that the county's water use isn't causing it.
Fahey said DWR's study next spring will help with addressing the issue.
"We will keep subsidence on our radar, but we don't want to wait until 2022," Fahey said. "If we know it's happening now, we can start taking some types of actions to address it."
SGMA could also influence DWR's survey by requesting the department adds more points throughout the county to more regularly monitor for subsidence.
"Obviously, as part of SGMA, this is something that will need to be dealt with," said Denise Carter, county supervisor. "This is something none of us had expected. The NASA study is what led us to push for the GPS survey, it's another data point. I don't know if we can infer everything at this point, but I think it's going to be important to have more regular monitoring."