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San Benito
July 13, 2024

County releases landslide report blaming water district pond

A consultant’s report funded by the county and obtained by San Benito Live concludes that a Sunnyslope Water District waste pond likely caused a series of 2018 landslides on Southside Road.

That 90-page report details the investigation done by Earth Systems, a geotechnical engineering firm, in the aftermath of at least five landslides along Southside Road near Blossom Lane starting in May 2018.

Those landslides led to nearby road closures, a seven-figure repair and research cost, and tensions between the county and Sunnyslope Water District over the cause and communication on the matter.

The county earlier this month initially declined to release the report, but ultimately provided the document to San Benito Live after County Supervisor Mark Medina and open records attorney Jim Ewert urged for public access to the information.

The report funded by San Benito County includes a variety of analysis and concludes that water from Sunnyslope’s Pond #5, adjacent to the site of the landslides, caused destabilization of the slope and the resulting landslides. Earth Systems senior geologist Brett Faust prepared the document for County Counsel Barbara Thompson, according to the report.

The involvement of prospective litigation was the reason behind the county’s initial reluctance to initially release the report, according to officials. There is no litigation filed, however, and the document was compiled as an engineering analysis to make technical conclusions on landslides’ cause.

The open records attorney for the California Newspaper Publishers Association and Medina supported the document’s release due to the fact that tax dollars funded the work and that it was technical in nature while performed to find a cause for the landslides. Ewert also underscored how a lack of litigation filed on the matter also points to a necessary release.

The county has spent over $1.2 million on its response to the landslides, which have created logistical problems for residents and the Southside School District located on the rural road.

Some of the conclusions in the report include the following:

  • Earth Systems assessed geologic conditions, local seismic activity, groundwater conditions, past grading activity and nearby land uses in the study.
  • The firm reported no apparent slope instability between 1939 and minor landsliding that occurred sometime between 2006 and 2009, “suggesting that the addition of Pond #5 in 1984 likely accelerated the process of dissolving cementation.”
  • The report does, meanwhile, acknowledge “limited available data” in coming to a conclusion.
  • There was no obvious correlation with seismic activity.
  • Continued loading of Pond #5 will likely continue to weaken slopes adjacent to the plant.
  • Changes to drainage patterns associated with the nearby dirt path, and road maintenance that has removed soli from the toe of slopes, may have contributed to the landslide.
  • San Benito Formation rocks are heavily cemented at the site. The calcium carbonate cement has been dissolved by subsurface water, decreasing slope stability in the area of the landslides.
  • Subsurface water appears to be migrating along a fault trace that passes “very close” to the west end of Pond #5.
  • Earth Systems found that groundwater was likely not the cause.
  • Still, saturated soil samples from the landslide area showed inconclusive results, partly due to the limited amount of liquid sample that could be extracted.
  • The setting of the landslides has been the north side of Southside Road and northeast of Blossom Lane where the elevation is 355 feet to the slope crest. About 80 feet north of the slope crest is Pond #5 of the Sunnyslope Water District Treatment Plant. Ridgemark is northeast and east of the plant.
  • The report refers to apparent, probable impacts to slope stability resulting from percolation of treated wastewater directly above the landslide area.

As for next steps, the county’s Resource Management Agency Director John Guertin was not immediately available Wednesday on the matter.

Sunnyslope Water District General Manager Don Ridenhour, though, said the county has communicated about setting up a meeting on the matter. He said he received a copy last week after the county previously declined to provide it.

“I’ve certainly gone through it, read it and gone through it a couple times,” Ridenhour said. “We’ve sent it off to our attorney who has sent it off to a geotechnical expert our attorney has hired on our behalf to review it.”

His initial feeling after reviewing the report is that it wasn’t “very conclusive” in nature.

“I’ve read the conclusions several times now, and boy it doesn’t read very conclusive to me,” Ridenhour said.

He said he feels like those conclusions have been based more on the district pond’s proximity to the landslides rather than actual data.

“I’ve yet to see the evidence that shows that’s really certain at all,” he said about the concept that the pond caused the slides. “But time will tell. I don’t know that it isn’t (the cause).”

Out of precaution, he said, Sunnyslope shut down Pond #5 in May after the initial landslide and plans to keep it that way for the time being.

“Until this is resolved, we’re not going to use that pond,” he said.

He said Sunnyslope also has to wait on its own experts to have a good idea on next steps.

“It’s a little frustrating to get the report this late in the game when the accusations have been made for months now,” he said. “It’s going to take some time.”

Ridenhour earlier this month told San Benito Live he had been disappointed after Supervisor Anthony Botelho broached the report’s findings in a meeting of the Local Agency Formation Commission when the panel had been considering annexation of a 90-home subdivision, near Ridgemark, into the Sunnyslope Water District.

“I’m a little disappointed and frustrated with them that it could come out in a LAFCO meeting but they refuse to share it publicly,” he told the local media outlet, which had been pushing for the document’s release as well.

He was also upset the matter was being discussed by county officials in closed session due to the prospects for litigation.

“It’s all about money,” Ridenhour said at the time. “But how are you ever going to resolve it unless you release the information?”

To read analysis in the report, excluding attachments, link below:

Southside Road file photo Courtesy of Medina