According to a recent report, the Sunnyslope County Water District Wastewater Treatment Plant’s pond No. 5 is likely the cause of recent landslides on Southside Road.
The report, conducted by Earth Systems, concludes that the stability of the slope where at least five landslides occurred earlier this summer was weakened by groundwater traveling along a fault trace near the water retention pond at the top of the slope. The report was produced using aerial shots of the landscape, review of available data and geological testing.
In June, San Benito County hired Don Chapin Co. to clean the landslide that forced the closure of a portion of Southside Road. With the recurring landslides, the county added a second contract with Don Chapin on Aug. 21 Supervisors regular meeting, bringing the total to over $1 million, according to the meeting agenda packet.
According to the report, the construction and use of pond No. 5 contributed to and accelerated the destabilization of the slope that is made of cemented fine-grained materials, sand and small amounts of gravel.
The report states the water can be “directly attributable” to the Sunnyslope wastewater treatment plant as there are no other water sources nearby, since groundwater is about 23 feet below Southside Road.
“Continued loading of pond No. 5 will likely continue to weaken slopes adjacent to the plant,” the report states, adding that soil removed from the bottom of the slope for drainage and road maintenance also likely contributed to the landslides.
While the county suspected that water from the treatment plant was the cause of the landslides, Lynn Hilden, owner of an adjacent property, told the San Benito County Board of Supervisors at multiple meetings that pond No. 5 was the problem.
“They knew . . . and they started putting water into it again,” Hilden said at the Aug. 21 supervisors meeting. “I’ve lost a couple acres. It’s no question what the problem is. It’s Sunnyslope Water District.”
Hilden spoke out against Sunnyslope again in the supervisors’ Aug. 28 special meeting. He said the water district began using pond No. 5 two years ago after it had been non-operational following a landslide in 1984. Sunnyslope treats water for the communities of Ridgemark, Quail Hollow and Oak Creek and has been a special district since 1954.
According to the Sunnyslope website, the district "operates as a special district to provide local service in accordance with the California State Water Code. The District is responsible directly to the customers of the District. It is not an arm of the federal, county, or city governments, or other water agencies."
Besides testing the soils and moisture of the slope on Southside Road where the landslides occurred, the study included research on seismic activity. The report concludes that the landslide was not a result of earthquakes.
Earth Systems found four earthquakes with a magnitude higher than 1 and within 10 kilometers from the landslide location during a two-week span, a week before and after the initial landslide.
Of the four earthquakes registered, the highest, a 2.7, occurred on May 24. The first landslide occurred in mid-May.
“Because of the relative low magnitude of these earthquakes, their distance from the site, and and because none occurred on or the day before initiation of the [Southside Road Landslide], there was no apparent evidence that these earthquakes destabilize slope at the subject site,” the report states.
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