A safety advisory on Wolf Creek was lifted Tuesday morning, following a sewage spill at the peak of a series of storms that dropped more than 11 inches of rain in four days on western Nevada County.
Of the estimated 450,000 gallons of untreated or partially treated wastewater that spilled into the creek, 136,500 gallons were recaptured, said Tim Kiser, director of the public works department that oversees the Grass Valley Waste Water Treatment Plant located at 355 Freeman Lane.
Grass Valley will report the spill as required to the Regional Water Quality Control Board, arguing that the overflow was an “act of God,” said City Manager Dan Holler.
During Sunday’s peak, 20 million gallons came into the treatment facility over 155 minutes, Kiser said, however the facility was designed to handle 10 million gallons per day. Those waters came amid mass flooding in western Nevada County, with sewers around Grass Valley backed up.
“Any municipality cannot design a wastewater treatment plant to deal with these act-of-God situations,” Kiser said. “It would be way too costly.”
Nearly 600 residents along Wolf Creek, from Grass Valley into southern Nevada County, were warned of the spill Sunday through Nevada County’s Environmental Health Organization activating a phone-warning system.
The city previously made the same “act of God” argument to the Water Quality Control Board after nearly 4 million gallons of untreated or partially treated wastewater spilled into Wolf Creek amid a March storm that dropped 5 inches of rain over several days.
The Water Quality Control Board slapped a $110,000 fine on Grass Valley after 10,000 gallons of wastewater spilled into the Wolf Creek in October 2011. That spill came from near a Joyce Street lift station, not the treatment plant.
In June 2009, an estimated 10,000 gallons spilled into Wolf Creek as a result of a plugged valve and a monitoring system that failed to sound an alarm.
According to the state Water Resources Control Board, Grass Valley had a total of three violations in five years.
Grass Valley isn’t the only facility that has had recent spills. South county’s Lake of the Pines wastewater treatment plant released about 37,500 gallons of raw sewage into Magnolia Creek in January.
While discharged sewage contains bacteria and other pathogenic micro-organisms that can cause illness in humans and animals, Kiser argued the impact of Sunday’s spill was mitigated from the high water flow through Wolf Creek, which was 10 times its normal capacity, he said.
“When water moves so fast, the waste is washed out of area very quickly,” Kiser said.
Another factor contributing to the overflow was the supersaturation of a waterway from the old Newmont Mine, Kiser said. The waterway adds 500,000 gallons to the treatment plant on a normal day, but it adds more than double that amount on high-rain days.
The Newmont Mine’s water was diverted Sunday from the treatment plant, putting it directly into the creek, officials said. Kiser said that although the state mandates the city treat the mine’s water, he said it is merely groundwater that is not any more dangerous than the creek’s natural water.
“It’s the same water as a house’s well above the Newmont Mine,” Kiser said.
Groundwater seepage into city’s water pipes also contributed to the overflow, Kiser said.
In January, Grass Valley officials approved $1.16 million to replace clay pipes laid beneath city streets decades ago and formed small cracks that have increased the rainwater that gets into sewer pipes, increasing the amount of fluid heading into the city’s sewage treatment plant — and costing ratepayers money.
That’s on top of $171,000 Grass Valley spent to reline sewer pipes in 2011.
Built in the 1940s, the wastewater treatment plant’s last major update was in 1995, Kiser said.
“I don’t think this city can replace it and I don’t think it would be a good use of city funds,” Kiser said. “When you are faced with the large amount of rainfall we received in a very short amount of time, unfortunately the plant is not designed to handle those kinds of flows. There isn’t much that, at this point in time, we can do to resolve that.”
To contact Staff Writer Christopher Rosacker, email email@example.com or call (530) 477-4236.