To protect pond levees and its water treatment infrastructure, the city of Modesto began releasing partially treated wastewater into the San Joaquin River on Thursday afternoon.
Wastewater that has been treated but not disinfected will be discharged at an estimated average daily flow of 30 to 45 cubic feet per second, the city said in a news release Monday morning. The release will “continue for as long as necessary to prevent pond levee failure. Completion could take weeks.”
With the San Joaquin’s current flow downstream of Vernalis higher than 32,000 cfs, the city said, the wastewater is greatly diluted and the potential hazard lessened. Still, because water that’s not been disinfected carries bacteria and viruses, the city warns that the public should avoid direct contact with the river during the discharge.
The water is being released at the city’s Jennings Road treatment plant.
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The city is required to sample the river and report the data to the state to assess the impacts of this discharge. The public will be notified if other risks should arise.
News release on discharge of partially treated wastewater
Early last week, Modesto noticed increased flows of wastewater into its Sutter Avenue treatment plant as the Tuolumne River continued to rise because a spillway gate at Don Pedro Reservoir was opened to reduce the reservoir level and protect the dam. The Tuolumne flows into the San Joaquin.
The city discovered Feb. 23 that a hole in a sewer trunk line that runs along a bank of the Tuolumne was the cause of the increased flow into the plant. But after crews built a temporary barrier around the hole to keep river water out, the city estimated that release of partially treated water could be weeks away.
What changed? As the city was working to secure the sewer line, “it took a lot longer for the flows to go down than initially thought, city Utilities Director Larry Parlin said Thursday. Working with the California Department of Water Resources’ dam-safety division, there was a shared concern that increased elevation in treatment ponds, combined with wind and wave action, could erode levees, he said.
“City staff has contacted the appropriate state agencies and they are fully informed of the actions the city will be taking to prevent pond levee failure and flooding at the Jennings Road facility,” the city’s news release said.
On the public health risk from the releases, Parlin said, “Obviously, you don’t want to be drinking floodwater, especially if it contains undisinfected treated wastewater.”
Modesto’s wastewater first is treated at the Sutter Avenue plant along the Tuolumne River and then is sent by pipeline to the city’s Jennings Road treatment plant along the San Joaquin River. The wastewater there receives additional treatment.
But given the high river flows and “all sorts of nasty stuff” flooding has added to the rivers – animal waste, leaching from septic tanks and much more – he said he believes what the releases add to the mix is “almost insignificant.”
The city will be monitoring river water quality upstream and downstream of the discharge, Parlin said, and is not expecting any measurable change in bacteria. “It’s probably not going to add to the current health hazard.”
Still, the discharge is a violation of the city’s permit, Parlin said, “and we’re not happy about it.” Now that flows into the Sutter Avenue plant have decreased, the city will monitor the situation and end the discharge as soon as possible, he said.
When the weather is clear and warm, there’s tremendous evaporation from the holding ponds, Parlin said, which helps. On the other hand, he said, if it gets too warm, the snowpack begins to melt and increases river flows.
Though the city never has had to discharge partially treated wastewater before, Parlin said, it doesn’t compare to the flooding of 1997, which submerged parts of Modesto’s treatment plant, carrying raw sewage into waterways.
Deke Farrow: 209-578-2327