Groundwater studies in the vicinity of wells that draw drinking water for Columbia indicate that one of the city’s wetlands units used to treat waste water could be leaking contaminants into the underground system.
Joseph Richards of the U.S. Geological Survey, which has been studying groundwater in the Missouri River bottoms south of Columbia since 1992, said increased levels of chloride, sulfate and sodium have been detected in monitoring wells since 1995. The highest levels of chloride were detected in a monitoring well 50 to 100 feet from a city wetlands treatment unit.
“A number of the wells showed increased chloride,” Richards said. “Some showed up to 10 to 20 times more than the background concentrations. That gives us an indication that the treated effluent is interacting with the groundwater.”
Matt Knowlton, an assistant research professor at MU who is knowledgeable about the groundwater studies, said the data leaves him with little doubt that one of the city’s treatment units is leaking. “It’s not supposed to leak, and it’s leaking,” Knowlton said. “There’s no other way to account for the chloride. It may not be a problem, but I don’t know.”
Knowlton said that while chloride, sulfate and sodium do not pose a health threat, they serve as good indicators that treated effluent is leaking into the groundwater. Richards agreed it’s unlikely that any leakage from the treatment units would pose health risks.
In response to inquiries about the USGS data, city spokesman Robert Ross on Friday issued a one-sentence prepared statement: “If a problem is identified, the city would isolate that problem and fix it.”
The city operates four wetlands treatment units in the McBaine bottoms south of Columbia. Waste water is treated at a mechanical plant on Gillespie Bridge Road and pumped to the wetlands units for additional treatment using cattails and natural biological processes. The bottoms of the treatment wetlands are lined with clay to prevent leakage.
After flowing through the city treatment units, treated waste water is sometimes mixed with river water and used to help flood Eagle Bluffs Conservation Area, a nearby state-managed wetlands. The ongoing groundwater studies are also designed to monitor any effects that the state wetlands might be having on the underground water system.
The well field used by the city to draw drinking water is also located in the McBaine bottoms. Fourth Ward Councilman Jim Loveless, who manages Eagle Bluffs for the Missouri Department of Conservation, said it’s improbable that the water tested by the USGS will infiltrate the municipal well field. The USGS took samples from wells about 30 to 60 feet deep, he noted, and city wells are more than 100 feet underground.
“We have tens of feet between where these samples were taken and where the city gets their water,” Loveless said.
Furthermore, Loveless said, there is reason to believe that much of the groundwater in McBaine runs laterally rather than vertically. Over the years, the Missouri River has deposited layers of silt, sand and clay, he said, and the clay layers act as an impermeable surface that may keep water near the surface from penetrating deep into the water table.
Dennie Pendergrass, chief of operations for the city Public Works Department, said work has been done on the city wetlands unit closest to the monitoring well that has recorded elevated levels of chloride, but not in direct response to any possible leakage. The unit began suffering from damaging dirt slides during the mid-1990s, Pendergrass said, which could disrupt the flow of water in the treatment wetlands. Work has begun to fix these problems, and Pendergrass hopes this will help reduce any possible leakage.
The dirt slides “could be part of the problem, but I don’t know,” Pendergrass said. “Once we finish the work, we’ll continue the monitoring of the wells and look if the levels” of chloride, sodium and sulfate decline.
The most recent USGS study, released in December, found that the seasonal flooding of Eagle Bluffs has created a dome under the conservation area in which the water table is raised. This means underground water can flow downward in several directions as opposed to simply following the normal slope of the land.
As surface water from Eagle Bluffs percolates down, Knowlton said, it might be heading in the direction of the city well field. But he said this is not a problem. The water from Eagle Bluffs takes six years to reach the city well field, he said, and by that time he considers it safer than the water that cities such as St. Louis and Kansas City take directly from the Missouri River.
Water from Eagle Bluffs might be contributing traces of chloride, sodium and sulfate, he said, but those levels are far less than those he believes are leaking from the city wetlands treatment unit.
“Other than the fact that ‘ooh, it used to be waste water,’” the water in the state conservation wetlands is good,Knowlton said.
Groundwater monitoring will continue in the McBaine bottoms. On Jan. 5, the Columbia City Council decided to give $31,000 of the $56,300 needed to allow the USGS to continue its studies using 16 monitoring wells.