Researchers achieve proper pH in streams undergoing culvert maintenance
Researchers at Morgan State University learned that a spike in the pH of streams caused by common culvert repairs could be tempered using a readily available material - peat moss.
The culverts, often made of galvanized steel, are subject to scour over time, and concrete grout is used to repair them. When water flows over the freshly paved culvert, the grout surface produces a dissolution effect that temporarily elevates the pH, which can spike as high as 12.5; most aquatic organisms prefer a pH of 6.5 to 8.0. Such repair work often requires a permit from the Maryland Department of the Environment and the U.S. Army Corps of Engineers, and Maryland regulations limit pH to 8.5.
"When water flows over the paved grout surface, the pH of that water will rise until the free lime content of the paved surface is dissolved," said Dr. Dong Hee Kang, one of the paper's authors and a lecturer in Morgan's School of Engineering. "The mechanism and rate of this reaction occurs most strongly in the first few hours, and higher flow rates result in shorter spikes."
Researchers at Morgan filtered the water through a sediment bag with common materials at different flow rates and pipe lengths to see if they could remediate that spike in pH. They tried straw, mulch and peat moss, both wet and dry. The tests were conducted in laboratory experiments and in the field at three sites undergoing Maryland State Highway Administration (SHA) paving in 2011-2012: Owings Mills, Crofton and Frederick. The research was funded by the SHA.
In laboratory tests, straw did not reduce the pH much, and while mulch proved a little better, the results were still higher than the Maryland regulations requiring a pH of 8.5. But wet peat moss reduced the pH to an average of 4.9 for both fast and slow rates. Researchers then tested nitrate and phosphate concentrations to ensure using wet moss wouldn't adversely affect streams.
At the field sites, researchers used a specially built wooden frame to filtrate the water through mulch, straw or peat moss and measured the pH before the culvert and at various distances afterward. Again, wet peat moss was found to be effective in quickly reducing pH.
"Following construction, any loose pieces of dried grout or dust should be removed if possible, either by sweeping or vacuuming, "Dr. Kang said. "A catchment area devised of sandbags should be used to capture the flush from the culvert and then the water should be pumped into a sediment bag on a bed of wetted peat at a minimum depth of 4 inches. This could potentially save time and cost when meeting water quality standards."
The complete research report is available at http://www.roads.maryland.gov/OPR_Research/MD-14-SP109B4D_pH-Standard_Report.pdf.