Environmental Health - Toxic Substances Hydrology Program
U.S. Geological Survey (USGS) scientists determined that the colmation layer (top 25 centimeters of lake sediments) was highly effective in removing cyanobacteria, viruses, and dissolved organic carbon during water passage through the lake bottom to aquifer sediments.
River-bank filtration has proven to be a sustainable and cost-effective method of removing microorganisms and chemicals from surface water during filtration through bottom and aquifer sediments. Lakes can also be important locations for bank filtration operations. The biologically active layer of sediments immediately beneath the sediment/water interface, sometimes referred to as the colmation layer, is considered to be important for contaminant and carbon removal, although its specific role(s) in the elimination of human pathogens poses a lingering question.
In this study, scientists focused on the importance of the upper 25 centimeters of lake sediments in the removal of cyanobacteria, bacteriophages (viruses of bacteria), and dissolved organic carbon during natural bank filtration in lakes. This study was done at two shallow (0.5 meter deep), sandy, near-shore sites at the southern end of Ashumet Pond, a waste-impacted, kettle pond on Cape Cod, Massachusetts , that is subject to periodic blooms of cyanobacteria and continuously recharges a sole-source drinking-water aquifer. Cultured cyanobacteria (Synechococcus sp. IU625), its cyanophage (virus, AS-1), a coliphage (MS-2), and synthetic microspheres were injected into a pond water/groundwater interface and tracked as they passed through the colmation layer and underlying aquifer sediments.
Scientists determined that more than 99 percent of the cyanobacteria and bacteriophages and approximately 44 percent of the pond-dissolved organic carbon were removed in the colmation layer. Scientists determined that the low relative breakthrough of the bacteriophages occurred through a combination of sorptive-filtration and inactivation. Although the physicochemical and biological parameters controlling microbial removal in the colmation layer vary from site to site, there is evidence to indicate that cyanobacteria populations are largely removed within the top few centimeters of bottom sediments during lake-bank filtration.
Lakes can contain large numbers of cyanobacteria that may produce harmful toxins, as well as viruses and high levels of dissolved organic carbon that can be of concern if the lakes are used as sources of drinking water. The findings of this study indicate that lake-bank filtration can contribute to removal of cyanobacteria and viruses, thus providing a method of pre-treatment of lake water used for drinking water.
Harvey, R.W., Metge, D.W., LeBlanc, D.R., Underwood, J., Aiken, G.R., Butler, K., McCobb, T.D., and Jasperse, J., 2015, Importance of the colmation layer in the transport and removal of cyanobacteria, viruses, and dissolved organic carbon during natural lake-bank filtration: Journal of Environmental Quality, v. 44, no. 5, p. 1413-1423, doi:10.2134/jeq2015.03.0151.