Importance of Lake Sediments in Removal of Cyanobacteria, Viruses, and Dissolved Organic Carbon

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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.

USGS scientists collecting water-quality samples from shallow groundwater under Ashumet Pond

U.S. Geological Survey (USGS) scientists collecting water-quality samples from shallow groundwater under Ashumet Pond, Cape Cod, Massachusetts. Photo Credit: Denis R. LeBlanc, USGS

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.

Experimental setup for small-scale transport test

Experimental setup for small-scale transport test designed to assess the effectiveness of the "colmation layer" at the bottom sediments of Ashumet Pond (Cape Cod, Massachusetts) for filtering out cyanobacteria and viruses. The green color in the collapsible bag containing the colloids and a bromide tracer is caused by the presence of microbial sized fluorescent microspheres that scientists added to act as colloidal tracers. Photo Credit: Denis R. LeBlanc, USGS

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.

Environmental Health Considerations

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.

This research was funded by the USGS Ecosystems Mission Area’s Environmental Health Program (Contaminant Biology and Toxic Substances Hydrology) and the Sonoma County Water Agency.