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New Method for Assessing Bed Sediment Contamination

USGS scientists have developed a method for estimating the contribution of contaminated bed sediments in lakes and reservoirs to the overall levels of contamination in water systems. Resource managers in the Nation’s mining-affected areas are faced with difficult challenges, such as what to do about fish contaminated with toxic metals, how to quantify the significance of different sources of metal contamination in watersheds, and how to assess the impacts of remediation and restoration plans. The USGS’s new method gives resource managers information on the rate that nutrients and trace metals, such as methylmercury or nickel, move between the sediments and overlying waters. This rate is known as the benthic flux. The method involves collecting bed sediment cores from the bottom of a body of water, collecting water-quality samples, and conducting laboratory test on the cores. The magnitude and variability of the benthic flux determines whether or not contaminants in the bottom of lakes and reservoirs represent a significant source of those biologically reactive solutes to the environment.

Collecting cores for benthic flux analysis from San Franciso Bay, California Click on image for larger version

USGS scientists used this method to determined the benthic flux of

• Mercury in the Camp Far West Reservoir, California
• Zinc, lead, copper, and cadmium in Lake Coeur d'Alene, Idaho
• Nickel in the South San Francisco Bay
• Mercury in the Carson River Basin

Resource managers are tasked with reducing toxic metal contamination in water bodies. USGS studies have shown that remediation efforts to decrease loads of toxic metals from rivers and/or point sources would most likely be compensated initially by increases in the benthic flux of the metals from bed sediments. This is because the management-induced change would result in a greater concentration difference between the contaminant concentration in the water body and within the bed sediments at the sediment-water interface. The potential long-term outlook for these managed water bodies is that concentrations of toxic metals are predicted to decrease slowly after loads from rivers and/or point sources are controlled and contaminated sediment is buried with sediments containing fewer contaminants.

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