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U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993, Water-Resources Investigations Report 94-4015

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Studies of the Molecular Size of Dissolved Organic Carbon Fractions Downgradient from the Oil Body at Bemidji, Minnesota

by

George Aiken (U.S. Geological Survey, WRD, 3215 Marine Street, Boulder, Colorado 80303)

Abstract

Preliminary results of equilibrium dialysis measurements on whole-water samples and isolates of the hydrophobic acid fraction of the dissolved organic carbon (DOC) in ground water at the Bemidji, Minn., toxic-waste study site are presented. Membranes with 2,000 (2K)-dalton and 12,000 (12K)-dalton cutoffs were used. Approximately 10 percent of the DOC in the whole-water sample from well 530 B was excluded from the 2K-dalton-cutoff membrane. Similar results were obtained with aqueous solutions of the hydrophobic acid fraction isolated from water from well 530 B. These results differed markedly from those obtained with aqueous solutions of the hydrophobic organic acid fraction from water from well 310, a background-water-quality well, where approximately 30 percent of the DOC was excluded from the 2K-dalton membrane. Measurements made on whole-water samples from two other wells in the contaminated plume indicate that the amount of organic matter excluded from the 2K-dalton membrane increases with decreasing distance to the oil body, possibly because of organic-matter/iron or organic- matter/organic-matter interactions. The results suggest that colloid-sized organic constituents are generally absent from the ground water but that interactions of DOC with other dissolved constituents could result in an increase in molecular size of DOC fractions.

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