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
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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