Distribution of petroleum hydrocarbons and toluene biodegradation, Knox Street Fire Pits,
Fort Bragg, North Carolina
Water-Resources Investigations Report 96-4066
By Stephen L. Harden and James E. Landmeyer
Full Report (PDF, 43 pages, 645 Kb)
Abstract
An investigation was conducted at the Knox Street fire pits, Fort Bragg,
North Carolina, to monitor the distribution of toluene, ethylbenzene,
and xylene (TEX) in soil vapor, ground water, and ground-water/vapor
to evaluate if total concentrations of TEX at the site are decreasing
with time, and to quantify biodegradation rates of toluene in the
unsaturated and saturated zones. Soil-vapor and ground-water samples
were collected around the fire pits and ground-water/vapor samples
were collected along the ground-water discharge zone, Beaver Creek, on
a monthly basis from June 1994 through June 1995. Concentrations of
TEX compounds in these samples were determined with a field gas chro-
matograph. Laboratory experiments were performed on aquifer sediment
samples to measure rates of toluene biodegradation by in situ micro-
organisms.
Based on field gas chromatographic analytical results, contamination
levels of TEX compounds in both soil vapor and ground water appear to
decrease downgradient of the fire-pit source area. During the 1-year
study period, the observed temporal and spatial trends in soil vapor
TEX concentrations appear to reflect differences in the distribution
of TEX among solid, aqueous, and gaseous phases within fuel-contaminated
soils in the unsaturated zone. Soil temperature and soil moisture are
two important factors which influence the distribution of TEX com-
pounds among the different phases. Because of the short period of data
collection, it was not possible to distinguish between seasonal fluc-
tuations in soil vapor TEX concentrations and an overall net decrease
in TEX concentrations at the study site.
No seasonal trend was observed in total TEX concentrations for ground-
water samples collected at the study site. Although the analytical
results could not be used to determine if ground-water TEX concen-
trations decreased during the study at a specific location, the data
were used to examine rate constants of toluene biodegradation. Based
on ground-water toluene concentration data, a maximum rate constant
for anaerobic biodegradation of toluene in the saturated zone was
estimated to be as low as 0.002 d-1 or as high as 0.026 d-1.
Based on analyses of ground-water/vapor samples, toluene was the prin-
cipal TEX compound identified in ground water discharging to Beaver
Creek. Observed decreases in ground-water/vapor toluene concentrations
during the study period may reflect a decrease in source inputs, an
increase in dilution caused by higher ground-water flow, and(or)
removal by biological or other physical processes.
Rate constants of toluene anaerobic biodegradation determined by
laboratory measurements illustrate a typical acclimation response of
micro-organisms to hydrocarbon contamination in sediments collected
from the site. Toluene biodegradation rate constants derived from
laboratory microcosm studies ranged from 0.001 to 0.027 d-1, which is
similar to the range of 0.002 to 0.026 d-1 for toluene biodegradation
rate constants derived from ground-water analytical data. The close
agreement of toluene biodegradation rate constants reported using both
approaches offer strong evidence that toluene can be degraded at
environmentally significant rates at the study site.
Citation:
Harden, S.L., and Landmeyer, J.E., 1996, Distribution of petroleum hydrocarbons and toluene biodegradation, Knox Street fire pits, Fort Bragg, North Carolina: U.S. Geological Survey Water-Resources Investigations Report 96-4066, 39 p.
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North Carolina Water Science Center
U.S. Geological Survey
3916 Sunset Ridge Road
Raleigh, North Carolina 27607
(919) 571-4000
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