Simulated Transport and Biodegradation of
Chlorinated Ethenes in a Fractured Dolomite
Aquifer Near Niagara Falls, New York
by Richard M. Yager
U.S. GEOLOGICAL SURVEY
Water-Resources Investigations Report 00-4275
ABSTRACT
Leakage of trichloroethene (TCE) from a
neutralization pond at a former manufacturing
facility near Niagara Falls, N.Y. during 1950-87
into the Guelph Formation of the Lockport Group,
a fractured dolomite aquifer, created a plume of
TCE and its metabolites that, by 1990, extended
about 4,300 feet south of the facility. A smaller
plume of dense, nonaqueous-phase liquids
(DNAPL) probably serves as a continuing source
of TCE. The presence of the TCE metabolites
cis-1,2-dichloroethene (DCE), vinyl chloride (VC),
and ethene in the plume, and the results of
previous laboratory microcosm studies, indicate
that the TCE is being degraded by naturally
occurring microorganisms. Biodegradation rates
of TCE and its metabolites were estimated
through simulation with BIOMOC, a solute-transport
model that represents multispecies
reactions through Monod kinetics. A fracture zone
in the Guelph Formation was represented as a
porous medium containing an extensive, 3-foot thick
layer with several interconnected fractures;
this layer is bounded above and below by
subhorizontal stratigraphic contacts. The Monod
reaction constants were estimated through
nonlinear regression to minimize the difference
between computed concentrations of TCE and its
metabolites, and the concentrations measured
before and during 5 years of pump-and-treat
remediation.
Transport simulations indicated that, by April
1998, the chlorinated ethene plume had reached a
dynamic equilibrium between the rate of TCE
dissolution and the rate of removal through
pumping and biodegradation. Biodegradation of
chlorinated ethenes at the site can be simulated as
first-order reactions because the concentrations
are generally less than the half-saturation
constants estimated for Monod kinetics (320 mg/L
for TCE, 10 mg/L for DCE, and 1 mg/L for VC).
Computed degradation rates are proportional to
the estimated ground-water velocity, which could
vary by more than an order magnitude at the site,
as indicated by the estimated range of fracture
porosity--3 to 0.3 percent. Half-lives
corresponding to first-order rate constants
estimated for the lower velocity (5 to 15 feet per
day) ranged from 21 to 25 days for TCE, 170 to
230 days for DCE, and 18 to 23 days for VC.
Chlorinated ethene concentrations of April
1998 were better reproduced when the TCE
source was represented as a constant
concentration than as a constant flux, because the
latter predicted that the plume would dissipate
after 5 years of pump-and-treat remediation. This
result indicates that the rate of TCE dissolution is
not limited by the mass of TCE in the DNAPL
plume. Simulation of diffusion by the transport
model MOC3D indicated that concentrations of
these contaminants within the rock matrix
surrounding the fracture zone were relatively
unchanged after 5 years of pump-and-treat
remediation. The principal sources of uncertainty
in the prediction of biodegradation rates and of
the fate of chlorinated ethenes at the site are the
fracture porosity and DNAPL mass in the
aquifer.
Citation: Yager, R.M., 2002, Simulated Transport and Biodegradation of
Chlorinated Ethenes in a Fractured Dolomite Aquifer Near Niagara Falls,
New York:
U.S. Geological Survey Water-Resources Investigations Report 00-4275, 55 p.
[Full Report, Acrobat PDF
(9.2M)]
Adobe Acrobat's .pdf (portable document file) format can be viewed using the
free Adobe Acrobat Reader
available for DOS, Windows, Macintosh, and UNIX.
Users with visual disabilities can visit this
site for conversion tools and information to help make PDF files accessible.
For more information, contact
U.S. Geological Survey
425 Jordan Rd
Troy, New York 12180
(518) 285-5602
E-mail
|
|
To order copies of printed reports, contact
U.S. Geological Survey
Information Services
Box 25286, Federal Center
Denver, CO 80225
1-888-ASK-USGS
E-mail
|
Return to the New York District Home Page
|
