U.S. Department of the Interior - U.S. Geological Survey
NUMBER 3, March 1995
The information on this website is for administrative use only.
It should not be quoted or cited as a publication.
Metal Fluxes Across the Sediment-Water Interface in Terrace Reservoir,
Colorado
by Laurie S. Balistrieri
U.S. Geological Survey, Geologic Division, Branch of Geochemistry,
School of Oceanography WB -10, University of
Washington, Seattle, WA 98195
Introduction
Terrace Reservoir was built in 1912 and is located along
the Alamosa River about 20 km downstream of the
confluence with the Wightman Fork. (See map, page 4) Water
is stored in this reservoir for irrigation downstream in
the San Luis Valley. As indicated in other articles in this
newsletter, the drainage basin upstream of Terrace Reservoir
is highly mineralized. Therefore, metals derived from mining
activities, including the Summitville mine, as well as
natural weathering of mineralized areas have entered the
reservoir and potentially have been deposited in the
sediments. Because water-column and sediment data were not
available for Terrace Reservoir, the Environmental
Protection Agency (EPA) funded the USGS to collect this
information as well as to evaluate the processes controlling
the transport and cycling of metals through and within the
reservoir. Our specific goals are to: 1) understand metal
transport into and out of the reservoir, 2) define the
limnological characteristics (e.g., stratification patterns,
development of anoxia) of the reservoir and how they affect
metal cycling in the water column, 3) determine metal
content, metal accumulation rates, and geochemical processes
(e.g., sorption, dissolution/ precipitation) affecting metal
distributions in reservoir sediments, and 4) determine the
exchange of metals between interstitial porewater and the
overlying water column as well as the processes affecting
that exchange. This information will be used by others to
make risk assessments and to develop remediation plans for
Terrace Reservoir.
Sampling of Terrace Reservoir occurred in the spring,
summer, and fall of 1994. A late winter/early spring
sampling is planned for 1995. Many of the analyses as well
as integration of the water-column, porewater, and
sediment results are not yet completed. Therefore, this
article presents preliminary results and will focus only on the
exchange of Cu across the sediment-water interface.
Methods for Determining Metal Fluxes Across the Sediment-Water Interface
The exchange of metals between interstitial porewater and overlying water is determined by two
methods. The first
method calculates the flux using Fick's First Law. This law defines the exchange of an element by
molecular
diffusion as:
F = - D ( C/ z)
where F is the flux, is the sediment porosity, D is the
molecular diffusion coefficient for the element in the
sediment, and C/z
is the concentration gradient of the
element across the sediment-water interface (Bemer, 1980).
The diffusion coefficient, D
is related to porosity and
tabulated diffusion coefficients in solution at infinite dilution
(e.g., Li and Gregory, 1974; Bemer, 1980). The concentration
gradient is determined from metal concentrations in
overlying water and porewater. Porosity is determined from
water content and density of the sediments. Sediment
cores and associated porewater were collected from the reservoir
at three sites per sampling period for these
determinations. The second method for determining fluxes is
to measure the flux directly using a benthic flux
chamber. Their design and operation are discussed by
Devol (1987). Briefly, a benthic flux chamber consists of a
box that isolates a volume of water in contact with the sediments.
This isolated water is gently stirred and sequentially
sampled as a function of time. Temporal changes in the
concentration of an element in the box are used to
determine the flux of the element across the sediment-water
interface. A benthic flux chamber was deployed at each site
in the reservoir and samples were collected at 5-hour
intervals for up to 15 hours.
Cu Fluxes in July 1994
The results for a site close to the entrance of the Alamosa River into Terrace
Reservoir indicate that dissolved Cu concentrations decreased as a function of
time in the benthic flux chamber in July 1994 (Fig.
1a). In a core from this site, dissolved Cu concentrations in the overlying
water (designated as 0 cm in Fig. lb) were larger than porewater Cu concentrations
just below the sediment-water interface. Both sets of data are consistent with
the interpretation that the flux of dissolved Cu was from the water column to
the sediments in July 1994 (Fig. 1), i.e.
the sediments during this time were acting as a sink rather than a source for
dissolved Cu. A combination of porewater and sediment data eventually will be
used to define the geochemical processes affecting this exchange across the sediment-water
interface. The flux of dissolved Cu into Terrace Reservoir sediment in July 1994
as calculated from Fick's First Law is 170 µg Cu/cm²/y compared to 420
µg Cu/cm²/y from the benthic flux chamber. These values agree within
a factor of 2.5. Carignan and Nriagu (1985) also found that sediments in an acid
lake (pH 4.5) in Ontario, Canada acted as a sink for dissolved Cu. However, their
measured concentrations of dissolved Cu (i.e., ~25-50 µg/L) in the
water column and fluxes of 3.5-5.1 µg Cu/cm2/y are significantly lower than
those determined in Terrace Reservoir.
Conclusions
Preliminary results for Cu fluxes in Terrace Reservoir indicate that the sediment acted as
a sink for dissolved Cu in July 1994. Fluxes from different sites within the reservoir and
at different times of the year eventually will be compared. The future integration of metal
(e.g., Fe, Mn, Cu, Co, Zn) data from the water column, porewater, and sediments will
provide an understanding of the cycling of potentially toxic elements in Terrace Reservoir.
Acknowledgements
This is a cooperative project between the Water Resources and Geologic Divisions of the
USGS. My co-investigators include Pat Edelmann and Art Horowitz. This team effort
included USGS personnel, summer students, and volunteers, specifically, Roger Ortiz,
Chuck Moore, Nicole Nelson, Melinda Wright, and Dawn Gordon-Perine. The owners of
South San Juan Field Camp, about two miles from Terrace Reservoir, provided laboratory
facilities. Dr. Al Devol of the School of Oceanography, University of Washington, kindly
let us borrow his benthic flux chamber. Paul Briggs of the Analytical Chemistry Services
Group of the USGS did the metal analyses. Jim Crock and Kathleen Stewart provided
helpful comments on an earlier draft of this article. The results of this work were
presented at the Summitville Forum in Fort Collins, Colorado, in January 1995.
References
- Berner, R.A., 1980, Early diagenesis: A Theoretical Approach: Princeton University
Press, 241 p.
- Carignan, R. and Nriagu, J.O., 1985, Trace metal deposition and mobility in the sediments
of two lakes near Sudbury, Ontario: Geochimica et Cosmochimica Acta, v. 49, p. 1753-
1764.
- Devol, A.H., 1987, Verification of flux measurements made with in situ benthic flux
chambers: Deep Sea Research, v.34, p. 1007-1026.
- Li, Y.-H., and Gregory, S., 1974, Diffusion of ions in seawater and deep-sea sediments:
Geochimica et Cosmochimica Acta, v. 38, p. 703-714.
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