U.S. Department of the Interior - U.S. Geological Survey
NUMBER 3, March 1995
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IMPACTS OF ACID DRAINAGE ON WETLANDS IN THE SAN LUIS VALLEY,
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
Open-pit mining activities at the Summitville mine in the San Juan Mountains of
southwestern Colorado have exposed pyrite and other metal (e.g., As, Cu, Zn) sulfides
to air and water resulting in the production of acidic and metal-enriched drainage. This
drainage flows into the Wightman Fork. The Wightman Fork and other tributaries draining
mineralized areas then flow into the Alamosa River, through Terrace Reservoir, and
downstream to the San Luis Valley (see map, page 4). Within the Valley, this water is
used for agricultural irrigation (e.g., alfalfa and barley crops) and is a source of surface
water to wetlands near and within the Alamosa National Wildlife Refuge (see overview
article by Plumlee and Edelmann in this newsletter, page 4).
As part of the USGS Environmental Geoscience Studies of the Summitville mine and in
cooperation with the U.S. Fish and Wildlife Service (USFWS), we examined the influence
of acid drainage from natural and mine sources on the biogeochemistry of wetlands near
and within the Alamosa National Wildlife Refuge in June 1993. These wetlands are
seasonal hosts to migratory fowl, including the endangered whooping crane. Information
about the toxic element content of these wetlands is necessary for properly managing
wildlife within this area.
Our approach for assessing the influence of acid drainage on the wetlands was to identify
signature elements for the drainage, evaluate the spatial extent of these signature
elements within the Alamosa River system, and compare the biogeochemistry of wetlands
in the San Luis Valley that receive surface water from different sources. This article
focuses on a portion of the sediment geochemistry results for river-bed and wetland
sediments. More details about the sediment geochemistry and information about the
composition of river and wetland water and rooted aquatic wetland vegetation are
discussed elsewhere (Balistrieri et al., 1995).
Signature Elements
The on-site work of Plumlee et al. (1994, 1995) at the Summitville mine indicates that
drainage from the mine is significantly enriched in Al, As, Cd, Co, Cr, Cu, Fe, Li, Ni, Zn,
rare earth elements (Ce, La, Nd), Th, U, V, Be, and Te. Are any of these elements
enriched in the bed sediments of the Wightman Fork, the main stream draining the
Summitville mine?
A comparison of the compositions of sediments collected at a site within the
Wightman Fork just before it enters the Alamosa River (site B) with sediments
in the Alamosa River upstream of the confluence with the Wightman Fork (site
C) indicates that As, Cr, Cu, Fe, Li, and Zn are enriched in Wightman Fork sediments
(Fig. 1). Of these elements, Cu and As show
the greatest enrichment (13-15 times), whereas Cr, Fe, Li, and Zn are less enriched
(1.3-2.0 times) in the Wightman Fork. Al, Ce, Co, Ni, Th, and V in Wightman
Fork sediments show no enrichment relative to Alamosa River sediments above
the confluence. Other elements identified by Plumlee et al. (1994, 1995) as
being enriched in Summitville mine drainage were either not measured or below
detection limits in Wightman Fork sediments.
The enrichment of certain elements (As, Cr, Cu, Fe, Li, and Zn) in Wightman Fork
sediments suggests their potential use as tracers of drainage from the Summitville mine.
However, these particular elements may not be definitive indicators of Summitville mining
activities because acidic, metal-enriched drainage from other naturally mineralized areas
enters the Alamosa River both above and below the confluence with the Wightman Fork.
In this article, a subset of these signature elements--As, Cr, Cu, and Zn--are used to trace
acidic, metal-enriched drainage within the Alamosa River system and within selected
wetlands in the San Luis Valley.
Spatial Extent of Signature Elements Within the Alamosa River System
How far downstream are the signature elements observed in the Alamosa River system?
Data indicate that 1) Alamosa River sediments downstream of the confluence with
the Wightman Fork tend to be enriched in As, Cr, Cu, and Zn relative to sediments
above the confluence and 2) this enrichment is observed at least 50 km downstream
of the confluence (Fig. 2).
Metal Accumulation in Wetland Sediments
The wetlands within and west of the Alamosa National Wildlife Refuge are approximately
80 km downstream of the confluence of the Wightman Fork and Alamosa River. These
wetlands receive surface water from either the Alamosa River, the Rio Grande River, or
a mixture of Alamosa River and La Jara Creek water. Are there differences in the
sediment geochemistry of wetlands receiving Alamosa River water from those that receive
water from rivers, such as the Rio Grande River, that do not drain mineralized areas?
The concentration of Cu in wetland sediments receiving surface water from
different sources is significantly higher in those wetlands receiving Alamosa
River water (Fig. 3). These differences are
also observed for Cr and Zn, but not As. Sediment metal concentrations, sedimentation
rates as determined by 210Pb, and bulk sediment densities were used to determine
metal accumulation rates in two wetlands--one receiving Alamosa River water
and one receiving primarily Rio Grande River water. Metal accumulation rates
for Cr, Cu, and Zn are two to four times larger in the wetland receiving Alamosa
River water as compared to the wetland receiving Rio Grande River water, whereas
accumulation rates for As are similar for the two wetlands (Fig.
4).
Conclusions
Certain metals (e.g., Cr, Cu, and Zn) derived from the weathering of mineralized areas
in the southern San Juan Mountains of Colorado appear to be transported throughout the
Alamosa River system and are enriched in downstream wetlands that receive surface
water from the Alamosa River water.
Acknowledgments
My co-investigators in this project are Larry P. Gough and R.C. Severson of the USGS
and Andrew Archuleta from the USFWS. Maria Montour assisted in collecting samples
and members of the Analytical Chemistry Services Group (Betty Adrian, Paul Briggs, Jim
Crock, Joe Curry, David Fey, Phil Hageman, and Clara Papp) did the analyses. Jim
Crock and Kathleen Stewart provided helpful comments on an earlier draft of this article.
Parts of this article were presented at the 1994 Annual Meeting of the Geological Society
of America, held in Seattle, Washington, during October 1994, and also were presented
at the Summitville Forum in Fort Collins, Colorado, in January 1995.
References
- Balistrieri, L.S., Gough, L.P., Severson, R.C., and Archuleta, A., 1995, The
biogeochemistry of wetlands in the San Luis Valley: the effects of acid drainage from
natural and mine sources, in, Summitville Forum Proceedings.*
- Plumlee, G.S., Ficklin, W.H., Smith, K.S., Montour, M., Gray, J., Hageman, P., Briggs,
P.H., and Meier, A.L., 1994, Geologic and geochemical controls on the composition of
acid waters draining the Summitville mine, Colorado: U.S. Geological Survey Circular
1103-A, p. 78.
- Plumlee, G.S., Smith, K.S., Mosier E.L., Ficklin, W.H., Montour, M. Briggs, P., and Meier,
A., 1995, Geochemical processes controlling acid-drainage generation and cyanide
degradation at Summitville, in, Summitville Forum Proceedings.*
- *Posey, H.H., Pendleton, J.A., and Van Zyl., D., eds., 1995, Summitville Forum
Proceedings, Colorado Geological Survey, Special Publication 38.
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