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<br />reservoir; (3) determine the mass of metals in the
<br />surficial reservoir sediments and the depth of metal-
<br />enriched sediments; (4) evaluate the potential for
<br />remobilization of metals from the sediments; and
<br />(5) assess the exchange of metals between the
<br />sediments and the overlying water column.
<br />
<br />Purpose and Scope
<br />
<br />This report describes the metals transported
<br />into and out of Terrace Reservoir between April 1994
<br />and March 1995. Specifically, it describes the spatial
<br />and temporal variations in metal concentrations in
<br />the Alamosa River immediately upstream and
<br />downstream from Terrace Reservoir and provides
<br />estimates ofloads into and out of Terrace Reservoir
<br />during the study period. A summary of the percentage
<br />of dissolved-metal concentrations relative to totsl-
<br />metal concentrations is presented; correlations of
<br />concentrations of the metals of concern, streamflow,
<br />specific conductance, and pH also are presented.
<br />The primary metals of concern in the Alamosa
<br />River were identified as aluminum, cadmium, copper,
<br />iron, manganese, and zinc (Morrison and Knudsen
<br />Corporation and ICF Keiser Engineers, 1994).
<br />Therefore, this report is limited to extensive discus.
<br />sion of these metals, but additionally provides a
<br />summary of concentrations for total arsenic, dissolved
<br />and total barium, dissolved and total cobalt, total lead,
<br />dissolved and total nickel, and total vanadium.
<br />
<br />Water Quality of the
<br />Upper Alamosa River Basin
<br />
<br />From 1985 through 1992, the Summitville Mine
<br />site produced highly acidic, metal-enriched water that
<br />drained from the mine site into Wightman Fork and
<br />flowed to the Alamosa River and Terrace Reservoir
<br />(King, 1995). Acid drainage forms during chemical
<br />weathering when sulfide minerals react with water and
<br />oxygen to generate large amounts of sulfuric acid.
<br />Metals are released into solution when the sulfuric
<br />acid comes in contact with rocks, and heavy metals are
<br />leached out, resulting in water high in metal content
<br />and low in pH. The Summitville Mine drainage water
<br />has been among the most acidic and metal-bearing
<br />water in Colorado; the drainage water has had a pH
<br />generally less than 3 and has contained high concen-
<br />trations of aluminum, copper, iron, zinc, and other
<br />metals (King, 1995).
<br />
<br />In addition to mining as a source of contamina-
<br />tion, natural degradation of the water quslity of the
<br />Alamosa River has occurred from tributaries and
<br />springs with low pH (Moran and Wentz, 1974;
<br />Hamilton, 1989; Miller and McHugh, 1994; Kirkham
<br />and others, 1995; Walton-Day and others, 1995).
<br />Previous studies of water quality indicated that
<br />Wightman Fork has been the predominant source of
<br />aluminum, copper, iron, manganese, and zinc during
<br />peak snowmelt and the post-peak snowmelt periods
<br />and has been the source of most of the copper,
<br />manganese, and zinc discharged annually into the
<br />Alamosa River (Walton-Day and others, 1995). Other
<br />metal-enriched sources downstream from Wightman
<br />Fork might include tributaries, material stored in the
<br />streambed, and ground water. The Alamosa River
<br />upstream from Iron Creek contains only moderate
<br />concentrations of iron and aluminum and very minor
<br />amounts of copper, zinc, and manganese (Walton-Day
<br />and others, 1995).
<br />Water quality in the AIamosa River Basin is
<br />variable. The pH has varied from about 3.5 to greater
<br />than 7, hardness has varied from about 40 to almost
<br />200 mgIL, and the alkalinity generally has ranged
<br />from 2 to about 40 mgIL (Mueller and Mueller, 1995).
<br />During 1994, pH in Wightman Fork ranged from
<br />about 5 to 7.5 during the early spring snowmelt period,
<br />subsequently decreased to between 3.5 and 5 during
<br />the peak snowmelt period, and remained low
<br />throughout the summer and winter (Mueller and
<br />Mueller, 1995). During 1994, pH in the Alamosa
<br />River upstream from Wightman Fork ranged from 3.8
<br />during the early spring snowmelt period to 7.5 during
<br />summer (Mueller and Mueller, 1995).
<br />
<br />Methods of Investigation
<br />
<br />Two water-quality sampling sites, AR34.5
<br />(station 08236000, Alamosa River above Terrace
<br />Reservoir) and AR3J.0 (station 08236500, Alamosa
<br />River below Terrace Reservoir) were selected to
<br />describe the metal chemistry and to evaluate metal
<br />transport into and out of Terrace Reservoir (fig. I).
<br />These sites were selected because of their proximity
<br />to the reservoir and because continuous streamflow
<br />data, which are required for load computations, are
<br />available for these sites. Both sites currently are
<br />operated as streamflow-gaging stations by the
<br />Colorado Division of Water Resources. The stream-
<br />flow data used in this report were collected by the
<br />Colorado Division of Water Resources.
<br />
<br />INTRODUcnON 3
<br />
<br />0031GO
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