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<br />
<br />Identification of Water-Quality Trends Using
<br />Sediment Cores from Dillon Reservoir,
<br />Summit County, Colorado
<br />
<br />By Adrienne I. Greve, Norman E. Spahr, Peter C. Van Metre, and Jennifer T. Wilson
<br />
<br />Abstract
<br />
<br />Since the construction of Dillon Reservoir,
<br />in Summit County, Colorado, in 1963, its
<br />drainage area has been the site of rapid urban
<br />development and the continued influence of
<br />historical mining. In an effort to assess changes in
<br />water quality within the drainage area, sediment
<br />cores were collected from Dillon Reservoir in
<br />1997. The sediment cores were analyzed for pesti-
<br />cides, polychlorinated biphenyls (PCBs), polycy-
<br />clic aromatic hydrocarbons (PAHs), and trace
<br />elements. Pesticides, PCBs, and PAHs were used
<br />to determine the effects of urban development,
<br />and trace elements were used to identify mining
<br />contributions. Water-quality and streambed-
<br />sediment samples, collected at the mouth of
<br />three streams that drain into Dillon Reservoir,
<br />were analyzed for trace elements.
<br />Of the 14 pesticides and 3 PCBs for which
<br />the sediment samples were analyzed, only 2 pesti-
<br />cides were detected. Low amounts of dichloro-
<br />diphenyldichloroethylene (DDE) and dichloro-
<br />diphenyldichloroethane (DDD), metabolites of
<br />dichlorodiphenyltrichloroethane (DDT), were
<br />found at core depths of 5 centimeters and below
<br />15 centimeters in a core collected near the dam.
<br />The longest core, which was collected near
<br />the dam, spanned the entire sedimentation history
<br />of the reservoir. Concentrations of total combus-
<br />tion PAH and the ratio of fluoranthene to pyrene
<br />in the core sample decreased with core depth and
<br />increased over time. This relation is likely due to
<br />
<br />growth in residential and tourist populations in the
<br />region. Comparisons between core samples gath-
<br />ered in each arm of the reservoir showed the
<br />hiahest PAH concentrations were found in the
<br />to
<br />Tenmile Creek arm, the only arm that has an
<br />urban area on its shores, the town of Frisco. All
<br />PAH concentrations, except the pyrene concentra-
<br />tion in one segment in the core near the dam and
<br />acenaphthylene concentrations in the tops of three
<br />cores taken in the reservoir arms, were below
<br />Canadian interim freshwater sediment-quality
<br />guidelines.
<br />Concentrations of arsenic, cadmium, chro-
<br />mium, copper, lead, and zinc in sediment samples
<br />from Dillon Reservoir exceeded the Canadian
<br />interim freshwater sediment-quality guidelines.
<br />Copper, iron, lithium, nickel, scandium, titanium,
<br />and vanadium concentrations in sediment samples
<br />decreased over time. Other elements, while no
<br />trend was evident, displayed concentration spikes
<br />in the down-core profiles, indicating loads
<br />entering the reservoir may have been larger than
<br />they were in 1997. The highest concentrations of
<br />copper, lead, manganese, mercury, and zinc were
<br />detected during the late 1970's and early 1980's.
<br />Elevated concentrations of trace elements
<br />in sediment in Dillon Reservoir likely resulted
<br />from historical mining in the drainage area. The
<br />downward trend identified for copper, iron,
<br />lithium, nickel, scandium, titanium, and vanadium
<br />may be due in part to restoration efforts in
<br />mining-affected areas and a decrease in active
<br />mining in the Dillon Reservoir watershed.
<br />
<br />Abstract
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