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Using Kendall's tau, the statistically significant (a = 0.05) correlations between core depth and some components of the PAH data were determined for core DLN. The ratio of fluoranthene to pyrene concentra- tions and total combustion PAH decreased with depth, or increased over time (table 2, figure 3). Total combustion PAH concentrations also increased with population (p-value = 0.044). Housing units and skier days increased substantially since construction of the reservoir, but direct relations to components of the PAH data were not identified. Because sites located near the mouth of rivers can be affected by flooding or scouring, only the top 2 cm of each of the cores from the arms of the reser- voir were analyzed for PAHs. The upper core values for each of the three arms had higher concentrations of total and combustion PAH than concentrations from the core collected at the dam site, with the largest values at Tenmile Creek. This may be because the Tenmile arm of the reservoir has the most development near its shores, which includes the town of Frisco. The PAH components expressed as ratios were similar to those for the core taken near the dam. Residential, commercial, and industrial energy use has been linked to combustion-derived PAH concentrations (Heit and others, 1988). It is likely that the increased concentrations of total combustion PAH and the increased ratios of fluoranthene to pyrene is due to the rapid growth in the area over the last 30 years. However, even though the concentrations of PAHs have increased in Dillon Reservoir, all core concentrations are well below concentrations detected in major urban areas and are within the range Table 2. Kendall's tau results for five polycyclic aromatic hydrocarbon components paired with core depth for core DLN, collected near the dam in Dillon Reservoir, August 1997 [--. not applicable] Parameter Tau p-Value Trend or ratio Direction 2- and 3-ringed -0.067 0.862 no compounds: 4- and 5-ringed compounds Fluoranthene : pyrene -0.511 0.046 yes Upward over time Phenanthrene: anthracene -0.111 0.728 no Total PAH -0.200 0.484 no Total combustion PAH -0.644 0.009 yes Upward over time typical of remote freshwater lakes in the Western United States (Heit, 1985). To evaluate the ecological significance of the concentrations of PAHs in sedi- ments, values were compared to the Canadian Sediment Quality Guidelines for freshwater lakes (Canadian Council of Ministers of the Environment, 1999); both the interim sediment-quality guidelines (ISQGs) and probable effect levels (PELs) were used. Canadian standards were used as a basis of compar- ison because similar standards in the United States were in review at the time of writing and subject to revision. Only two PAH constituent concentrations exceeded the ISQG: acenaphthylene concentrations from the top of three cores taken in the reservoir arms and pyrene concentration at the 2-4 cm depth in the core taken near the dam. In all cases, the concentra- tions were slightly above the ISQG and well below the PEL. Trace Elements in Reservoir-Sediment Cores, Stream-Sediment, and Water Samples Trace-element sediment-core concentrations also were compared to the Canadian interim sediment- quality guidelines. Guidelines have been set for arsenic, cadmium, chromium, copper, lead, mercury, and zinc. In all cases except mercury, each core, at all depths, contained concentrations that exceeded the ISQG. Zinc and lead concentrations in all seven cores were well above the PELs, and cadmium concentra- tions, in all cores except DLN, exceeded the PELs (fig. 4, table 3). Although concentrations of many trace elements in sediment samples exceeded the PEL for freshwater lakes established by the Canadian Council of Ministers of the Environment, limnological conditions (based on data from 1997 and 1998) do not appear to allow the elevated core-sediment concentra- tions to affect water quality in Dillon Reservoir. The concentrations of trace elements in the reservoir water column did not exceed the water-quality standards established by the Colorado Water Quality Control Commission (Colorado Department of Public Health and Environment Water Quality Control Commission, 1998). Copper, iron, lithium, nickel, scandium, tita- nium, and vanadium concentrations were identified, using Kendall's tau, as increasing with core depth, indicating that concentrations in the sediment column IDENTIFICATION OF WATER-QUALITY TRENDS 11