As population has increased in the Denver metropolitan area, many mountain tributary basins have changed from forested areas with little or no development to high-volume traffic corridors with suburban and recreational development.
Throughout the mountains, alteration of land use for the ever-increasing numbers of new homes may increase the probability of contaminants being introduced to the ground-water system. Wells are the primary drinking-water source for most mountain communities. Data from wells and stream sites indicated that ground- and surface-water quality in the mountains generally was representative of natural background conditions. Most chemical concentrations were lower in the forested mountains than in urban, agricultural, and rangeland areas. For example, pesticide compounds were not detected in ground water from the forested mountain areas, but often were detected in other land-use areas (Bruce and McMahon, 1998).
Among the mountain tributaries sampled, concentrations of dissolved solids, suspended sediment, nitrate, and phosphorus correlated with the degree of development in the basin. There were higher concentrations of these constituents in Clear Creek (a more developed area) and lower concentrations in the Cache La Poudre and Big Thompson Rivers (less developed areas)(Litke and Kimbrough, 1998). In ground water, only 1 of 27 mountain wells had a nitrate concentration above the drinking-water standard of 10 mg/L as nitrogen, and 5 of those 27 wells had nitrate concentrations that might be affected by septic systems (more than 2 mg/L) (Mueller and Helsel, 1996).
Volatile organic compounds (VOCs) are manufactured chemicals such as organic solvents, by-products of chlorinated disinfectants, and many petroleum-based derivatives that are components of gasoline, fuel oil, and lubricants. VOCs were detected in 8 of 27 ground-water wells sampled, but none of the concentrations exceeded drinking-water standards (Bruce and McMahon, 1998). The occurrence of VOCs in the mountain wells indicates that ground water is susceptible to contamination from development and human activity.
Median concentrations of selected constituents in forested mountain areas of the South Platte River Basin.
Undeveloped mountain drainage and residential development in mountain areas.
Geology and proximity to the source rocks are important factors affecting the concentration of trace elements in bed sediments. Higher concentrations of most elements were present in bed sediment in the mountains, and concentrations generally exceeded basin background levels (Heiny and Tate, 1997). Elevated concentrations of some elements in Clear Creek bed sediments were attributed to the effects of mining or development.
The numbers of invertebrate and fish taxa at forested mountain sites were related to past mining activity and the level of urban development. Clear Creek (mining/development-affected) had fewer invertebrate taxa (10) and lower invertebrate density (1,980 individuals per square meter) than the Cache La Poudre River above Fort Collins, Colo. (less developed; 19 taxa and 3,300 individuals per square meter) (Tate and Heiny, 1995). Clear Creek also had fewer fish species, more unstable banks, more channelization, and less available habitat than the Cache La Poudre or Big Thompson Rivers.
Water quality in the forested mountain setting generally represents background conditions; however, surface water, ground water, bed sediment, and fish communities have been minimally affected by mining activities and residential development. Because continued development is anticipated in mountain communities, long-term monitoring of water quality is important.