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<br />mountain communities in the headwaters of the study
<br />unit (Bureau of Census, 1992), In addition, the head-
<br />waters of the region are subject to increasingly intense
<br />year-round recreational activity, Point-source pollu-
<br />tion from urbanization includes discharge from wa~te-
<br />water-treatment plants, solid-waste disposal, leaking
<br />underground storage tanks, industrial discharges, and
<br />storm runoff, which also is a nonpoint source, A II these
<br />sources have a potential to affect the surface and
<br />ground water by adding nutrients, pesticides, various
<br />chemicals, hydrocarbons, trace elements, and salts
<br />depending on the specific point source, Because of the
<br />naturally low phosphorus concentrations that
<br />probably limit algal growth, reservoirs, such as Dillon,
<br />can be extremely sensitive to additional phusphorus
<br />loading, which leads to accelerated eutrophication
<br />(Colorado Department of Health, (992), Advanced
<br />wastewater treatment tiJrmany municipal facilities and
<br />contml of nonpoint sources of pollution fmm urban
<br />area~ that discharge into Dillon Reservoir and the
<br />Fraser River, Eagle River, Roaring Fork, and several
<br />other tributary stream~ have been necessary to maintain
<br />water quality, Many streams in the study unit tend to
<br />have higher pH values than in other basins of the
<br />State; therefore, strict un-ionized ammonia standards
<br />(0,02 mgIL) have been required of wastewater
<br />facilities to protect cold-water aquatic life (Colorado
<br />Depamnent of Health, 1992), In the moontain areas.
<br />communities and resorts generally are located in
<br />narrow valleys containing highly permeable gravelly
<br />sediments, which have some nitrate contamination in
<br />localized, shallow ground water, In areas where septic
<br />tanks are used for waste disposal instead ofcummunity
<br />waste-treatment systems, nitrate contamination is
<br />especially likely, Aquifers in fractured rocks of the
<br />mountainous areas also are vulnerable to nitrate
<br />contamination from individual septic systems, Nitrate
<br />contamination probably will continue to be the most
<br />widespread ground-water problem in Colorado
<br />(Colorado Department of Health, 1992),
<br />
<br />There are a number of potential agricultural
<br />nonpoint-source-pollution issues in the lower region
<br />of the basin, Agriculture in the basin can produce
<br />increased levels of salinity, sediments, nutrients, pesti-
<br />cides, selenium, and other trace elements, which have
<br />an adverse effect on the surfacc water, ground water,
<br />and biological habitats,
<br />
<br />Salinity is an imponant water-quality cuncern
<br />in the agricultural areas of the ba~in, High dissolved-
<br />solids concentrations occur in irrigated areas near the
<br />lower Gunnison and lower Colorado Rivers and their
<br />tributaries (Colorado Water Quality Control Division,
<br />19~9), Irrigation-return flows, as seepage from canals
<br />and reservoirs and from field irrigation, are the largest
<br />
<br />human source for dissolved,solids conccntrations in
<br />thc study unit. The sedimentary geologic units, which
<br />include a large part of the central and lower parts of the
<br />basin, contain soluble minerals that contribute to the
<br />dissolved-solids concentrations, Surface water that is
<br />divened for irrigation is applied to fields and a large
<br />amount is lost to evapotranspiration, This process
<br />concentrates the dissolved solids in the remaining
<br />water that eventually returns to the stream, Also, the
<br />reuse of water has a high potential to incrca~e salinity
<br />in the Colomdo River, Agricultural practices also can
<br />add to the salinity problem as a result of natural chem-
<br />ical processes, The chemical process involves trans-
<br />forming calcium sulfate (gypsum) to sodium sulfate by
<br />cation exchange in solution, Since sodium sulfate has
<br />a higher solubility in solution than calcium sulfate, thc
<br />sodium concentration wi II bc increased in the surface
<br />water, The cation-exchange process can result from
<br />irrigation practices or stann runoff,
<br />
<br />Sediments are formed as a result of erosion and
<br />runoff processes, These processes are affected by soil
<br />type, land slope, climate, and tillage practices, which
<br />all affect the movement of contaminant!; to surface and
<br />ground waters, Sediment erosion by wind and water
<br />can be increased by cultivation practices and by live-
<br />stock that trample stream banks, which occurs in many
<br />semiarid to arid climates like the Colorado Plateau,
<br />Soil permeability is a key factor affccting water quality
<br />in agricultural areas, In areas of well-drained soils,
<br />such as alluvial valleys in the basin, nitrate and pesti-
<br />cide concentrations can increase locally in alluvial
<br />aquifers beneath cropland that is fcrtilized, particularly
<br />where irrigated, Although phosphorus is at low levels
<br />throughout the basin, in areas of poorly drained soils,
<br />nitrogen and phosphorus from cropland fertilizers
<br />move to the surface water as surface runoff. Manure
<br />can contribute nitrogen to reservoirs and strcams,
<br />Nitrate generally is at low levels in the basin, but is at
<br />higher levels in the Roan Creek, Uncompahgre River,
<br />and lower Colorado River reachcs, because the reaches
<br />receive large quantities of drainage water from irriga-
<br />tion, Pesticide data are limited for surface and ground
<br />water, The areal distribution of data sites, the number
<br />of samples per site, and thc temporal distribution of
<br />samples are inadequate for an assessment of pesticide
<br />distribution and occurrcnce,
<br />
<br />Selenium concentrations in the Gunnison and
<br />Uncompahgre Rivers exceed U,S, Environmcntal
<br />Protection Agency criteria for protection of aquatic
<br />life, and the concentrations can be of concern for fish
<br />and waterfowl (V,S, Environmental Protection
<br />Agency, 1987), The Colorado River downstream from
<br />the Gunnison River and the lower reaches of the
<br />Gunnison River provide habitat for endangered fish
<br />
<br />30 Environmental Selling end Implications on Wate, Quality, Upper Coloredo Rive, Basin, Colorado and Utah
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