Evaluation of water quality and bulk atmospheric deposition in the Guanella Pass area, Clear Creek and Park Counties, Colorado, water year 1995

A study of water quality and bulk
atmospheric deposition in the Guanella Pass
area during water year 1995 (October 1994 to
September 1995) has provided information to
characterize conditions in the South Clear Creek
and Geneva Creek Basins about 40 miles west
of Denver, Colorado, prior to a possible roadreconstruction
project.
Analytical results of water samples
collected from streams, ground water, and lakes
and reservoirs indicate that water in the study
area generally contained small concentrations
of dissolved solids (low-flow stream sample
median, 43 milligrams per liter; ground-water
median, 104 milligrams per liter) and suspended
sediment (high-flow stream sample median,
115 milligrams per liter). Nutrient, trace-element,
and organic-carbon concentrations also were
small. Acidic pH and large trace-element concentrations
were associated with areas affected
by natural acid-sulfate weathering or mining.
Increases in particulate nutrients, total recoverable
trace elements, and suspended sediment
were related to seasonal snowmelt and thunderstorms.
Benthic macroinvertebrate and periphyton
(algae) samples collected from selected sites in
August 1995 indicated large variability in characteristics
among sites. Macroinvertebrate density
ranged from 6.5 to 1,300 organisms per square
meter, and periphyton biovolume ranged from
0.023 to 4,700 x 10⁶ cubic microns per square
centimeter.
The discharge of road runoff from
any single drainage feature (ditch or culvert)
was small compared to streamflows. Analytical
results from samples of road runoff indicated a
large range in specific conductance, pH in a
neutral range, large values of turbidity, and
calcium bicarbonate type water. Sodium or
magnesium and chloride, however, were predominant
in some samples and were probably related
to applications of sodium and magnesium chloride
to the roads. Although dissolved nutrients
and trace elements were detected in samples
of road runoff, particulate forms had the largest
concentrations. Suspended-sediment concentrations
in road runoff were large, and the sediment
was primarily fine grained.
Large concentrations of dissolved and
total recoverable nutrients, total recoverable
trace elements, total organic carbon, and
suspended sediment in road runoff could cause
increased concentrations in streams, lakes, and
reservoirs. Ground-water concentrations of chloride,
nitrite plus nitrate, and dissolved iron and
manganese·could increase if the large concentrations
present in road runoff infiltrate shallow
aquifers.
Bulk atmospheric deposition was collected
at three sites with a paved road surface, five sites
with a gravel or dirt surface, and five sites with
collectors at least 500 feet away from a road. The
median bulk -atmospheric-deposition rate for
gravel (or dirt) roads was 106 times the median
rate at a reference site (located at least 500 feet
away from a road) and 39 times the median rate at
a typical paved road site. If a stream, lake,
or reservoir is located near the road, bulk-atmospheric-
deposition products may settle
directly onto open water or be washed in by
surface runoff.
Because the predominant road-runoff
trace-element phase is particulate, potential
toxic effects on receiving water may be minimal
when compared to aquatic-life standards and
regulations in the State of Colorado, which are
primarily based on dissolved concentrations.
U.S. Environmental Protection Agency primary
maximum contaminant levels for drinking water
were exceeded for dissolved uranium (proposed)
in one ground-water sample.