Assessment of water quality, road runoff, and bulk atmospheric deposition, Guanella Pass area, Clear Creek and Park counties, Colorado, water years 1995-97
The Guanella Pass road, located about 40 miles west of Denver, Colorado, between the towns of Georgetown and Grant, has been designated a scenic byway and is being considered for reconstruction. The purpose of this report is to present an assessment of hydrologic and water-quality conditions in the Guanella Pass area and provide baseline data for evaluation of the effects of the proposed road reconstruction. The data were collected during water years 1995-97 (October 1, 1995, to September 30, 1997). Based on Colorado water-quality standards, current surface-water quality near Guanella Pass road was generally acceptable for specified use classifications of recreation, water supply, agriculture, and aquatic life. Streams had small concentrations of dissolved solids, nutrients, trace elements, and suspended sediment. An exception was upper Geneva Creek, which was acidic and had relatively large concentrations of iron, zinc, and other trace elements related to acid-sulfate weathering. Concentrations of many water-quality constituents, especially particle-related phases and suspended sediment, increased during peak snowmelt and rainstorm events and decreased to prerunoff concentrations at the end of runoff periods. Some dissolved (filtered) trace-element loads in Geneva Creek decreased during rainstorms when total recoverable loads remained generally static or increased, indicating a phase change that might be explained by adsorption of trace elements to suspended sediment during storm runoff. Total recoverable iron and dissolved zinc exceeded Colorado stream-water-quality standards most frequently. Exceedances for iron generally occurred during periods of high suspended-sediment transport in several streams. Zinc standards were exceeded in about one-half the samples collected in Geneva Creek 1.5 miles upstream from Grant. Lake-water quality was generally similar to that of area streams. Nitrogen and phosphorus ratios calculated for Clear and Duck Lakes indicated that phytoplankton in the lakes were probably phosphorus-limited. Measures of trophic status (secchi depth, total phosphorus, and chlorophyll- a ) indicated that Duck and Clear Lakes were oligotrophic in 1997. Ground water had relatively low specific conductance (range 24 to 584 microsiemens per centimeter) and did not exceed U.S. Environmental Protection Agency drinking-water standards, except for samples collected from a single well, which exceeded the Proposed Maximum Contaminant Level for uranium. Runoff from the Guanella Pass road enters streams through surface channels connected to culverts and roadside ditches. Fifty-six percent of the total number of culvert and roadside-ditch drainage features on the Guanella Pass road showed evidence of recent surface runoff connection to an adjacent stream. Road runoff is generated during snowmelt and during summer rainstorms. At a road cross-drain culvert monitored continuously for discharge (water years 1996-97), most runoff (77 to 96 percent) was a result of snowmelt, and runoff from the road preceded the basinwide peak streamflow, resulting in sediment and water-quality constituent inputs to the stream when the stream’s capacity for dilution of the road runoff was low. Specific conductance of road-runoff samples ranged from 14 to 468 microsiemens per centimeter. Major-ion composition of some samples indicated effects from deicing salt (sodium chloride) and dust inhibitor (magnesium chloride) applied to sections of the road, but changes in the stream concentrations that might be attributed to the runoff were brief and relatively small. Nutrients were commonly measured in road-runoff samples at larger concentrations than in streamflow. Concentrations of nitrate and ammonia, especially during rainfall-generated road runoff, were more similar to the concentrations in precipitation than to the concentrations in stream water. Concentrations of ammonia plus organic nitrogen (total as N) (range less than 0.2 to 24 milligrams per liter) and total phosphorus (range 0.024 to 7.2 milligrams per liter) in road runoff were generally large in snowmelt and rainstorm samples and were related to abundant particulate organic material and suspended sediment in the road runoff. Trace-element data indicated that total recoverable trace-element concentrations in road runoff were substantially larger (by several times) than dissolved trace-element concentrations, indicating that most trace elements were particulate. Most of the stream trace-element standards would not likely be exceeded as a result of road-runoff constituents discharged into streams because the standards apply primarily to dissolved-phase (filtered) constituents, and the predominant phase associated with Guanella Pass road runoff is particulate. The suspended-sediment concentrations in snowmelt-generated road runoff ranged from 66 to 7,360 milligrams per liter. Rainstorm-runoff concentrations ranged from 34 to 38,800 milligrams per liter. The sediment was primarily fine grained, which facilitated transport of the sediment. The median percentage of silt and clay (finer than 0.062 millimeter) in road-runoff samples analyzed for size fractions was greater than 90 percent for snowmelt samples and 88 percent for rainstorm samples. Dilution of estimated road runoff to South Clear Creek upstream from Naylor Creek as a result of snowmelt on the road in early May (prior to peak watershed streamflows) ranged from approximately 10 to 100 times. During peak snowmelt in the watershed (early June), dilution of road runoff was estimated to be greater than 1,000 times. Dilution factors for late-summer-rainstorm road runoff were in the range of 10 to 500 times. Given the large concentrations of suspended sediment in road runoff, the potential water-quality effects of road runoff could be substantial during the low streamflows of early snowmelt and late summer base flow (low dilution capacity) but inconsequential during high streamflows (high dilution capacity). The median bulk deposition rate of filterable solids is probably related to dust deposition from roads. The median deposition rate at sites near unpaved roads was 105 times the median rate at an undisturbed reference site (located at least 500 feet away from a road), and the rate at sites near paved roads was 3 times the undisturbed reference site median rate. During August to September 1997, bulk atmospheric deposition directly on the active channel of a reach of Geneva Creek was estimated to be 1.7 tons, or about 1 percent of the estimated 165 tons of suspended sediment transported past the nearby stream-monitoring site near Grant during the same period. The estimate of 1.7 tons indicates little potential effect from direct settling of bulk atmospheric solids on suspended-sediment transport in Geneva Creek, a stream that closely parallels the Guanella Pass road.