|
<br />Las Animas. Temporally, fecal-streptococci concentra-
<br />tions in the Arkansas River were similar to that of
<br />fecal coliform. Median total-coliform concentrations
<br />in the upper basin were low and ranged from 7.5 colo-
<br />nies/] 00 rnL at Parkdale to 107 colonies/] 00 mL at
<br />Portland. The higbest concentrations occurred during
<br />snowmelt runoff.
<br />
<br />SUMMARY
<br />
<br />This report describes the water-quality condi-
<br />tions in the Arkansas River Basin in southeastern
<br />Colorado between April 1990 and March 1993.
<br />Discussions focus on the site-to-site variability of
<br />water quality in the main stem of the river in the upper
<br />basin (upstream from Pueblo) and in tbe lower basin
<br />(from Pueblo to the Colorado-Kansas State line).
<br />Water-quality data commonly are presented by stream-
<br />flow regime. In general, the data were separated into
<br />three streamflow regimes that are defined as: low flow
<br />(October-April), snowmelt runoff (May-June), and
<br />post-snowmelt runoff (July-September). For trace
<br />elements in the upper basin, data were further sepa-
<br />rated into a pre-snowmelt runoff period in April;
<br />therefore, the low-flow regime for trace elements in
<br />the upper basin is defined as October through March.
<br />Generally, the waters of the Arkansas River
<br />were well oxygenated. The pH of the river was near
<br />neulral to slightly alkaline. Dissolved-solids concen-
<br />trations in the Arkansas River showed distinct spatial
<br />variability and were influenced by many factors,
<br />including runoff from snowmelt or rainfall, geology,
<br />land and water use, mine drainage, and ground-water
<br />inflow. Overall, median dissolved-solids concentra-
<br />tions in the main stem of the river ranged from
<br />62.5 mgIL at Granite to 3,825 mgIL at Coolidge. The
<br />downstream increase in dissolved-solids concentra-
<br />tions was caused by a transition in geology from
<br />igneous and metamorphic rocks to sedimentary rocks
<br />and by the concentrating effects associated with the
<br />use and reuse of water for irrigation.
<br />In general, concentrations of all six major ions
<br />increased downstream. In the upper basin, calcium and
<br />bicarbonate were the dominant major ions. From Port-
<br />land to Below Pueblo Reservoir, the water changed
<br />from a calcium bicarbonate type to a calcium sulfate
<br />type, probably as a result of contact with marine shales
<br />in the area. In the lower basin, the water changed from
<br />
<br />\
<br />,
<br />,
<br />
<br />a calcium sulfate type to a mixed water type al
<br />Coolidge.
<br />The primary cause of elevated trace-element
<br />concentrations in the upper Arkansas River is metal-
<br />laden drainage from abandoned mines and mine
<br />tailings. The spatial and temporal variability of
<br />trace-element concentrations at main-stem sites in
<br />the upper Arkansas River probably was influenced by
<br />(I) distance from the Leadville area, (2) dilution by
<br />tributaries, (3) degree of partitioning between tbe
<br />dissolved and particulate phases, and (4) deposition
<br />and resuspension of fluvial sediment. Generally,
<br />eoncentrations of dissolved trace elements in the
<br />Arkansas River decreased from Leadville to Portland.
<br />Dissolved cadmium, copper, lead, manganese, and
<br />zinc concentrations did increase between Leadville
<br />and Malta, but these elevated concentrations were
<br />subsequently diluted by inflow from Twin Lakes
<br />Reservoir. Temporally, dissolved eadmium, iron,
<br />manganese, and zinc concentrations typically were
<br />largest during early-snowmelt runoff; total-recover-
<br />able cadmium, copper, iron, lead, manganese, and zinc
<br />concentrations generally were largest during snowmelt
<br />runoff when resuspension of fluvial sediments
<br />occurred. Concentrations of cadmium, copper, manga-
<br />nese, and zinc decreased significantly (u=0.05) at
<br />several sites in the upper Arkansas River Basin
<br />following the completion of water-treatment facilities
<br />at two mine-drainage tunnels in the Leadville area.
<br />Chronic and acute stream-water-quality standards
<br />were exceeded in samples collected at several upper-
<br />basin sites; however, the vast majority of exceedances
<br />occurred prior to the installation of tbe mine-drainage
<br />water-treatment facilities. Tributaries downstream
<br />from Granite generally did not contribute substantial
<br />loads to the river, although several tributaries
<br />upstream from Granite (California Gulch, Lake Fork,
<br />and Lake Creek) frequently contributed substantial
<br />trace-element loads to the river.
<br />Sources of trace elements in tbe lower basin
<br />include inflow from the mining-affected upper basin,
<br />point and nonpoint sources associated with urban land
<br />use, irrigation return flows associated with agricultural
<br />land use, and tributary inflow. Concentrations of
<br />dissolved cadmium, chromium, copper, lead, and
<br />nickel generally were near or less than their reporting
<br />limits at most sites. Dissolved-selenium concentra-
<br />tions, however, comprised about 90 percent of the
<br />total-recoverable selenium concentration at most sites.
<br />Total-recoverable selenium concentrations increased
<br />
<br />66 Water-Quelity Assessment of the Arkansas River Basin, Southeastern Colorado, 199M3
<br />
|