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<br />002351 <br /> <br />Samples collected in November, January, and March from the Gunnison River <br />were collected at normal stream discharges (fig. 4) and the sample collected <br />in July was collected at less than normal stream discharge. The largest con- <br />centrations of many constituents in the samples collected at sites 3 and 11 <br />occurred in July, when small releases from Blue Mesa Reservoir resulted in <br />less dilution of constituent concentrations by upstream water. <br /> <br />Site 2 (below the Gunnison Tunnel) was considered the reference site for <br />the Gunnison River. Many trace-element concentrations, including selenium, <br />were near Dr less than reporting limits in the samples collected at site 2 <br />(table 16). For comparison to the selenium data for site 11, dissolved- <br />selenium concentrations at gaging station 09152500 near Grand Junction were <br />2 ~g/L in April 1988, 10 ~g/L in August 1988, and 8 ~g/L in September 1988 <br />(fig. 9). <br /> <br />Pesticide concentrations in the Gunnison River at site 11 were not <br />unusual except for the organophosphate insecticide parathion. The parathion <br />concentration of 0.18 ~g/L (table 17) exceeds the standard of 0.04 ~g/L for <br />protection of aquatic life. Some of the parathion in the Gunnison River at <br />site 11 came from the Uncompahgre River based on the parathion concentration <br />at site 9 of 0.33 ~g/L (table 17). Acute toxicity to certain invertebrates <br />was reported for parathion concentrations as small as 0.4 ~g/L (U.S. <br />Environmental Protection Agency, 1986). Parathion does not persist in water <br />in substantial concentrations and does not accumulate in biota to toxic <br />amounts. There was no positive identification of carbamates (the last six <br />pesticides, aldicarb through sevin, listed in table 17) in the sample col- <br />lected at site 11. <br /> <br />Uncompahgre River <br /> <br />Most concentrations of dissolved solids (fig. 10), major ions (especially <br />sodium and sulfate), nitrite plus nitrate, selenium (fig. 11), boron, and <br />uranium (fig. 12) in the Uncompahgre River were larger at site 9 (Delta) than <br />at site 4 (Colona) (table 16). Most of the increases in constituent con- <br />centrations between sites 4 and 9 probably can be attributed to irrigation <br />drainage from the Uncompahgre Project area. A comparison of constituent loads <br />for the four sets of samples for sites 4 and 9 indicate that, for most of the <br />samples, at least 75 percent of the dissolved-solids, magnesium, sodium, <br />sulfate, nitrite plus nitrate, selenium, boron, and uranium loads at site 9 <br />can be attributed to inflow between sites 4 and 9. For example, on <br />November 18, the Uncompahgre River gained 638 tons of dissolved solids between <br />sites 4 and 9, which is 79 percent of the dissolved-solids load at site 9 for <br />that day. A large quantity of selenium is contributed to the river from the <br />project area; for example, on November 18 the Uncompahgre River gained about <br />29 lbs of selenium between sites 4 and 9, which is about 98 percent of the <br />selenium load at site 9 for that day. Most of the inflow between sites 4 and <br />9 during the sampling periods in November, January, and July was irrigation <br />drainage and return flow from the Uncompahgre Project. A substantial part of <br />the inflow in March was snowmelt runoff from tributaries, however, snowmelt <br />runoff tends to dilute constituent concentrations. The nitrite plus nitrate <br />concentration for site 4 in January (25 mg/L) seemed anomolous, but there were <br />cattle-feeding areas adjacent to and upstream from site 4 during the winter. <br /> <br />41 <br />