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<br />median dissolved and suspended total gross beta
<br />concentrations were less than 50 pCiIL, and most were
<br />less than 10 pCiIL.
<br />
<br />Pesticides
<br />
<br />Pesticides are widely used in agricultural areas
<br />and are of concern because of the potential to degrade
<br />water quality. Even in small concentrations, some
<br />pesticides are probable or possible carcinogens
<br />(U.S. Environmental Protection Agency, 1987). The
<br />distribution and concentration of pesticides in surface-
<br />water systems are affected by sorption, bioaccumula-
<br />tion, and transformation processes (Smith and others,
<br />1988). The characteristics of a compound, such as
<br />solubility or resistance to degradation, affect its
<br />mobility and occurrence in a given area. General pesti-
<br />cide usage, including insecticides and herbicides, has
<br />increased in the Arkansas Ri ver Basin in recent years.
<br />In 1989, an estimated 843,000 Ib of herbicides,
<br />357,000 Ib of insecticides, and 43,000 Ib of fungicides
<br />and nematocides were applied in the southeast region
<br />of the State (Bohmont, 1991). A summary of agricul-
<br />tural pesticides commonly applied in the Arkansas
<br />River Basin is presented in table 26.
<br />Pesticide samples were collected periodically at
<br />four sites on the Arkansas River during the study
<br />period. Five samples at each site were collected at
<br />Portland, Avondale, Catlin Dam, and Las Animas.
<br />One sample was collected at the Below John Martin
<br />Reservoir site. Pesticide sampling was scheduled
<br />during irrigation periods. The samples were analyzed
<br />for 35 insecticides, 29 herbicides, and polychlorinated
<br />biphenyls. Concentrations in nearly 97 percent of the
<br />samples analyzed were less than the analytical
<br />reporting limit. A summary of the detected pesticides
<br />in surface-water samples from the Arkansas River are
<br />presented in table 27. Of the seven compounds
<br />detected, two are organophosphorus insecticides and
<br />five are herbicides.
<br />Organophosphorus insecticides are among the
<br />least environmentally persistent pesticides because of
<br />their relatively rapid chemical and biological degrada-
<br />tion in both soil and surface-water systems. Diazinon
<br />was detected more frequently than malathion (nine
<br />detections compared to two detections), which is
<br />likely related to the fact that diazinon is one of the
<br />most persistent organophosphorus insecticides (Smith
<br />and others, 1988). Diazinon was detected in all five
<br />
<br />samples at Avondale, in three of the four samples at
<br />Catlin Dam, and in one of four samples at Las Animas.
<br />Diazinon is used primarily on onions and com;
<br />malathion is used on alfalfa, com, and beans and
<br />for mosquito control (Linda Coulter, Colorado
<br />Department of Agriculture, oral commun., 1994).
<br />Herbicides are, in general, nonpersistent in the
<br />environment and are rapidly degraded in surface-water
<br />systems (Smith and others, 1988). The most
<br />commonly used herbicide in the lower Arkansas River
<br />Basin is 2,4--D (Linda Coulter, Colorado Department
<br />of Agriculture, oral commun., 1994). It was detected
<br />at each of the five surface-water sites but at levels less
<br />than the MCL of70 IlgIL (U.S. Environmental Protec-
<br />tion Agency, 1993). The other herbicides---bromacil,
<br />cyanazine, prometon, and 2,4--DP-were detected
<br />intermittently at concentrations at or near the detection
<br />level. Two of the herbicides, bromacil and prometon,
<br />are used almost exclusively on rights-of-way (Linda
<br />Coulter, Colorado Department of Agriculture, oral
<br />commun., 1994).
<br />
<br />Suspended Sediment
<br />
<br />A network of 12 sampling sites was established
<br />to determine the spatial variability of suspended-
<br />sediment concentrations and loads in the basin. All the
<br />sites were located on the main stem of the Arkansas
<br />River with the exception of Fountain Creek (table I).
<br />Twenty-four to 28 water samples were collected at
<br />each of the 12 sites. Three other sites were sampled for
<br />suspended sediment only in response to various hydro-
<br />logical conditions observed at the time. In addition,
<br />suspended-sediment data collected periodically at
<br />Lower Badger Creek and Halfmoon Creek (pI. 1) were
<br />available for analysis. Instantaneous suspended-sedi-
<br />ment load was computed by multiplying streamflow
<br />by concentration.
<br />In the upper Arkansas River Basin, suspended-
<br />sediment concentrations showed downstream
<br />increases and large temporal variability but were rela-
<br />tively low in comparison to concentrations in the
<br />lower basin (fig. 30). Median suspended-sediment
<br />concentrations did not exceed 10 mgiL from Leadville
<br />to Wellsville but increased significantly (u=0.05)
<br />downstream from Wellsville to Portland. The median
<br />suspended-sediment concentrations at Parkdale and
<br />Portland were 26 and 56 mgIL, respectively. The large
<br />increase in suspended-sediment concentrations down-
<br />
<br />60 W.t.r-Qu.Uly A..e..m.nt olth. Ark.n... River B..ln, Southe..t.rn Color.do, 199G-93
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