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02;ll is derived from these irrigation return flows. The extensive application of irrigation water can leach selenium from the seleniferous soils and rocks in the study area. Thus, local geology and irrigation return flows are the two most likely causes of the increasing selenium concentrations between Pueblo and Las Animas (fig. 23). Selenium concentrations were largest during the low-flow regime when streamflow and irrigation return flows were less diluted by the river. The apparent decrease in selenium concentrations between the Las Animas and Below John Martin Reservoir sites (fig. 23) seems to have been caused by a process that removed dissolved selenium from the water column and stored it in the biota or bottom sediments. Oremland and others (1989) have demonstrated the possibility of such a process involving the bacterial reduction of selenate, the most mobile inorganic sele- nium species, under anoxic conditions. Selenite, whicb is relatively insoluble, is expected to be the most common selenium species under anoxic condi- tions. Selenite has been shown to readily sorb to ferric oxides, thus controlling the selenium concentration in solution (Brown, 1991). Total-recoverable selenium concentrations frequently exceeded the chronic stream-water-quality standard of 10 ilg/L at several sites in the lower basin (table 24). The frequency of exceedance was largest at Catlin Dam and at Las Animas, where about Table 24. Number of samples that exceeded chronic selenium water.quality standards in the lower Arkansas River, April 199o-March 1993 Number of samples that exceeded chronic standard for total. recoverable selenium 1 Below Pueblo Reservoir 3 0 Puebto t6 t Highway 227 3 2 Avondale t 6 7 Callin Dam 24 J 4 Las Animas 24 t 7 Below John Martin Reservoir 23 1 IChronic selenium standard (10 micrograms per Iller) is based on total-recoverable selenium concentration (Colorado Department of Health. 1994). Site name (see table 1) Number of samples analyzed S8 percent and ahout 71 percent of the samples exceeded tbe standard, respectively (table 24). Sele- nium concentrations exceeded the stream-water- quality standard during all three flow regimes, although the standard was exceeded more frequently during the low. flow regime. Sliver Dissolved and total-recoverable silver concen- trations were less than the reporting limit (l ilg/L) in neariy all of the samples lhat were collected in the lower basin. The analytical reporting limit was too large to indicate if silver occurred at concentrations that might be toxic to aquatic life. Zinc A comparison of the median dissolved- and total-recoverable zinc concentrations at Portland (fig. 15) and Below Pueblo Reservoir (fig. 24) indi- cates that very little zinc was transported through Pueblo Reservoir from the upper basin. In the lower basin, the median dissolved-zinc concentrations ranged from aboul 2 to 7 ilg/L and varied extensively (fig. 24). The median total-recoverable zinc concentra- tion ranged from 7 to 200 ilg/L; the maximum total- recoverable zinc concentration (200 ilg/L) occurred at Catlin Dam during the post-snowmelt runoff regime (fig. 24). Total-recoverable zinc concentrations varied little at most sites during the low-flow regime but tended to increase downstream from Avondale during the snowmelt-runoff and post-snowmelt runoff regimes (fig. 24). This increase probably is attributable to resuspension and tributary inflow. In the two stonn- runoff samples that were collected from the Apishapa River and in the one sample collected from the Purga- toire River, the total-recoverable zinc concentrations were 1,300, 850, and 1,000 ilg/L, respectively (Dash and Ortiz, 1996). The total-recoverable zinc concen- trations tended to decrease between Las Animas and the site just downstream from John Martin Reservoir (fig. 24), probably because of deposition within the reservoir. Zinc concentrations did not exceed the stream-water-quality standards at any main-stem site in the lower Arkansas River Basin. WATER QUALITY 49