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Pueblo site (table I). The confluence of the Arkansas River and Fountain Creek is downstream from the main-stem site at Pueblo and just upstream from the main-stem site at Highway 227 (table I). A substantial percentage of streamflow in Fountain Creek is derived from tbe city of Colorado Spring's municipal waste- water effluent. Downstream from Avondale to the Colorado-Kansas State line, land use predominantly is agricultural, and the primary trace elements of poten- tial concern are copper, iron, manganese, selenium, and zinc. Secondary trace-elements of potential concern in this reach include cadmium, chromium, lead, and silver, whicb were analyzed in six samples collected at each of the following Arkansas River main-stem sites: Catlin Dam, Las Animas, and Below John Martin Reservoir (table I). Concentrations of some trace elements in the lower Arkansas River have considerable temporal variability because of the variability of streamflow. Water storage and release operations in the two main-stem reservoirs in the lower basin, Pueblo Reservoir and John Martin Reservoir, cause much of the variability in streamflow in the lower Arkansas River. The downstream temporal variability of trace- element concentrations in the lower basin is descrihed in tenns of three flow regimes, which are slightly different from those described for the upper basin trace elements. The streamflow regimes in the lower basin are low flow (October-April), snowmelt runoff (May-June), and post-snowmelt nmoff(July- September). This definition of streamflow regimes serves as a general guideline for the interpretation of trace-element concentrations and is subject to some degree of variability due to year-to-year variability in weather conditions and reservoir operations. Cadmium Downstream from Pueblo Reservoir to Avon- dale, dissolved- and total-recoverable cadmium concentrations typically were less than their respective reporting limits, 0.1 i1glL and I i1glL, during all three flow regimes. Two exceptions were measured during the low-flow regime at the sites Below Pueblo Reser- voir and Pueblo; tbe median dissolved-cadmium concentrations were 0.18 and 0.14 i1g1L, respectively. Cadmium concentrations were analyzed in six samples collected at each of three main-stem sites from Catlin Dam downstream to Below John Martin Reservoir. At these sites, all dissolved-cadmium concentrations were less than the reporting limit (1 i1g1L). Conversely, total-recoverable cadmium concentrations at Catlin Dam and at Las Animas typically were greater than the reporting limit (I i1g1L) and ranged from I to 5 i1glL. Total-recoverable cadmium concentrations in samples collected at Below John Martin Reservoir were all less than the reporting limit. Only one sample (collected in August 1991 at the site Below Pueblo Reservoir) exceeded the chronic water-quality stan- dard for dissolved cadmium (table 21). No samples exceeded the chronic standard for total-recoverable cadmium (table 21). Table 21. Number of samples that exceeded chronic cadmium water-quality standards in the lower Arkansas River April 199G-March 1993 ' Site neme (see table 1) Number of samples analyzed Below Pueblo Reservoir 26 Pueblo 25 Highway 227 24 Avondale 24 Catlin Dam 6 Las Animas 6 Below John Martin Reservoir 6 IColorado Depanment of Health, 1994. 2Chronic standard == e{O.7852[ln(hardn,ss)].3.490) 3Unless noted: Acute standard = e(1.128[In(hardness)].Z.905), 4 Acute standard = e( I. 128Qn(hardncS3)J.J.490l. Number of samples that exceeded standard I Chronic Chronic stlndlrd for stlndlrd for totll-recoverable dissolved cldmlum2 cadmfum3 I O' o 0 o 0 o 0 o 0 o 0 o 0 38 Weler-Quellly As..ssmenl ollhe Arkensas River Besln, Southeestern Colorado, 1991Hl3