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/' --- ,;f"~' - . - . . - . --.--- .-..-:';' '::"".::-...:-=--=:::-..:.-=-~:..- - - - - - - -- , lP ~",p 0233 Chromium About 90 percent or more of the samples collected at the four sites from Pueblo Reservoir to Avondale had dissolved-chromium concentrations that were less than the reponing limit of I j.1g/L. Total- recoverable chromium concentrations typically were less than or equal to the reponing limit (I j.1g/L) at the main-stem sites, Below Pueblo Reservoir and Pueblo (fig. 17). Between Pueblo and Highway 227, the median total-recoverable chromium concentration increased to 1.4 j.1g/L and 2.0 j.1g/L during the post- snowmelt runoff and low-flow regimes, respectively (fig. 17). Inflow of chromium from Fountain Creek presumably caused this increase; the median total- recoverable chromium concentration in Fountain Creek at Pueblo was 6 j.1g/L (Dash and Ortiz, (996). The median total-recoverable chromium concentration generally increased between Highway 227 and Avon- dale (fig. 17); the resuspension of chromium with fluvial sediment and unsampled tributary inflow between Highway 227 and Avondale are possible causes of the increased concentrations. Dissolved and total-recoverable chromium concentrations were analyzed in six samples collected Z 4.5 o ;:: r:; 4.0 l- I- Z W u 8a: 3.51- ~~ ::E~ 3.0f- Ow a:a. :I:", u" ~[2 2.5f- "'<!l "'0 a: a: ~(J 2.0:'" 8~ wZ ~- 1.5- g I- ~ 1.0- i5 w " 0.5 , ~o~ '1-'4' ,po ",.~ ~ at three main-stem sites from Catlin Dam downstream to Below John Manin Reservoir. Seventeen of these 18 samples had dissolved-chromium concentrations that were less than the reponing limit (\ j.1g/L). Conversely, total-recoverable chromium concentra- tions at Catlin Dam and at Las Animas typically were greater than the reporting limit and ranged from 2 to 170 j.1g/L. It was not possible to distinguish temporal patterns in total-recoverable chromium concentrations at these three sites because of the small number of samples that were collected. The median total- recoverable concentrations at Catlin Dam and at Las Animas were 16.5 and 11.5 j.1g/L, respectively. Between Las Animas and Below John Martin Reser- voir, the median total-recoverable chromium concen- tration decreased to 0.85 j.1g/L, presumably due to deposition in the reservoir. The resuspension of chro- mium with fluvial sediment and unsampled tributary inflow are possible causes of increased total-recover- able chromium concentrations at Catlin Dam and at Las Animas. No samples collected at any site in the lower basin had dissolved-chromium concentrations that exceeded water-quality standards. : - /' - - ;'- - - - - - --- - /' /' --- -- -- - ~ ,."' -?-4' "'~. ...",0 EXPLANATION - - SNOWMELT RUNOFF POST-SNOWMELT RUNOFF - . - LOW FLOW (May-June) (July-September} (October-April) Figure 17. Median total-recoverable chromium concentrations by streamflow regime in the lower Arkansas River, Apnl199o-March 1993. WATER QUALITY 39