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<br />reporting limits (5 ~g/L) in all samples but could have exceeded the aquatic- <br />life standard for chronic exposure if hardness was low. However, the only <br />site where hardness was low enough for the standard to be less than 5 ~g/L was <br />site RI (Pueblo Reservoir), which is upstream from the irrigated' area. The <br />maximum concentrations of arsenic, chromium, copper, and zinc in all water <br />samples were no more than 50 percent of the most restrictive standards for <br />these trace elements. <br /> <br />Because data collection for the study occurred during a 4-month period, <br />short-term hydrologic conditions could have had a substantial effect on water <br />quality. To evaluate this effect, historical data from several surface-water <br />sites were compared to data collected during the study. The largest record of <br />data available in the study area was for specific conductance, which can be <br />a surrogate measure of the dissolved-solids concentration in water. The <br />historical distributions of specific-conductance data from eight sites are <br />plotted in figure 12. Values measured at these sites during the study also <br />are shown. At Pueblo Reservoir (site RI) and Lake Meredith (site R2), the <br />June values were about equal to the lower quartile of the historical data, but <br />the October values were as high or higher than the historical maximums. <br />Specific conductance generally varies inversely with streamflow; therefore, <br />the greater than normal inflow to the study area during 1983-87 (fig. 5) could <br />have diluted specific conductance throughout the study area in the beginning <br />of the sampling period. As inflow to the study area decreased to less than <br />normal during the sampling period, specific conductance could have increased. <br />However, most specific-conductance values for stream samples collected in <br />August were about the same as the historical medians for the stream sites (AI, <br />TI, T2, A2, and A4). In the Arkansas River near Coolidge (site A6), the value <br />was much less than the historical median. This site is downstream from <br />John Martin Reservoir, from which releases were greater than normal during the <br />study period. These releases could have maintained a dilute condition in the <br />eastern part of the study area, even though inflow at the western end (site AI) <br />was less than normal. <br /> <br />Similar effects can be seen in plots of dissolved-selenium data (fig. 13) <br />for sites with at least eight historical values. Between June and October, <br />selenium concentration at Pueblo Reservoir (site RI) increased from the <br />historical 25th percentile to the 75th percentile. Concentrations were about <br />the same as the historical median in the Arkansas River upstream from Pueblo <br />(site AI) and downstream from John Martin Reservoir (site A4) but less than <br />the 25th percentile farther downstream near Coolidge (site A6). In general, <br />the constituent concentrations in samples collected during the study upstream <br />from John Martin Reservoir may be representative of normal conditions, but <br />those in samples collected at downstream sites may be representative of dilute <br />conditions. <br /> <br />36 <br />