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<br />1923 <br />INTRODUCTION <br /> <br />?RCUMIN,c.RY RE'PC';:"~ <br /> <br />. <br /> <br />The lower Arkansas River in Southeastern Colorado extends about 200 mi downstream from Pueblo Reser- <br /> <br />voir to the Colorado-Kansas Stateline (fig. I). The Arkansas River serves as the primary municipal water supply <br /> <br />for most of the 165,580 people who live in the five counties that comprise the lower Arkansas River valley and as <br /> <br />the primary agricultural irrigation supply for about 300,000 acres of irrigated land in the lower basin. Because of <br /> <br />the dependence on the Arkansas River as a domestic and agricultural water supply, the quality of water in the <br /> <br />Arkansas River is very important. <br /> <br />The quality of water in the Arkansas River changes dramatically in the study area. Specific conductance. <br /> <br />which is directly related to dissolved-solids concentration. increases downstream from a median of about 500 !-,SI <br /> <br />cm near Pueblo to about 3,900 !-IS/cm at Lamar; this range in specific conductance corresponds to a range in the <br /> <br />total dissolved-solids concentration of about 340 [0 3,600 mglL (Cain, 1984). Ele,.ated specific conductance and <br /> <br />dissolved-solids concentrations can cause problems for domestic and agricultural water supplies. The secondary <br /> <br />. <br /> <br />maximum contaminant level (SMCL) for dissolved solids in drinking water is 500 mg/L (U.S. Environmental Pro- <br /> <br />tection Agency, 1988). Additionally, elevated dissolved-solids concentrations (>2.000 mgIL) can cause reduced <br /> <br />crop yields for salt sensitive crops. The downstream increase in specific conductance and hence, dissolved solids, <br /> <br />is largely due to the consumptive use of surface water and ground water used for agricultural irrigation (Miles. <br /> <br />1977). <br /> <br />Figure 1 near here. <br /> <br />Discharge in the lower Arkansas River is regulated by the storage and release operations of two mainstem <br /> <br />reservoirs, Pueblo Reservoir and John Martin Reservoir. Both reservoirs can increase or decrease the dilution <br /> <br />potential in the river through streamflow manipulation and thus can affect specific conductance. Changes in spe- <br /> <br />cinc conductance might affect the suitability of Arkansas River water for domesti: or agricultural use. Although <br /> <br />. the specific-conductance conditions and relations between specific conductance and discharge in tbe Arkansas <br /> <br />4 <br />