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USGS Scientific Investigations Report 2004 -5024 <br />Pagel of 2 <br />Water Resources of Colorado <br />Methods to Identify Changes in Background Water - Quality Conditions <br />Using Dissolved - Solids Concentrations and Loads as Indicators, <br />Arkansas River and Fountain Creek, in the Vicinity of Pueblo, Colorado <br />by Roderick F. Ortiz <br />Available from the U.S. Geological Survey, Branch of Information Services, Box 25286, Denver Federal Center, <br />Denver, CO 80225, USGS Scientific Investigations Report 2004 -5024, 20 p., 11 figs. <br />This document also is available in pdf format: M SIR2004 -5024 (1.9MB) <br />(Requires Adobe Acrobat Reader) <br />This document also is available in HTML format: SIR2004- 5024.html (85KB) <br />The citation for this report, in USGS format, is as follows: <br />Ortiz, Roderick F., 2004, Methods to Identify Changes in Background Water - Quality Conditions Using Dissolved - <br />Solids Concentrations and Loads as Indicators, Arkansas River and Fountain Creek, in the Vicinity of Pueblo, <br />Colorado: U.S. Geological Survey Scientific Investigations Report 2004 -5024, 20 p. <br />Abstract <br />Effective management of existing water- storage capacity in the Arkansas River Basin is anticipated to help <br />satisfy the need for water in southeastern Colorado. A strategy to meet these needs has been developed, but <br />implementation could affect the water quality of the Arkansas River and Fountain Creek in the vicinity of Pueblo, <br />Colorado. Because no known methods are available to determine what effects future changes in operations will <br />have on water quality, the U.S. Geological Survey, in cooperation with the Southeastern Colorado Water Activity <br />Enterprise, began a study in 2002 to develop methods that could identify if future water - quality conditions have <br />changed significantly from background (preexisting) water - quality conditions. A method was developed to <br />identify when significant departures from background (preexisting) water - quality conditions occur in the lower <br />Arkansas River and Fountain Creek in the vicinity of Pueblo, Colorado. Additionally, the methods described in this <br />report provide information that can be used by various water - resource agencies for an internet -based decision - <br />support tool. <br />Estimated dissolved- solids concentrations at five sites in the study area were evaluated to designate historical <br />background conditions and to calculate tolerance limits used to identify statistical departures from background <br />conditions. This method provided a tool that could be applied with defined statistical probabilities associated with <br />specific tolerance limits. Drought data from 2002 were used to test the method. Dissolved - solids concentrations <br />exceeded the tolerance limits at all four sites on the Arkansas River at some point during 2002. The number of <br />exceedances was particularly evident when streamflow from Pueblo Reservoir was reduced, and return flows and <br />ground -water influences to the river were more prevalent. No exceedances were observed at the site on <br />Fountain Creek. These comparisons illustrated the need to adjust the concentration data to account for varying <br />streamflow. As such, similar comparisons between flow- adjusted data were done. At the site Arkansas River near <br />Avondale, nearly all the 2002 flow- adjusted concentration data were less than the flow- adjusted tolerance limit <br />which illustrated the effects of using flow- adjusted concentrations. Numerous exceedances of the flow- adjusted <br />tolerance limits, however, were observed at the sites Arkansas River above Pueblo and Arkansas River at Pueblo. <br />These results indicated that the method was able to identify a change in the ratio of source waters under drought <br />conditions. Additionally, tolerance limits were calculated for daily dissolved - solids load and evaluated in a similar <br />manner. <br />Several other mass -load approaches were presented to help identify long -term changes in water quality. These <br />included comparisons of cumulative mass load at selected sites and comparisons of mass load contributed at the <br />Arkansas River near Avondale site by measured and unmeasured sources. <br />Table of Contents <br />http: / /water.usgs.gov /pubs /sir/2004/5024/ 5/5/2005 <br />