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<br />. <br /> <br />. <br /> <br />. <br /> <br />Q <br />~ <br />~ <br />Ul <br /> <br />CHAPTER II <br /> <br />NEED FOR ACTION <br /> <br />of deep percolation of irrigation water believed to be the primary <br /> <br />contributor; and secondarily, infiltration of precipitation and runoff <br /> <br />within the drainage basin and upward movement from the underlying deep <br /> <br />aquifer system. <br /> <br />The SCS estimates of 29,590 acre-feet of return flows from irriga- <br /> <br />tion show that most of the salt loading results from deep percolation of <br /> <br />irrigation waters. <br /> <br />Some of this return flow is surface runoff and a <br /> <br />portion is lost to phreatophyte consumptive use. While this exceeds the <br /> <br />Reclamation estimate of 19,550 acre-feet of seep flow, it serves to <br /> <br />reinforce the belief that most of it is due to irrigation. <br /> <br />Problem Identification and Quantification <br /> <br />Problem identification <br /> <br />Investigation of the Big Sandy River salt loading problem was ini- <br /> <br />tiated by Reclamation in 1972 with the installation of a U.S. Geological <br /> <br />Survey (USGS) gaging station at Gasson Bridge, downstream of all known <br /> <br />seeps. <br /> <br />In 1981, a USGS gaging station was installed below the <br /> <br />confluence of the Big Sandy River and Little Sandy Creek above the seep <br /> <br />area. <br /> <br />Continuous flow, electrical conductivity, and temperature <br /> <br />measurements have been taken at these two stations to better define seep <br /> <br />inflows and salt loading. Flow and quality data are also available at <br /> <br />other USGS stations along the river. <br /> <br />An extensive drilling program began in 1975 to determine aqui- <br /> <br />fer location and characteristics associated with the seep flows. <br /> <br />28 <br />