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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />3755 <br /> <br />It was determined that irrigation return flow was responsible for <br />690,800 tons per year (66%) of the total and natural groundwater <br />inflow was responsible for the remaining 349,200 tons per year <br />(34%). <br /> <br />Summary <br /> <br />The Grand Valley Subbasin is one of the more studied salt <br />loading source areas in Colorado. The USGS, EPA, BuRec, SCS, and <br />BLM have all developed salt load by source estimates for that <br />portion of the Colorado River that includes the Grand valley. <br />The studies cover different periods of time and the areas <br />included in the studies vary slightly. All these things limit <br />the value of any direct comparison of results. <br /> <br />USGS Man Caused 85.7% <br /> Natural 14.3% <br />EPA Man Caused 95.8% <br /> Natural 4.2% <br />BuRec Man Caused 98.7% <br /> Natural 1.3% <br />*BLM Man Caused 66.0% <br /> Natural 34.0% <br /> <br />* Refers to percentage of salt load contributed by <br />groundwater sources only. <br /> <br />All salt loading information on the Colorado River between <br />Cameo and the state line identify man caused sources, irrigated <br />agriculture in particular, as making up the largest portion of <br />the increase through this stretch of river. <br /> <br />The Bureau of Reclamation study does not consider the effect <br />of municipal and industrial use of water, and their estimate of <br />10,000 tons per year from natural surface runoff above irrigated <br />lands is not consistent with other estimates. <br /> <br />-23- <br />