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<br />J <br />J <br />J <br />J <br />J <br />J <br />. <br />J <br />J <br />J <br />J <br />J <br />J <br />J <br />J <br />J <br />J <br />J <br />J <br /> <br />3760 <br /> <br />groundwater sources. As was done in the Grand Valley, water <br />quality data were collected from drainage channels in the Lower <br />Gunnison that were thought to be yielding only return flows. <br />From this data, the BLM estimaten that of the total salt load <br />contributed by groundwater, 344,400 tons per year were from <br />irrigation return flows and the remaining 381,800 tons per year <br />were from natural base flow. <br /> <br />Summary <br /> <br />The USGS, EPA, SCS, and BLM have all developed estimates of <br />salt yields by source for the Gunnison River Basin. With the <br />exception of the BLM estimate of salt loading from irrigation <br />return flows, the portions of total basin salt load attributed to <br />the various man caused and natural sources do not differ <br />significantly from one study to the next. <br /> <br />USGS <br /> <br />Man Caused 64% <br />Natural 36% <br />Man Caused 67% <br />Natural 33% <br />Man Caused 58% <br />Natural 42% <br /> <br />EPA <br /> <br />SCS <br /> <br />The BLM estimated that 344,400 tons of salt per year were <br />contributed by irrigation return flows in the Gunnison River <br />Basin. All other agency estimates of salt loading from <br />irrigation in the Basin were more than twice as high. <br /> <br />-28- <br />