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<br />UIJI"'~ I <br />The Montrose Arroyo <br />Demonstration Project <br /> <br />A demonstration project was initiated in <br />1998 to determine if replacing five open. <br />ditch laterals with 7.5 miles of buried pipe in <br />the Montrose Arroyo basin would reduce <br />selenium loads and to examine the relation <br />between salt and selenium loading. <br />Montrose Arroyo is a small basin south of <br />Montrose that has high selenium and salt <br />concentrations and is significantly affected <br />by irrigation drainage. The project was <br />funded by the Colorado River Basin Salinity <br />Control Program and NIWQP. Workers <br />from the Uncompahgre Valley Water Users <br />Association completed the project in 2000 <br />at a cost of about S 1.3 million which is <br />about 24% less than originally anticipated. <br /> <br /> <br />Biweekly sampling was done by the USGS <br />from 1998 through October 2000 to monitor <br />five sites in the basin. Data were separated <br />into pre-project and post-project sets for <br />each site. <br /> <br />Changes in selenium and salt loads in the <br />ground-water were then used to determine <br />the effects of the project. Placing the five <br />laterals in pipe caused a significant <br />reduction in selenium loads in the post- <br /> <br />... <br /> <br />project period at all monitoring sites except <br />at the control site, where selenium loads did <br />not change. At the outflow monitoring site at <br />Niagara Road in Montrose, the selenium load <br />decreased by about 28 percent of the pre- <br />project load, and more than 90 percent of that <br />decrease was in the ground.water load. Salt <br />load also decreased in the post-project period <br />at all monitoring sites except the control site. <br />The percentage decreases in salt loads were <br />smaller than the percentage decreases in <br />selenium loads. <br /> <br />It should be noted that ail of the potential <br />effects of the project on selenium and salt <br />loads were not measured at the monitoring <br />sites. Part of the seepage that originated from <br />some of the laterals prior to their being <br />replaced with pipe probabiy did not discharge <br />into Montrose Arroyo upstream from the <br />outflow monitoring site at Niagara Road. <br />Taking into consideration the extent of the <br />drainage not captured at the Niagara Road <br />site and using the USGS data from that site, <br />the Bureau of Reclamation has estimated the <br />project's total salt load reduction at about <br />2,500 tons per year and the selenium load <br />reduction at 210 pounds per year. <br /> <br />One interesting observation was that the <br />project caused a greater percentage decrease <br />in selenium loads than in salt loads. This may <br />have been partially the result of decreases in <br />selenium concentrations in the ground-water <br />in some portions of the project area in addition <br />to the decrease in selenium load caused by <br />the reduction in ground-water discharge. The <br />USGS plans to continue sampling two sites in <br />the Montrose Arroyo to monitor the long-term <br />effects of future residential and golf.course <br />development on selenium and salt loads. <br /> <br />In addition to piping the laterals, the project <br />also included a component to replace lost <br />wildlife habitat resulting from eliminating the <br />seepage from the laterals. The "habitat <br />replacement plan" involved constructing a <br />pipeline to provide a water supply for <br />development of new habitat at the Escalante <br />State Wildlife Area near Delta. <br /> <br />Page 3 <br />