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More attention to the selenium issue came in 1997, when the Colorado State Water Quality Control Commission revised chronic aquatic-life criterion for dissolved selenium from 17 µg/L to 5 µg/L. Imple- mentation of the 5-gg/L standard for the Gunnison River Basin meant that the Uncompahgre River and the lower Gunnison River (downstream from the Uncompahgre River to the mouth), along with some other water bodies in the lower Gunnison River Basin, were out of compliance for selenium. Temporary modifications were put in place for the affected water bodies to allow time for measures to be taken at the local level to address the selenium issue. A local initia- tive began in 1998, with formation of the Gunnison Basin Selenium Task Force (Task Force). The Task Force is a group of private, local, State, and Federal interests who examined numerous projects, methods, and ideas for feasibility for reducing selenium levels in the Uncompahgre and Gunnison Rivers. One participant in the Task Force is the NIWQP because of a common interest in searching for ways to reduce selenium concentrations in the Uncompahgre and Gunnison Rivers. A public meeting was held by the Task Force in November 1999 to obtain input from the local community about what could be done to reduce selenium. The Task Force and the NIWQP analyzed more than 40 remediation alternatives to identify feasible methods for reducing selenium in the Uncompahgre and Gunnison Rivers to meet the State standard of 5 gg/L. The remediation alternatives will be evaluated for their effectiveness on reducing sele- nium concentrations in the main-stem rivers, their cost effectiveness, and their possible negative effects on the local economy and lifestyle. A number of remediation alternatives were considered by the NIWQP prior to formation of the Task Force. Some alternatives involved improving irri- gation-distribution systems or improving on-farm irri- gation; these are the type of projects that have been used by the Salinity Control Program to reduce salinity. Because the Mancos Shale has been identified as the major source of selenium and salt in the lower Gunnison River Basin, it was logical to assume that a salinity-control type project such as canal and lateral lining might provide dual benefits of reducing sele- nium and salt loading. For many years, numerous studies were done by government and university researchers on salt-loading mechanisms and methods to estimate salinity reductions attributable to canal and lateral lining (for example, Duke and others, 1976; Bureau of Reclamation, 1978, 1982). In general, there is a correlation between sele- nium load and salt load (Butler and others, 1996) in the Grand and Uncompahgre Valleys; however, the accuracy of estimating selenium reductions for a. canal or lateral-lining project based on the salt load is not known. A demonstration project to measure directly how lateral lining or piping affected selenium loads was proposed by the NIWQP. Such a project would require a small basin in a high-selenium area, where monitoring could be done rather easily, and where there were a few miles of open laterals that were assumed to be contributing a significant selenium load (from leakage) that was measurable in that basin. Such an area exists southeast of Montrose in an area drained by Montrose Arroyo (fig. 1). Because there would be salinity reductions from such a project, the Salinity Control Program was approached along with the NIWQP to provide funds for a demonstration project. Amendments to the Salinity Control Act in 1995 allowed the Salinity Control Program to share costs on a project. Thus, with funds provided by the Bureau of Reclamation (BOR) through the Salinity Control Program and the NIWQP and with construction labor and machinery supplied by the Uncompahgre Valley Water Users Association (UVWUA), the Montrose Arroyo demonstration project was initiated in 1998. The U.S. Geological Survey (USGS) collected sele- nium and salinity data from June 1998 through October 2000 to document the effects of the project. The Montrose Arroyo demonstration project had two objectives: 1. For the Salinity Control Program, to prevent an esti- mated 2,300 tons per year of salt from entering the Colorado River. 2. For the NIWQP, to demonstrate the effect of a salinity-control type project, such as lateral lining, on selenium loads. This report describes the Montrose Arroyo demonstration project and its effect on selenium and salt loads in the Montrose Arroyo Basin. The report also describes the monitoring plan and the method of analysis used for determining the effect of the project on selenium and salt loads in Montrose Arroyo. INTRODUCTION