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Montrose Arroyo Demonstration Project Montrose Arroyo is a tributary of Cedar Creek, which is tributary to the Uncompahgre River in the Montrose area (fig. 1). Montrose Arroyo drains mixed land uses of urban, suburban, agricultural, and unde- veloped desert areas. The study area (fig. 2) occupies about the middle one-third of the basin southeast of Montrose. Historically, the study area was primarily agricultural and rural, but as population has increased, some of the area has been developed with residential areas ranging from higher density subdivisions to small acreages. Also, a golf course development began about 1997-98 in the area between sites MA3 and MA2 (fig. 2). One reason Montrose Arroyo was selected for a demonstration project was because the area included several laterals of moderate size that could be lined or placed in pipe at a cost that was acceptable to the Salinity Control Program and the NIWQP. Five open- ditch laterals in the Montrose Arroyo Basin were part of the project: the AME, AMF, AMD, AMB, and D1.60 (fig. 2). The UVWUA replaced 8.5 miles of open laterals with about 7.5 miles of PVC pipe. The PVC pipe was placed in trenches and buried, and the original laterals were filled in. About 80 percent of the lateral construction was done in the winter of 1998-99, and the remainder was completed in December 1999. Monitoring Program and Method of Analysis Three sites on the main stem of Montrose Arroyo (MA4, MA3, and MA2) and two sites on drainage ditches (D2MA and DIMA) (fig. 2) were used to monitor the effects on selenium and salt loads from replacing the open laterals with pipe. A control site was needed to monitor inflow loads upstream from the project area. Site MA4 was upgradient from suspected ground-water inflow that could be caused by lateral leakage and was used as the control site. Site MA3 at 6700 Road was selected primarily because it was upstream from new golf course development and also to document changes in the upper part of the project. Site MA2 at Niagara Street was the outflow site and was assumed to be downstream from all or nearly all of the surface and subsurface inflows related to the five laterals. A substantial amount of pre-project 0 selenium and salinity data (31 samples) was available for site MA2. Those samples had been collected by the USGS as part of the NIWQP studies and monitoring in the Uncompahgre Valley in 1993-97. Monitoring for the lateral demonstration project began in June 1998 to collect pre-project data. Water samples were collected biweekly at the five sites for selenium and major-ion analysis. Salinity concentrations were determined using the sum of constituents method, which computes the salinity (dissolved solids) concentration by summing the major-ion concentrations. Stream discharge was measured with every sample collected to enable computation of loads. The biweekly sampling was continued through October 2000. Monthly samples were collected from April through September in 1999 and 2000 at two sites on the AM lateral (sites AM1 and AM3 in fig. 2) and one site on the Loutzenhizer Canal (site LZC). Data from the AM lateral and Loutzenhizer Canal were used to determine selenium and salinity concentrations in the irrigation water for use in the loading analysis. Water-quality data collected by the USGS in the Montrose Arroyo Basin before 1998 are published in Butler and Osmundson (2000). Water-quality data collected in 1998-2000 are unpublished but are on file at the USGS office in Grand Junction, Colorado. All water- quality data for the Montrose Arroyo area also can be obtained from the USGS National Water Information System. Selenium and salt loads for each sample were computed using the selenium and salinity concentra- tions from the chemical analysis and the streamflow that was measured at the time samples were collected. Instantaneous load is computed by multiplying the streamflow times the concentration times a conversion factor. Selenium loads are in pounds per day, and salt loads are in tons per day. The lateral construction was done during the nonirrigation season, when there is no water in the laterals, and began in November 1998. Because about 80 percent of the lateral work was completed in winter 1998-99, the first effects from the lateral project were expected to occur once the irriga- tion water was turned back into these laterals about April 1, 1999, and all data collected before April 1999 would be pre-project data. For the loading analysis, samples for each site were separated into a pre-project period (all data prior to April 1999) and a post-project period (April 1999-October 2000). It would seem that a simple before-and-after comparison of average loads 4 Effects of Piping Irrigation Laterals on Selenium and Salt Loads, Montrose Arroyo Basin, Western Colorado