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in the lower Gunnison River Basin, but the results might indicate that canal and lateral improvements have a greater effect on selenium loads than on salt loads in the Mancos Shale areas of the lower Gunnison River Basin. All of the decreases in selenium and salt loads that might have resulted from the demonstration project probably were not measured at site MA2. The AMB lateral is on a ridge between the Dry Cedar Creek and Montrose Arroyo Basins (fig. 2); therefore, not all ground water resulting from leakage from the AMB lateral would have necessarily discharged to Montrose Arroyo. For the other four laterals that were piped, it was assumed that all subsurface flow resulting from lateral leakage eventually discharged to Montrose Arroyo or tributary drainage ditches upstream from site MA2. Because of a lack of detailed information about the shallow ground-water flow paths in this area, that assumption might not be entirely valid. Because of the extensive pre-project data for site MA2, that site was used as the outflow site instead of a location farther downstream where no sampling had previously been done. Using the load reductions at site MA2 determined by this study and an estimate of the volume of lateral leakage that was not accounted for at site MA2, the BOR has estimated the project's total selenium-load reduction at about 210 lb/yr and the total salt-load reduction at about 2,500 tons/yr (Michael Baker, Bureau of Reclamation, written commun., 2001). FACTORS AFFECTING SELENIUM AND SALT LOADS The goal of lining a canal or lateral, or removing open-ditch laterals and placing them in PVC pipe as was done in the Montrose Arroyo project, is to reduce or eliminate canal and lateral leakage into shallow ground water. Canal or lateral improvements would have the effect of reducing the volume of shallow ground-water discharge, which should directly reduce selenium and salt loads because load is a function of concentration and discharge. The decreases in the salt loads in the ground-water component for the Montrose Arroyo area (table 5) are assumed to be related to the amount of ground-water recharge that was caused by leakage from the five laterals that were piped. Except for a small decrease at site D2MA (table 1), the salinity concentrations in shallow ground-water discharge (the nonirrigation season, or November through March samples) in the study area did not change significantly from the pre-project to the post- project period; therefore, decreases in ground-water discharge must be the primary reason for decreases in salt loads. However, changes in discharge cannot explain all of the decreases in the selenium loads. As discussed in the previous section and shown in tables 1 and 2, the relative decreases in selenium loads were considerably greater than the decreases in salt loads, which implies that selenium concentrations, at least in some areas, must also have decreased along with decreases in discharge. At three sites, selenium concentrations in the nonirrigation-season samples were lower in the post-project period than in the pre- project period (table 1). At site D2MA, the mean sele- nium concentration in the nonirrigation-season samples decreased from 149 µg/L in the pre-project period to 89 µg/L in the post-project period. In some areas, such as the area drained by ditch D2MA, the decrease in ground-water flow might have affected the geochemical environment in the shallow ground water, which could cause changes in selenium concentra- tions. It was assumed that the decreases in selenium and salt loads described in this report were solely attributed to the lateral project. Home construction was an ongoing activity in the study area, and work at the golf course between sites MA3 and MA2 (fig. 2) also was done during the study period. Because no large tracts of land were removed from irrigation, it does not seem likely that development would have caused lower selenium and salt loads in Montrose Arroyo in 1999-2000. Work at the golf course area proceeded slowly in 1998-99 and the area was not irri- gated. The only potential effect at the golf course in 1999 was the filling of a pond in June with water from Montrose Arroyo, which may have leached some sele- nium and salt from salt crusts that were inundated. Limited sprinkler irrigation began on a few of the fair- ways in late summer in 2000, but it is unlikely that the new irrigation had much effect on the post-project loads, which were computed through October 2000. In addition to human activities, there could be other factors that could have affected the results of this study. One factor that raised some concern was the unusually high rainfall in July and August 1999 in the Montrose area. At Montrose, the total precipitation in July and August 1999 was 6.07 inches, which is 288 percent above the 1961 to 1990 normal rainfall of 12 Effects of Piping Irrigation Laterals on Selenium and Salt Loads, Montrose Arroyo Basin, Western Colorado