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the entire irrigation season by multiplying the esti- <br />mated mean loads times 214 days. The means of the <br />individual sample loads collected in the nonirrigation <br />season (November through March) were multiplied by <br />151.25 days (allowing for leap years) to estimate the <br />nonirrigation season loads. The nonirrigation season <br />loads were assumed to be derived primarily from <br />ground-water sources in the Montrose Arroyo Basin, <br />so the sum of the nonirrigation-season load and the <br />estimated ground-water load in the irrigation season is <br />the annual ground-water load. The total annual sele- <br />nium and salt loads are the sum of the surface-water <br />load and the ground-water load (tables 4 and 5). The <br />pre-project selenium and salt loads were compared to <br />the post-project loads to examine the effects of the <br />lateral improvements. The change in the ground-water <br />load between the pre-project and post-project periods <br />should reflect changes caused by the lateral work. <br />The total loads shown in tables 4 and 5 can be <br />approximated by using the data in table 3; the total <br />loads are equal to the sum of the mean irrigation- <br />season load times 214 and the mean nonirrigation <br />season load times 151.25. The loading analysis was <br />done by using a computerized spreadsheet program, <br />and the computations were done using unrounded <br />numbers. The mean loads in table 3 are rounded <br />values. Slight differences in the total loads computed <br />using the values in table 3 when compared to the total <br />loads listed in tables 4 and 5 may be obtained because <br />of rounding. However, computing total loads using <br />data in table 3 does not separate the total load into <br />surface-water and ground-water load. The component <br />analysis of the irrigation-season loads was done <br />because of the need to know the effects of the lateral <br />project on ground-water loads. <br />Table 4. Selenium loads for the Montrose Arroyo demonstration project <br />[Loads are annual loads, in pounds; Pre, pre-project period; Post, post-project period; Change, the post-project load minus the pre-project load; <br />Percent, percent change; --, not calculated] <br />Site Total load Ground-water load Surface-water load <br />(fig. 2) <br /> pre Post Change Percent Pre Post Change Percent Pre Post Change Percent <br />MA4 48 45 -3 -6 - - - -- -- <br />D2MA 144 64 -80 -56 135 58 -77 -57 9 6 -3 -33 <br />MA3 206 130 -76 -37 195 121 -74 -38 11 9 -2 -18 <br />D1MA 163 125 -38 -23 158 121 -37 -23 5 4 -1 -20 <br />MA2 690 496 -194 -28 653 475 -178 -27 37 21 -16 -43 <br />I Control site <br />Table 5. Salt loads for the Montrose Arroyo demonstration project <br />[Loads are annual loads, in tons; Pre, pre-project period; Post, post-project period; Change, the post-project load minus the pre-project load; <br />Percent, percent change; --, not calculated; the sum of ground-water and surface-water loads may not equal total loads in some cases because of rounding] <br />Site Total load Ground-water load Surface-water load <br />(fi <br />2) <br />g. Pre Post Change Percent Pre Post Change Percent Pre Post Change Percent <br />MA4 2,780 2,960 180 6 -- - - - - <br />D2MA 3,400 2,300 -1,100 -32 3,030 2,070 -960 -32 370 230 -140 -38 <br />MA3 7,030 5,850 -1,180 -17 6,550 5,460 -1,090 -17 480 390 -90 -19 <br />D1MA 4,610 4,170 -440 -10 4,170 3,840 -330 -8 440 330 -110 -25 <br />MA2 18,200 15,300 -2,900 -16 15,700 13,900 -1,800 -11 2,490 1,420 -1,070 -43 <br />Control site. <br />8 Effects of Piping Irrigation Laterals on Selenium and Salt Loads, Montrose Arroyo Basin, Western Colorado