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Table 2. Mean selenium and salinity concentrations in irrigation water from the AM lateral and Loutzenhizer Canal, 1999-2000 [<,less than] Selenium Salinity Site (micrograms (milligrams (fig. 2) per liter) per liter) 1999 2000 1999 2000 AM1 2.2 1.5 209 188 AM3 3.6 2.4 262 246 LZC <1 0.9 261 250 the selenium and salinity concentrations (table 2) in monthly samples collected from April to October in 1999-2000 from two sites on the AM lateral and one site on the Loutzenhizer Canal (fig. 2). The ground- water concentrations (Cgw) were the mean concentra- tions of the samples collected in November through March at each site (table 1) and were assumed to approximate selenium and salinity concentrations in the ground-water component during the irrigation season. The Qsw and Qgw variables are unknowns. Equation (1) was solved for Qsw; Qsw = Qi - Qgw, and after substituting that expression for Qsw into equation (2), there was only one unknown, Qgw, remaining in equation (2). Once Qgw was calculated, Qsw was computed as Qi - Qgw. After computing Qsw and Qgw, the mean selenium and salt loads in the surface-water and ground-water components for the irrigation season were computed. Load equals flow times concentration times a conversion factor (0.00539 for selenium; 0.0027 for salinity). For example, the mean selenium load in surface water equals Qsw times Csw times 0.00539. This process was done for the pre-project and post-project periods for all sites except MA4, the control site. The mean loads derived by this method for the irrigation season (sum of mean surface-water load and mean ground- water load) and the nonirrigation season are listed in table 3. The mean loads for site MA4 were computed using the periodic samples collected at that site for each season. Readers are advised not to use the data in table 1 to compute the mean loads shown in table 3. A mean load computed using the mean streamflow and concentration data from table 1 may or may not equal the loads in table 3 because computing mean loads by that method is not equivalent to the method used to compute the mean loads listed in table 3. Once the irrigation-season analyses were done, the mean surface-water and ground-water loads were converted to annual loads. The means of the surface- water and ground-water loads for the irrigation season (April through October) were extrapolated to loads for Table 3. Mean selenium and salt loads for irrigation-season (April through October) and nonirrigation-season (November through March) samples at the five monitoring sites [Pre, pre-project period; Post, post-project period] Site (fig 2) Selenium load (pounds per day) Salt load (tons per day) . Pre Post Pre Post Irrigation season MA4 0.09 0.07 7.3 8.3 D2MA .38 .21 9.4 7.8 MA3 .57 .38 21.2 18.7 D1MA .60 .43 16.1 14.3 MA2 2.30 1.65 62.1 52.8 Nonirrigation season MA4 .19 .19 8.0 7.9 D2MA .41 .12 9.2 4.2 MA3 .55 .32 16.5 12.2 D1MA .23 .22 7.7 7.3 MA2 1.31 .95 32.6 26.9 INTRODUCTION 7