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Concentrations of DO were higher in water from wells IA and IB than in water from wells 2A and 2B at Hcnderson and Road 8 (fig. 8). The median concentra- tion of DO in water from wells 2A and 2B at those sites wa~ 0.4 mgIL, whereas the median DO concentration in water from wells IA and IB was 2.7 mglL. The highest DO concentrations, greater than 4 mgIL, were mea~ured in water from welllA at Road 8. Based on ongoing regional studies of ground water in parts of the alluvial aquifer overlain by agricultural land-use set- tings (U.S. Geological Survey, unpublished data, 1993), the relatively high DO concentrations in water from thc IA and IB wells at Henderson and Road 8 are more characteristic of DO concentrations in the large- area flow system than the low concentrations of DO in the 2A and 2B wells. The differences in DO concentra- tions between the IAIlB and 2A/2B well pairs at the Henderson and Road 8 sites could indicate differences in the degree of mixing between the large-area and small-area flow systems. As will be discussed in the "Small-Arca Flow System" section, water in the small- area flow system wa~ more depleted in DO than water in the large-area fl ow system. The median instantaneous daytime concentration of DO in the South Platte River, based on measure- ments at the McKay Road, Henderson, and Road 8 sites, was 8.4 mglL, which means that even though ground water in the large-area flow system was oxy- genated, discharge of this water to the river would have lowered surface-water DO concentrations under con- servative mixing conditions. For example, in reach 2 ofthe South Platte River extending from 104th Avenue to just upstream from the Brighton Ditch headgate (pI. I), if the concentration of DO in surface water entering the reach is assumed to be 8.5 mg/L, if the concentration of DO in ground-water discharge from the large-area flow system is assumed to be 2,5 mgIL, and if 17 percent ofstrearnflow in the reach comes from ground-water discharge (17 percent is the median value for reach 2 from table 4), the concentration of DO in the river at the downstream end of the reach would be about 7.5 mg/L. If7.5 mg/L represented theconcentra- tion of DO in surface water entering reach 3 of the South Platte River extending from 160th Avenue to Highway 52, mass-balance calculations indicate that the concentration of DO in the river at the downstream end ofthe reach would be. about 7.0 mg/L, assuming that II percent of the river flow is from ground-water discharge (II percent is the median value for reach 3 from table 4). Concentrations of dissolved nitrite plus nitrate in ground water from the monitoring wells ranged from less than 0.01 to 37.93 mg/L a~ nitrogen, and concen- trations of dissolved ammonium ranged from less than 0.1 to 9.2 mglL as nitrogen (table 10). Based on sepa- rate nitrite analyses, essentially all of the dissolved nitrite plus nitrate was as nitrate, Concentrations of nitrite plus nitrate and ammonium varied greatly between sites, between wells at a given site, and with depth (table 10 and figs. 9 and 10). Concentrations of dissolved nitrite plus nitrate generally increased going from ground water at McKay Road to ground water at Road 8, whereas dissolved-ammonium concentrations decreased. Water from welllA at Road 8 had the high- est measured concentration of nitrite plus nitrate, and water from welllA at McKay Road had the highest measured ammonium concentration (table 10). The large variability in concentrations of nitrite plus nitrate and ammonium between wells at a given site and between sites indicates that ground-water dis- charge as a source of dissolved-nitrogen species to the river also was spatially variable. For cxample, median concentrations of nitrite plus nitrate in the IA/IB and 2A/2B wells at McKay Road were less than 0.01 mgIL as nitrogen, whereas the median concentration in the river was 4.40 mglL as nitrogen. This indicates that ground-water discharge from the large-area flow sys- tem was not a substantial source of nitrite plus nitrate to the river at McKay Road. In contrast, the median concentrations of nitrite plus nitrate in the IAIlB wells and surface water at Road 8 were 24.50 and 5.23 mglL as nitrogen, indicating that ground-water discharge from the large-area flow system on the east side of the river at Road 8 was probably a substantial source of nitrite plus nitrate to surface water. However, the low median concentration of nitrite plus nitrate in the 2A/2B wells at Road 8 (0.29 mg/L as nitrogen) indi- cates that the flow system on the west side of the river was not a substantial source of nitrite plus nitrate to the river. Median concentrations of ammonium in the monitoring wells at McKay Road were about the same as the median concentration of ammonium in the river (5.3 mg/L at the IA/IB wells, 5.4 mglL at the 2A/2B wells, 5.1 mgIL in the river, all as nitrogen). Concentrations of dissolved phosphorus in water from the monitoring wells ranged from less than 0.0 I to 7.24 mglL as phosphorus (fig, 11). The highest con- centrations were measured in water from wells at McKay Road, and the lowest concentrations were in water from wells at Road 8. McKay Road was the only site at which water from the monitoring wells had, dissolved-phosphorus concentrations higher than dissolved-phosphorus concentrations measured in sur- face water at the same site (table 10), indicating that the site at McKay Road was the only one of the three sites having monitoring wells where the large-area flow system could be an important source of dissolved phos- phorus to the river. 22 Quantity and Quality 01 Ground.Walar Dlacharge 10 the Soulh Plalle Rlvar, Denver 10 Fori Lupton, Colorado, Augusl1992 Through July 1993