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<br />C:OOl <br /> <br />Based on water-balance considerations and <br />the cyclic nature of daily changes in discharge at <br />the sediment/water interface (fig. 5), the small-area <br />flow system probably did not provide a net addition <br />or loss of water to the river. For example, integration <br />of the discharge-rate data at the Road 8 site over <br />24 hours results in an estimate of instantaneous <br />ground-water discharge from the small-area flow <br />system to the river of about 1.49 (Mfrl /d)/mi. Based <br />on the November 30, 1992, mass-balance measurement <br />of incremental ground-water discharge from the large- <br />area flow system to the reach of river encompassing <br />the Road 8 site (the closest mass-balance measurement <br />in space and time to the 24-hour measurement), <br />incremental ground-water discharge was about <br />0.48 (Mfrl /d)/mi. Considering the time difference <br />between the two sets of measurements and other possi- <br />ble sources of error, the estimates of ground-water <br />discharge are very similar. The data indicate, therefore, <br />that the small-area flow system probably provided no <br />net gain or loss of water to the river, but that it was <br />superimposed on the large-area flow system that <br />did provide water to the river [at a median rate of <br />4.6 (frl/s)/mi]. In essence, the small-area flow system <br />represented a zone of mixing between the river and the <br />"large-area flow system. White (1993) has labeled this <br />mixing zone between surface and ground water as the <br />hyporheic zone. The short-term movement of water to <br />and from the small-area flow system, with no net <br />change in the amount of water in the river, explains <br />why measurements of ground-water discharge at the <br />sediment/water interface indicated high rates of water <br />movement into the aquifer even though water-table <br />altitudes in wells adjacent to the river were always <br />higher than the river stage. It also explains why the <br />median value [-5.8 (frl/s)/mi] of incremental discharge <br />from the small-area flow system was relatively close <br />to zero even though the magnitude of the minimum <br />[-1,360 (frl/s)/mi] and maximum [1,'000 (ff/s)/mi] <br />measured values were large (fig. 4 and table 9). <br /> <br />QUALITY OF GROUND-WATER <br />DISCHARGE <br /> <br />The discussion on the quality of ground-water <br />discharge to the South Platte River between Denver <br />and Fort Lupton will focus on pH, dissolved oxygen, <br />dissolved-nitrogen species, and dissolved-phosphorus <br />species because they are critical water-quality issues in <br />the study area (Camp Dresser and McKee, Inc., 1992). <br />Other water-quality data are reported in tables 9 and 10 <br />in the "Hydrologic and Water-Quality Data" section <br />at the back of this report. Quality-control data were <br />collected in the field and also are reported in the <br />"Hydrologic and Water-Quality Data" section in <br />table 11. The "Quality of Ground-Water Discharge" <br />section contains two subsections: the "Large-Area <br />Flow System" and the "Small-Area Flow System." <br /> <br />Water from the monitoring wells screened in the allu- <br />vial aquifer adjacent to the South Platte River was <br />assumed to be representative of water from the large- <br />area flow system, and water from piezometers screened <br />below the sediment/water interface was assumed to be <br />representative of water from the small-area flow <br />system. However, the degree to which these assump- <br />tions are true depended on the spatial extent of mixing <br />between the two flow systems, which was not fully <br />defined in this study. <br /> <br /> <br />Large-Area Flow System <br /> <br />Values of pH in water from the monitoring wells <br />screened in the alluvial aquifer adjacent to the river <br />ranged from 6.63 to 7.89, with a median value of7.16 <br />(fig. 6 and table 10). There was no apparent relation <br />between pH and the screened-interval depth of the <br />12 monitoring wells. Nor was there a relation between <br />pH and whether the well was located on the east side of <br />the river or the west side of the river. Values of pH in <br />water from the monitoring wells generally were lower <br />than values in the river (fig. 7 and table 10). Lower <br />values of pH in ground water compared to surface <br />water probably indicate: (I) Uptake of carbon dioxide <br />(C02) by photosynthesizing organisms in the river, <br />which would increase surface-water pH values; and <br />(2) equilibration of ground water with soil-zone CO2 or <br />CO2 produced by microbial processes in the saturated <br />zone. These data indicate that the discharge of ground <br />water from the large-area flow system to the river <br />would lower the pH of river water under conservative <br />mixing conditions. <br />Concentrations of DO in water from the monitor- <br />ing wells ranged from less than 0.1 to 5.2 mg/L, with a <br />median value of 0.9 mg/L (table 10). Concentrations <br />were variable between wells at each site, but at a given <br />well, were relatively constant (fig. 8). An exception to <br />the variability was at McKay Road where there was lit- <br />tle variability in the DO concentrations from well to <br />well. Concentrations of DO in wells at McKay Road <br />were less than I mgIL during each sampling period, <br />except during November 1992 when concentrations <br />were between 1.4 and 1.8 mgIL at three of the four <br />sites, The consistently low DO concentrations in water <br />from the McKay Road wells may indicate that there <br />was more organic carbon available to react with DO in <br />aquifer sediments at that site than in aquifer sediment <br />from the other sites. The median percentage of <br />organic-matter content in aquifer sediment collected <br />from well 2B at McKay Road was about four times <br />higher than the median organic-matter content in aqui- <br />fer sediment collected from well sites at Henderson or <br />Road 8 (table 5). It is not known whether the low con- <br />centrations of DO in ground water at McKay Road was <br />characteristic of all ground water underlying the more <br />urbanized parts of the study area, or whether it was a <br />local condition. <br /> <br />QUALITY OF GROUN[)..WATER DISCHARGE 17 <br />