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WSP08349
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Last modified
1/26/2010 2:47:50 PM
Creation date
10/12/2006 2:55:23 AM
Metadata
Fields
Template:
Water Supply Protection
File Number
8407
Description
Platte River Basin - River Basin General Publications
State
CO
Basin
South Platte
Water Division
1
Date
12/6/1995
Author
USGS
Title
Quantity and Quality of Ground-Water Discharge to the South Platte River - Denver to Ft Lupton - Colorado - August 1992 through July 1993
Water Supply Pro - Doc Type
Report/Study
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<br />from l04th Avenue to Brighton Ditch Diversion and <br />11 percent for the reach extending from 160th Avenue <br />to Highway 52. The median percentage for these two <br />reaches combined was 15 percent. Based on these esti- <br />mates, ground-water discharge between l04th Avenue <br />and Highway 52 would have accounted for a total of <br />about 30 percent of the flow in the river at Highway 52. <br /> <br />The quantity of ground-water discharge from the <br />small-area flow system to the South Platte River was <br />estimated from direct measurements of incremental <br />discharge made at 30 cross-section sites. Incremental <br />ground-water discharge from the small-area flow <br />system ranged from -1,360 to 1,000 (ft3/s)/mi, with a <br />median value of -5.8 (ft3/s)/mi at the 30 cross-section <br />sites. The maximum incremental ground-water <br />discharge rates measured for the small-area flow <br />system are 50 to 60 times higher than previously esti- <br />mated incremental ground-water discharge rates for <br />this reach of the South Platte River and are higher by a <br />similar magnitude than maximum incremental ground- <br />water discharge rates measured by mass balance in this <br />study. Furthermore, measurements of the large-area <br />flow system indicated that ground water continuously <br />discharged to reaches 2 and 3, whereas measurements <br />of the small-area flow system indicated that surface <br />water recharged the aquifer as frequently as ground <br />water discharged to the river. The reversals in flow <br />direction and the high rates of incremental ground- <br />water discharge from the small-area flow system are <br />plausible if they reflect transient conditions in the flow <br />system, Measurements of river stage and discharge <br />from the MWRD plant indicate that changes in river <br />stage were related to hourly changes in effluent- <br />discharge rates from the MWRD plant. These <br />frequent changes in river stage, although typically <br />less than about 0.25 ft, may have provided a stress on <br />the small-area flow system large enough to, cause the <br />transient discharge conditions that were measured at <br />the sediment/water interface. <br /> <br />The transient nature of incremental ground- <br />water discharge rates in the small-area flow system <br />were analyzed in a series of 24 hourly measurements of <br />discharge. In general, the rate and direction of water <br />movement from the small-area flow system were <br />related to changes in river stage, with the highest rates <br />of ground-water discharge corresponding to the lowest <br />river stages and the lowest rates of discharge occurring <br />at higher river stages, The data indicate that the small- <br />area flow system probably provided no net gain or loss <br />of water to the river, but that it was superimposed on <br /> <br />the large-area flow system that did provide water to the <br />river [at a median rate of (4.6 ff/s)/mi]. In essence, the <br />small-area flow system represented a zone of mixing <br />between the river and the large-area flow system. <br />The median value of pH in water samples from <br />the large-area flow system was 7,16. Values of pH in <br />water from the large-area flow system generally were <br />lower than values in the river, indicating that the dis- <br />charge of ground water from the large-area flow system <br />to the river would lower the pH of river water under <br />conservative mixing conditions. The median concen- <br />tration of DO in water from the large-area flow system <br />was 0.9 mgIL, which is less than the median instanta- <br />neous daytime concentration of DO (8.4 mgIL) in the <br />river, indicating that the discharge of ground water . <br />from the large-area flow system to the river also would <br />lower DO concentrations in the river water under con- <br />servative mixing conditions. <br />Concentrations of dissolved nitrite plus nitrate, <br />ammonium, and phosphorus in water from the large- <br />area flow system were highly variable between wells at <br />a given site and between sites. In general, concentra- <br />tions of nitrite plus nitrate were higher in ground water <br />along the downstream river reach than in ground water <br />along the upstream river reach. The opposite concen- <br />tration distribution was observed for ammonium and <br />phosphorus. The data indicate that ground-water dis- <br />charge from the large-area flow system may have been <br />a source of nitrite plus nitrate to the downstream river <br />reach and a source of ammonium and phosphorus to the <br />upstream river reach. <br />The median concentration of DO in water from <br />the small-area flow system was 0,2 mgIL, The DO data <br />indicate that sediments in the smaIl-area flow system <br />were a sink for DO in water discharging from the large- <br />area flow system and for surface water infiltrating into <br />the bed sediments. Organic carbon in water and sedi- <br />ments from the river provided the reduced carbon <br />needed to reduce DO entering the small-area flow sys- <br />tem, Even though the small-area flow system provided <br />no net addition of water to the river, it had the capabil- <br />ity of lowering DO concentrations in the river as sur- <br />face water cycled in and out of the sediments in the <br />small-area flow system. <br />Chemical and isotopic data and laboratory stud- <br />ies indicate that sediments in the small-area flow sys- <br />tem were a source of ammonium and phosphorus and a <br />sink for nitrite plus nitrate to water in the small-area <br />flow system. Whether that water was mostly from the <br />large-area flow system or from the river would not mat- <br />ter. Organic-rich sediments in the upstream reach of <br />the river appeared to be a source of ammonium and <br />phosphorus to water in the small-area flow system. <br />These sediments also had a high potential for removing <br /> <br />36 QUlntl1y Ind QUlll1y 01 Ground-Wltlr Dllchlrgl to thl South PlItt, River, Denver to Fort Lupton, Colorldo, <br />Auguet 1992 Through July 1993 <br />
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