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<br />n fj ~1!, tJI <br />ut)u A. A . <br /> <br />The presence of sources and sinks of dissolved- <br />nitrogen and -phosphorus species in the small-area <br />flow system is important because all of these constitu- <br />ents can contribute indirectly to the depletion of DO in <br />the river by promoting the growth of aquatic vegetation <br />that eventually die and decay, thereby adding an oxy- <br />gen demand to the river. Aquatic vegetation also <br />decreases DO concentrations in the river when they <br />respire at night. In addition, the oxidation of ammo- <br />nium to nitrate consumes DO in the river. In general. <br />the widespread distribution of sources and sinks of dis- <br />solved nitrite plus nitrate, ammonium, and phosphorus <br />in the small-area flow system makes it difficult to deter- <br />mine how much nitrogen and phosphorus is entering <br />the river from ground-water discharge. <br /> <br />SUMMARY <br /> <br />Concentrations of DO in reaches of the South <br />Platte River between Denver and Fort Lupton fre- <br />quently fall below the minimum concentrations estab- <br />lished by the Colorado Department of Health and <br />Environment to maintain the health of aquatic life in <br />the river. As a follow-up to preliminary studies con- <br />ducted by the USGS that indicated that the discharge <br />of anoxic ground water may contribute to low DO <br />concentrations in the river, an assesstuent was made <br />of the quantity and quality of ground-water discharge <br />to the South Platte River from Denver to Fort Lupton, <br />Colorado. The quantity of ground-water discharge <br />was determined by (1) mass balance of surface- <br />water inflows and outflows from August 1992 <br />through January 1993 and in May and July 1993, and <br />(2) measurements of ground-water discharge across <br />the sediment/water interface in the river channel from <br />August 1992 through January 1993. Samples of sur- <br />face water and ground water used for water-quality <br />analyses were collected from August 1992 through <br />January 1993 and in May and July 1993. Specific con- <br />ductance, pH, T, DO, alkalinity, and concentrations <br />of dissolved major ions and nutrients were measured <br />in surface-water samples collected at three locations <br />in the river, in ground-water samples collected from <br />12 monitoring wells screened in the alluvial aquifer <br />adjacent to the river, and in ground-water samples col- <br />lected from 9 piezometers screened in riverbed sedi- <br />ments underlying the active river channel. Specific <br />conductance, pH, T, and DO were measured in surface- <br />water samples collected at 27 of the 30 cross-section <br />sites in the river. Specific conductance, pH, T, DO, and <br />concentrations of dissolved nutrients were measured <br />in ground-water samples collected from an additional <br />159 temporary wells screened in the riverbed sedi- <br />ments underlying the active channel of the river. <br /> <br />The ground-water flow system was divided into <br />a large-area flow system and a small-area flow system. <br />The precise boundaries of the two flow systems are not <br />known. However, the large.area flow system was con- <br />sidered to incorporate all alluvial sediments in hydro- <br />logic connection with the South Platte River. The <br />small-area flow system was considered to incorporate <br />the alluvial aquifer in the vicinity of the river. Flow- <br />path lengths in the large-area system were considered <br />to be on the order of hundreds of feet to more than a <br />mile, whereas flow-path lengths in the small-area sys- <br />tem were considered to be on the order of feet to hun- <br />dreds of feet. The quantity of ground-water discharge <br />from the large-area flow system to the South Platte <br />River was estimated by calculating a mass balance of <br />all measured surface-water inflows and outflows to <br />three reaches of the river (64th Avenue to 88th Avenue, <br />l04th Avenue to upstream from the Brighton Ditch <br />headgate, and I60th Avenue to Highway 52) and by <br />attributing the difference between inflows and outflows <br />to ground-water discharge. Mass-balance estimates of <br />incremental ground-water discharge ranged from -27 <br /> <br />to 17 (rt3/s)/mi for the three reaches studied; the <br /> <br />median rate was 4.6 (rt3/s)/mi. Incremental ground- <br />water discharge rates determined by mass balance were <br />most variable in the reach extending from 64th to <br />88th Avenue and were partially related to the amount <br />of effluent being released from the MWRD. There <br />generally was a net addition of water to the river from <br />ground-water discharge when the rate of effluent dis- <br />charge was low (for example, September, November, <br />and December 1992 and July 1993), whereas there was <br />a net loss of water from the river to the aquifer when the <br />rate of effluent discharge was high (for example, <br />August 1992 and January 1993) or when flow in the <br />river upstream from the plant was high (for example, <br />May 1993). <br /> <br />There was less variability in the incremental <br />ground-water discharge rates determined by mass <br />balance in reaches of the river downstream from <br />88th Avenue than there was in the reach upstream <br />from 88th Avenue, possibly due to increased irrigation <br />return flows from agricultural fields overlying the allu- <br />vial aquifer downstream from Denver and to a dampen- <br />ing of discharge fluctuations with distance downstream <br />from the MWRD plant. Median rates of incremental <br />ground-water discharge in the reaches extending <br />from l04th Avenue to upstream from the Brighton <br />Ditch headgate and from 160th Avenue to Highway 52 <br />were within about 10 percent of each other [5.1 and <br />4.6 (ft3/s)/mi]. The fI\edian percentage of flow in the <br />river at the downstream end of each reach from ground- <br />water discharge was 17 percent for the reach extending <br /> <br />SUMMARY 35 <br />