WSP08349

• 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

depth. Ifwater depth was greater than 2.5 ft, velocity
was measured at a depth of 0.2 times the total depth and
at a depth of 0.8 times the total depth, and the velocities
were averaged. Velocity was multiplied by the area of
the incrementto determine volumetric discharge (cubic
feet per second) for each increment. Incremental dis-
charges were summed to detennine total surface-water
discharge. Accuracy of the discharge measurements
was about:1:5 percent.

Ground-water discharge to the South Platte
River was measured using indirect and direct tech-
niques, Indirect measurements of ground-water
discharge were obtained by calculating a mass balance
of all measured surface-water inflows and outflows to
three reaches of the river and by attributing the differ-
ence between inflows and outflows to ground-water
discharge. This indirect measurement of ground-water
discharge is called the incremental ground-water
discharge in the following discussion. The three
reaches of the South Platte River used in the mass
balance extend from 64th Avenue to 88th Avenue,
104th Avenue to upstream from the Brighton Ditch
headgate, and 160th Avenue to Highway 52 (pI. I).
Measurement~ of surface-water discharge were
made at the sites listed in table I. In addition, the
reaches were floated by boat to locate intennittent
flows not identified on maps. These flows usually were
from gravel-mine dewatering operations or were
ungaged irrigation return flows. These measurements
did not take into account surface-water losses from
evapotranspiration, which is an unknown quantity for
this study area. Indirect measurements of ground-
water discharge were made from August 1992 through
January 1993 and in May and July 1993.

River stage needs to be constant, or fluctu-
ations in stage need to be accounted for, to use the
ma.~s-balance technique for estimating ground-water
discharge. To minimize change in storage in the river,
discharge measurements made on the South Platte
River were timed using time-of-travel data provided
by the MWRD so that measurements made at the
upstream and downstream ends of the reach occurred
at about the same time on the hydrograph. For exam-
ple, if the upstream measurement was made at the
peak of daily stage fluctuations, the downstream
measurement wa.~ timed so that it also was made
at the peak of daily stage fluctuations. The time
lag between discharge measurements made at the
upstream and downstream ends of each segment were:
(I) 3 hours between 64th Avenue and 88th Avenue;
(2) 4.5 hours between 104th Avenue and Brighton
Ditch; and (3) 3.5 hours between 160th Avenue
and Highway 52 (Todd Harris, Metro Wastewater
Reclamation District, written commun., 1992).

Direct measurements of ground-water discharge
were made from August 1992 through January 1993 at
the 30 cross sections listed in table 2 using the follow-
ing techniques:

1. Five piezometers were installed at each of the
30 cross sections so that one piezometer was
located in the river channel 10ft from each
bank, one piezometer was located in the middle
of the channel, and one piezometer was located
between each of the outside piezometers and
the piezometer located in the middle of the
channel. The piezometers had carbon-steel
casings with 1 ,5-in inside diameters and 6-in
screens; the piezometers were installed so that
the middle of the screen was 1 ft below the
sediment/water interface. Each piezometer
was pumped at a rate of 0.25 gal/min until the
discharge water was clear (usually within
5 minutes), at which time pumping was
stopped and water levels allowed to equili-
brate. Water levels were allowed to equilibrate
for at least 15 minutes even though preliminary
tests indicated that equilibration was usually
achieved within 5 minutes.

Table 2. Looation of cross-section sites at which ground-
water discharge was measured

Distance Distance
downstream downstream
from the from the
Cros&- Metro Cross- Metro
section Wastewater section Waeteweter
number Reclematlon number Reclamation
District District
effluent outfalls effluent outfalls
(mllee) (miles)
1 0.2 16 11.7
2 1.2 17 12.4
3 2.1 18 13.8
4 2.5 19 14.2
5 2.8 20 14.8
6 3,1 21 16.8
7 3.4 22 18.0
8 4.3 23 19.0
9 4.9 24 19.9
10 5.5 25 21.6
11 6.7 26 22.0
12 7.5 27 22.5
13 8.5 28 23.1
14 10.5 29 23.7
15 ILl 30 24.6

6 Quantity and Quality of Ground.Water Discharge to the South Platte River, Denver to Fort Lupton, Colorado,
August 1992 Through July 1993