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

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QUANTITY OF GROUND-WATER
DISCHARGE

The discussion on the quantity of groUlTd-water
discharge to the South Platte River between Denver
and Fort Lupton is divided into two subsections: the
quantity of ground-water discharge from the large-
area flow system and the quantity of ground-water
discharge from the small-area flow system. The pre-
cise boundaries ofthe two flow systems are not known.
However, the large-area flow system was considered
to incorporate all alluvial sediments in hydrologic
connection with the South Platte River (Hurr and
Schneider, 1972). The small-area flow system was
considered to incorporate the alluvial aquifer in the
vicinity ofthe river. Flow-path lengths in the large-
area system were considered to be on the order of hUIT-
dreds offeet to more than a mile, whereas flow-path
lengths in the small-area system were considered to be
on the order of feetto hundreds of feet. The small-area
flow system is superimposed on the large-area system.

Large-Area Flow System

The quantity of ground-water discharge from
the large-area flow system to the South Platte River
was estimated by calculating a mass balance of all
measured surface-water inflows and outflows to
the three reaches of the river and by attributing the
difference between inflows and outflows to ground-
water discharge (table 4). This discharge is referred
to as the incremental ground-water discharge in
the discussion to follow. Surface-water discharge
measuremenl~ used in the mass-balance calculations
are listed in table 8 in the "Hydrologic and Water-
Quality Data" section. Mass-balance estimates of
incremental groUlTd-water discharge ranged from
-27 to 17 (ftl/s)/mi for the three reaches studied

(table 4); the median rate was 4.6 (ftl/s)/mi. Incremen-
tal ground-water discharge rates determined by mass
balance were most variable in reach I extending from
64th to 88th Avenue and were partially related to the
amount of effluent being released from the MWRD
(tables 4 and 8). There generally was a net addition of
water to the river from groUlTd-water discharge when
the rate of effluent discharge from MWRD was rela-
tively low (for example, September, November, and
December 1992 and July 1993), whereas there was a
net loss of water from the river to the aquifer when the
rate of effluent discharge was relatively high (for exam-
ple, August 1992 and January 1993) or when flow in
the river upstream from the plant was relatively high
(for example, May 1993). The total groUlTd-water
discharge in reach I on August 28, 1992, was about

the same magnitude as the accuracy of surface.water
discharge measurements. It is not certain, therefore,
that reach I was losing water to the aquifer on that date.
The effect of high flows from the MWRD on the
amoUlTt of ground-water discharge to reach 1 was to
decrease ground-water inflows, and the effect probably
was a daily occurrence based on the record oflarge
daily fluctuations in discharge from the MWRD plant
(fig. 2).

There was less variability in the incremental
ground-water discharge rates determined by mass
balance in reaches of the river downstream from
88th Avenue than there was in the reach upstream from
88th Avenue (table 4), possibly due to increased irriga-
tion return flows from agricultural fields overlying the
alluvial aquifer downstream from Denver and to a
dampening of discharge fluctuations with distance
downstream from the MWRD plant. Median rates of
incremental ground-water discharge in the reaches
extending from 104th Avenue to upstream from the
Brighton Ditch headgate and from I 60th Avenue to
Highway 52 were within about 10 percent of each other
[5.1 and 4.6 (ftl/s)/mi]. The median percentage offlow
in the river at the downstream end of each reach from
ground-water discharge was 17 percent for reach 2
extending from 104th Avenue to Brighton Ditch
Diversion and II percent for reach 3 extending from
I 60th Avenue to Highway 52. The median percentage
for these two reaches combined was 15 percent. The
median percentage of surface-water discharge derived
from groUlTd-water discharge for all three reaches was
13 percent. Based on these estimates, groUlTd-water
discharge between 104th Avenue and Highway 52
would have accoUlTted for a total of about 30 percent
of the flow in the river at Highway 52. Note that the
highest rate of incremental ground-water discharge
measured between I 60th Avenue and Highway 52,
16 (ftl/s)/mi, occurred on August 27, 1992 (table 4),
about 4 days after a large rainstorm that increased river
stage by about 3 ft at the Henderson gaging station.
This high rate of ground-water discharge probably
resulted from the return of storm-derived bank-storage
water to the river. The net inflow of ground water to the
river downstream from 88th Avenue is consistent with
continuous water-level measurements made in water-
table wells adjacent to the river at the Henderson and
Road 8 sites (fig. 3). At these two sites, ground-water
levels were higher than river stage during the period of
record, indicating that ground water was discharging to
the river rather than surface water infiltrating into the
large-area flow system, regardless of the daily varia-
tions in discharge from the MWRD plant (fig. 2).

QUANTITY OF GROUND-WATER DISCHARGE 11