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

water to the river. In general, mass balance and
instantaneous measurements of ground-water
discharge indicated that the ground-water flow
system in the vicinity of the river consisted of a
large-area flow system that provided a net addition
of water to the river and a small-area flow system
that cycled water in and out of the riverbed sedi-
ments, but provided no net addition of water to the
river. The small.area flow system was superim-
posed on the large-area flow system.

The median values of pH and dissolved
oxygen in ground water from the large-area flow
system were 7.16 and 0.9 milligrams per liter.
These values were lower than values for river
water, indicating that the discharge of ground
water from the large-area flow system would
lower values of pH and concentrations of dis-
solved oxygen in the river. Concentrations of
dissolved nitrite plus nitrate, ammonium, and
phosphorus in ground water from the large-area
flow system varied between monitoring-well sites.
The highest concentrations of nitrite plus nitrate
(maximum concentration of37,93 milligrams per
liter as nitrogen) were measured in water from
wells along the downstream river reach where rel-
atively low concentrations of dissolved oxygen
occurred in the river. The highest concentrations
of ammonium and phosphorus (maximum concen-
trations of9.2 milligrams per liter as nitrogen and
7.24 milligrams per liter as phosphorus) were
measured in water from wells along the upstream
river reach where relatively low concentrations of
dissolved oxygen occUlTed in the river. The water-
quality data indicate that ground-water discharge
from the large-area flow system may have been a
source of nitrite plus nitrate to the downstream
liver reach and a source of ammonium and phos-
phorus to the upstream river reach.

The median values of pH and dissolved
oxygen in ground water from the small-area flow
system were 7.33 and 0.2 milligrams per liter. The
dissolved-oxygen data indicate that sediments in
the small-area flow system were a sink for dis-
solved oxygen in water discharging to the river
from the large-area flow system and in surface
water infiltrating into the bed sediments. These
data indicate that even though the small-area flow
system was not a net source of water to the river, it
did have the potential to lower concentrations of
dissolved oxygen in the river as river water cycled

in and out of the bed sediments. Chemical and iso-
topic data indicate that organic-rich bed sediments
in the upstream river reach were a source of
ammonium and phosphorus to water in the small-
area flow system; therefore, the small.area flow
system may have been a source of ammonium
and phosphorus to the river even though it was
not a net source of water to the river. Laboratory
measurements of denitrification in bed sediments
from below the river indicate that sediments in the
small-area flow system also were a potential sink
for nitrate in the large-area flow system and in the
river as a result of microbial denitrification in the
sediments. The widespread distribution of sources
and sinks of dissolved-nitrogen and -phosphorus
species made it difficult to detennine how much
nitrogen and phosphorus were entering the river
from ground-water discharge.

INTRODUCTION

The South Platte River between Denver and
Fort Lupton has been designated by the Colorado
Department of Health and Environment, Water Quality
Control Commission (WQCC), as an Aquatic Life
Wann Water Class 2 river (Colorado Department of
Health and Environment, 1993). As such, the WQCC
has set minimum 24-hour average dissolved-oxygen
(DO) concentrations of 5.0 mg/L from May through
July and 4.5 mg/L from August through April to sus-
tain aquatic life in the river. Water-quality studies con-
ducted by the Metro Wastewater Reclamation District
(MWRD) have indicated that segments ofthis reach
do not meet the DO standards during some periods of
low flow. Although effluent discharged from the
MWRD central plant has been identified as an impor-
tant contributing factor in the DO depletion (Camp
Dresser and McKee, Inc., 1992), other unrelated pro-
cesses also may affect DO concentrations in the river
during low flow. Other sources of water to the river
include ungaged surface runoff, tributary inflows, and
ground-water discharge. Previous work conducted by
the U.S. Geological Survey (USGS) indicates that
ground-water discharge to this reach ofthe South Platte
River is a substantial source of water to the river when
runoff and tributary inflows are at a minimum (Hurr
and Schneider, 1972) and that biological reactions
in riverbed sediments at the interface between surface
water and ground water are an important sink for
DO in ground-water discharge (P.B. McMahon,
U.S. Geological Survey, unpublished data, 1993).
Discharge of DO-depleted ground water may contrib-
ute to the low DO concentrations in the South Platte

2 Quantlly and Quallly of Ground.Watar Dlacharga to tha South Platta River, Danvar to Fort Lupton, Colorado,
Auguat 1992 Through July 1993