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

Chemical and isotopic data indicate that bed
sediments in the upper reaches of the study area
were a source of ammonium to the small-area flow
system. For example. chemical and isotopic analy-
ses of dissolved ammonium in ground water below
the sediment/water interface at McKay Road on
November 2, 1992, represent ammonium production in
'the sediments under the river at this site (fig. 18).
Ammonium concentrations were higher at a depth of
12 in. below the sediment/water interface than at other
sampled depths, and the l)lsN composition of the
ammonium at a depth of 12 in. was about 9 to 12 per
mil lighter than the l)lsN composition of ammonium at
other depths (table 6). Ammonium produced in-situ
generally is isotopically lighter than ammonium that
has been transported and subjected to reactions like
nitrification that cause the residual ammonium to
become isotopically enriched. Although the hydraulic-
head difference at the time of measurement was
directed from the river into the sediments, indicating no
advective flux to the river of ammonium at the 12-in
depth, the 24-hour data indicate that an upward flux of
ammonium sometime during the day was likely. The
relatively high organic-matter contentofbed sediments
at McKay Road indicates that the ammonium probably
was derived from the mineralization of organic nitro-
gen in sedimentary organic matter. The concentration
of orthophosphate in ground water below the sediment!
water interface also was higher at a depth of 12 in. than
at other sampled depths on the same date. indicating
that the mineralization of organic matter may have pro-
vided dissolved phosphorus to the small-area flow sys-
tem.
Sedimentary organic matter probably was
the source of high concentrations of dissolved ammo-
nium and orthophosphate in ground water beneath the
river at cross-section site 6 (table 9). At this site, the
sedimentary organic matter may have been excrement
from waterfowl (table 6). This reach of the river was
densely populated by waterfowl during the fall. The
nitrogen-isotope composition of ammonium in ground
water at the site on December 22. 1992, was the same
as the isotopic composition of fresh waterfowl excre-
ment (table 6). The isotopic composition of ammo-
nium in weathered waterfowl excrement was about
4 per mil heavier than that in the fresh material, indicat-
ing that the ammonium in the weathered material
had mineralized and lost isotopically light nitrogen.
Although there was not a net discharge of ground
water to the river at site 6 at the time of sampling on

December 22, 1992 (table 9), hydraulic head at the
piezometer from which the isotope sample was col-
lected was higher than the head in the river.

In addition to the bed sediments being sources
of dissolved-nitrogen species to ground water in
the small-area flow system, they also were sinks for
dissolved-nitrogen species. The combination of
low DO concentrations in ground water below the
sediment!water interface and the presence of nitrate
provided suitable conditions for denitrification to occur
in the small-area flow system. Laboratory incubations
indicated a measurable potential for denitrification in
the bed sediments (table 7). Rates of denitrification
(as indicated by N20 production) in riverbed sediments
from 88th Avenue and McKay Road sites increased
substantially when nitrate concentrations in pore water
in those sediments were increased from low in-situ
values, whereas the increase in denitrification rate in
sediments from Road 8 site was much smaller when
nitrate concentratiQns were increased from high in-situ
values. These results indicate that (1) denitrification
rates in bed sediments from 88th Avenue and McKay
Road sites were more limited by the availability of
nitrate than denitrification rates in sediments from
Road 8 site; and (2) any nitrate entering bed sediments
in the vicinity of 88th Avenue or McKay Road sites,
3 to 6 mi. downstream from the MWRD, would proba-
bly be reduced to N2, which is consistent with the low
concentrations of dissolved nitrite plus nitrate in
ground water from the small-area flow system at those
sites (fig. 17). Rates of denitrification in sediments
from the Henderson and Road 8 sites increased propor-
tionally more than denitrification rates in sediments
from 88th Avenue and McKay Road sites when glucose
was added to the sediments, which indicates that
denitrification rates in riverbed sediments from the
Henderson and Road 8 sites were more limited by the
availability of organic carbon than denitrification rates
in sediments from the 88th Avenue and McKay Road
sites. This interpretation is consistent with measure-
ments of organic-matter content that indicated a larger
percentage of organic matter in riverbed sediments
from the McKay Road site than in sediments from the
Henderson or Road 8 sites (table 5). In general, the
presence of a denitrifying potential in sediments in the
small-area flow system indicates that the small-area
flow system could be a sink for nitrate in ground water
from the large-area flow system and for nitrate in river
water.

32 Quantity and Quality of Ground-Water Dlscharga to the South Platte Rlvar, Denver to Fort Lupton, Colorado,
AUgU8t1992 Through July 1993