Sources and Transformations of Nitrate from Streams ... · in the Lisha Kill watershed, and about 4% urban land. Salmon Creek is a tributary to Cayuga Lake, one of the Finger Lakes.
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1149
Knowledge of key sources and biogeochemical processes that aff ect the transport of nitrate (NO
3−) in streams can inform
watershed management strategies for controlling downstream eutrophication. We applied dual isotope analysis of NO
3− to
determine the dominant sources and processes that aff ect NO
3− concentrations in six stream/river watersheds of diff erent
land uses. Samples were collected monthly at a range of fl ow conditions for 15 mo during 2004–05 and analyzed for NO
3− concentrations, δ15N
NO3, and δ18O
NO3. Samples from
two forested watersheds indicated that NO3− derived from
nitrifi cation was dominant at basefl ow. A watershed dominated by suburban land use had three δ18O
NO3 values greater than
+25‰, indicating a large direct contribution of atmospheric NO
3− transported to the stream during some high fl ows. Two
watersheds with large proportions of agricultural land use had many δ15N
NO3 values greater than +9‰, suggesting an animal
waste source consistent with regional dairy farming practices. Th ese data showed a linear seasonal pattern with a δ18O
NO3:δ
15NNO3
of 1:2, consistent with seasonally varying denitrifi cation that peaked in late summer to early fall with the warmest temperatures and lowest annual streamfl ow. Th e large range of δ 15N
NO3 values (10‰) indicates that NO
3− supply was likely
not limiting the rate of denitrifi cation, consistent with ground water and/or in-stream denitrifi cation. Mixing of two or more distinct sources may have aff ected the seasonal isotope patterns observed in these two agricultural streams. In a mixed land use watershed of large drainage area, none of the source and process patterns observed in the small streams were evident. Th ese results emphasize that observations at watersheds of a few to a few hundred km2 may be necessary to adequately quantify the relative roles of various NO
3− transport and process patterns
that contribute to streamfl ow in large basins.
Sources and Transformations of Nitrate from Streams Draining Varying Land Uses:
Evidence from Dual Isotope Analysis
Douglas A. Burns* U.S. Geological Survey
Elizabeth W. Boyer Pennsylvania State University
Emily M. Elliott University of Pittsburgh
Carol Kendall U.S. Geological Survey
The eff ects of human activities on the nitrogen (N) cycle
at regional and global scales is the focus of much research
and concern because humans have more than doubled N fl uxes
and storage and the rates of many N cycling processes. Th is
human-induced acceleration of the N cycle is linked to myriad
environmental concerns, including soil acidifi cation, tropospheric
ozone, acute ground water pollution, and estuarine eutrophication
(Galloway et al., 2003). At the regional scale, much work has
focused on the control of nitrate (NO3−) concentrations and
fl uxes in riverine environments that range in scale from small
streams to large rivers (Boyer et al., 2002; Smith et al., 2005).
Major uncertainties remain in our understanding of how N from
various sources moves through landscapes to rivers and the extent
to which N cycling processes alter these sources during transport
(Schlesinger et al., 2006).
Dual isotope analysis of NO3− in surface waters can provide
meaningful insights into sources, sinks, and transport processes of
the N cycle that are diffi cult to obtain through more traditional
measurements of solute concentration and fl ux. Dual isotope stud-
ies of NO3− have proven useful for distinguishing sources such as
atmospheric deposition, human and animal waste, and fertilizer
and for tracing the eff ects of processes such as nitrifi cation and
denitrifi cation on NO3− pools in waters where measurements of
δ15NNO3
alone cannot distinguish among various sources and pro-
cesses (Burns and Kendall, 2002; Mayer et al., 2002; Lehmann et
al., 2003; Anisfeld et al., 2007). For example, δ18ONO3
values in at-
mospheric deposition are quite high, generally greater than +60‰
(Elliott et al., 2007). Th ese high values are believed to result from
reaction of atmospheric NOx with O
3 of δ18O greater than +90‰
during formation of HNO3 in the atmosphere (Michalski et al.,
2003). Once atmospheric NO3− is deposited onto the landscape,
the N is readily taken up by biota where it may be mineralized and
nitrifi ed to appear again as NO3−. Nitrifi ers are believed to obtain
two oxygen atoms from H2O (δ18O, −25 to −5‰) and one from
O2 (δ18O ~ +23‰) (Amberger and Schmidt, 1987). As a result,
δ18ONO3
derived from nitrifi cation in soils is generally in the range
of −10 to +10‰, and δ18ONO3
is a powerful tracer of the transfor-
Abbreviations: WWTP, wastewater treatment plant.
D.A. Burns, U.S. Geological Survey, Troy, NY; E.W. Boyer, Pennsylvania State Univ., State
College, PA. E.M. Elliott, Univ. of Pittsburgh, Pittsburgh, PA. C. Kendall, U.S. Geological
ton), and American beech (Fagus grandifolia Ehrh.) and minor
amounts of conifers including balsam fi r (Abies balsamea (L.)
Burns et al.: Land Use and Nitrate Isotopes in Streams 1151
Mill), red spruce (Picea rubens Sarg.), and eastern hemlock (Tsu-ga Canadensis (L.) Carrière). Additionally, Buck Creek has about
28% wetland area in its drainage, almost all of which is classifi ed
as woody wetland (Table 1; NLCD Class C90). Both forested
watersheds are dominated by well drained soils (Table 2).
Sample and Field Data CollectionStream water samples were collected approximately month-
ly at each stream for approximately 15 mo during 2004–05
and analyzed for NO3− concentrations and isotopes. Samples
were collected over a range of fl ow conditions because streams
commonly show fl ow-related changes in NO3− concentrations,
and several samples were collected at high fl ow when sources
may diff er from those at low fl ow (Fig. 2A). For example, the
median probability that daily mean fl ow was exceeded for sam-
Fig. 1. Map of New York State with hydrography and the locations of the six stream sampling sites.
Table 1. Physical and land cover/use characteristics of the six study stream watersheds. Land cover was determined from the 2002 National Land Cover Data.† Footnotes describe the NLCD classes used for each category in the table.
Stream Drainage area Average elevation Percent forested‡ Percent wetlands§ Percent urban¶ Percent agricultural#
km2 m
Lisha Kill 40 105 14.2 29.2 50.9 4.7
Mohawk River 8849 375 50.3 12.1 7.4 24.8
Salmon Creek 229 325 14.5 4.8 3.8 72.0
Fall Creek 327 415 34.0 7.4 6.1 44.6
Biscuit Brook 9.7 871 99.8 0.2 0 0
Buck Creek 3.2 651 69.4 28.4 0 0
† Data from http://www.mrlc.gov/mrlc2k_nlcd.asp.
‡ Sum of Classes C41, C42, and C43.
§ Sum of Classes C11, C90, and C95. All of the watersheds included <2% open water (C11).
¶ Sum of Classes C21, C22, C23, and C24.
# Sum of Classes C81 and C82.
Table 2. Geology and soil drainage properties of the six study stream watersheds. Data refl ect the dominant geology and soil type in each watershed as described in the references cited in the table.
StreamBedrock
type†Surfi cial
geology‡Soil drainage properties§
Lisha Kill shale sand dunes excessively drained
Mohawk River –¶ glacial till moderately well drained
Salmon Creek shale glacial till well drained
Fall Creek shale glacial till poorly drained
Biscuit Brook sandstone exposed bedrock well drained
Buck Creek gneiss glacial till well drained
† Fisher et al. (1971).
‡ Cadwell et al. (1986).
§ Soil Survey Staff (2006)
¶ The Mohawk River has wide variation in bedrock type with sedimentary
rock in the central and southern parts of the basin and igneous and
metamorphic rock in the northern part of the basin.
ples analyzed for NO3− concentrations was <0.50 at all sites,
and several samples were collected at all streams at daily mean
fl ow conditions that were exceeded <25% of the days based
on an analysis of the 2004–05 fl ow data at each stream (Fig.
2A). Only about half the samples collected in the forested set-
tings were analyzed for NO3− isotopes because previous studies
at Biscuit Brook in the Catskills (Burns and Kendall, 2002)
and at Archer Creek in the Adirondacks (in a similar mixed
wetland-forested watershed <50 km from Buck Creek; Piatek
et al., 2005) documented the sources and seasonal patterns of
stream NO3− isotopes in these settings.
Samples were collected by rinsing a 250-mL polyethylene
bottle three times with stream water before fi lling. Samples
were collected near the middle of the channel, except in the
Mohawk River, where water was collected by an automated
sampler through a fi xed line with an intake located close to
the stream bank. Samples were stored on ice in a cooler, trans-
ferred to a refrigerator (4°C) on returning from the fi eld, and
passed (generally within 24 h of collection) through a 0.4-μm
polycarbonate fi lter. A 100-mL aliquot of each sample was then
shipped on ice to the U.S. Geological Survey Menlo Park, CA
Stable Isotope and Tritium Laboratory for isotope analysis, and
the remaining sample was stored at 4°C until NO3− analysis.
Monthly samples of precipitation were pooled from weekly
samples collected during 2004 and 2005 by wet only collectors
at 10 National Trends Network sites operated by the National
Atmospheric Deposition Program (NADP/NTN). Th e proce-
dures for compositing these samples and a discussion of quality
assurance of results from a similar study of archived precipita-
tion samples can be found in Elliott et al. (2007). Th e 10 sites
chosen for analysis included all of the NADP/NTN sites in New
York that were operated throughout 2004–05 (see location in-
formation at http://nadp.sws.uiuc.edu/sites/sitemap.asp?state =
ny). Nitrate isotope data for dry deposition were not available. A
previous study at Biscuit Brook found that neither δ18ONO3
nor
δ15NNO3
values in throughfall (often considered a surrogate for
wet plus dry deposition) were signifi cantly diff erent than those
in wet deposition, suggesting similar isotope values among wet
and dry deposition in rural forested settings (Burns and Kendall,
2002). Despite a lack of data for oxidized nitrogen species in dry
deposition, indirect measurements, such as δ15N in the tissue of
mosses collected near roadways, suggests that vehicle NOx emis-
sions have isotope values that diff er from those of NO3− in wet
deposition (Pearson et al., 2000); however, the possible infl uence
of dry deposition of NOx on the isotopic composition of stream
NO3− could not be evaluated in the current study.
Streamfl ow data for these streams/rivers were obtained by
gages operated by the U.S. Geological Survey at fi ve of the six
study streams (see data and information at http://waterdata.
usgs.gov/ny/nwis/sw). Salmon Creek had no operating stream
gage until July 2006. To estimate daily discharge data for the
2004–05 study period at Salmon Creek, a linear regression re-
lationship was developed between mean daily discharge at Fall
Creek and Salmon Creek from July 2006 through September
2007 (r2 = 0.83; p < 0.001). Th is relationship was used to pre-
dict a mean daily discharge at Salmon Creek.
Analytical MethodsSamples collected at Biscuit Brook, Buck Creek, Lisha Kill,
and Mohawk River were analyzed for NO3− concentrations by
ion chromatography according to methods described in Law-
rence et al. (1995). Quality assurance/quality control methods
and results are described in biannual reports available at http://
ny.water.usgs.gov/publications. Th e data quality objective for
analytical precision is ±10%, and these objectives are generally
achieved for >90% of analyses of quality assurance samples (Lin-
coln et al., 2006). Th e samples collected at Fall Creek and Salm-
on Creek were also analyzed by ion chromatography at the labo-
ratory of E.W. Boyer at the State University of New York College
of Environmental Science and Forestry in Syracuse, NY.
A denitrifying bacteria, Pseudomonas aureofaciens, was used
to convert 20 to 60 nmoles of NO3− into gaseous N
2O before
isotope analysis (Casciotti et al., 2002; Sigman et al., 2001).
Samples were analyzed for δ15N and δ18O in duplicate using
a Micromass Instruments IsoPrime Continuous Flow Isotope
Ratio Mass Spectrometer (use of brand names is for identifi ca-
tion purposes only and does not imply endorsement by the U.S.
Government) and values are reported in parts per thousand (‰)
relative to atmospheric N2 and Vienna Standard Mean Ocean
Water, for δ15N and δ18O, respectively, using the equation:
δ (‰) = sample standard
standard
(R) (R)
(R)
− × 1000 [1]
where R denotes the ratio of the heavy to light isotope (e.g., 15N/14N or 18O/16O).
Samples were corrected using international reference stan-
dards USGS34, USGS35, and N3 (Böhlke et al., 2003); linear-
ity and instrument drift were corrected using internal standards.
Sample replicates had an average standard deviation (σ) of 0.2‰
for δ15N (n = 204) and 0.7‰ for δ18O (n = 204). Analytical pre-
cision for δ15N was 0.3‰ based on an internal reference stan-
dard that was analyzed at least once per 40 samples.
Data AnalysisStreamfl ow data from the Lisha Kill were separated into a
basefl ow and a direct runoff component using a local mini-
mums method of daily fl ow as calculated by the Base Flow
Index program (http://www.usbr.gov/pmts/hydraulics_lab/
twahl/bfi /). Th ese hydrograph separations were performed
only for days when samples were collected, and the resulting
data were used to assist with interpretation of δ18ONO3
values.
Multivariate stepwise linear regression was used to explore
the role of NO3− availability, stream runoff , and temperature on
δ15NNO3
values at Fall and Salmon Creeks where isotope data
supported a role for denitrifi cation in seasonal NO3− evolution.
Th ese independent variables were chosen because they are among
the dominant environmental controls on denitrifi cation rates for
which data were available in this study. Th e air temperature data
for this analysis were obtained from the Freeville, NY National
Weather Service site (http://climod.nrcc.cornell.edu).
Pairwise statistical comparisons of isotope data from various
sites were performed with a t test if the data passed tests for
Burns et al.: Land Use and Nitrate Isotopes in Streams 1153
normality and equal variance or with the Mann–Whitney rank
sum test (Mann and Whitney, 1947) if the data failed either of
these tests.
Results and Discussion
Stream NO3− Concentrations
Salmon Creek, the watershed with the highest proportion
of agricultural land, had the highest NO3− concentrations
throughout the sampling period, with values that ranged from
about 2 to 7 mg L−1 as NO3−–N (mean, 4.5 mg L−1; Fig. 2B).
Th e lowest concentrations at Salmon Creek were found in ear-
ly- to mid-summer, but values otherwise varied little through-
out the year.
Fall Creek, with the second greatest proportion of agricul-
tural land, had the second highest NO3− concentrations among
the six streams/rivers with values that ranged from about 0.6 to
1.8 mg L−1 as NO3−–N. Th e mean NO
3− concentration in Fall
Creek during the study was 1.1 mg L−1, close to the value of
1.16 mg L−1 reported for Fall Creek during 2000 through 2005
by Johnson et al. (2007). Fall Creek showed a general tendency
for lower NO3− concentrations during the May–October grow-
ing seasons (0.9 mg L−1 as NO3−–N) and higher concentra-
tions in the November–April dormant season (1.3 mg L−1 as
NO3−–N), similar to Salmon Creek, although the seasonal de-
clines from late spring to mid-summer were not as sharp and
distinct as those of Salmon Creek.
Discharge from the two wastewater treatment plants
(WWTPs) in Fall Creek was not likely to have greatly aff ected
NO3− concentrations in the stream. Combined discharge from
these two plants was provided by New York State (Dept of En-
vironmental Conserv., unpublished data), and the average con-
tribution to streamfl ow at Fall Creek during the study was 0.35
± 0.21% (1 SD of the mean). Domestic sewage typically has
total N concentrations of 21 to 42 mg L−1, which are reduced
to about 1 to 5 mg L−1 through biological treatment (Sedlak,
1991), such as that found in the largest of the two plants (NY-
SDEC, 2004). Assuming an average WWTP discharge of total
N into Fall Creek of 5 mg L−1 and complete conversion of this
waste N to NO3− (fairly liberal estimates), the load of NO
3− in
Fall Creek from WWTP discharge averaged about 1.6 ± 0.9%
of the total annual load, with a high monthly value of 3.2%.
Additionally, septic systems serve many rural areas of the Fall
and Salmon Creek watersheds and likely contribute additional
NO3− to streams that originate from human waste (Genesee/
Finger Lakes Regional Planning Council, 2001).
Among the other four streams/rivers studied, NO3− con-
centrations were about 0.5 to 1 mg L−1 as N during the non-
growing season (November–April) despite the wide diff erences
in land use/landscape cover among these watersheds (Fig.
2B). During the May through October growing season, lower
NO3− stream concentrations were evident in the two streams
(Biscuit Brook and Buck Creek) that drain forested or mixed
forested/wetland landscapes. In contrast, NO3− concentrations
remained fairly constant throughout the sampling period at the
Mohawk River. Th e suburban Lisha Kill showed greater varia-
tion in NO3− concentrations than the Mohawk but less than
that of the two forest-covered watersheds.
General Source Indications from NO3− Isotope Data
Isotope source diagrams such as shown in Fig. 3 indicate the
ranges of δ15NNO3
and δ18ONO3
values reported for various sources
in published studies, and here we use a slight modifi cation of the
fi gure shown in Kendall et al. (2007). Some general patterns and
diff erences among the sites are immediately evident on examining
Fig. 3A and 3B. Th e precipitation data generally lie within the
range reported in previous studies (Elliott et al., 2007; Kendall
et al., 2007), with a mean δ15NNO3
value of –0.8 ‰ (SD 2.7)
and mean δ18ONO3
value of +77.8‰ (SD 8.1). Th e δ18ONO3
value
is slightly higher than the mean value of +72.0‰ reported for
bulk deposition, throughfall, and snowpack samples collected in
the Adirondacks during 2001–02 (Piatek et al. (2005), about 50
km from Buck Creek) and much higher than the mean value of
+46.0‰ in wet deposition in the Catskills during 1995–97 (Burns
and Kendall, 2002). Th e higher δ18ONO3
values obtained with the
denitrifi er method compared with the previously common closed-
tube AgNO3 method have also been noted by previous investiga-
tors (Revesz and Böhlke, 2002; Kendall et al., 2007).
Fig. 2. Samples collected and analyzed for nitrate concentrations in the six study streams during 2004–05. (A) Flow duration curve with sample fl ow exceedances marked. (B) NO
waste source. Th ree high fl ow samples from the suburban stream
had δ18ONO3
values greater than +25‰, indicating that about
50% of the NO3− was from a direct atmospheric source and re-
fl ected some bypassing of soil contact via impermeable surfaces
and the network of storm sewers in the watershed.
Two of the streams with large proportions of agricultural
land had the highest NO3− concentrations in this study and a
source signature consistent with animal waste from dairy farm-
ing in the watersheds. Th ese isotope data showed a linear rela-
tionship of δ18ONO3
:δ15NNO3
of about 1:2, consistent with vari-
able amounts of denitrifi cation that likely occurred outside of
the stream environment. Th e eff ects of denitrifi cation appeared
greatest (based on multivariate regression of δ15NNO3
values)
when air temperature was warmest and streamfl ow was low-
est during summer and early fall, coincident with the lowest
stream NO3− concentrations. Th e NO
3− in these streams may
also have been transported from two or more distinct sources,
but this possibility could not be fully evaluated with these data
and would require additional hydrometric and/or tracer data.
Nitrate isotope data from the Mohawk River, a basin of nearly
9000 km2 that drains agricultural, urban, and forested land in
proportions approximately equal to the average of the other fi ve
study watersheds, showed neither the high δ18ONO3
values of the
suburban stream nor the high δ15NNO3
values and 1:2 sloping line
of the agricultural streams. Th is fi nding suggests that in mixed
land use watersheds at large basin scales, the complex mixture of
NO3− sources combined with additional travel time may obscure
the process- and source-level information that can be observed in
NO3− dual isotope patterns at smaller drainage scales. Th e small to
medium drainage area scale in streams dominated by one form of
land use is likely the most eff ective scale for distinguishing the role
of N sources and hydrologic and biogeochemical processes on the
transport of NO3− in riverine environments.
AcknowledgmentsTh is work was supported by a grant from the New York State
Energy Research and Development Authority. Th e authors
thank Elizabeth Nystrom, Heather Golden, and Michael
Brown for help with sample collection and Gregory Lawrence
and Michael McHale for sharing data.
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