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Isotope Ratio Mass Spectrometry (IRMS):
$10 to $20 per sample analysis, and an additional
$30 to $50 for sample preparation.
Numerous laboratories analyze for stable isotopes
using IRMS;see ISOGEOCHEM
(an Internet discussion list) for links to many
New resin sampling:
Stable Isotopes Laboratory web site
There are two stable isotopes of nitrogen: 14N
All nitrogen compounds contain both isotopes,
but because of isotopic fractionation they are
incorporated into compounds in differing ratios
depending on the nature of the reactions that
produce the compounds. For example, as nitrogen
compounds are passed up the food chain, the lighter
isotopes are excreted in urine and the heavier
isotopes are retained. Nitrogen in animal waste
is hydrolyzed to ammonia and then converted to
nitrate. During this process more of the heavy
isotope is concentrated in the resulting nitrates.
When various sources of nitrogen compounds are
mixed together in surface runoff or in a body
of water, the ratio of light to heavy nitrogen
isotopes in the water can be used to estimate
the relative contributions of the various sources.
Commercial fertilizers, animal or human waste,
precipitation, and organic nitrogen within the
soil are common sources of nitrate in groundwater.
Each of these nitrate source categories has a
distinguishable isotopic signature (i.e., 15N/14N
(Modified from Hoefs 1997 and Clark and Fritz
1997 with data from Amberger and Schmidt 1987,
Böttcher et al. 1990, and Létolle
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Samples can be analyzed for d15N
or gaseous N2.
Generally, samples are filtered in the field through
0.1 micron filters, put in rinsed bottles, preserved
(with sulfuric acid, mercuric chloride, or chloroform),
chilled, wrapped in insulating packing material,
and sent to the laboratory in ice chests. Sample-size
requirements are in the range of 10-100 µM
of N, however sample size requirements also vary
between laboratories. In addition to measuring
value of nitrate, a few labs are able to measure
value as well.
the USGS Water Resources Division, Western Region
for detailed information on the collection and
recovery of d15N
New resin sampling
Samples with high NO3-
concentrations are collected as bulk water samples;
however, low concentrations often require the
use of ion exchange resins. This alternate method
concentrates the NO3-
on anion or cation exchange resins. Collection
of nitrate on anion exchange resins eliminates
the need to send large quantities of chilled water
back to the laboratory, eliminates the need for
hazardous preservatives, makes it easier to archive
samples, and allows analysis of extremely low-nitrate
Nitrate analyses are routinely performed with
isotope ratio mass spectrometry (IRMS). IRMS separates
the ions of the element (14N/15N)
on the basis of their differing mass/charge ratio.
Sample preparation consists of converting solid
or liquid material to a gas (N2)
and isolating the particular gas that must be
also our IRMS web
page for a further description of the technology)
The use of stable nitrogen isotopes in environmental
and ecological studies, plant nutrition, and soil
fertility has increased considerably during the
past three decades. Nitrogen isotopes are often
useful in determining sources of nitrate in groundwater
and surface water. This is especially true in
regions that have sandy, well drained soils (where
of the nitrate has less opportunity to be transformed
by biological activity).
Biologically-mediated reactions (e.g., assimilation,
nitrification and denitrification) strongly control
nitrogen and nitrogen isotopic compositions in
both soil and water. Nitrification is a chemical
process that produces nitrate (NO3-)
through the oxidation of ammonium (NH4+).
After gaseous N2,
nitrate is the most stable form of nitrogen and
is present in most groundwater. The nitrification
reaction, below, occurs under aerobic conditions,
whereas denitrification occurs under anaerobic
+ 2O2 = NO3-
A bacterium known as Thiobacillus denitrificans
is responsible for most of the denitrification
in groundwater. However, other types of bacteria
can denitrify in the absence of carbon, using
electron sources such as Mn2+,
sulfide and methane. Fractionation during denitrification
causes the d15N
of residual nitrate to increase exponentially
as nitrate concentrations decrease due to fractionation.
Denitrification processes can also be identified
by the amount of gaseous N2.
produced by denitrification results in excess
contents in groundwater. The total N2
(which consists of air N2
trapped during recharge plus N2
produced by denitrification) can be collected,
analyzed for d15N,
and used to estimate the extent of denitrification,
initial composition of the nitrate, or the mixing
history of the water.
In addition to denitrification, NO3
may be removed from water by plant assimilation,
as such plants can assist in the remediation of
surface and groundwater, for example. Riparian
zones are assumed to buffer NO3-
in surface waters. During assimilation, nutrients
are incorporated in the plant, where they remain
until they are released by mineralization during
decay. The plants do not remove N from ecosystem,
but increase the residence time of nutrients through
a reduction of the mobility of N compounds. However,
there are limits, and large amounts of nutrients
cause increased plant growth, resulting in eutrophication
and anoxia. In this way, increased nutrients lead
to a shorter retention time of nutrients in the
riparian buffer zone.
The effects of denitrification and assimilation
can be distinguished with the use of d15N
analyses combined with d18O
analysis. If plant uptake alone is responsible
remediation, the isotopic composition of the remaining
remains unchanged. If both denitrification and
assimilation are occurring, the isotopic composition
of the residual nitrate is enriched and the overlying
plants reflect the isotopic composition of the
source. The isotopic composition of the plants
will remain the same and the water will become
more enriched if denitrification is the only process
Analysis of d18O
in combination with d15N
provides additional information about nitrates
in water and soils, specifically on the relative
contributions of fertilizers vs. soil NO3
or manure/septic waste, and on the relative contributions
of atmospheric NO3
vs. fertilizer, soil NO3,
or manure/septic waste.
- Amberger, A. and H.-L. Schmidt, Natürliche
isotopegehalte von nitrat als indicatoren für
dessen herkunft. Geochimica et Cosmochimica
Acta, 51: 2699-2705, 1987.
- Bohlke, J.K. and J.M. Denver, Combined use
of groundwater dating, chemical and isotopic
analyses to resolve the history and fate nitrate
contamination in two agricultural watersheds,
Atlantic Coastal Plain, Maryland, Water Resour.
Res. 31(9), 2319-2339, 1995.
Böttcher, J., O. Strebel, S. Voerkelius,
and H.-L. Schmidt, Using isotope fractionation
of nitrate nitrogen and nitrate oxygen for evaluation
of denitifcation in a sandy aquifer, J. of
Hydrol., 114: 413-424, 1990.
- Clark, I., and P. Fritz, Environmental
Isotopes in Hydrogeology, Lewis Publishers,
Boca Raton, 1997.
- Cook. P.G., and A.L. Herczeg, editors, Environmental
Tracers in Subsurface Hydrology, Kluwer
Academic Publishers, Boston, 2000.
- Hoefs, J., Stable Isotope Geochemistry,
4th ed., Springer, Berlin, 1997.
- Kendall, C., and J.J. McDonell, eds., Isotope
Tracers in Catchment Hydrology, Elsevier
Science, Amsterdam, 1998.
- Létolle, R., Nitrogen-15 in the natural
environment; chapter 10 in Handbook of Environmental
Isotope Cheochemistry, vol. 1: The Terrestrial
Environment, ed. by P. Fritz and J.-Ch.
Fontes, Elsevier, Amsterdam, 407-433, 1980.
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State Water Survey, Nitrogen
Nolan, B.T.., B.C. Ruddy, K.J. Hitt, and D.R.
Helsel, A national Look at nitrate contamination
of ground water, Water Conditioning and Purification
39(12), 76-79, 1998. (Republished on USGS
Carolina State University, Rivernet
Stable Isotope Tracer Program, Annual Report 2001.
of Physical Geography, Ch. 9: Introduction
to Biogeography and Ecology, The Nitrogen Cycle.
G.S., K.C. Hackley, H.H. Hwang, and T.M. Johnson,
of nitrogen and oxygen isotopes to identify sources
of nitrate, Illinois Groundwater Consortium
G.S., K.C. Hackley, H.H. Hwang, and T.M. Johnson,
of nitrogen and oxygen isotopes to identify denitrification
in a shallow aquifer with a variable influx of
nitrate, Geological Society of America Annual
in groundwater of the Upper Snake River Basin,
Idaho and Western Wyoming, 1991-95, USGS Report.
Grant North Carolina, Study
of Excess Nitrogen Sources in Neuse River Estuary
J.L., and S.M. Bernasconi, A
century-long record of anthropogenic nutrient
loading provided by d15N
values in sediment from a eutrophic lake,
9th Annual V.M. Goldschmidt Conference.
Environmental Protection Agency, U.S.
Map: Risk of Groundwater Nitrite Contamination.
Periodic Table - Nitrogen.
T.C., J.W. Harvey, O.L. Franke, and W.M. Alley,
water and surface water: a single resource,
USGS Circular 1139.
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