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By Alan
Jeffrey, Isaac Kaplan, Dachun Zhang, Shan-Tan Lu, and Jesper
Nielsen
Background
Nitrate contamination of groundwater supplies is an increasing
concern as urban areas continue to expand into rural areas.
Farming, intensive livestock operations, septic sewage
systems, and other practices in rural areas release organic
and inorganic nitrogen into the environment, which ultimately
show up in the groundwater as dissolved nitrate.
Elevated nitrate concentration in drinking water is a known
health concern. Concentrations above 10 mg-N/L can cause
methemoglobenemia, a form of anemia, in infants, and the
public drinking water Maximum Contaminant Level for nitrate is
established at the 10 mg-N/L level. Carcinogenic nitrosamine
compounds may be formed in humans from ingested nitrates.
Elevated nitrate concentrations are also a concern for the
health of aquatic biota, and most water quality guidelines
limit organic and inorganic nitrogen to 1 mg/L.
If elevated concentrations of nitrate are discovered in a
groundwater aquifer, an immediate concern is to identify the
source, to limit future nitrogen inputs. The most common
sources of groundwater nitrate are:
1. synthetic nitrogen fertilizers
2. animal waste
3. human septic waste.
Fortunately, the three sources can often be distinguished by
differences in their stable nitrogen isotope ratios.
Stable Isotope Ratios - Explanation
Many elements exist as two or more stable isotopes. For
example, all nitrogen atoms have 7 protons and 7 electrons.
The number of protons and electrons defines the way nitrogen
reacts chemically. Most nitrogen atoms also have 7 neutrons
and have an assigned atomic weight of 14 (14N). However, a
small number, around 1 in every 270 has 8 neutrons and has an
atomic weight of 15 (15N). This difference in mass does not
affect the way it reacts chemically. However, because 15N has
a greater mass, the 15N isotope forms a stronger bond with
other elements and may react at a reduced rate in physical,
chemical and biological reactions. If a nitrogen-containing
compound undergoes a reaction process where 14N reacts faster
than 15N, then the product of the reaction will be enriched in
14N, whereas the residual unreacted compound will be enriched
in 15N. This is what happens when nitrogen passes through the
food chain in animals (and humans).
In the food cycle, nitrogen compounds are assimilated with
little change in the relative amounts of 14N and 15N, and
plants and soil microorganisms that can use atmospheric
nitrogen or can metabolize nitrogen from man-made ammonium
nitrate fertilizer, retain only a small enrichment of 15N over
the starting 15N/14N ratio in N2 or NH4NO3. However, metabolic
processes involved in protein breakdown result in a
preferential release of 14N-enriched products and retention of
15N-enriched cellular chemicals. Herbivores further enrich
their tissues in 15N and each consumer up the food chain eats
nitrogen progressively enriched in 15N. The body mass and
excreted waste of carnivores at the top of the food chain,
including humans, are most enriched in 15N.
Source Identification
Because of difficulties in measuring the precise concentration
of the minor isotope 15N, early studies (circa 1950) proposed
the use of isotope ratios (i.e. 15N/14N, also expressed as
15N) to measure the enrichment or depletion of a particular
isotope. 15N values are conventionally expressed in parts per
thousand (o/oo) deviation from the ratio in an internationally
assigned standard, and can be measured with a precision of
+0.15 o/oo. By convention, atmospheric N2 has been adopted as
the international standard for nitrogen isotope ratios, and is
given a 15N value of 0 o/oo. The nitrogen in synthetic
nitrogen fertilizer is derived from atmospheric N2 by high
temperature catalytic reaction with hydrogen to form ammonia,
which is subsequently oxidized to nitrate. This process
results in very little change in the original atmospheric
nitrogen isotope ratio. So ammonia and nitrate derived from
such a fertilizer source has a 15N value close to 0 o/oo, the
value of atmospheric N2 (Figure 1). Nitrate derived from
sewage sources can have values greater than +20 o/oo
signifying a 15N enrichment of 2% over the original 15N/14N
ratio in starting N2. This is significantly different from the
fertilizer nitrogen isotopic value.
Whereas it is generally straight forward to differentiate the
waste released by herbivores from that of carnivores, waste
from animals in the food chain that consume both plant and
animal protein, such as free-ranging poultry, and hogs, may
have intermediate values. This may be useful in cases where it
is important to distinguish nitrate derived from different
livestock animal waste. Also, nitrogen in the environment is
involved in a large number of reactions as it cycles through
the food chain. In particular, dissolved nitrate can undergo
denitrification under anaerobic conditions to N2O and N2 by
bacteria in the soil. In this process, the products (N2O and
N2) are enriched in 14N, while the residual nitrate is
enriched in 15N. This could lead to nitrate derived from a
fertilizer input having a 15N values close to sewage
nitrogen. Usually there are other indicators that can
differentiate bacterial denitrification from sewage or septic
tank contributions, and a more detailed study will identify
this and allow the actual source of the nitrate to be
determined.
References
Environmental Isotopes in Hydrogeology, Clark, I. and P.
Fritz, Lewis Publishers, Boca Raton, FL, 1997.
Alan W.A.
Jeffrey, Isaac R. Kaplan, Dachun Zhang, Shan-Tan Lu, and
Jesper Nielsen are with ZymaX forensics Inc., San Luis Obispo,
California.
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