The study of the origin, path, interactions and fate of natural waters and its conduits is called hydrology. As we come to the realization that fresh water is a precious resource in limited supply, the importance of understanding its dynamics becomes critical. Stable isotope ratio analysis is a powerful tool for helping the hydrologist track a body of water and study the processes that effect surface and subsurface waters.
Water is a simple molecule of hydrogen and oxygen. Both elements have multiple stable isotopes we can use for ratio measurements. It is possible to track changes in an aquifer that are related to recharge events (rain) and explore the turnover times or residence times of the water. It is also useful and sometimes necessary to know if multiple subsurface waters are related to each other or hydrologically separate. Isotopes ratios provide a fingerprint of a water mass that can used to identify and distinguish between separate waters.
Of increasing importance is the quality of
our fresh water and an understanding of the
processes that can effect that quality. The
presence of nitrogen in an aquifer either
as nitrate
(oxidized) or ammonia (reduced) has a direct
effect on the quality of our drinking water
and the ecological health of our rivers and
lakes. The major sources of this increasing
problem are 
agricultural fertilzers and industrial and
residential sewage. Agricultural fertilizers
are produced with high temperature reactions
from air nitrogen and have an isotope ratio
similar to air (defined as 0). Nitrogen that
has been transfered through several trophic
levels, as sewage nitrogen has, can be as
high as +20 or more.
While it is helpful to measure the concentrations
of nitrate in groundwater, it doesn't always
help determine the origin of the problem.
By measuring the stable isotope ratios of
nitrogen species in natural waters we can
often differentiate between sources. Nitrogen
from a feed lot or residentail septic system
looks completely different than the runoff
from a heavily fertilized golf course, for
instance.
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