studies show that they work their way into the very fabric of life on Earth.
The integrity of the coating could be monitored by using either a single isotopic tracer
such as [.sup.68.Zn] incorporated into the coating or an "overkill" that could employ a multiple isotopic approach, as Cd has eight naturally occurring isotopes, Se has six, Te has eight, and Zn has five (Table 1).
It was in this respect that the work in Marion's laboratory differed from most of the isotopic tracer
work going on in other laboratories.
We introduced an isotopic tracer
([N.sup.15]-[[NO.sub.3].sub.-]) for 27 days in August 1996 to help determine how watershed-derived nitrogen moves through the upper estuary.
This method involves labelling a mineral nutrient pool with an isotopic tracer
and then measuring the changes with time of the amounts of the labelled nutrient and the isotopic ratio in the labelled nutrient pool.
After a time, a rat could be killed and its body fat analyzed for hydrogen-2, which would have acted as an isotopic tracer
, indicating the types of reactions that fat molecules underwent.
We introduced a stable isotopic tracer
([N.sup.15]-labeled nitrate) into the oligohaline reach of the Parker River estuary (Massachusetts, USA), with the overall goal of determining the biogeochemical fate of watershed-derived nitrate during the biologically productive period of summer and early fall.
Further, their comment regarding a concern for using an isotopic tracer
close to its expiration date only emphasizes the advantages of nonisotopic signals.
Zouari, "The use of hydrochemical and environmental isotopic tracers
to understand the functioning of the aquifer system in the Bou Hafna and Haffouz regions, Central Tunisia," Quaternary International, vol.