underlain by a highly permeable shallow vadose layer. Kreitler and Jones
(1975) report the successful use of this technique to identify sources of
nitrate in ground water in Texas, and Kreitler et al. (1978) report that
differences in 615N with depth in the ground water of Long Island, New York
were related to a change in land use. They suggested that the lighter 615N in
the deeper aquifer (Magothy) was associated with recharge from cultivated
fields during the agricultural period (pre-1950), while heavier 615N in the
shallow aquifer reflected inputs from existing septic systems. Wolterink et
al. (1979) collected over 300 soil and ground water samples for analysis of
15  140
N/ N ratios from around the United States and representing a variety of
environmental conditions. Standard statistical techniques were used to
analyze the observed variations in 61SN with respect to several nitrate
sources and various environmental factors. It was concluded that nitrates
from feedlots, barnyards, and septic tanks can be distinguished from natural
soil nitrate on the basis of their 6 15N.  They cannot, however, be
distinguished from each other. Spalding et al. (1982) suggest tha't low 615N
in ground water in the Burbank-Wallula area of Washington indicate the primary
source of nitrate contamination is from agricultural leachates. Flipse, Jr.
15N
and Bonner (1985) report on 61N from two heavily fertilized sites in Suffolk
County, New York. The purpose of their study was to determine whether the
61S N of fertilizer is increased during transit from land surface to ground
water to an extent which would preclude use of this ratio to distinguish
agricultural from animal sources of nitrate in ground water.  They state that
the nitrogen-isotope ratios of fertilizer-derived nitrate were not altered to
an extent that would make them indistinguishable from animal-waste derived
nitrates in ground water.