Figure 5.  Nitrogen and oxygen isotopic composition of nitrate sources based on the work of previous researchers (Kreitler, 1975; Kreitler, 1979; Heaton, 1986; Amberger and Schmidt, 1987; Bottcher et al., 1990; Aravena et al., 1993; Durka et al., 1994; Wassenaar, 1995; Boehlke et al., 1997).