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Fig. 4. Trends of stable isotopes in ground water. Values of 
15N and 18O are given with respect to air N2 and Vienna standard mean ocean water, respectively, in parts per thousand. (A) 15N[NO3–] versus 18O [NO3–]. Lines are least squares regressions of aerobic and anaerobic ground water data. (B) Change in 15N[NO3–] during transport (15N[NO3–]- 15N[NO3–]0) versus denitrification progress, 
denit. For comparison, curves show hypothetical Rayleigh fractionation based on apparent fractionation factors (
) of –5 and –20
. (C) 18O values of coexisting NO3– and H2O in aerobic ground water and near stream samples. Dotted lines are given for reference and are defined by the isotopic composition of the major potential O sources during nitrification: 18O[NO3–] = a 18O[H2O] + (1–a) 18O[air O2], with a = 0, 0.67, and 1. For (A), (B), and (C), "aerobic" samples (gray symbols) are defined by O2 > 0.016 mmol L–1 and water age < 25 yr. "Near stream" samples (unfilled symbols shown here for comparison and not included in other analyses) are ground water from near-river piezometers (Puckett et al., 2007) that show minimal interaction with surface water.
