Fig. 7. Nitrogen and oxygen isotope ratio data. Open circles represent tile drain samples. Closed circles, diamonds, triangles, and squares represent samples from Wells 3, 5, 6, and 11, respectively. The solid line represents the evolution of a hypothetical packet of water with initial ¹⁵N = ¹⁸O = 5.0 as it undergoes denitrification with = 12.0 for N and 5.7 for O. Uncertainties are smaller than the sizes of the symbols.