Using Stable Water Isotopes to Evaluate the Changing Contribution of Water Age in Three Southern Ontario Agricultural Streams

Annual and event scale water age contributions were analyzed using δ¹⁸O for three agricultural sub-catchments in southern Ontario: Nissouri Creek catchment (loamy soils, high tile drainage), North Creek catchment (heavy clay soils, low tile drainage) and Big Creek catchment (loamy clay soils, high tile drainage). The local meteoric water line was δ²H = 6.94 δ¹⁸O + 2.51 (r²=0.97, n=158, p-value<0.0001). The annual young water contribution was determined for Nissouri (0.3108), North (0.9895) and Big (0.4907). Under a two-component mixing model, hydrograph separations determined the mean total event contribution of old water for Nissouri (57%), North (41%) and Big (55%). This indicates that the catchment with the lowest presence of tile drainage had the highest contribution of young water, but also that soil type may have a stronger control on dominant flow pathways than tile drainage presence alone. Antecedent moisture had no significant correlation to event water age contributions.