Laboratory Investigation of Boundary Condition Impacts on Nitrate Anion Exclusion in an Unsaturated Soil

Abstract

Transient unsaturated horizontal column experiments were conducted with a loam soil, under variable boundary conditions, to obtain added insight on nitrate (NO3−) anion exclusion processes. The boundary conditions evaluated were column inlet soil water content (θ0), initial soil water content (θi), column inlet soil solution NO3−–N concentration (C0), and the initial soil solution NO3− concentration (Ci). Results consistent with significant anion exclusion of NO3− were exhibited in all experiments. For tests with NO3− solution injected at the column inlet, the soil solution NO3− concentration at the inlet was found to be 12 to 19% less than the injected NO3− concentration. For these same tests, where initial soil conditions were dry with no NO3− originally present, the NO3− concentrations at the wetting front edge were substantially greater than the injected NO3− concentration, while for initially wetter conditions, with or without NO3− originally present, the NO3− front was found to extend well beyond the theoretical “piston front” separating the injected solution from displaced initial soil water. For displacement tests, in which water was injected at the inlet of a column initially wetted with a NO3− solution, the soil solution NO3− front also extended beyond the piston front. Inlet NO3− excluded water content ranged from 0.03 to 0.06 and was not affected by pore water velocity, but did have a modest positive correlation with total water content (r = 0.60 and r = 0.67). The findings of this investigation provide a better overall understanding of nitrate movement through the soil profile.