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Long-term Movement of a Chloride Tracer under Transient, Semi-Arid Conditions

M. F. Dyck, R. G. Kachanoski, E. de Jong
Soil Science Society of America journal 2003 v.67 no.2 pp. 471-477
application rate, drainage, groundwater contamination, potassium chloride, prairies, rhizosphere, risk, semiarid zones, soil, soil sampling, solutes, tracer techniques, vadose zone, variance, water resources, Saskatchewan
Understanding the magnitude and variability of water and solute fluxes in the vadose zone is required to assess the risk of potential contamination of ground water resources. In this study, the long-term (>30 yr) movement of a Cl tracer applied to the soil surface is quantified. In 1966, granular KCl was applied to the surface of nine 6 by 90 m plots at a field site near Saskatoon, Saskatchewan. Additional KCl was applied to two of the plots in 1970. Application rates varied between 0.11 to 2.24 kg KCl m⁻² Moment analysis was used to calculate the mean travel depth (E[z]) and variance (V[z]) of depth breakthrough curves compiled from soil samples taken 1, 2, 3, 4, 28, and 34 yr after the initial KCl application from the 2.24-kg KCl m⁻² plot. Spatial variability of Cl transport was assessed on 51 cores taken along a 10-m transect, 34 yr after application. Initial movement of the tracer through the root zone was relatively quick (E[z] = 1.34 m, 4 yr after application), followed by slow movement (E[z] = 1.68 m, 34 yr after application). The change in E[z] between 4 and 34 yr after application was used to calculate a velocity below the root zone of 11 mm yr⁻¹ and a deep drainage estimate of 3 mm yr⁻¹ (θ = 0.3) which is within the range of other estimates for the Canadian Prairies. Variability in E[z] along the 10-m transect, after 34 yr of transport was low (coefficient of variation [CV] = 4%), suggesting relatively uniform deep drainage. This contrasts sharply with the high variation in convective fluxes measured in other steady-state high-flow field-tracer experiments.