Spatial variability of long-term chloride transport under semiarid conditions: pedon scale

Knowledge about the magnitude and variability of water and solute fluxes in semiarid environments is required to assess the environmental risk associated with contaminants in the vadose zone. The purpose of this study was to examine the pedon-scale spatial variability in the transport of a chloride tracer after 34 yr at a site 50 km southwest of Saskatoon, SK. Soil cores were taken from two, pedon-scale transects (transects encompassing many pedons), and solute transport was quantified for each depth breakthrough curve using the method of moments. Modal transport depth (depth of peak concentration) did not vary significantly over the length of the transects (average = 1.34 m), indicating a relatively uniform soil water balance at the pedon scale. Therefore, trends in central moments (mean travel depth (E[z]), and variance about E[z]) were attributed to spatial variations in soil layering. Transport variance was negatively correlated (P < 0.05) to the thickness of a fine-textured, varved layer. The high water content (transport volume) of the varved layer decreased the velocity of the leading edge of the solute depth breakthrough curve which, over time, decreased V[z]. The two-dimensional distribution of the chloride shows that the estimated horizontal velocity of the chloride pulse (approximately 25 mm yr-1) is more than twice the estimated downward, vertical velocity (approximately 11 mm yr-1). The large horizontal flow component is attributed to anisotropy in soil hydraulic properties. The significant influence of layers on convective and dispersive solute fluxes in a low flow, semiarid environment under natural conditions, complements previous observations in high flow field experiments where two- and three-dimensional flow occurred along layer boundaries, and suggests anisotropy should be incorporated into transport models.