Transport of conservative and reactive tracers through a naturally structured upland podzol field lysimeter

Compared to agricultural soils, descriptions of solute transport in upland soils at scales smaller than catchments are scarce. This study evaluates data from three consecutive field tracer experiments in which Cl, Na and Ca (stepped rates of 4.5, 9 and 4.5 mm h−1) were irrigated onto a ferric podzol monolith (3×2 m2 and 0.6 m depth) isolated from the surrounding soil except at the base. For each individual experiment a one hour pulse of tracer was applied and subsequently washed through with water for approximately 1 pore volume and monitored using suction cup samplers located in the O and B horizons. Breakthrough curves showed considerable heterogeneity in the response of both horizons and inferred that downward leaching would result in considerable temporal spreading of solute mass across wide time scales. Patterns of Cl and Na breakthrough were similar and were characterised by low peak concentrations (maximum C/C0 of 0.05 Cl and 0.03 Na). Minimal Ca breakthrough was observed in the O horizon, although a higher breakthrough was observed in the B horizon. These factors suggest that isolated highly active pathways were controlling the O horizon response, although these were not captured by the sampler network. This was supported by more rapid mean times to peak in the B horizon. Lateral drainage, intercepted at the base of the O horizon on the downslope face, accounted for approximately 45% of solute volume and 25, 21 and 14% of Cl, Na and Ca tracer mass, respectively. This rapid drainage pathway comprised surface flow and flow along the organic mineral interface, likely fed from preferential flow pathways in the O horizon. The breakthroughs observed in the mineral soil, although heterogeneous, indicated a response for this horizon of sustained leaching to shallow ground waters.