Suction cup sampler bias in leaching characterization of an undisturbed field soil

An accurate assessment of leaching losses in the vadose zone requires measurement of both solute and water flux to compute flux concentrations (C-F). Leachate collected at a depth of 1.2 m in 32 passive capillary samplers (PCAPS), which sample soil-pore water continuously at tensions of 0-50 cm H2O was compared to that collected in 32 suction cup samplers operated under a falling head vacuum of 530-cm H2O over a 2-year period. There was evidence that PCAPS collected C-F and suction cup samplers collected resident concentrations (C-R) as shown by the earlier breakthrough of a bromide tracer in the PCAPS as compared to the suction cup samplers. C-R was up to 100% lower than C-F during the rising branch of Br tracer breakthrough and up to 78% greater; during the declining branch of breakthrough. Br content and water flux into PCAPS were correlated with correlation coefficients changing from positive to negative values with the advancement of the tracer breakthrough peak through the profile indicating the declining importance of preferential flow on Br transport. C-R and C-F differed significantly (P < 0.05) for 35% of the sampling events for NO3, but seasonal means were mostly insignificantly different for this regularly applied and therefore more uniformly distributed anion. The early breakthrough of Rhodamine WT and Brilliant Blue FCF, which was applied with the Br, was very low with 0.15% and 0.08% of the initial concentration C-0, respectively, with C-R differing from C-F by up to -100%. For all tracers, mass leached using C-R is therefore prone to bias for short-term (<0.6 pore volumes) monitoring.