Field evaluation of passive capillary samplers

Soil solution samplers have certain inadequacies that limit their range of possible applications. Passive capillary samplers (PCAPS), which apply suction to the soil pore water via a hanging water column in a fiberglass wick, have shown promising results in preliminary experiments in regard to collection efficiency of water and Br tracers. The objectives of this study were to evaluate PCAPS with respect to (i) effect of installation procedure and operational characteristics; and (ii) ability to estimate the soil-water and solute flux. We installed 32 PCAPS and 32 ceramic suction cup samplers at a depth of 1.2 m in an undisturbed silt loam soil. For the first year, water flux, NO3-N and Br concentrations in the part of the PCAPS closest (0.3 m) to the refilled trench were 15, 14, and 34% less, respectively, than in the part farthest (0.9 m) away from the refilled trench. This was attributed to installation procedure, since the bias ceased for the second year. Soil-water flux measurements of PCAPS underestimated those determined by a water balance by 20 to 34%. Some of the discrepancy is thought to be due to the lack of accounting for runoff, which was observed but not quantified. The recovery of a Br tracer was low with an average of 29%, ranging from 5 to 77%, which was attributed to lateral water movement due to prominent lateral stratification. To estimate the mean Br concentration with a 30% bound at the 0.05 confidence level on this 1-ha held site, 25 PCAPS and 37 suction cup samplers are necessary. Sampling a more evenly distributed solute such as residual fertilizer NO3 requires only eight PCAPS for this level of confidence.