Mini Suction Cups and Water-Extraction Effects on Preferential Solute Transport

Soil water suction cups used in laboratory solute transport studies may cause undesired perturbations of the flow field. The twofold objective of this study was to test mini suction cups, and to assess the effects of pore water-extraction rate and cup size on preferential water flow and bromide (Br⁻) breakthrough. Porous ceramic cups of 0.25-cm outer diameter and 1-cm length could extract 1 cm³ of pore water within 17 (34, 38) min from 100% (85%, 70%) saturated loam soil by applying 0.1 (0.3, 0.5) bar suction. The corresponding sampling times for larger cups (0.6-cm diam., 2-cm length) were 3.3, 4.6, and 5.7 min. The smaller cups were subsequently tested for solution extraction of 1-cm³ samples every 20 min out of the matrix and preferential flow path (PFP) of a large soil column (24-cm diam., 80-cm high) during a Br⁻ transport experiment. Numerical simulations were used (i) to describe the Br⁻ transport experiment and (ii) to evaluate how preferential Br⁻ transport would be affected by pore solution extraction at different locations in the matrix and the PFP of a loam soil block (20 by 20 by 20 cm³) subject to wet and dry initial conditions. Three-dimensional water flow and solute transport were simulated using the Richards and convection–dispersion equations. The experimental and simulation results revealed a dilemma: while fast solution extraction using larger cups altered the flow field and preferential Br⁻ breakthrough for wet and particularly for dry initial conditions, slower solution extraction using small cups caused negligible perturbation of the flow field, but yielded insufficient resolution of the preferential Br⁻ breakthrough. Sampling in the matrix did not considerably affect Br⁻ transport, and gave sufficient resolution of the matrix Br⁻ peak. This study showed that the use of suction cups for measuring solute transport may be problematic in case of preferential flow.