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ORIGINAL RESEARCH

Quantifying Minimum Monolith Size and Solute Dilution from Multi-Compartment Percolation Sampler Data

^a Wageningen Univ., Dep. of Environmental Science, Sub-Dep. Water Resources, Soil Physics, Ecohydrology, and Groundwater Management Group, Nieuwe Kanaal 11, 6709 PA The Netherlands
^b Swiss Federal Inst. of Aquatic Science & Technology (Eawag), Dep. of Water Resources and Drinking Water, Überlandstr. 133, 8600 Dübendorf, Switzerland
^c School of Ecology & Environment, Office J228 Warrnambool Campus, P.O. Box 423, Warrnambool, Victoria 3280, Australia
^d Dep. of Biological & Agricultural Engineering, Univ. of Idaho, P.O. Box 442060, Moscow ID 83844-2060
^* Corresponding author (ger.derooij{at}wur.nl)

Preferential flow affects solute transport in natural soils, leading to high spatiotemporal variation of concentration. A multicompartment solute sampler (MCS), yielding multiple breakthrough curves at a given depth, can monitor tracer movement in a heterogeneous soil. We present a technique to estimate from MCS data whether a soil monolith is sufficiently large to capture preferential flow, which is a necessity for tracer breakthrough curves to be representative. For several soils, we estimate that an MCS should be larger than 0.1 to 0.2 m². We also expand dilution theory to analyze the concentration variations of a tracer passing the control plane monitored by the MCS, in addition to the conventional plume spreading analysis. We characterize the set of locally observed breakthrough curves by the entropy-based dilution index. For given first and second-central moment, the spatially uniform log-normal breakthrough curve maximizes the dilution index. The ratio between observed and maximum dilution index is denoted reactor ratio. For a 300-compartment solute sampler, covering an area of 0.75 m², we compute a reactor ratio of 0.665, compared with 0.04 for stochastic-convective and 1 for convective-dispersive transport. With a single, large collector the reactor ratio would be 0.958, severely underestimating concentration variations. Large collector areas are clearly inadequate to estimate dilution. Values of the dilution index and the reactor ratio for individual sampling compartments indicate efficient longitudinal mixing in most but not all cases, and considerable spatial variation of the leaching process.

Abbreviations: BTC, breakthrough curve • MCS, multicompartment sampler • SSDC, spatial solute distribution curve