『Abstract
　Organic substrates in reactive barrier systems are often heterogeneous material mixtures with relatively large contrasts in hydraulic conductivity and porosity over short distances. These short-range variations in material properties imply test preferential flow paths and diffusion between regions of higher and lower hydraulic conductivity may be important for treatment efficiency. This paper presents the results of a laboratory column experiment where denitrification is investigated using a heterogeneous reactive substrate (sawdust mixed with sewage sludge). Displacement experiments with a non-reactive solute at three different flow rates are used to estimate transport parameters using a dual porosity non-equilibrium model. Parameter estimation from breakthrough curves produced relatively consistent values for the fraction of the porosity consisting of mobile water (β) and the mass transfer coefficient (α), with average values of 0.27 and 0.42 d^-1, respectively. The column system removes ＞95％ of the influent nitrate at low and medium flow, but only 50-75％ of the influent nitrate at high flow, suggesting that denitrification kinetics and diffusive mass transfer rates are limiting the degree of treatment at lower hydraulic residence times. Reactive barrier systems containing dual porosity media must therefore consider mass transfer times in their design; this is often most easily accommodated by adjusting flowpath length.

Keywords: Dual porosity; Heterogeneity; Tracer; Diffusion; Dipersivity; CXTFIT』

1. Introduction
2. materials and methods
　2.1. Column experiment
　2.2. Displacement tests and dual porosity model
　2.3. Analyses
3. Results and discussion
　3.1. Hydraulic properties with single porosity model
　3.2. Hydraulic properties with dual porosity model
　3.3. Denitrification experiment
4. Implications for barrier systems
Acknowledgments
References