Method of Estimating the Travel Time of Noninteracting Solutes Through Compacted Soil Material

The pollutant travel time through compacted soil material (i.e., when a pollutant introduced at the top first appears at the bottom) cannot be accurately predicted from the permeability (saturated hydraulic conductivity) alone. The travel time is also dependent on the effective porosity of the material; i.e., the portion of the total porosity that contributes significantly to fluid flow. Once permeability and effective porosity are determined for a selected material, the travel time of noninteracting pollutants through specified thicknesses of compacted material at specified hydraulic gradients can be predicted easily. Pollutant travel time is directly proportional to effective porosity and thickness of a compacted layer and inversely proportional to permeability and hydraulic gradient. This paper presents a straightforward method of determining the effective porosity of compacted soil materials. The determination of effective porosity is based upon the total porosity and the spread on a log scale in the pore sizes of a compacted sample. The total porosity is calculated from measurements of bulk and particle density. Pore size distribution information is obtained from the cumulative porosity curve of the sample as measured by a mercury‐intrusion porosimeter. Once the total porosity and pore size distribution information are obtained for a particular sample, the effective porosity can be determined directly by using a graphical relationship. This paper also compares measured and predicted solute breakthrough times for three compacted soil materials. Predicted travel times through compacted samples of glacial till, loess, and paleosol materials were reasonably close to measurements of CL ‐ travel time.