[1] Numerical models were used to evaluate how aquifer and barrier heterogeneity affect influent and effluent concentrations for permeable reactive barriers (PRBs). Spatial variability in the reaction rate constant k r and hydraulic conductivity K P of the PRB and hydraulic conductivity of the aquifer (in terms of variations in the mean m lnK , standard deviation s lnK , and correlation scale l of the logarithm of hydraulic conductivity (ln K)) were considered. Spatial variability of k r and K P was found to change influent and effluent concentrations by less than an order of magnitude. Spatial continuity in hydraulic conductivity parallel to flow, described by the correlation length l x , has a modest effect, with greater continuity yielding higher effluent concentrations. Decreasing the hydraulic conductivity of the aquifer (i.e., decreasing m lnK ) does not affect influent concentrations but decreases the median effluent concentrations and broadens the distribution of effluent concentration due to increased residence time in the PRB. Increasing the variability of the aquifer hydraulic conductivity (i.e., increasing s lnK ) decreases the median influent concentration and broadens the distribution of influent concentrations due to additional dispersion caused by greater heterogeneity. Greater variability in aquifer hydraulic conductivity also results in a higher median effluent concentrations and a broader distribution of effluent concentration. Comparison of effluent concentrations predicted using a one-dimensional deterministic model and the threedimensional numerical model shows that longer residence times and lower effluent concentrations are usually predicted by the one-dimensional model. The results indicate that designers should carefully consider factors of safety used for design, perhaps opting for a more conservative approach until more guidance on designing amidst aquifer heterogeneity is available.

Characterizing mass transfer during in situ air sparging requires knowledge of the size, shape, and interfacial area of air channels. These characteristics were determined by analysis of digital images of air channels passing through submerged glass beads having particle size in the sand range. Pore‐scale channeling occurred in all cases. The analysis showed that the air channels were narrower, more tortuous, more closely spaced, and moved nearly vertically through the coarser media. In the finer media, air channels had larger diameter, were spaced further apart, and passed nearly horizontally through the media. The mean diameter of the channels varied between 2.8 and 8.1 mm, and the mean spacing varied between 8.3 and 19.4 mm. Estimates of the area of the air‐water interface per unit volume of soil (a0), computed using data from the digital images and an assumed arrangement of channels, ranged from 0.02 to 0.2 mm2/mm3. Larger a0 were obtained for coarser media and uniformly graded media. These estimates of a0 compare well with published values for common packed‐column materials and for unsaturated soils.

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