Low-k zones (i.e., low permeability zones such as silt, clay layers) can serve as indirect, low-level sources of contamination to transmissive zones due to matrix diffusion. If you can apply several simplifying assumptions about heterogeneity at your site coupled with its concentration history, the Matrix Diffusion Toolkit (Toolkit) can provide planning-level estimates of:  mass discharge (sometimes called mass flux, in grams per day) and/or concentrations in the transmissive zone caused by matrix diffusion; and  mass of contaminants and concentrations in the low-k zone. Why is this important? Understanding and evaluating matrix diffusion can provide information regarding a variety of key questions, such as: