The term exposure, as applied to a Source, has a specific meaning within GoldSim. In particular, species mass in a Source is considered to be exposed if it is available for mass transfer within and away from the Source. If a contaminant is present within an intact barrier, or bound within intact matrix material of some sort (e.g., grout, metal, glass), it is not available for mass transport, and hence is considered to be unexposed. The mass cannot be transported until it is physically liberated from barriers and/or matrix material present at the Source. Hence, within a Source, exposure is controlled by two processes:
•Loss of Containment. The mass can be specified to be present in the Source in discrete packages (e.g., 100 separate drums). For each individual package, two layers of containment (barriers) can be explicitly considered to exist (e.g., a drum and a liner within the drum). The mass can be distributed in various places within the barrier system inside a package (e.g., between the outer and inner barriers, inside both barriers). Barrier failure rates are described in terms of density functions of failure frequency (e.g., the rate of drum failures).
•Degradation of waste matrix. All or portions of the species mass within the Source can be specified to exist within one or more matrix materials, such as cement, grout, solidified glass, or metal. Species which are bound in such a matrix are not released until the matrix itself degrades (e.g., dissolves, corrodes or alters) in some manner. Release of species mass from the matrix is then assumed to be congruent with the degradation of the matrix.
These two processes together determine the rate of exposure of contaminant mass within a source.