If a number of steel drums (or some other type of container) were buried in the ground (or left sitting at the ground surface), at some point in time they would begin to fail (i.e., leak). It is highly unlikely, however, that the containers would all begin to leak at the same time. Rather, there would be temporal variability in their failure times (i.e., some containers would fail early, and some would fail at later times). If there were a large number of containers considered, the distribution of failure times would likely be relatively continuous. This variability in failure times could be quantified by defining a failure distribution.
Such a failure distribution is shown below:
A failure distribution is a density function of failure frequency. The figure above shows two views of a failure distribution. The solid line (left axis) shows the fraction of containers failing at a given time. This can be thought of equivalently as a failure rate plotted versus time. The dashed line (right axis) represents the integral over time of this curve, which is the actual fraction of containers that had failed by a given time.
You represent barrier failure (the loss of containment) in GoldSim by defining failure distributions for the barrier(s) in the packages within your Source. A package in a Source can be specified to have no barriers, a single barrier, or double barriers (one inside the other).
For a particular barrier, multiple failure modes can be defined, with each mode having a different failure distribution. For example, one failure mode may be assigned to represent one type of corrosion (e.g., uniform corrosion), while another may be assigned to represent another type of corrosion (e.g., pitting corrosion). GoldSim combines the failure modes in an appropriate manner to obtain the total failure distribution. It is assumed that the different modes operate independently and without synergism. Appendix E of the Contaminant Transport Module User’s Guide describes in detail the manner in which the separate failure modes are mathematically combined.
The figure below illustrates the combination of two failure modes.
Note that within GoldSim, a barrier has only two states: unfailed or failed. The definition of “failure” is specified by the user. Typically, failure is defined as the initial breaching of the container such that material inside the barrier (i.e., contaminants) is exposed to the environment (e.g. flowing water).