Linear sources or sinks are either zero or •rst-order processes.* The GEM includes •ve types of linear sources and sinks: a zero-order exogenous sink, (2) a •rst-order exogenous sink, (3) a •rst-order cross-compartment source or sink, (4) a •rst-order cross-chemical source or sink, and (5) a •rst-order retardation sink that is commonly used in groundwater modeling. These are described below.