Within the GEM’s Environmental System mode, for dynamic problems the GEM builds and solves partial differential equations describing the concentrations of chemical constituents in environmental media in space and time subject to transport by advection and/or dispersion/diffusion, direct loadings, and internal sources and sinks. For example, for a single constituent in a single spatial dimension (x) and constant coef•cients, the governing partial differential equation is

R c t

v c x

E c x

∂ ∂ =

∂ ∂ +

∂ ∂ ±

2 sources

sinks (3.1)

where c = concentration (M/L3) dependent variable v = advective transport velocity coef•cient (L/T) E = dispersive or diffusive transport coef•cient (L2/T) R = optional retardation term used in modeling porous media t = time (T) independent variable x = space (L) independent variable

A more general case may involve multiple interacting dependent variables and multiple spatial dimensions. The coef•cients may be functions of space or time. The source and sink terms may also be functions of space or time and may be linear or nonlinear with respect to the dependent variables.