Consider that the contaminated source zone is a pond or surface impoundment on the surface of the vadose zone soil. The source-term release module conceptualizes this zone to be a rectangular parallelopiped with volume V, horizontal cross-sectional area A, and depth dp.
It is assumed that the pond or surface impoundment consists primarily of water (i.e., it is primarily an aqueous phase rather than being a NAPL-phase mixture). However, there may be small globules of NAPL phase suspended in the water, if the masses of any of the contaminants that may partition into a NAPL phase are above the saturation limit of the pond water. (In other words, the pond may be a sparse emulsion of NAPL in water.) Furthermore, it is assumed that the pond contains suspended sediment, which can act as a solid-sorbent phase for the contaminants. In addition, it is also assumed that the aqueous solubilities of all contaminants are low enough that contaminants present in the aqueous solution always exist as "dilute" constituents of that solution (and therefore behave that way with respect to volatilization theory). The source zone is assumed to be a so-called "well-mixed reactor," which means that its properties are spatially uniform throughout.
The contaminated pond/surface impoundment source zone is considered to not include a layer of contaminated sorbing sediment at the bottom of the pond. Any layer of contaminated sediment at the bottom of the pond should be treated as separate source zone for the current version of the source-term release module.(a)
Water is assumed to leave the pond by leaching into the vadose zone below the pond and by runoff from the top of the pond (and also possibly by evaporation from the pond surface). Water is assumed to be added to the pond by natural precipitation or run-on. No anthropogenic additions of water are assumed to occur. Furthermore, it is assumed that the net rate of water addition is just equal to the net rate of water loss, which means that V, A, and dp can be considered constant in time.
It is assumed that suspended sediment particles do not leave the pond along with the water that leaches into the vadose zone (i.e., sediment particles would be filtered out of the water as it percolates into the soil at the bottom of the pond, and they would subsequently be resuspended into the pond rather than build up as a bottom sediment layer). However, suspended sediment particles are assumed to leave the pond with the runoff water. Similarly, run-on water is assumed to carry additional sediment particles into the pond. Furthermore, we will assume that the net rate of sediment particle addition is just equal to the net rate of sediment particle loss; which means that the concentration of suspended sediment in the pond, ßss, can be considered constant in time.
The water lost by leaching and runoff will contain contaminants. (Water evaporating from the pond surface will be pure.) It is assumed that the water additions to the pond will not contain contaminants. It is further assumed that the water being lost to runoff will contain contaminated suspended sediment and suspended NAPL-phase globules, in addition to the dissolved contaminant. Water that leaches into the vadose zone from the bottom of the pond is assumed to contain only dissolved contaminants (i.e., NAPL-phase globules would also be filtered out of the water as it percolates into the soil at the bottom of the pond, and they would subsequently be resuspended into the pond rather than build up as a continuous pool).
Section 2.2.2 describes how the source-term release module tests to determine if a NAPL phase exists (based on the current masses of contaminants in the source zone) at the beginning of each time step. Equation 2.1 in Section 2.2.2 is the test criterion in general form. The volume of the contaminated pond/surface impoundment source zone is given by
where
A is the horizontal cross-sectional area of the contaminant source zone (cm2)
dp is the depth of the pond (cm).
Substituting Equation 4.1 into Equation 2.1 the test criterion can be rewritten as
Note that because the source zone is a pond/surface impoundment (where the volumetric air content is zero and the bulk density of the solid sorbent is more appropriately defined as the suspended sediment concentration), Equation 4.2 is equivalent to
where ßss is the concentration of suspended sediment in the pond water (g cm-3).
Equation 4.3 is the test criterion that the module actually uses for contaminated pond/surface impoundment simulations.
Because all contaminant masses and loss fluxes are updated within a time step (before the numerical solution algorithm proceeds to the next time step) the module also has a complete record of contaminant mass produced in a time step based on chain decay of some parent species. This new mass is included in the total mass for that contaminant for the next time step.
(a)
Note that this is different than the way previous versions of MEPAS conceptualized a pond.
