GoldSim allows you to control where mass enters an Aquifer by specifying a Source Zone Length. This represents a length along the flow direction in the pathway. Hence, if your pathway had a length of 100m, and you specified a Source Zone Length of 40m, mass would enter the pathway uniformly over the first 40m (rather than at the beginning of the pathway):
This is useful, for example, if your Aquifer represents an aquifer which is being loaded from above with mass from source which is of such an areal extent (e.g., a landfill) that it extends over a significant portion of the pathway and therefore cannot be treated as a point source.
By default, the Source Zone Length for an Aquifer is zero. In this case, any mass entering the Aquifer is treated as a point source which is applied at the beginning of the Aquifer (the first Cell). If the Source Zone Length for an Aquifer is greater than zero, the mass is distributed uniformly over the specified length.
A number of points regarding the use of the Source Zone Length are worth noting:
•The Source Zone Length must be static (it cannot change with time).
•If a non-zero Source Zone Length is defined, the Aquifer Length and Area cannot change with time (doing so would generate a fatal error).
•The Source Zone Length is applied to all forms of mass that enter the Aquifer: mass that enters as an initial or boundary condition, mass that enters the Aquifer from other pathways via a mass flux link, and mass that enters the Aquifer as a specified discrete change via the Discrete Changes field.
•If the Source Zone Length is larger than the Aquifer Length, the Source Zone Length is assumed to be equal to the Aquifer Length (and a warning message is written to the Run Log).
Warning: The Source Zone
Length should only be used if the total flow in the Aquifer is substantially
larger than the flow associated with the incoming mass. This is because
that the Source Zone Length does not change the flow rate along the Aquifer (all
of the pathway’s inflows are assumed to enter at the beginning of the pathway);
it only impacts where the mass is input. Hence, it is equivalent to
assuming that the amount of flow associated with the mass entering the pathway
is negligible relative to the total flow in the pathway (e.g., associated with
“clean” upgradient water).