5.1.9 Infiltration Generation
The leachate generated in any month is the amount of maximum percolation
in excess of AET and soil moisture recharge. If all the maximum percolation
in a given month is used for AET and soil moisture recharge, or if the
adjusted temperature for the month is equal to or below freezing, then
no leachate is generated:
where Ij is the leachate generation for the j-th month
(cm).
The module then calculates an annual infiltration rate (i.e. Darcy flux
density) by summing the monthly values of leaching
where j is the index on the month (unitless).
Like natural soil, waste-site material exhibits a certain water-holding
capacity. Fenn et al. (1975) note that the amount of water that can
be added and stored in the waste material depends on the composition of
the waste and its initial moisture content (which can vary widely) when
delivered to the site. Theoretically speaking, water movement through
a waste layer will act in a similar manner as water movement through a
soil layer; the field capacity must be exceeded before leachate movement.
Practically speaking, some channeling of water, from the heterogeneities
associated with the waste, will occur before the attainment of field capacity.
In an effort to avoid the complex nature of various waste forms,
the module assumes that surfacial waste sites act like surfacial natural
soil, and that the moisture content of the buried waste sites equals field
capacity. Therefore, the moisture that percolates through the bottom
of the waste does so at a rate equal to qw as calculated above.