Page 360 - Materials Chemistry, Second Edition

CAT3525_C10.qxd 1/31/2005 12:00 PM Page 331
The Sanitary Landfill 331
the soil and from plant uptake and transpiration are typically combined into a single term, evapo-
transpiration. Design requirements under RCRA make the quantities of surface water run-on and
water entering through the sides and bottom of the fill negligible.
The hypothetical water balance is represented by the equation

L P R U E R off (10.10)
on
where
L leachate
P precipitation
R run-on surface water
on
U underflow of groundwater into the cell
E evapotranspiration
R off run-off surface water
If the landfill is designed and operated properly, surface water will be diverted from the waste and
therefore R 0. Additionally, a landfill constructed above the water table and possessing an
on
impermeable liner will give U 0 (i.e., there is no underflow).
The equation can thus be simplified to:

L P E R off

The integration of these concepts is shown in Figure 10.22.
The amount of runoff depends upon the soil permeability, the slope of the surface, the type of
vegetation, duration and frequency of precipitation, and whether the precipitation is in the form of
rain or snow. The fraction of precipitation that becomes runoff is expressed by a runoff coefficient.
The fraction of precipitation that is converted into runoff is in the range 0.05 to 0.35 (Table 10.16).
In utilizing the water-balance approach for predicting leachate at landfills, a number of references
are available (Thornwaite and Mather, 1957; Fenn et al., 1975; Bagchi, 1994); for estimating evap-
otranspiration rates.

EXAMPLE 10.6

Calculate the annual volume of leachate generated per hectare for a sanitary landfill located in
the northcentral United States. The climate is temperate, average annual rainfall is 1.07 m/year
(42 in./year), and evapotranspiration is estimated at 55%. The wastes are covered with soil and

Precipitation (P)
Evapotranspiration (E)
Runoff (R )
off
Infiltration

Leachate (L)
FIGURE 10.22 Mass balance of moisture in a sanitary landfill.