CE 527 Solid Waste Management
Leachate Generation and Estimation
Dr. S.K. Ong
Soil and Waste Characteristics
 ______________ is the ratio of the total volume of voids to the total volume of the material
 ______________ = volumetric water content remaining after a prolonged period of gravity drainage without
additional supply, soil water suction of 0.33 bars
 _____________ - lowest volumetric water content that can be achieved by plant transpiration, at a water suction of
15 bars.
0.6
Field
Capacity
Water content
(vol/Total vol)
Wilting Point
0.0
Sand
Sand
loam
Loam
Silty
loam
Clay
loam
Silty
clay
Clay
Water Balance Approach
 Summing the amounts of water entering the landfill and subtracting the amounts of water consumed in chemical
reactions and the quantity leaving as water vapor
 Assume a typical cell
As per book:
Incoming - outgoing = moisture stored in cell
Equation will be more useful if it is written in terms of time and that when storage is greater than field capacity water will flow and will be equal to WBL.
The water balance equation at period i is given by:
moisture
in SW
moisture
in sludge
water
in cover
moisture
infiltration
water loss
- gas
water loss
- water vapor
water loss
-surface
evaporation
let this equal to the initial storage SW i-1
where Y = storage at i + water out, if any
Then we have
If the amount of water available (Y) for period i, is greater than SFC (field capacity storage) then water will flow
until field capacity is reached.
Therefore
HELP
 Stands for Hydrologic Evaluation of Landfill Performance
 A quasi two-dimensional deterministic water routing model for determining water balances
 Quasi - one-dimensional vertical percolation
- one-dimensional lateral
 Consists of several models developed for specific processes
 Simulates daily water movement through into and out of a landfill
 Examines water fluxes throughout the complete vertical profile
HELP Model Requirements (http://www.wes.army.mil/EL/elmodels)
Climate Data
Daily Rainfall data
1. Input by User
2. Generated stochastically - have parameters to generate synthetic precipitation
3. Historical data base (5 years) for 102 US cities
Daily temperature and solar radiation generated stochastically or input of data by user
Soil Data
Porosity
Field capacity
Wilting point
Saturated hydraulic conductivity
Soil conservation service (SCS) runoff curve number
Initial soil water content
Evaporation coefficient
Vegetation data
Crop type
Crop cover
Ground biomass data
evaporative zone depth
Design data
number of layers
layer thickness
layer slope
surface area of landfill
maximum drainage distance for lateral drainage layer
information on geomembrane
leachate recirculation procedure
Layers (see Figure for a typical profile of a landfill)
 can model 4 types of layers
- ________________________
- ________________________
- ________________________
- ________________________
Vertical Percolation Layers
- downward flow due to gravity or upward/extracted flow by evaporation
- downward drainage is assumed to occur by gravity drainage whenever soil moisture is greater than field
capacity
- rate of drainage is assumed to be independent of conditions in adjacent layers
- only vertical flow - no lateral flow
Lateral Drainage Layers
- lateral drainage to collection systems at or below the surface liner systems
- allows lateral drainage
- vertical drainage modeled in the same manner as vertical percolation layer
- restricted to a lateral drainage layer that is underlain by only a liner or another lateral drainage layer
- slope of bottom of layer may vary from 0 to 40%
- hydraulic conductivity > 1 x 10-3 cm/sec needed for substantial flow
Barrier Soil Liners
- intended to restrict vertical flow
- saturated hydraulic conductivity typically < 1 x 10 -6 cm/sec
- only downward flow
- leakage depends on depth of water-saturated soil above the base of the layer, thickness of liner and
hydraulic conductivity
- properties - assumed saturated and do not change with time
Geomembrane Liners
- nearly impermeable
- restricts significant leakage to small areas around defects
- leakage - 3 sources
- vapor diffusion - across active area as a function of head on the surface liner
- manufacturing flaws (pinholes)
- installation defects (puncture, cracks, tears, etc.)
Notes on Major Equations Used for HELP
Two categories of Processes
- __________________________
- __________________________
Surface Processes
- precipitation
- surface runoff
- surface evaporation
- snow melt
- interception of rainfall by vegetation
- infiltration
Surface Water Balance
(assume no surface water storage, i.e., no puddles of water.
Precipitation
- input by users
- generates stochastically
- use of historical data base for 102 US cities
Surface Runoff
- modeled using the Soil Conservation Service (SCS) curve number
- developed for large storms on small watershed
- an empirical approach in computing runoff for different types of surfaces and vegetation
- runoff computed using a single parameter - runoff curve number, CN
where
Q = runoff (ins)
P = precipitation (ins)
S = retention (ins)= 1000/CN - 10
Curve numbers (CN) dependent on types of soils and vegetation (see Table)
HELP used antecedent moisture condition II.
Snow Accumulation and Melt
- uses SNOW-17 sub-routine of the National Weather Service River Forecast System (NWSRFS)
- when daily mean temperature is below 32o F program stores precipitation on the surface as snow
- melt process divided into non-rain periods or periods with rainfall
Evaporation
- rate of evaporation is a function of solar radiation, temperature, humidity, vegetation cover, growth stage,
surface wetness and soil water content
- divided into 3 components
- surface evaporation, evaporation of water retained on surface and/or foliage of vegetation
- evaporation from the soil
- transpiration by plants
Subsurface Processes
Subsurface Water Routing
Flow can be computed by considering various segments as shown
The equation at a given time i can be written as
SM ( j) 
DRi(j)
SMi(j)
ETi(j)
RCi(j)
SIi(j)
DR i( j)  DR i 1( j)
2

DR i( j1)  DR i 1( j1)
2

ET i( j)  ET i 1( j)
2

RC i( j)  RC i 1( j)
2

SI i( j)  SI i 1( j)
2
= drainage into segment j from above during time step i (inches)
= soil water storage of segment at the midpoint of time step i (inches)
= evapotranspiration from segment j during time step i (inches)
= lateral drainage recirculated into segment j during time step, i (inches)
= subsurface inflow into segment j during time step i (inches)
The vertical drainage DR is modeled using Darcy's Law
qK
dh
dL
note that for unsaturated soil, K is dependent on the soil water content
ET is modeled by assuming that there is an evaporation zone (need to specify depth in the model or use
default values). This zone is divided into 7 layers with each layer providing a different amount of
evaporation. The most evaporation will come from the top layer - decreasing as one goes deeper. To
model the amount evaporated, a weighting factor is used as shown.
The above numerical equation is solved iteratively and the solution is accepted if it is within 0.3 percent of
the preceding value.