Section Four
WASTE MANAGEMENT
B. LANDFILLS, SOLIDIFICATION
31    MODELING OF INITIAL DISCHARGES FROM HYDRAULIC
BARRIERS UNDERLYING SOLID AND HAZARDOUS
WASTE LANDFILLS
Zuhdi Y, Al-Jobeh, Assistant Professor
Department of Civil Engineering
Valparaiso University, Valparaiso, Indiana
INTRODUCTION
The disposal of hazardous wastes is increasingly becoming one of the most important areas in the
study of environmental contamination. The usual places to dispose of these wastes are hazardous
waste landfills, which are specially designed, constructed, and operated landfills. One important
problem with disposing of hazardous wastes in landfills is the leaching of fluids from the bottoms of
landfills into the underlying soils and then into the groundwater. This movement of fluids in the
subsurface involves the combined flow of water, gases, and toxic chemicals.
To minimize groundwater contamination, the U.S. Environmental Protection Agency (EPA) established a set of minimum technological requirements for hazardous waste landfill liners design and
construction.' These liners are usually composite systems made up of multiple layers of low permeability clay soil and impermeable high density polyethylene (HDPE) geomembranes.
The clay liners are often constructed of native soils that contain appreciable amounts of clay-sized
particles. In some cases other materials, such as bentonite, are mixed with poor clayey soils to reduce
their permeabilities. When these liners are constructed under changing weather conditions, they often
retain higher moisture content than their optimum moisture content. As the overlying disposal cells
are filled with waste, the imposed load can induce flow of this trapped moisture content. It is very
important to identify the origin of this flow. In newly constructed landfills the flow can be a result of
clay liners compression and deformation if the fluid quantities observed are within some pre-
estimated range and the system does not contain geosynthetic liners. For liners systems containing
impermeable geosynthetics, underdrain discharges would normally be interpreted as evidence of liner
leakage, and would generally prompt corrective action. For "mature" landfills, discharges generally
indicate leakage, and corrective actions need to be investigated. It is clear that discharges can result
from overburden loading, and it would be a mistake to initiate corrective actions if the observed
discharge quantities are within a pre-estimated range.
Unfortunately, there is no accurate mean to estimate the discharge from clay liners at early stages of
a landfill cell's life. The EPA's standard computer model used to simulate a variety of designs for
landfills, including natural clay and geosynthetic liners is the Hydrologic Evaluation of Landfill
Performance (HELP). The HELP model cannot be used to predict the initial discharges from landfill
liner systems. The first two years of a cell's life are treated as periods of equilibrium for both the
HELP simulations and the cells and, therefore, are not included in the comparisons between collected
data and model predictions.2
The main purpose of the work here is to present a mathematical model that will be used to develop a
numerical computer code to estimate the initial discharges from newly constructed landfill cells. In
order for this model to be effective, it must be based on sound physical principles. A system of partial
differential equations describing the problem's underlying physics was derived. These equations are
based on theories of multiphase (air, water, and solid) flow in unsaturated deforming soils.
FORMULATION OF THE MATHEMATICAL MODEL
Water movement through porous media has been the interest of many people in a large number of
disciplines. Many models and partial differential equations have been derived to describe unsteady
water flow.3,4 These models or flow equations generally assume the soil structure of the unsaturated
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