Simulation of Natural and Engineered Hydraulic Trap Systems in a Landfill, Using one- dimensional Three-Layer Laboratory Models

The role of unsaturated drainage layer and hydraulic control systems in sanitary-engineered solid
waste landfills were simulated using three-layer one-dimensional laboratory models. In the models,
from top to bottom, a sodium chloride solution as a contaminant source reservoir, a first compacted silt
layer as a primary liner, a coarse sand layer as a secondary leachate collection system or a hydraulic
control layer, a second compacted silt layer as a secondary liner, and a bottom water reservoir as a
groundwater aquifer, were used. In the first two tests, the model simulated the secondary leachate
collection system and natural hydraulic trap system (upward flow through the second clayey silt
layer). In this case, the contaminant transport mechanisms through the first silt layer were downward
advection and diffusion, and through the second silt layer, diffusion was downward and advection was
upward. The results showed that the implementation of the natural hydraulic control system could
effectively reduce the contaminant transport to the underlying groundwater reservoir. In the third test,
the natural and engineered hydraulic trap systems were simulated (upward flow from the bottom
reservoir to the upper reservoir). In the fourth test, the model simulated the engineered hydraulic trap
system (downward flow through the first silt layer and upward flow through the second silt layer). The
results showed that natural and engineered hydraulic trap systems have important effects on reducing
the contaminant transport toward the underlying aquifer. In all experiments the chloride concentration
in silt and coarse sand layers and top and bottom reservoirs were measured and the observed
concentrations were compared with theoretical concentrations calculated by computer code POLLUTE
V6. The results showed that there is good agreement between theoretical and observed data.