Mathematical modeling of leachate routing in a leachate recirculating landfill

Full-scale application of leachate recirculation has been hampered by an inability to uniformly apply leachate to the waste mass. Much of this problem is due to the varied nature of the disposed waste which promotes preferential routing and short-circuiting of leachate flows. The United States Geological Survey’s Saturated-Unsaturated Flow and Transport (SUTRA) model [Voss C. I. (1984) SUTRA, Saturated–Unsaturated Transport, A Finite-Element Simulation Model for Saturated–Unsaturated, Fluid-Density-Dependent Ground-Water Flow with Energy Transport or Chemically Reactive Single-Species Solute Transport, US Geological Survey, National Center, Reston, VA.] was used to model leachate application to homogeneous anisotropic and heterogeneous waste masses. The homogeneous anisotropic scenario was modeled using vertical hydraulic conductivities of 10−3 and 10−4 cm/s while the horizontal hydraulic conductivity was simulated as one order of magnitude higher than these values. The heterogeneous waste mass was simulated by applying statistical relationships to the local hydraulic conductivities. Normal, exponentially increasing, and exponentially decreasing probability density functions (PDFs) were used to model the frequency-hydraulic conductivity relationship. Median hydraulic conductivities of 10−3 and 10−4 cm/s with hydraulic conductivity ranges of 10−1–10−5 cm/s and 10−2–10−6 cm/s, respectively, were simulated for each probability distribution. Leachate was applied 8 h/day via a horizontal trench at a daily rate of 4 m3/m of trench. Saturation iso-clines are used to illustrate the effect of the waste mass hydraulic parameters on leachate routing. The ability to develop predictions and evaluate a variety of different scenarios without the effort and expense of physical experimentation is a great strength of mathematical models. However, a model’s predictive capabilities must be verified by comparing the model results with field studies. In an attempt to verify model estimations, SUTRA was used to simulate the site recirculation system and operational procedures used at the Delaware Solid Waste Authority Test Cells and the Yolo County Leachate Recirculation Demonstration Project. Cumulative measured and simulated leachate generation were compared. Results from this verification effort indicate that channeled flow is the major leachate movement mechanism that is not well understood at this time.