By Hydrosimulatics INC  

Leachate from a Landfill – Local and Regional Analysis 
Figure 1: Landfill and local hydrogeology. (Top) Conceptual representation, cross-section view and (bottom) plan view.

Originally a network of sloughs, lakes and wetlands, a piece of land has been converted into an open-pit landfill to dispose of refuse of a nearby population center. Solid waste continues to pile up, creating a leachate mound in the landfill that serves as a driving force for vertical and lateral loading of contaminants into a sand and gravel aquifer and the sloughs, respectively. Local residents are concerned about the impact on the surface water (used for fishing and recreational purposes) and groundwater (used for drinking water supply).

1. Using an analytical approach, calculate the flux of leachate into the slough (surrounding the landfill on all 4 sides) and into the aquifer. Also calculate the travel time from the landfill to the sloughs and underlying aquifer. Sub-steps include:

  • Calculate the effective hydraulic conductivities for flow between the landfill and sloughs (horizontal directions) and between the landfill and aquifer (vertical directions)
  • Calculate the specific discharges in the horizontal and vertical directions
  • Calculate the total volumetric flux in the horizontal and vertical directions, Qx and Qz
  • Calculate leachate (or seepage) velocity in the horizontal and vertical directions
  • Calculate travel times in the horizontal and vertical directions

2. Develop a MAGNET groundwater model to simulate the transport of the leachate that has entered the aquifer after a long time. Treat the landfill as a continuous source (constant concentration of 300,000 mg/L), and assume that the flux into the aquifer does not change with time. Sub-steps include:

  • Use a time-step of 365 days and 80 model cells in the horizontal direction (NX=80). Make the model simulation length very long (e.g., 365000 days).
  • To properly represent the leachate flux into the aquifer from the landfill above, use the ‘Recharge—Quantity & Quality’ option in the Prescribed Sources and Sinks tab of the Zone Attributes menu.
    • The rate of leachate ‘recharge’ can be assigned from your computed leachate flux in part 1. The leachate concentration is given.
  • Use the model to determine where the leachate moves after entering the aquifer
  • Use the model estimate the leachate travel time from the landfill to River 2.
  • Use the model to estimate the concentration of leachate in the groundwater when it first reaches River 2.

Additional information/instructions:

  • Leachate water level: 30 ft above mean sea level (amsl)
  • Slough water level: 10 ft amsl
  • Bottom of the slough and landfill: 2 ft amsl
  • Aquifer head beneath the landfill: 10.5 ft (inferred from the regional flow patterns)
  • Silt dike width: 30 ft
  • Silt thickness beneath the landfill: 50 ft
  • Effective porosity of the silt: 0.1
  • The silt is highly stratified in the vertical direction; each individual “microlayer” is isotropic
  • Hydraulic conductivities measured in bottom silt vertical borehole profile (every 5-feet intervals):
  • 1 ft/d
  • 10 ft/d
  • 0.1 ft/d
  • 0.01 ft/d
  • 5 ft/d
  • 0.2 ft/d
  • 35 ft/d
  • 2.0 ft/d
  • 0.03 ft/d
  • 5 ft/d
  •  Hydraulic conductivities measured in dike silt vertical borehole profiles (every 4-foot intervals)
  • 0.8 ft/d
  • 42 ft/d
  • 0.02 ft/d
  • 0.3 ft/d
  • 8 ft/d
  • 1.5 ft/d
  • 4.2 ft/d