By Hydrosimulatics INC  

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.    calculate the flux of leachate (seepage flux and contaminant flux, assuming a  constant "indicator" contaminant concentration of 300 mg/l) into the sloughs surrounding the landfill (on all 4 sides). Also calculate the travel time from the landfill to the sloughs.

2. calculate the vertical flux of leachate and travel time into the underlying aquifer

3. simulate leachate offsite migration in the deep aquifer using MAGNET 


For saturated flows, it is well known that the equivalent parallel hydraulic conductivity is equal to the arithmetic mean of all individual hydraulic conductivities of the layers parallel to flows, and the equivalent perpendicular hydraulic conductivity is equal to the harmonic mean of all individual hydraulic conductivities of the layers perpendicular to flows.

Figure 1: Landfill and local hydrogeology. (Top) Conceptual representation, cross-section view and (bottom) plan view.

Additional field information:

  • 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

MAGNET/Modeling Hints:

  1. 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).
  2. 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.
    1. The rate of leachate ‘recharge’ can be assigned from your computed leachate flux in part 1. The leachate concentration is given.
  3. Use the model to determine where the leachate moves after entering the aquifer
  4. Use the model estimate the leachate travel time from the landfill to River 2.
  5. Use the model to estimate the concentration of leachate in the groundwater when it first reaches River 2.