Effect of Perched Water Conditions in MSW Landfills: Considerations - PowerPoint PPT Presentation

Effect of Perched Water Conditions in MSW Landfills: Considerations for Landfill Operators Timothy Townsend and Pradeep Jain Department of Environmental Engineering University of Florida 2005 SWANA Landfill Symposium Bolder, Colorado

Motivation • At last year’s landfill symposium, several presentations and audience comments described the issue of saturated waste layers in the deeper parts of landfills. • The presence and cause of these saturated layers can be interpreted differently. • The question that we asked: “what should one expect?”

Consider a MSW Landfill MSW LCRS Liner

Install Gas Wells

Consider a MSW Landfill MSW LCRS Liner

Landfill Gas Well MSW LCRS Liner

Landfill Gas Well Water Surface LCRS Liner

Implications of Perched Water • Problems with gas recovery? • Slope stability concerns? • Leachate collection system problems? • Future side slope seepage issues?

Landfill gas well equipped with liquid pumping system

Pump repair and maintenance

Landfill Gas Slope stability Well concerns LCRS Liner

Landfill Gas Slope stability Well concerns LCRS Liner

Landfill Gas Slope stability Well concerns LCRS Liner

The implications of the perched liquids depend on their true nature within the landfill

Landfill Gas Well Water Surface LCRS Liner

Landfill Gas Well Phreatic surface LCRS Liner

Landfill Gas Well LCRS Liner

Landfill Gas Slope stability Well concerns LCRS Liner

Landfill Gas Slope stability Well concerns LCRS Liner

Landfill Gas Slope stability Well concerns LCRS Liner

Let’s examine the scenario where only waste around the well is saturated Some source of water is added to the well at a rate greater that it can drain out. Possible sources: • Gas condensate • Perched zones of leachate in the landfill • Short circuiting from liquids addition

Gas Well

Perched Liquids Soil Layer

Vertical Injection Wells at New River Regional Landfill Flow 2 Flow 3 Flow 1

Modified Version of Richard’s Equation ∂ ∂ ψ ∂ ψ ∂ ∂ ψ     kK + + + =     r K k K k k ∂ ∂ ∂ ∂ ∂ r z     r r r r z z ∂ ψ ∂ ψ ( ) = ψ + C S ∂ ∂ s t t Richard’s equation was solved using a USGS program called SUTRA

r 0 5 10 15 20 0 S=0.2 5 S=1 10 z 15 20

r 0 5 10 15 20 0 5 ψ=0 ψ=2 m 10 ψ=4 m z 15 20

r (ft) 0.0 1.0 2.0 3.0 4.0 0 Simulation Parameters K = 10 -5 cm/sec 10 Q = 17 gallons/day z (ft) Duration of Moisture Addition = 20 10 days Head in the well ~ 0 ft 8 ft 1 ft 2 ft 3 ft 4 ft 30 5 ft

Landfill Gas Well Phreatic surface LCRS Liner

Let’s examine the scenario where saturated conditions will develop in the landfill even if barrier layers are not present • If the liquids are added to the landfill at a rate greater than the hydraulic conductivity, saturated conditions will result

Consider a Liquids Infiltration Pond • The waste underneath the pond will become saturated • In the absence of cover soil layers, a saturated zone will extend to the leachate collection system

Consider a MSW Landfill with an Infiltration Pond MSW LCRS Liner

Consider a MSW Landfill with an Infiltration Pond MSW LCRS Liner

Consider a MSW Landfill with an Infiltration Pond h + h d = i d d LCRS Liner

Consider a MSW Landfill with an Infiltration Pond MSW LCRS Liner

Consider a MSW Landfill with an Infiltration Pond MSW LCRS Liner

Consider a MSW Landfill with an Infiltration Pond Water Level MSW LCRS Liner

Can saturated conditions develop if the liquids are added at a rate less than the permeability of the waste? Decreasing Permeability • Yes, if the permeability of the waste is reduced with depth

10 -3 At ~1400 pcy Hydraulic Conductivity (m/sec) 10 -4 K = 8x10 -5 cm/sec 10 -5 Col 21 vs Col 22 10 -6 10 -7 10 -8 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00 Density (t/m 3 )

Air permeability of waste at NRRL at different depths 15 Number of Locations 12 9 6 3 0 <0.1 0.1-2.5 2.5-5.0 5.0-7.5 7.5-10.0 10.0-12.5 12.5-15.0 15.0-17.5 17.5-20.0 20.0-22.5 22.5-25.0 >25.0 Air Permeability, k (X10 -12 m 2 )

Air permeability of waste at NRRL at different depths 15 Number of Locations 12 9 6 3 0 <0.1 0.1-2.5 2.5-5.0 5.0-7.5 7.5-10.0 10.0-12.5 12.5-15.0 15.0-17.5 17.5-20.0 20.0-22.5 22.5-25.0 >25.0 Air Permeability, k (X10 -12 m 2 )

Air permeability of waste at NRRL at different depths 15 Number of Locations 12 9 6 3 0 <0.1 0.1-2.5 2.5-5.0 5.0-7.5 7.5-10.0 10.0-12.5 12.5-15.0 15.0-17.5 17.5-20.0 20.0-22.5 22.5-25.0 >25.0 Air Permeability, k (X10 -12 m 2 )

27 Gpd K=10 -4 cm/s Compacted MSW 60 ft K=5X10 -5 cm/s Leachate Collection System Bottom Liner

0 10 20 Depth (ft) 30 40 50 60 -8 -6 -4 -2 0 2 4 6 8 Pressure (ft of w.c.)

r (ft) 0.0 1.0 2.0 3.0 4.0 0 Simulation Parameters Decreasing K = 10 -5 cm/sec (top) to 5X10 -6 cm/sec (bottom at 60 ft 10 deep) Q = 8.5 gallons/day z (ft) Duration of Moisture Addition = 10 days 20 Head in the well ~ 5 ft 0 ft 1 ft 2 ft 30 3 ft 4 ft

Review • The existence of standing liquids in gas wells in landfills does not necessarily result from a phreatic liquid surface in the landfill. • Liquids added to wells as a result of perched layers in the landfill, gas condensate or other sources can result in relatively large depths of water in the well.

Review • The decreasing permeability of landfilled waste with depth should have impact. • Saturated waste conditions may be present, but the pressure of this water may not be accurately reflected by the depth of water that would be measured if a well was installed. • At large liquid addition rates, saturated conditions in deeper layers may develop.

Implications • The presence of liquids in gas wells in “dry” landfills should not automatically assumed to represent a phreatic surface. • In “wet” landfills, the liquid levels in wells may result from both situations. • When evaluating slope stability, careful thought must be given to the pressures that truly occur. • Leachate collection systems need to be designed and operated correctly.

New Experiment in Florida Bury piezometers in waste vertical well and horizontal trench

Well #1 Well #2 Injection Wells 10’ 5’ VW Piezometer Well 25’ Between Wells Current Bioreactor

Injection Injection Well #1 Well #2 Data Station Cover Soil 10’ MSW 35’ 20’ 30’ 5’ 40’ 10’ VW piezometers 20’

Contact Info • Tim Townsend – ttown@ufl.edu • Pradeep Jain – pjain@ufl.edu