Effect of Perched Water Conditions in MSW Landfills: Considerations for Landfill Operators

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1 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

2 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?

3 Consider a MSW Landfill MSW LCRS Liner

4 Install Gas Wells

5 Consider a MSW Landfill MSW LCRS Liner

6 Landfill Gas Well MSW LCRS Liner

7 Landfill Gas Well Water Surface LCRS Liner

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

9 Landfill gas well equipped with liquid pumping system

10 Pump repair and maintenance

11 Slope stability concerns Landfill Gas Well LCRS Liner

12 Slope stability concerns Landfill Gas Well LCRS Liner

13 Slope stability concerns Landfill Gas Well LCRS Liner

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

15 Landfill Gas Well Water Surface LCRS Liner

16 Landfill Gas Well Phreatic surface LCRS Liner

17 Landfill Gas Well LCRS Liner

18 Slope stability concerns Landfill Gas Well LCRS Liner

19 Slope stability concerns Landfill Gas Well LCRS Liner

20 Slope stability concerns Landfill Gas Well LCRS Liner

21 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

22 Gas Well

23 Soil Layer Perched Liquids

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

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27 Modified Version of Richard s Equation ( ) t S t C k z k z K r r kk r k r K s z r r + = = ψ ψ ψ ψ ψ ψ Richard s equation was solved using a USGS program called SUTRA

28 0 r S=0.2 z 10 S=

29 0 r ψ=0 ψ=2 m z 10 ψ=4 m 15 20

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

31 Landfill Gas Well Phreatic surface LCRS Liner

32 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

33 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

34 Consider a MSW Landfill with an Infiltration Pond MSW LCRS Liner

35 Consider a MSW Landfill with an Infiltration Pond MSW LCRS Liner

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

37 Consider a MSW Landfill with an Infiltration Pond MSW LCRS Liner

38 Consider a MSW Landfill with an Infiltration Pond MSW LCRS Liner

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

40 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

41 10-3 Hydraulic Conductivity (m/sec) Col 21 vs Col 22 At ~1400 pcy K = 8x10-5 cm/sec Density (t/m 3 )

42 Air permeability of waste at NRRL at different depths < >25.0 Number of Locations Air Permeability, k (X10-12 m 2 )

43 Air permeability of waste at NRRL at different depths < >25.0 Number of Locations Air Permeability, k (X10-12 m 2 )

44 Air permeability of waste at NRRL at different depths < >25.0 Number of Locations Air Permeability, k (X10-12 m 2 )

45 27 Gpd K=10-4 cm/s Compacted MSW 60 ft Leachate Collection System K=5X10-5 cm/s Bottom Liner

46 Depth (ft) Pressure (ft of w.c.)

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

48 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.

49 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.

50 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.

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

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

53 Injection Well #1 Data Station Injection Well #2 Cover Soil MSW VW piezometers

54 Contact Info Tim Townsend Pradeep Jain