Water Balance and the Influence of Soil Structural Changes on Final Covers for Landfill Closure

1. Water Balance and the Influence of Soil Structural Changes on Final Covers for Landfill Closure Melissa Salt, University of Adelaide Mark Jaksa, University of Adelaide Jim Cox, CSIRO Paul Lightbody, Tonkin Consulting

2. Research Objectives • Measure and compare the drainage to determine if phytocovers reduce drainage to the same extent as conventional covers • Correlate changes in drainage patterns over time from the phytocover and conventional cover changes with changes in bulk density, soil water characteristic curve and permeability • Assess the tendency for anthropomorphic soil to tend towards the natural profile of the borrow source or toward a new stable profile • Determine the effect of changes in bulk density, soil water retention curve and permeability on the predictability of the water balance as estimated from pre-construction laboratory testing

3. Water Balance • P = ET + R + L + D + ΔS Precipitation (P) Evapotranspiration (ET) Runoff (R) Soil moisture storage (S) COVER Lateral flow (L) Drainage (D) WASTE Leachate

4. Earthen berms Runoff collection Soil cover layer Root barrier 1.5 mm LLDPE geomembrane Drainage layer Drainage collection Interim cover Monitoring nest not to scale Methodology – Field Scale • Precipitation - Weather station • Runoff - Flow meters • Drainage and lateral flow - Tipping bucket rain gauges • Soil moisture content – MP406 • Soil suction – CS229

10. Methodology – Small Scale • 1 m x 1 m x 1.5 m deep • Replicate conventional and phytocover from Adelaide, including plants • Irrigation and measure drainage • Destructively sample 1 box of each cover type every 6 months • Analyse samples for bulk density, soil water characteristic curve and permeability

11. Methodology – Core samples • To measure any change in permeability or bulk density as a result of alternate saturation and then drying the soil layers proposed to use in the Adelaide A-ACAP trial. • Prepared core samples at known bulk density of phytocover soil and clay barrier • Wet using falling head permeability apparatus until saturated and hydraulic conductivity measured • Dry in oven at 30 oC until equilibrated and observe shrinkage and cracking • Repeat process

12. Water Balance Modelling Moisture content at depth through conventional profile over first 3 years modelled

13. Water Balance – Adelaide Site

14. Water Balance Predictions Predicted annual runoff volumes when surface soil permeability is reduced by one order of magnitude

15. Results – Adelaide Soil Moisture

16. Results – Adelaide Topsoil Moisture Content

17. Results – Melbourne Soil Moisture

18. Outcomes • Water balance comparison of phytocaps for Australian environment • Variability of water balance predictions from selected models • Quantification of soil structural changes in the short to medium term • Impact of soil structural changes on sustainability of the phytocaps • Determination of best soil input parameters for pre-construction modelling