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18-Env-A6 Solid Waste Engineering and Management · May 2014

Question 16 of 22: Definition Sketch for a Landfill Water Balance

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

National Examination, May 2014 — 04-Env-A6 / 18-Env-A6, Solid Waste Engineering and Management. 3 hours duration, closed book, no calculator, one letter-sized aid sheet permitted. All 22 questions constitute a complete paper (100 marks total).

Reference texts: Tchobanoglous, Theisen & Vigil, Integrated Solid Waste Management: Engineering Principles and Management Issues; Vesilind, Worrell & Reinhart, Solid Waste Engineering; Davis & Cornwell, Introduction to Environmental Engineering (6th ed.); CCME, Guidance Document on Landfill Gas Management; Canadian Environmental Protection Act, 1999.

Question 16: Definition Sketch for a Landfill Water Balance (5 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Leachate formation is assessed with a water balance drawn around the refuse mass as a control volume: precipitation falling on the final cover ($P$) either runs off the cover surface (surface runoff, $RO$), is returned to the atmosphere through evaporation and plant uptake (evapotranspiration, $ET$), or infiltrates through the cover soil into the refuse below. That infiltrating water is first held in storage as the refuse's moisture content rises; only once the refuse reaches its field capacity ($FC$, the maximum moisture it can retain against gravity) does any further infiltration pass through as percolation to the base of the fill, where it is collected as leachate ($L$).

Refuse Mass (Water-Balance CV)(CV boundary — dashed)RO: SurfacerunoffP: Precipitationon coverET: Evapo-transpirationL: Leachate(once fieldcapacityexceeded)
Definition sketch: water balance around the refuse mass. Precipitation (P) enters at the cover; surface runoff (RO) and evapotranspiration (ET) leave without ever reaching the refuse; the remainder infiltrates and is stored until the refuse's field capacity is exceeded, after which percolation is collected as leachate (L).

In balance form: $P = RO + ET + \Delta S + L$, where $\Delta S$ is the change in moisture storage within the refuse (zero once the refuse is at steady-state field capacity, so that all further infiltration reports directly as leachate).