Landfill Leachate Seepage Capture by Barrier Wall and Recovery Wells Next to the North Saskatchewan River

Heidi Lovett, Kathryn Wilneff, Paul Morton

Dans les comptes rendus d’articles de la conférence: GeoNiagara 2021: 74th Canadian Geotechnical Conference; 14th joint with IAH-CNC

ABSTRACT: In 2017, the City of Edmonton was notified to halt an uncontrolled release of landfill leachate-impacted groundwater, originating from the closed Clover Bar Landfill, observed as an iron-stained, 40-metre seepage line along the lowermost riverbank of the North Saskatchewan River. Operating between 1975 and 2009, the closed landfill is now part of the Edmonton Waste Management Centre, a 230-hectare, liquid and solid waste processing and research facility. A groundwater pathways investigation was completed by historical aerial photographs and geophysics (EM31 and ERT), that identified river terrace mining for aggregates within the landfill's pre-construction footprint. Historical trenches were identified between open-pit areas and the riverbank, apparently for pit drainage, potentially representing preferential pathways for leachate-impacted groundwater leaving the landfill and moving to the riverbank. A four-well trial groundwater capture system was implemented in 2017-2018, uphill of the riverbank seepage. The trial yielded a low capture volume, attributed to a conventional well design being mismatched to sheet-like, hillside, groundwater flow, along the overburden-bedrock contact. Recognizing the uppermost groundwater flow regime as being mostly interface flow became key to the successful leachate interception and reduction in riverbank seepage. A full-scale capture system was commissioned in 2019, hydraulically re-designed to capture interface flow by a combined barrier-and-well approach, extended horizontally and vertically to minimize bypass flow and underflow. A foundations caisson-type auger rig 'rock socketed' concrete piles and boreholes into competent shale bedrock, completed as a 60-metre long secant-pile wall and companion line of metre-diameter capture wells. Each capture well was winterized and provided with a level-controlled automated pump-out system to maintain drawdown-capture performance. Groundwater disposal records for 2020 quantified daily interception rates as being 12 cubic metres (summer) and two cubic metres (winter). Only minor riverbank seepages were observed in 2020; capture system optimization and riverbank monitoring are continuing into 2021.

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