Bamboo bridge access for drill pad formation
In Hong Kong, rainfall patterns appear to be changing, with more frequent and longer rainfall events. Rainfall infiltration has increased within the Pillar Point Valley Landfill, along with leachate production. With more than 7.5 million people, solid waste management is a complex undertaking for Hong Kong’s Environmental Protection Department. The department manages three open and 13 closed landfills.
The Pillar Valley Landfill in Tuen Mun in northwest Hong Kong received municipal solid waste from 1983 until 1996. This landfill is now closed and restored. Its operator, SUEZ NWS, continues to collect and treat leachate--water that has percolated through the landfill and leached out contaminants.
With more frequent and longer rainfall events, the surrounding catchment area of Pillar Point Valley Landfill was producing more groundwater, which subsequently resulted in an increase in weak leachate. SUEZ NWS retained Associated Engineering to undertake a multi-phase hydrogeological assessment of the landfill, and develop strategies to remove groundwater upgradient of the landfill to minimize leachate generation.
“The landfill generates both strong and weak leachate. The strong leachate is driven by the movement of fluids through the waste mass, and the weak leachate originates from the mixing of groundwater and leachate at the landfill’s base, where the original leachate and groundwater collection systems are closely located." - Project Manager, Norman Di Perno
The project consisted of five phases. Phase I involved visiting the site, and analyzing a past hydrogeological study as well as historic rainfall and leachate and groundwater chemistry. Norm advises, “We recommended using level loggers within select monitoring wells to help establish how quickly the fractured bedrock aquifer responds to rainfall events.”
Typical output from geophysics survey with drilling targets
During Phase II, we conducted a site-wide geophysics program, including magnetics, electro-magnetics, and resistivity surveys. This analysis of the subsurface bedrock structures identified the areas of structural weakness which had the highest probability of conducting groundwater into the downgradient landfill mass. The results helped to improve the placement of groundwater extraction wells.
Phase III involved the design and installation of 10 groundwater extraction wells (each are 200 millimetre diameter and 145 metres deep), camera logging, pump tests to establish sustainable well yields, and the design of the well pumping systems and controllers. Phase IV included ordering and arranging transportation of the equipment, and training staff to install the equipment.
Development of extraction well
“The data acquired from the telemetric logger installation helps us to understand the dynamics around rainfall, groundwater level rise and the response time or lag between them, and if pumps should be adjusted to pump at lower or higher rates during wet and dry seasons.” - Senior Hydrogeologist, Jacques Groenewald
Phase V involved additional geophysics (magnetics), eight additional groundwater extraction wells, and support for installation of the pumping systems. Associated is also supporting SUEZ NWS to install an InSitu down-hole logger system with a satellite connection allowing near real time review of the groundwater levels in select pumping wells.