TABLE OF CONTENTS Apr 2010 - 0 comments

Landfill Technology: Natural Attenuation

Managing natural attenuation landfills

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By: Darren MacNeil
2010-04-01

The swamp reluctantly released its grip on my steel-toe rubber boots as I lurched my way to a tiny yellow flag implanted on the bank of the wetland. The flag marked a 2" diameter clear plastic tube that conveyed a barely perceptible flow of groundwater, known as a seep, from the bank. In this mosquito infested location, it took a painfully slow 47 seconds to fill a 40 ml vial. This record will be used in a pilot study to evaluate how the recently installed leachate collection trenches are impeding the seep flow into the wetland. As an employee of the Region of Durham, it's my responsibility to oversee the implementation of a remedial action plan to mitigate the effects the Brock Landfill is currently having on the wetland. Dealing with past landfill siting mistakes to ensure a positive outcome is a difficult feat, especially in the field of waste management.

When the Region of Durham was formed in 1974, it assumed ownership of seven landfills that can be defined as natural attenuation sites. One common characteristic of these types of landfills is that they operated for decades before current regulatory requirements for landfill design and operation were enacted. The legacy of these historical landfill siting decisions presents many challenges to municipalities today as they assume responsibility for these landfills, whether active or inactive.

The primary challenge with managing a natural attenuation landfill is created at the very inception of the landfill itself. Historically, landfill operations were situated in areas that, at the time, were considered to have little economic and/or ecological value: areas in or near wetlands, streams, abandoned gravel/sand pits, and rural areas far removed from developed communities. The proximity to wetlands and streams would later prove to be an environmental liability because the gravel and sand pits used as landfills provided porous subsurface pathways for leachate migration.

The Brock Landfill is a prime example of the lack of concern for the environmental issues that existed when dumping began in the former gravel pit in the 1950s. Data from boreholes at the Brock Landfill indicate that sections of the waste fill area are directly underlain by a layer of fine to coarse sand and/or are directly submerged in the water table. One could compare this soil profile to be as effective at attenuating leachate as a coffee-maker without a filter. In addition, nearly a third of the approved one hundred acre site is covered by the Gibson Hill Swamp, identified by the Ministry of Natural Resources as a Provincially Significant Wetland. It's reasonable to assume that the ecological importance of the wetland was not recognized at that time.

Another problem facing many small "legacy" landfills is development encroachment. Landfills originally situated in remote areas have slowly acquired neighbours in the form of farms and subdivisions. The buffer areas required for the natural attenuation process have been reduced or have entirely disappeared. The Brock Landfill site abuts a wetland to the south and east, and a former farming property to the north. To obtain more breathing room, the region purchased the property to the north to eventually establish a contaminant attenuation zone. The site is now located in a predominantly agricultural area. Again, one of the legacies of an older landfill is that no consideration was given as to how the surrounding area would change over time. The proximity of other land uses presents municipalities with additional engineering challenges of modifying natural attenuation landfills to mitigate off-site impacts and protect adjacent properties and their occupants.

To address the deficiencies in design and operation of these landfills, municipalities can implement voluntary groundwater monitoring programs, purchase adjacent properties to re-establish buffer zones, and initiate remedial work to mitigate leachate and landfill gas migration. However, it's almost certain that the usual challenges associated with the operation and post-closure care of modern landfills will be amplified by the deficiencies inherent in historical landfill siting practices.

In 2006, Durham was issued a Provincial Officer's Order to address off-site impacts to adjacent properties originating from the Brock site. In response, the region retained Conestoga-Rovers & Associates to prepare a Remedial Action Plan (RAP) and a Site Operations Report (SOR).

The RAP contains five primary remedial components, all of which are interdependent and interconnected: land acquisition to provide additional land space for natural abatement and future control measures; shaping and contouring of the site to effectively maximize storm water runoff; an engineered cover system to minimize infiltration and reduce the volume of leachate generated; a surface water system to convey storm water to the wetland, control sediment discharge, and prevent erosion; and a leachate management system to intercept, collect, and treat leachate. The objective of the RAP is to augment a natural attenuation site with engineered controls.

Currently, a temporary re-circulation system, consisting of two collection trenches terminating at one end to a pump, and connected to a retention pond by forcemains, has been constructed in order to conduct a pilot study to determine how much leachate is generated and how it will be treated. Based on field data, a report that evaluates the leachate treatment options available will be submitted to the environment ministry for approval and eventual amendment to the CofA. Including consulting fees and construction costs, the pilot study to date has cost nearly $450,000. The property north of the site was purchased by the region to establish a contaminant attenuation zone, for approximately $950,000, and the budget estimate to implement the remaining components of the RAP is over $10 million.

Since each of the remedial components of the RAP are interconnected, it's necessary to establish how the leachate will be treated before the overall design can be finalized. The current RAP schedule allows for a year for design finalization, required ministry approvals, and the procurement process, to precede actual construction. Construction is scheduled over a three-year period to accommodate the current proposed plan to contour the site by importing 80,000 tonnes of waste or engineered fill over that period.

There are also financial implications to be considered given that projects cannot proceed without the capital to support them. Fortunately, Durham is able to support the active and inactive landfills in its jurisdiction with responsible voluntary groundwater monitoring, consultant reports, and dedicated employees committed to the perpetual care of the landfills. Smaller municipalities may not have the resources or funding to provide the appropriate and necessary care of the landfills for which they're responsible.

Provincial requirements that old landfills be brought up to current standards should be accompanied by provincial funding to municipalities struggling to meet these standards. In April 2009, a program was announced by Ontario's environment ministry to assist six municipalities with the upgrades required to meet mandatory regulations in the process of capturing landfill gas emissions. Although this is a step in the right direction, many more municipalities require assistance.

When one considers the impact of the Brock Landfill in isolation, it may not seem like much; however, the cumulative effects of hundreds of similar landfills are significant. Durham has seven landfills in its perpetual care program, and all of them to various degrees have off-site impacts.

Darren MacNeil, B.Sc., works in the Waste Management Department of the Regional Municipality of Durham, Ontario. Contact Darren at darren.macneil@durham.ca

Photos


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Contractors examine the clear stone backfill in the leachate collection trench constructed for the pilot study.
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Contractors covering a leachate collection pipe with clear stone and a geotextile.
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A pilot-scale leachate collection system under construction at the Region of Durham's Brock Landfill.
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A leachate seep entering the wetland area adjacent to the Brock Landfill's inactive fill area, identified with a yellow flag as No. 4 for the pilot study
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