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Closed Landfill Management Plans Robertson, R; Simeonov, J; Willmot, C

Closed Landfill Management Plans

Martin Robertson, Senior Environmental Scientist Montgomery Watson Harza Johan Simeonov, Solid Waste Planning and Performance Manager, Wellington City Council

Charles Willmot, Senior Policy Advisor, Ministry for the Environment Abstract The New Zealand Waste Strategy calls for contaminated sites, including closed landfills to have been identified and for at least half of them to have been subject to a rapid screening system by 2008. It also requires the development of a remediation programme for those considered to be high risk. The intention of this paper is to give a generic overview of the issues involved in generation of Management Plans for Existing Closed Landfills and addresses specific topics that are considered necessary to achieve environmental compliance. The components of a successful management and closure plan could assist in the development of a Management Plan for closed landfills or form part of an application for resource consent. The topics covered contain background information in support of the requirement for a closure plan for existing waste disposal sites. The document is expected to assist in the inspection, monitoring, control, maintenance and remediation activities of closed landfills including water, air and soil pollution. It also presents an after care management plan for maintaining the integrity and effectiveness of the final cover, operating and maintaining the leachate and gas collection system, and ongoing monitoring of the groundwater quality. Introduction The MfE landfill survey identified over 320 operating landfills in 1995 and in 2002 this has dropped to 115. By 2010, it is predicted that there will be only 43 operating landfills left in New Zealand. These will tend to be large and servicing regional catchments. The trend to regionalisation of landfill capacity has resulted in a large number of landfill closures resulting in a number of potentially contaminated sites. It is recognised that many sites in New Zealand have not yet been adequately characterised and assessed. The New Zealand Waste Strategy calls for contaminated sites, including closed landfills, to have been identified and for at least half of them to have been subject to a rapid screening system by 2008. Landfill “closure” involves the completion of all engineering aspects of the original design and or measures to meet standard best practice for landfill completion. The level of engineering required to attain “closure” has increased in recent years and many older sites will need to undergo a characterisation, risk assessment and “closure” process before post-closure management can truly commence.


To ensure the proper performance and post-operational control of a closed landfill, it is important to ensure that the landfill is "closed", and maintained to meet all regulatory and statutory requirements. This can be achieved during the post-closure period through continuous monitoring, maintenance and asset improvement. Post-closure care involves the preservation of the integrity and effectiveness of the final cover and any amenity plantings, operation and maintenance of the leachate collection system, monitoring groundwater quality and maintaining and operating the gas collection system. Post-closure plans must also allow for the monitoring of proposed after-use of the site, physical investigation and checking of any structures which have been erected on or near the boundary if the closed landfill and provide an emergency management plan in case of adverse environmental effect or natural disaster. Plan Objectives It is important for each closed landfill to have a Post Closure Management Plan which meets the following minimum objectives:

• To manage each landfill in accordance with RMA and HSNO act. • To ensure compliance with the relevant Resource Consent. • To manage the site in a manner consistent with MfE closed landfill guidelines. • To provide specific measures for full reclamation and reestablishment of vegetation • To establish long term site specific monitoring, maintenance and improvement

programme • Integrated Emergency Management Plan

To address this and to meet the above objectives it is essential that each closed landfill have a site specific management plan addressing environmental compliance and gauging its risk to human health and the environment. Our experience in assessment, closure and post-closure management leads us to conclude that a plan should have the general format described in the following sections. Owner / Operator (Management) Responsibility Once a landfill is closed, it is the owners responsibility to manage and address all issues related to the post closure care as long as the risk of potential adverse environmental and health issues remain. It is the closed landfill’s owner responsibility under the Health and Safety in Employment Act to ensure that any activities, which might be carried out with the predetermined area, are preceded by proper occupational health and safety instructions. Priority needs to be given to compliance with the health and safety rules to ensure:

(i) The protection of human life on and around the site


(ii) Working conditions to ensure that any civil engineering or building activities within the identified area may require a special mitigation measures to prevent subsidence, fire and explosion, or the release of dangerous substances

(iii) Setting of work safety rules for hazardous tasks (iv) Emergency first aid procedures, and (v) Adequate training is available

Relevant signage regulating the access or referring to site related hazards must form part of the overall safety measures and are the sole responsibility of the closed landfill owner/operator. It must be recognised that the Management Plan will have to be reviewed periodically to meet any new performance procedures and standard or regulatory and statutory changes throughout the post-closure period for the landfill. Accordingly the organisation responsible for the landfill will need to ensure that appropriate expertise is involved in future reviews. General Site Characteristics: Natural and Built Environment This section of the plan needs to include information on natural features and values. Sensitive receptors and receiving environments must be identified and considered in the closure planning process. These include site geology, hydrogeology, groundwater resources, marine environments, fresh water streams and other water bodies, as well as active users of former landfills such as contact sports, and the users of buildings on or near former landfills, particularly those with potential gas accumulation areas. The design of any structures on the landfill needs to be documented in terms of their purpose, design life and fitness for purpose. Specific discussion must be included on how recognised hazards have been mitigated or avoided in design.. The plan should carefully document the location and nature of all current and abandoned services crossing the site. Landfill Engineering Information A detailed description of the design, function, and operation of all environmental control systems needs to be included, together with as-built plans and specifications where appropriate. In particular, information should be available on the design and operation of the stormwater collection and treatment system, leachate collection and treatment system and gas collection and treatment system if provided. Issues often occur long after the staff involved have left the employment of the owner or operator. The detail included in the plan needs to be sufficient for someone who has no previous connection with the site, to quickly gain familiarity and understand the reasons why certain design parameters were used and decisions were made.


Resource Consents Under Section 15 of the RMA, the discharges of contaminants, including the discharge of water into water, require a resource consent unless such discharges are specifically provided for (as a permitted activity) in a regional plan. The need for a consent will depend on the requirements of individual regional plans. Most authorities consider that a discharge is still occurring from closed landfills if the owner has control over the discharge. As an example the discharge of leachate from a landfill could clearly be mitigated by an impermeable cap and thus it can be argued that the site owner is continuing to allow a discharge to occur. Potential Hazards (Risks and Risk Mitigation) Potential hazards at closed landfills include:

• Landfill gas – explosion and asphyxiation risks in confined spaces or depressions • Sharp objects – cuts and infection from exposed refuse • Toxic and ecotoxic effects from any groundwater or surface water contamination from

leachate • Toxic and ecotoxic effects from any soil contamination which may affect the health of

site occupiers, utility workers and vegetation growing on site or near to the site • Caving in of poorly compacted material in excavations • Subsidence and collapse of capping over voids (eg rusted out car bodies or pockets of

organic wastes). • Landslides / Slope failures

The level of risk at a site is a function of the nature of the potential hazard and the degree of exposure. The degree of exposure is tied strongly to site sensitivity and presence of receptors. If there is no exposure pathway or no receptor there is no risk. Accordingly risk can be managed by physical mitigation measures or institutional controls. Physical mitigation measures need to be identified in the closure planning process and documented in the management plan. A realistic timetable for action and a review period should be identified. Institutional Controls Service, maintenance and excavation workers are most often the most significant receptors. It is clear that they are exposed to a high level of risk when working on landfill sites. Strict controls must be placed on work practices, workplace monitoring and health and safety. Service trenches and any confined spaces close to landfills or connected via the conduit of a service trench near to landfills should be routinely monitored for flammable or asphyxiating gases. Specific procedures should be appended to the management plan to provide guidance to service, maintenance and excavation workers.


Guidance should be provided to Utilities on how to avoid the creation of gas migration pathways. A work permit system put in place to ensure that services crossing landfills do not collect and focus gas discharges. Redevelopment of landfill sites has occurred in the past due to a lack of knowledge of the risks associated with closed landfills. These sites typically present the highest level of risk. They are also the most difficult to mitigate. There are numerous examples of houses located on landfills that have been affected by settlement, soil contamination or gas migration. Often in the past, landfills have been built on the outskirts of town. As the town has expanded and land has become more expensive, development has spread beyond the town limits and infill development has meant that the risk profile of the site has changed substantially. It is to be expected that this trend will continue, and that landfills being closed today, in favour of larger, more regional facilities, will be subjected to the same pressures of infill development as those already closed. Any redevelopment is beyond the scope of the management plan and should be subject to a specific risk assessment, development of appropriate mitigation measures and updating of the management plan to reflect the new risk profile. Engineering Closure Requirements Few closed landfills will meet all of the recommendations contained in recent landfill guidelines. Accordingly the management plan must set out the engineering requirements to achieve “closure”. A site-specific risk assessment should be carried out to ensure that resources are utilised efficiently to achieve the objectives for post closure management and if necessary to prioritise mitigation measures. Some examples of typical deficiencies that may need to be addressed are outlined below: Insufficient cover is a very common issue at older closed landfills. Cover tends to be to be highly variable and contaminated with refuse and sharp material. This is less common where the sites have been redeveloped for sports fields and have often been recapped several times to correct differential settlements. Cover problems are more common where the sites have been retired to passive recreation areas or left as waste land. For example we have seen situations where excavators tracking across a closed landfill in wet weather brought broken glass to the surface in a children’s playing area. It is sometimes possible to source large volumes of cleanfill from one-off greenfield developments and achieve a win-win situation with developers by allowing free tipping in return for shaping and topsoiling, although care must be taken with such arrangements that the sites are not turned into defacto landfills.


If the cover is deficient the management plan will need to provide specifications and plans showing where cover must be supplemented. A timetable for the work should also be outlined. In the photo to the right the cap is relatively thin and has been eroded along the coast due to inadequate slope protection. Leachate discharges are common at older sites but were also highlighted in the latest MfE landfill inspection and audit. It was identified that of 115 operational landfills only 55 (47%) had leachate collection systems in operation. A risk assessment will need to be conducted and consents sought to cover any ongoing discharges to the environment, whether this is from a formal leachate collection system or not. It is critical that the risk assessment consider the geology, hydrogeology, any groundwater resource values and potential impact on receptors or the environment. The management plan must set out the engineering requirements needed to mitigate the discharge, if required under the consent.

The photo on the left shows a common scenario for older unlined gully fill type landfills. Leachate in an anaerobic state contains high levels of iron in solution. The Eh/Ph regime changes drastically when the leachate is aerated and bright orange iron-oxy-hydroxides precipitate. There has been considerable research into low cost on-site leachate treatment systems in recent years and this

work has shown that even remote sites can be retrofitted with effective low cost systems. Cascade aeration systems and sand filters are effective in mitigating this type of contamination. Gas discharges can occur for many years beyond the theoretical gas production window. The need for gas control measures will again be a function of the sensitivity of the site and the rate of gas production. All gas control measures should be documented clearly in the management plan and an operations and maintenance plan appended where appropriate. The picture on the right illustrates the gas production potential remaining in a 30 year old landfill. The partially decomposed paper is from 1972. Clearly a dry tomb landfill could be reactivated by changes in landfill moisture content. For example settlement of


stormwater services could result in increased water infiltration leading to renewed gas production. It is suggested that a buffer distance around the site be identified to ensure that future development near the site boundary is not impacted by landfill gas. Any existing development must be addressed at this time too. An example of such an issue, identified during the landfill closure risk assessment is illustrated below.

Landfill gas is present in the area where the photograph on the left was taken giving an unintended significance to the advertising sign. The area in the foreground was inadvertently zoned residential. This inappropriate zoning was detected before the land was sold and the land will now be placed in reserve status. This example highlights the importance of accurately recording fill boundaries as subsequent earthworks can confuse the situation.

Subsidence and slope issues may develop over time particularly in response to changes in drainage. Differential settlements can cause drainage problems but do not tend to be critical. Of more concern are stormwater systems beneath fills. These may deteriorate over time and in the illustrated case lead to a piping failure that opened a large hole on a sports field. Similar, although less drastic scenarios are possible where pockets of degradable waste are present for example former liquid waste soakage pits, offal pits and the like. The structural integrity of the landfill form should be addressed and in particular the manner in which landfill faces were formed should be documented. For example the difference between structural bunds and hardfill facings on refuse slopes can have significant stability implications. The sites should be checked for static and dynamic (including seismic) stability and internal water levels may need to be determined if an initial screening assessment indicates that this could be critical. Permitted and Restricted Activities The presence of landfill material at a site necessitates restrictions on future development. Consideration needs to be given to the age of the landfill when assessing restrictions. A precautionary approach is warranted in the absence of records or investigations. The Closed


Landfill Guidelines provide some guidance but may not prove conservative enough with respect to gas issues. The following activities should generally not be permitted until a risk assessment has been completed:

• Smoking • Habitable structures • Camping • Greenhouses • Use of tools that are sparking or not

intrinsically safe • Open fires

• Slab on grade or other structures without a crawl space or subfloor ventilation

Specific landfill hazard assessment should be carried out before finalising a site-specific list of restricted activities. Monitoring The MfE closed landfills guidelines provide an indicative monitoring schedule. The management plan should contain a succinct set of field procedures, GPS co-ordinates, location plans and photographs of sampling locations. A comprehensive monitoring plan can be finalised only after the final management closure plan has been compiled and all relevant parameters related to gas, leachate and groundwater model for pollutants have been assessed. Key receptors need to be identified and the monitoring designed to be protective of these receptors and provide sufficient warning of increased gas flux or contaminant breakthrough before it affects a receptor. Closed sites need to be inspected on a regular basis. The frequency of inspection will depend on the level of engineering and attendant maintenance requirements, the land use and the age of the site. Initially inspection frequency should be monthly and progressively reduced commensurate with the frequency with which issues are identified by inspections. Significant earthquake events and storm events should also be triggers to a site inspection. The monitoring programme would typically include:

• Monitoring of the integrity of the site engineering (gas vents, leachate collection, stormwater etc)

• Construction and other disturbances that occur on the landfill during the post-closure period

• Water quality monitoring (leachate, streams and or groundwater) • Monitoring for gas migration (e.g. site offices and buildings, monitoring bores,

electrical or pipe conduits, manholes and utility service corridors) • Regular assessment of capping integrity and vegetation.


The monitoring frequency will be based on the degree of landfill stabilisation and gradually reduce with time if warranted. Reports on the inspections and monitoring will need to be prepared for action by the owner/operator, when necessary. A simple Site Inspection Checklist could ensure that the closed landfill is properly maintained with the appropriate preventive measures or follow up action. The requirements and timing of any inspections should be detailed in the Management Plan. A monitoring report should be prepared at an appropriate frequency as specified in the management plan. It should contain all monitoring data consistently tabulated and formatted electronically. The information should be published in a manner that will aid interpretation of data. All relevant quality assurance protocols, data records, charts, maps and diagrams should be attached and form an integral part of the overall report Any closed landfill monitoring programme should be subject to regular review in order to limit or increase the monitoring parameters, redefine the purpose and goals and optimise the cost of monitoring. Consideration should be given to independent review. Maintenance and After Care It is important that a clear distinction should be made between the terms “aftercare” and “maintenance”. Aftercare refers to the post closure phase of a landfill and involves monitoring and maintenance procedures and systems. However some aspect of the monitoring process are used to identify maintenance needs to maintain the landfill at an appropriate standard. The main objective of a long term aftercare and maintenance plan is to preserve the integrity and effectiveness of those features of the site that protect the environment and communities from adverse effects. Maintenance schedules should be included in the management plan and should be adjusted where site monitoring identifies a need. Emergency Response Plan It is the duty of owner/operator to ensure the integrity of the closure measures and to detect and prevent any adverse impacts of the site on public health, safety and the environment. An Emergency Response Plan is specifically structured to formulate the appropriate response procedures in case of events, which might result in an emergency situation. The basic aim is to have the relevant response measures which will mitigate adverse environmental and health effects By establishing procedures to respond to any emergency such as fire the owner/operator should be in a position to deal with an environmental and health emergency without undue delay.


Typically in carrying out this evaluation each potential failure event which can create an adverse environmental or health impact is analyse in the Failure Event Tree for the following critical areas:

• failure mode • environmental and health impact • response action plan.

Failure modes are based on Risk Assessment and are site specific but some are common to all closed landfills as they are identified with the properties and environmental effects related to closed landfill sites. Typically therefore the following failure modes should be included in any Emergency Response Plan:-

a) Slip failure. b) Escape of leachate/Contaminant escape to surface water. c) Leachate infiltration of groundwater. d) Unacceptable level of Gas migration to external receptors

To illustrate the process a typical Slip Failure Event Tree could be constructed in the following pattern:

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By conducting those assessments, the owner/operator can demonstrate ‘due diligence’ in its effective management of specific emergency event. This will be crucial in case of any adverse environmental or health impact as a result of failure if the owner faces charges related those impacts. The proper application of the failure event tree assessment methodology for emergency response measures could also result in the implementation of various accident preventive measures including perimeter safety enhancement. Records All site changes need to be documented and accurately recorded. Wherever possible the management plan itself should be updated to reflect these changes and act as a repository for all critical information about the site. Monitoring records should be kept in an electronic data base so that long term trends can be evaluated and exceedances of “trigger levels” can be quickly acted upon. Summary The following steps are recommended to ensure that a post closure management plan is effective:

• Characterise the site and document all critical features (engineering, sensitive receptors etc) • Undertake a risk assessment and determine “closure” requirements • Prioritise and schedule identified mitigation and maintenance measures such as capping

supplementation and correction of differential settlements, leachate control and treatment, gas migration control, subsidence repairs and slope stability

• Communicate requirements and protocols to any significant receptors such as neighbours, site workers and maintenance/service workers

• Identify an appropriate inspections schedule, peer review schedule and reporting framework • Obtain “closure consents” • Document all requirements in a post closure management plan • Provide for regular review of the management plan and ensure that risk assessments are

updated where land uses or community expectations change

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