In general, the measures taken for the restoration and aftercare of the Shuen Wan Landfill and the control of landfill gas and leachate should not be relied upon to ensure the safety of adjoining developments; however, it must also be acknowledged that the restoration works undertaken will have the effect of lowering the potential for an incident to occur off-site when compared to the historical situation.  Allowance for this has been made in the qualitative assessment undertaken.

The qualitative hazard assessment undertaken in the EIA has concluded that the potential for landfill gas and leachate to affect the proposed Project during the construction and operational phase is very low.  During the construction period, similar hazards may also arise related to the flammability or the potential asphyxiating properties of landfill gas and/or the potentially toxic nature of leachate.  Given the results of the qualitative risk assessment, it should be assumed that the risks due to the landfill gas or leachate during construction and operational phases is very low.  No mitigation measures are considered necessary.

This section of the Manual provides advice and recommendations for the avoidance of environmental impacts related to landfill gas and leachate/contaminated groundwater during the construction and operational phases.  The recommendations are made on a precautionary basis.

5.1.1 General Hazards Related to Landfill Gas and Leachate

Landfill Gas

All contractors or Towngas personnel participating in the construction and operation of the Pipelines and Towngas complex should be aware that potential of methane and carbon dioxide present in the soil and all works should be undertaken on the basis of an "assumed presence of landfill gas".  In addition the following properties of landfill gas should be noted.

* Methane is odourless and colourless, although in landfill gas it is typically associated with numerous highly odoriferous compounds, which gives some warning of its presence.  However, the absence of odour should not be taken to mean that there is no methane.  Methane levels can only be reliably confirmed by using appropriately calibrated portable methane detectors.

* Methane is a flammable gas and will burn when mixed with air between approximately 5 and 15% (v/v).  If a mixture of methane and air with a composition between these two values is ignited in a confined space, the resulting combustion may give rise to an explosion.  Methane is also an asphyxiant.

* Carbon dioxide, the other major component of landfill gas is an asphyxiating gas and causes adverse health effects at relatively low concentrations.  The long-term Occupational Exposure Limit (OEL) is 0.5% (v/v).  Like methane, it is odourless and colourless and its presence (or absence) can only be confirmed by using appropriately calibrated portable detectors.

* Gas density.  Methane is lighter than air whereas carbon dioxide is heavier than air.  Typical mixtures of landfill gas are likely to have a density close to or equal to that of air.  However, site conditions may result in a ratio of methane to carbon dioxide which may make the gas mixture lighter or heavier than air.  As a result, landfill gas may accumulate in either the base or top of any voids or confined spaces.   


The main problem associated with leachate is its potential for the corrosion of steel and concrete structures and pollution of receiving waters.  Leachate also presents a potential health risk to anyone who comes into contact with it.  In particular, it may cause severe irritation if there is contact with skin or eyes due to its acidic nature.  Many of the compounds likely to be present in the leachate are toxic, if present at a sufficiently high concentrations. 

5.1.2 General Recommended Precautionary Measures

Notwithstanding the development of other safety and protection measures and procedures, which will be required through the construction contracts or according to site conditions, the following recommendations should also be considered when works are to be carried out within the Consultation Zone of the Shuen Wan Landfill (Figure 5.1a).

* During all works, safety procedures should be implemented to minimise the risks of fires and explosions, asphyxiation of workers and toxicity effects resulting from contact with contaminated soils and groundwaters.

* Safety officers, specifically trained with regard to landfill gas and leachate related hazards and the appropriate actions to take in adverse circumstances, should be present on the site throughout the works, in particular, when works are undertaken below ground.

* All personnel who work inside the pipeline trench should be made aware of the possibility of ignition of gas in the vicinity of the works, the possible presence of contaminated water and the need to avoid physical contact with it.

* Monitoring for landfill gas should be undertaken when underground works (i.e. excavation below 2m ground level) are to be undertaken through the use of an intrinsically safe portable instrument, appropriately calibrated and capable of measuring the following concentrations:

– methane     0 to 100% (v/v)

– carbon dioxide   0 to 100% (v/v); and

– oxygen      0 to 21% (v/v)

* Enhanced personal hygiene practices including washing thoroughly after working and eating only in "clean" areas should be adopted where contact may have been made with any groundwater which is thought to be contaminated with leachate.


The construction of the Pipelines and Gas Receiver Station (GRS) will be undertaken in accordance with the construction contract, which specifies the relevant statutory and other recommended environmental, health and safety guidelines to be followed.  In addition, and specifically, the tenderers will be made aware of hazards associated with landfill gas and other issues relevant to construction in the vicinity of a landfill through the Materials and Workmanship Specification and further, will be provided with copies of this report in order to inform them of the likely significance of these issues and any additional recommended safety and precautionary measures that may need to be implemented during the construction phase.

Prior to works commencing, the Towngas should ensure through review and verification of the Contractors Safety Plan that adequate consideration has been given by the Contractor to the occurrence and management of emergency situations and that emergency plans have been developed to cover contingencies relating to landfill gas and leachate.

The construction contract also calls for Contractors to submit "method statements" or statements of working procedures to the Client prior to commencing specific activities on site. 

The method-of-working statement should cover, inter alia:

* monitoring proposed;

* emergency procedures, including fire fighting;

* supervisors responsibilities;

* storage and use of safety equipment;

* safety procedures; and

* signs, barriers and guarding.

It is recommended that the Towngas's Environmental and Safety Manager provides a review of all method statements, where there is a potential for landfill gas and leachate impacts to occur, to ensure that adequate measures are incorporated in the procedures to counter any possible adverse effects.  The Contractor should not be allowed to commence work prior to review by these persons and where recommendations are made, until appropriate adjustments to the work procedure as required by these persons have been incorporated and agreed.

Monitoring for methane, carbon dioxide and oxygen should be undertaken during the construction phase at work areas within the Consultation Zone of the Shuen Wan landfill where works involve deep excavations or entry into confined spaces (i.e. manhole).  The frequency and areas to be monitored should be documented in the Contractor's safety plan prior to commencement of the works.

All monitoring should be carried out by trained technicians and equipment calibrated according to the manufacturers instructions.  A standard record form, detailing the location, time of monitoring and equipment used, together with the gas concentrations measured, should be used to ensure all relevant data are recorded for future reference. 

Depending on the results of gas monitoring and the environment under which it is conducted (ie confined space or atmospheric works), appropriate actions will vary and should be stated as part of the Contractor's safety plan.

In emergencies, the following bodies should be contacted:

* Fire Services Department;

* Hong Kong Landfill Restoration Group Ltd; and

* EPD.


Periodically during the excavation works within the Consultation Zone of the Shuen Wan Landfill.   All measurements in excavations should be made within the monitoring tube located not more than 10mm from the exposed ground surface.  Monitoring should be undertaken as follows:

* At the ground surface before excavation commences;

* Immediately before any worker enters the excavation;

* At the beginning of each working day for the entire period the excavation remains open; and,

* Periodically through the working day whilst workers are in the excavation.


Depending on the results of the measurements, actions required will vary and should be set down in advance by the Safety Officer or other appropriately qualified person.  As a minimum these should encompass those actions specified in Table 5.1.

Table 5.1 Actions in the Event of Gas Being Detected in Excavations






< 19 %

Ventilate trench/void to restore O2 to > 19 %


< 18 %

Stop works

Evacuate personnel/prohibit entry

Increase ventilation to restore O2 to > 19 %


> 10 % LEL

Post “No Smoking” signs

Prohibit Hot Works

Ventilate to restore CH4 to < 10 % LEL


> 20 % LEL

Stop works

Evacuate personnel/prohibit entry

Increase ventilation to restore CH4 to < 10 % LEL


> 0.5 %

Ventilate to restore CO2 to < 0.5 %


> 1.5 %

Stop works

Evacuate personnel/prohibit entry

Increase ventilation to restore CO2 to < 0.5 %