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Advisory Council on the Environment

STRATEGIC ASSESSMENT AND SITE SELECTION STUDY FOR CONTAMINATED MUD DISPOSAL

 

(ACE-EIA Paper 4/2001)
For information




INTRODUCTION

1. The paper presents the findings of Agreement CE 105/98 entitled Strategic Assessment and Site Selection Study for Contaminated Mud Disposal (hereafter referred to as the Study") commissioned by the Civil Engineering Department (CED). Members' views are sought on the major recommendations. The Study was commissioned to provide a strategic assessment of potential contaminated mud management options and to recommend a suitable site or sites for the preferred options. The major findings and recommendations are summarised below, for detailed evaluation of environmental, engineering and planning factors please refer to Attachment 2, Strategy Selection Report dated 7 December 1999.

BACKGROUND

2. Notwithstanding the efforts to minimise contaminated mud arisings, the capacity of Contaminated Mud Pit (CMP) IV at East Sha Chau will be exhausted by late 2007 and hence, a new disposal facility must be provided, on or before that time, to ensure adequate capacity for contaminated dredged material disposal to the year 2010.

ARISINGS OF CONTAMINATED DREDGED MATERIAL

3. Hong Kong's new sediment assessment framework (Works Bureau Technical Circular No. 3/2000 - Management of Dredged/Excavated Sediment) incorporates an expanded list of contaminants of concern coupled with toxicity testing of sediments proposed for removal. Dredged materials will be classified for open sea disposal (Type 1 disposal), confined marine disposal (Type 2 disposal) or as requiring special treatment/disposal arrangements (Type 3 disposal). Implementation of the new framework will ensure that environmentally protective disposal practices are pursued and that the limited space in confined disposal facilities is managed efficiently. Type 2 disposal is currently available at the East Sha Chau disposal facility.

4. Since the commencement of this Study a number of factors have changed. As a result there is now too much uncertainty to identify a disposal option for the long term. However, there is a need to identify now, an intermediate disposal facility for the period 2007 - 2010 to accommodate approximately 8 Mm3 of contaminated marine sediments requiring Type 2 disposal. The changed factors include:
 

  • Several large projects have been scaled back or are being reviewed;
  • Contaminated Mud Pit IVc will be deepened to increase capacity and consequently is expected to receive material for a longer period; and,
  • Arisings of contaminated mud after 2010 are subject to many uncertainties. These include uncertainties about which projects will be occurring and whether they will generate material requiring Type 2 disposal. Another uncertainty is that it is unclear at present what the impact of the new sediment classification framework will be to arisings of material requiring Type 2 disposal. Although the findings of the Study have taken into account the likely impact of the technical circular it has yet to be tested in practice on a large scale.

5. Planning for contaminated mud disposal past 2010 is complicated by the uncertainties surrounding estimates of arisings, which, at this time (early 2001) are not clear enough to make an estimation of the size of facility required.

DISPOSAL OPTIONS

6. Contaminated mud disposal options including contained aquatic disposal (CAD), confined disposal facility (CDF), special treatment/disposal, and disposal outside of Hong Kong waters were considered.

7. Contained Aquatic Disposal (CAD) sites are those, such as the current Type 2 disposal facility at East Sha Chau, which involve filling a seabed pit with contaminated mud and capping it with uncontaminated material such that the original seabed level is restored and the contaminated material is isolated from the surrounding marine environment. The main environmental issues to consider when proposing particular sites for CAD are the dispersive characteristics of the site and its proximity to sensitive receivers. If materials are placed in the CAD through simple bottom dumping from barges, sediments will form plumes and disperse down-current possibly outside the pit and affecting sensitive receivers. Hong Kong's experience with the East Sha Chau CAD facility is substantial and given the extensive track record of monitoring, can be considered one of the most comprehensively documented programmes in the world. Other countries, such as the United States, Germany and Japan also have successfully employed CAD facilities for disposal of contaminated dredged material. As revealed from the available results of monitoring studies and other related assessments, there are no indications of unacceptable adverse environmental impacts caused by the CAD facility in Hong Kong. It was thus considered that new CAD facilities, engineered using similar principles, would have a similar level of environmental performance and cost-effectiveness if recommended as a Type 2 disposal option.

8. Confined Disposal Facilities (CDFs) are nearshore or island diked containment structures which, similar to CADs, serve to isolate contaminated dredged material, but extend up to and possibly above sea level. The environmental impacts of CDFs do not involve dispersion of plumes but relate primarily to the degree of containment in the adopted CDF design. In general, designs with greater control over contaminant migration pathways will have higher associated costs of construction, operation and maintenance. Features such as installing a liner to gather leachate, implementing a system to treat leachate and effluent, and/or controlling placement using an enclosed pipeline system can be used to mitigate contaminant release to the environment. CDFs have been developed in the Netherlands for both highly and moderately contaminated dredged material. All CDF options were considered appropriate for Hong Kong sediments based on the ranges of material types which have been disposed under similar operations in the Netherlands, the United States and Japan.

9. For the small predicted volumes of material requiring Type 3 special treatment/disposal material (less than 0.1% of the total arisings of contaminated mud), physical or chemical stabilisation and subsequent disposal at a CAD or CDF seemed the most practical and cost effective option for dredged material. Based on this assessment, CAD and CDF were the only options which were recommended for consideration as part of the overall strategy. Although other treatment and disposal options may be suitable for particular dredged materials, they should be considered on a case-by-case basis. For example, incineration with subsequent solidification and landfill disposal of the resulting ash may be suitable for materials with a high organic contaminant content.

10. A further option that was examined was disposal of contaminated dredged material in sites outside of Hong Kong waters. The only sites which could be cost-effectively utilised as disposal options for contaminated dredged materials from Hong Kong are those located within Mainland waters. Since the Mainland's preferred option for the disposal of dredged materials is ocean disposal, this assessment focused on the use of existing open sea disposal sites. The present Mainland practice of disposal of contaminated sediments at uncontained/dispersive marine sites would be inconsistent with Hong Kong's practice for waste disposal and is expected to be actively discouraged over the life of the strategy.

11. Three initial screening criteria designed to eliminate disposal options which are unsuitable or impractical for Hong Kong regardless of siting consideration were utilised, as follows:
 

  • implementation at the required scale;
  • appropriate, given the characteristics of Hong Kong's contaminated dredged material; and
  • consistent, if implemented, with all applicable Hong Kong legislation, regulations and policies.

12. The outcome of the screening process resulted in the following list of options for Hong Kong's future contaminated dredged material disposal strategy:
 

  • CAD - Constructed from an Existing Seabed Pit left after sand removal (Disposal Types 2 and 3 - treated);
  • CAD - Purpose-dredged Seabed Pit (Disposal Types 2 and 3 - treated);
  • Nearshore CDF (Disposal Types 2 and 3 - with or without treatment); and
  • Island CDF (Disposal Types 2 and 3 - with or without treatment).

For a full description of the disposal options selection process please refer to Attachment 1, Report on Assessment of Disposal Options dated 30 July 1999.

VIABLE SITES

13. Existing, potential and future incompatible uses within HKSAR waters were identified as screening criteria to exclude unsuitable areas. These included marine traffic constraints, depth constraints, future developments/uses, and sensitive environmental receivers (eg fish culture zones, known areas of high coral abundance or diversity, gazetted Marine Parks and Reserves, artificial reef areas, Country Parks, SSSIs, and gazetted beaches). Once the unsuitable areas were excluded the remaining areas were examined to see if it were possible to locate operable units that were of sufficient size for a disposal facility.

14. Evaluation of viable disposal options necessitated making an estimate of the area required for a contaminated mud disposal facility. Where water depth is between 5m and 20m a footprint of 2.4 km2 is required, and where water depth is greater than 20m a footprint of 7.3 km2 is needed. A total of 20 sites which have the potential to locate one of the contaminated mud disposal options were subsequently identified within Hong Kong waters (Figure 1).

SUITABLE ALTERNATIVES

15. The 20 sites were examined to determine their suitability for various options (ie existing pit CADs, purpose built CADs, island CDFs, and nearshore CDFs). The resulting site-option combinations (referred to as Alternatives) were subject to further evaluation in the suitability assessment. The criteria used in the suitability assessment covered environmental, engineering and planning issues:
 

water quality interference with marine traffic and risk of collision
dispersal characteristics ability to isolate contaminants as a function of cost
sediment characteristics ability to receive arisings
cumulative effects cost of construction and management
ecological characteristics ease and practicality of use and management
conflicts with beneficial uses technical uncertainty and risk of failure
procedural impacts potential environmental benefits
placement/berthing degree of compatibility with development plans

16. A categorisation system was applied to reflect the degree of suitability of the alternative for contaminated mud disposal. Alternatives which were assigned an unsuitable rating under any of the criteria were excluded from further consideration resulting in a total of thirteen suitable alternatives (Table 1).

Table 1 - Alternatives considered suitable for further evaluation as disposal strategies
 

Site Existing Pit CAD Purpose Built CAD Island CDF
North Brothers
tick
   
East Tung Lung Chau
tick
   
Airport East  
tick
tick
East Sha Chau  
tick
 
Airport West  
tick
tick
Hei Ling Chau  
tick
tick
Shek Kwu Chau  
tick
tick
Southern Waters  
tick
tick

STRATEGY SELECTION

17. Given that the thirteen suitable alternatives could be combined into a large number of potential strategy permutations, a series of principles were developed to guide the formation of strategies by excluding certain combinations as follows:
 

  • The strategy, whether it be composed of single or multiple alternatives, should be able to accommodate 8 Mm3 of mud and be operational, year round, by late 2007;
  • If two or more alternatives must be combined to form a strategy then they should be geographically close to each other;
  • Single alternative strategies are preferred to multiple alternative strategies on an environmental and administrative basis since they minimise disruption to the marine environment and its uses, and entail economies of scale in construction, management and monitoring.

18. Although the expectation is that CMP IVc will be operational until 2007, it is possible that the pit may be filled either earlier or later. Consequently, the intermediate facility must have a degree of flexibility. CAD facilities are more flexible than CDF, as a series of pits can be created within a CAD designated area, allowing for incremental provision of capacity, as the need arises, whereas for a CDF, which is a bunded facility that has to be constructed for the start of disposal, the ability to either expand (horizontally or vertically) or contract the capacity is limited and costly. A further issue is that CED are conducting a study (CE 46/2000) examining the feasibility of accommodating dredged and inert construction and demolition materials. Should an artificial island facility be selected under CE 46/2000, it could become a competitor with a CDF for public fill material and for a suitable site. Thus, although CDFs do have potential as a means of managing contaminated sediments in the long term, the preferred option for intermediate disposal is a CAD facility.

19. The application of these principles resulted in a list of seven CAD strategies: North Brothers, East Sha Chau, Airport East, Airport West, Hei Ling Chau, Shek Kwu Chau, and Southern Waters. These seven were further evaluated against the environmental, engineering, and planning criteria used for the suitability assessment, to generate a relative ranking amongst the seven strategies.

DISCUSSION

20. Southern Waters was disadvantaged because of the deep and exposed waters which are undesirable environmentally, affect cost and present technical difficulties in implementation. Airport West was disadvantaged because of its proximity to a newly designated nature reserve in Mainland waters. The CAD strategies at Shek Kwu Chau and Hei Ling Chau would be located close to the Cheung Sha Wan Fish Culture Zone (FCZ) and mariculturists could thus be expected to object strongly to either of these strategies. Both of these strategies have further constraints imposed upon them by marine traffic. Consequently, neither of these strategies are considered as preferred for future study and implementation.

21. The East Sha Chau and North Brothers strategies were both disadvantaged by their lying within what is now considered critical Hong Kong habitat for the Indo-pacific Humpbacked Dolphin. Although the monitoring results at East Sha Chau indicated that there were no direct or indirect impacts on the Indo-pacific Humpbacked Dolphin, strong resistance could be expected if further expansion is planned in this area.

RECOMMENDATION

22. The strategy of developing a purpose built CAD facility at Airport East is less constrained, and in terms of environmental rankings is regarded as the most suitable among the seven CAD options under consideration. A preliminary assessment based on the monitoring programmes and studies conducted for the general area has not revealed any insurmountable problems for this strategy. Nevertheless, its environmental acceptability would need to be confirmed via an Environmental Impact Assessment (EIA) under the EIA Ordinance (EIAO) before the commencement of detailed design, construction and operation. By containing operations to within the general vicinity of the existing contaminated mud disposal site for Hong Kong, the Airport East CAD strategy avoids the proliferation of disposal sites and builds on the existing knowledge base established through over seven years of site management and environmental monitoring. The site which is very shallow, is not regarded as critical habitat for the Indo-pacific Humpbacked Dolphin. The Airport East CAD thus emerged as the preferred strategy for material requiring either Type 2 or Type 3 (treated) disposal. The site does, however, lie outside the existing gazetted mud disposal area and so, after completion of the EIA process, gazetting would be required under the Foreshore and Seabed (Reclamations) Ordinance and a Dumping at Sea (Exemption) Order. It is likely that public consultation during the EIA process would identify and address most of the concerns likely to emerge during the gazettal procedures.

THE WAY FORWARD

23. This strategy for contaminated mud disposal from 2007 to 2010 will now be subject to an Environmental Impact Assessment (EIA) and Engineering Feasibility Study. The tentative programme is to initiate the EIA in accordance with the EIA Ordinance in December 2001.

ADVICE SOUGHT

24. Members are invited to note and comment on the recommended strategy for the disposal of contaminated mud from 2007 to 2010.

ATTACHMENTS

Attachment 1 - Report on Assessment of Disposal Options dated 30 July 1999
Attachment 2 - Strategy Selection Report dated 7 December 1999
Attachment 3 - Strategy Development Report dated 22 May 2001

Civil Engineering Department
June 2001

 

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