7. Landfill Gas Hazard Assessment

7.1 Introduction

7.1.1 This section aims to present a qualitative assessment of landfill gas (LFG) hazard posed by Tseung Kwan O Stage I Landfill (TKO Stage I) on Road D1, Road D8, Road D10 and Extended Road P2. These roads form part of the major access road network of Tseung Kwan O New Town.

7.2 Government Legislation and Standards

7.2.1 Environmental Protection Department (EPD) has issued two guidance notes regarding LFG hazard assessment, namely ProPECC PN 3/96 - Landfill Gas Hazard Assessment for Development Adjacent to Landfill and Landfill Gas Hazard Assessment - Guidance Note.

7.2.2 These two guidance notes set out the conditions under which a LFG hazard assessment should be carried out and provide guidance on undertaking LFG hazard assessment. The guidance notes recommend that in general, assessment of LFG hazard is required for proposed developments that lie within the 250-m Consultation Zone around a landfill.

7.2.3 Since part of Road D1 and the whole of Road D10 are located within the 250-m Consultation Zone of TKO Stage I, a landfill hazard assessment is required to be undertaken for these two roads.

7.2.4 The two guidance notes recommend that at the planning stage of a development (when no detailed site investigation at the development site has been carried out), the LFG hazard assessment should be of qualitative nature.

7.3 Description of the Environment

Proposed development

7.3.1 Road D1, approximately 1.5 km, runs from south of the western roundabout of Road D4, alongside the eastern boundary of Area 65 (immediately west of the Eastern Drainage Channel) and the southern boundary of Areas 65, 66 and 67 and is connected to the northern roundabout of Road P2. Road D1 would also separate Areas 65, 66 and 67 from the possible future development to the south of Road D1.

7.3.2 Road D10, approximately 1.3 km, connects Road D1 to Wan Po Road, running on structure across the Eastern Drainage Channel, along the southwestern boundary of TKO Stage I (Area 77). Road D10 separates Area 86 from Area 77.

7.3.3 The locations of Road D1, Road D10 and TKO Stage I (together with the 250-m Consultation Zone) are shown on Figure 7.1.

TKO Stage I Landfill

History

7.3.4 TKO Stage I constitutes a marine reclamation covering approximately 68 ha. Landfilling commenced in 1979 directly onto marine sediments. Northern and southern sections of seawall were then constructed from quarry rock, without prior dredging. The remaining connecting section of seawall was constructed some years later, and was intended to be more permanent in nature. Separating the seawall from the landfill is a 17.5-m wide soil margin.

7.3.5 Between 1979 and 1989, the landfill received approximately 11.9 million tonnes of waste material. The site was then covered over its northern and western sections with a restoration cap of 2.6 m thick. The cap composed of topsoil, completely decomposed volcanic (CDV) material, and granular drainage material. However, the southern and eastern sections of the site were covered only by a thin layer of CDV material in anticipation of further waste deposition. Both planted and self-seeded vegetation was established across the site. The vegetation showed no signs of stress from LFG before a large part of it was removed for additional waste input.

7.3.6 In 1992 the Civil Engineering Department (CED) let a contract for further on-site development at TKO Stage I. The works comprised the stripping of the previous capping material for re-use, and further reclamation to the south of the site. An embankment was also constructed along the southern boundary to allow further waste disposal. A permanent seawall was also constructed approximately 100 m from the previous seawall providing capacity for a further 3.2 million m³ of waste. Areas of this new section were lined in an attempt to prevent direct discharges of leachate to the sea. Following the completion of these works, the landfill was to be capped with 1 m of inert cover and hydro-seeded.

Restoration Works

7.3.7 The restoration works for TKO Stage I commenced in August 1997 and completed in January 1999. The works include installation of a covering cap, a landfill gas collection system with flaring and electricity generation, a leachate collection and treatment system, surface and sub-surface drainage systems, and works to increase geotechnical stability. It has been proposed that the reinstated landfill would be developed to a golf course and some rugby/ soccer fields.

Landfill Capping

7.3.8 The components of the landfill restoration capping system include the following (from top to bottom):

· A 850 mm thick soil layer comprising completely decomposed volcanic (CDV) or completely decomposed granite (CDG) and an additional 650 mm CDV is also provided in the location where trees or shrubs are provided (Figure 7.2);

· A subsoil drainage layer comprising a synthetic drainage medium, surrounded by suitable geotextile filters.

· An impermeable layer (anchored in CDV at perimeter) comprising a 1 mm thick low density polyethylene (LDPE) geomembrane; and

· The final intermediate layer of 500 mm thick soil above the waste is well compacted and free from stones or other sharp particles.

Landfill Gas Management System

7.3.9 The landfill gas management system consists of active extraction wells, electricity generation with LFG, flaring system for LFG, passive vent trenches/ pipes, and monitoring of LFG both on and off-site. The LFG gas extraction system is integrated with the leachate management system.

7.3.10 Active gas extraction from the waste is used to relieve the pressure build-up in the waste to prevent LFG from migrating off-site laterally or vertically. The extracted LFG with over 45% methane content is used to generate electricity for on-site usage. Other extracted LFG is collected to a flare stack and burnt. The gas flare is situated to the southeast of the landfill (Figure 7.2).

Leachate Management System

7.3.11 TKO Stage I is an old landfill without a line system and leachate collection system under most of the landfill. A capping system was installed on the landfill to prevent any infiltration of surface runoff into the landfill. A leachate extraction system was installed to the west of the landfill to control the level of leachate on the landfill slope. The leachate extraction/ management system is integrated with the LFG management system and consists of catchpits, soakaway pits, trapezoidal channels (filled with granular material), subsoil drains, leachate drains, and outfall pits. The collected leachate is stored temporarily at the gas/ leachate compound at the south of the landfill (Figure 7.2). It is then transferred to a leachate treatment facility built at the TKO Stage II/III Landfill for treatment before discharge to the sewage treatment plant at Area 85.

Geological Assessment

Geology and Hydrogeology at TKO Stage I Landfill

7.3.12 TKO Stage I is a marine reclamation landfill occupying an area of about 68 hectares. According to Hong Kong Geological Survey, Sheet 12, Series HGM20, Edition 1 – 1989 (scale 1:20,000) (shown in Figure 7.3), the bedrock of the bay comprises tuffs, ash and lavas of the Upper Jurassic age. The landfill was extensively reclaimed from the sea, with waste deposited above marine mud.

7.3.13 Groundwater flows westward and south-westward from the ridge of Clear Water Bay Country Park. Groundwater levels fall significantly from the ridge to the landfill, and finally to the west of the landfill (Eastern Drainage Channel) and the southwest of the landfill (i.e. Junk Bay). The hydraulic conductivity of the strata is moderate, at approximately 10^-6 – 10^-8 ms^-1.

7.3.14 Leachate depth is greatest towards the middle of the landfill where the deposits of waste material are thickest. The hydraulic gradient within the landfill slopes downward towards the sea.

Geology and Hydrogeology at Proposed Development

Road D1

7.3.15 Since Road D1 will be on reclaimed land, the geological formation underneath this road is anticipated to be fill composed of natural earth overlying marine mud.

Road D10

7.3.16 Again, since Road D10 will situate on reclaimed land, the geological formation underneath this road is anticipated to be fill/sanitary fill composed of natural earth and waste.

7.4 Assessment methodology

7.4.1 This qualitative LFG hazard assessment was undertaken following the method recommended in Landfill Gas Hazard Assessment - Guidance Note. This method is based on the “Source – Pathway – Target” model. The meaning of these three components are as follows:

· Source – location, nature and likely quantities/ concentrations of LFG which has the potential to affect the development.

· Pathway – the ground and groundwater conditions, through which LFG must pass in order to reach the development.

· Target – elements of the development that are sensitive to the effects of LFG.

7.4.2 The LFG source, identified pathway(s), and identified target(s) are then categorised in order to facilitate the assessment process.

7.4.3 The present landfill gas assessment based on the “Source – Pathway – Target” model only focuses on the hazard associated with landfill gas migration for Roads D1 and D10. Another potential impact posed by landfill gas in relation to impact of heat from the gas flare to motorists using Road D10 is not covered by the present assessment. As such, a further specific assessment for this potential impact is required at the detailed design stage of Road D10.

7.5 Qualitative Source-Pathway-Target Analysis

Landfill Gas Source

7.5.1 Landfill gas generation is dependent upon a number of factors, including: temperature, pH, substrate availability, moisture content and oxygen level. Calculations by others[1] have indicated that the LFG generation rate peaked in 1988, producing a collectable volume of some 120 million m³yr^-1. LFG generation was predicted to decline rapidly after this peak. Assuming moisture content of 25%, the annual production rate by 1999 was predicted to be less than a third of the peak rate, at less than 40 million m³yr^-1.

7.5.2 As part of the Restoration Contract for TKO Stage I, a post-restoration monitoring programme commenced in late 1997 on a monthly basis. LFG (including carbon dioxide) and oxygen have been monitored at some monitoring wells installed to the east and the south of the landfill (within the site boundary of the landfill). The locations of these monitoring wells are shown on Figure 7.4. The available gas monitoring data are tabulated in Tables 7.1 to 7.14. It should be noted that monitoring well D165 had been destroyed prior to the restoration works for TKO Stage I and is no longer in use. Besides, monitoring well 1DG7 has not been constructed. Therefore, no monitoring data are available for these two wells.

7.5.3 Tables 7.10 to 7.14 show that elevated levels of methane (maximum 18.0 % v/v) have been detected at few of the monitoring wells installed along the southern boundary of the landfill (i.e. monitoring wells 1DG1 to 1DG6) in 1998, indicating LFG migration to the south of the landfill (i.e. Road D10). However, the methane levels detected at these wells have dropped dramatically to 0.0 % in most cases, with the highest only 0.7 % (at 1DG3 in October 1999).

7.5.4 From July 2000 to July 2001, significant levels (> 5%) of carbon dioxide were recorded for more than one occasion in monitoring wells 1DG3, 1DG4, 1DG5 and 1DG6, all of which are located very close to or along Road D10. In accordance with sections 3.9 and 3.10 of the Landfill Gas Hazard Assessment Guidance Note, the source of LFG at TKO Stage I Landfill is therefore considered as major.

Table 7.1 LFG Monitoring Data for Monitoring Well D113

Date	CO2 % (v/v)	CH4 % (v/v)	O2 % (v/v)	Pressure (mbar)	Temperature (^oC)
9 Feb 98	4.4	0.0	17.7	2.0	21.6
20 Apr98	5.4	0.0	15.3	0.0	26.5
10 Jul 98	1.1	0.0	14.5	2.0	31.5
23 Jul 98	1.2	0.0	14.5	1.0	32.9
6-7 Aug 98	2.0	0.0	18.2	1.0	31.6
19 Aug 98	1.4	0.0	14.5	2.0	32.5
11 Sep 98	2.4	0.0	18.1	1.0	32.3
24 Sep 98	1.2	0.0	14.5	1.0	32.9
14 Oct 98	2.1	0.0	17.9	1.0	29.8
29 Oct 98	1.4	0.0	14.5	2.0	29.4
10 Nov 98	2.3	0.0	17.8	0.0	30.4
25 Nov 98	3.4	0.0	15.2	1.0	25.2
12 Dec 98	3.2	0.0	10.3	0.0	20.4
23 Dec 98	2.8	0.0	11.3	0.0	22.6
12 Jan 99	2.0	0.0	16.9	0.0	24.0
22 Apr 99	4.5	1.1	13.8	0.0	23.4
14 Jul 99	4.2	0.6	14.5	1.0	31.2
26 Oct 99	6.1	0.0	11.8	1.0	36.2
10 Jan 00	6.1	0.0	16.1	0.0	21.8
15 Apr 00	7.0	0.0	0.7	0.0	25.9
25 Jul 00	6.5	0.1	2.7	0.0	28.6
26 Oct 00	7.4	0.0	3.4	0.0	29.3
26 Dec 00	5.4	0.0	12.4	0.0	26.9
16 Feb 01	5.9	0.0	13.1	0.0	27.6
14 Apr 01	7.9	0.0	0.2	0.0	22.5

Table 7.2 LFG Monitoring Data for Monitoring Well D114

Date	CO2 % (v/v)	CH4 % (v/v)	O2 % (v/v)	Pressure (mbar)	Temperature (^oC)
9 Feb 98	0.0	0.0	20.2	3.0	21.7
10 Mar 98	0.0	0.0	19.9	0.0	18.0
28 May 98	0.0	0.0	20.3	1.0	26.6
19 Jun 98	0.0	0.0	20.4	1.0	29.1
26 Jun 98	0.0	0.0	20.4	1.0	29.3
10 Jul 98	0.0	0.0	20.2	3.0	33.3
23 Jul 98	0.0	0.0	20.4	2.0	33.6
6-7 Aug 98	0.0	0.0	20.3	0.0	31.4
19 Aug 98	0.0	0.0	20.2	1.0	33.5
11 Sep 98	0.0	0.0	20.4	1.0	29.3
24 Sep 98	0.0	0.0	20.7	0.0	33.6
14 Oct 98	0.0	0.0	20.8	0.0	30.0
29 Oct 98	0.0	0.0	20.4	0.0	30.1
10 Nov 98	0.0	0.0	20.4	0.0	26.8
25 Nov 98	0.0	0.0	20.4	0.0	25.6
12 Dec 98	0.0	0.0	20.1	1.0	20.3
23 Dec 98	0.0	0.0	20.3	0.0	23.4
14 Jul 99	0.0	0.0	20.4	0.0	29.2
24 Sep 99	0.0	0.0	20.2	0.0	29.4
16 Dec 99	0.3	0.0	19.9	0.0	18.5
21 Mar 00	0.0	0.0	20.7	0.0	28.2
21 Jun 00	0.0	0.0	28.4	0.0	-
23 Sep 00	0.0	0.0	20.2	0.0	33.1
26 Dec 00	0.0	0.0	19.8	0.0	28.5
16 Feb 01	0.0	0.0	19.6	0.0	26.2
14 Apr 01	0.3	0.0	18.8	0.0	25.5

Table 7.3 LFG Monitoring Data for Monitoring Well D115

Date	CO2 % (v/v)	CH4 % (v/v)	O2 % (v/v)	Pressure (mbar)	Temperature (^oC)
10 Mar 98	4.8	0.0	12.5	0.0	18.1
28 May 98	4.9	0.0	11.8	0.0	25.7
19 Jun 98	0.0	0.0	20.4	0.0	28.3
26 Jun 98	0.0	0.0	20.4	0.0	28.4
10 Jul 98	2.6	0.0	17.8	0.0	30.2
23 Jul 98	4.3	0.0	13.4	1.0	32.0
6-7 Aug 98	4.9	0.0	11.8	0.0	31.2
19 Aug 98	2.6	0.0	17.8	1.0	32.4
11 Sep 98	2.0	0.0	18.0	0.0	28.4
24 Sep 98	3.6	0.0	14.3	2.0	32.4
14 Oct 98	3.3	0.0	16.2	2.0	29.2
29 Oct 98	2.4	0.0	18.0	0.0	29.8
10 Nov 98	2.2	0.0	18.2	0.0	25.4
25 Nov 98	4.5	0.0	15.5	1.0	24.8
12 Dec 98	0.1	0.0	20.0	0.0	16.1
23 Dec 98	0.0	0.0	20.4	0.0	21.3
27 Feb 99	2.8	0.0	16.8	0.0	24.9
21 May 99	2.8	0.0	16.9	3.0	26.2
12 Aug 99	3.4	0.0	14.3	0.0	26.2
21 Nov 99	2.0	0.0	17.1	2.0	21.6
16 Feb 00	3.5	0.0	16.6	0.0	17.4
9 May 00	4.4	0.0	11.7	0.0	25.8
15 Aug 00	5.9	0.0	10.5	0.0	31.4
6 Nov 00	1.3	0.0	18.6	0.0	24.6
23 Jan 01	1.9	0.0	20.3	0.0	20.2
2 Mar 01	0.2	0.0	19.5	0.0	23.1

Table 7.4 LFG Monitoring Data for Monitoring Well D118

Date	CO2 % (v/v)	CH4 % (v/v)	O2 % (v/v)	Pressure (mbar)	Temperature (^oC)
9 Feb 98	0.6	0.0	19.1	4.0	22.8
10 Mar 98	3.6	0.0	13.4	0.0	17.7
14 May 98	4.1	0.0	12.6	0.0	27.1
28 May 98	1.6	0.0	17.9	0.0	27.0
19 June 98	3.4	0.0	13.4	2.0	39.8
26 June 98	2.0	0.0	16.8	1.0	29.4
10 July 98	4.1	0.0	12.5	1.0	29.3
23 July 98	4.1	0.0	12.6	1.0	33.4
6-7 Aug 98	1.6	0.0	17.9	0.0	30.8
19 Aug 98	4.4	0.0	12.5	0.0	30.3
11 Sept 98	1.6	0.0	16.8	1.0	29.4
24 Sept 98	4.1	0.0	12.8	2.0	33.4
14 Oct 98	2.1	0.0	17.4	2.0	28.5
29 Oct 98	3.6	0.0	16.4	1.0	29.6
10 Nov 98	3.2	0.0	15.8	1.0	25.8
25 Nov 98	3.8	0.0	15.4	2.0	25.0
12 Dec 98	0.0	--	20.3	0.0	24.3
23 Dec 98	4.2	0.0	12.6	1.0	22.1
12 Jan 99	3.2	0.0	16.8	1.0	23.6
22 Apr 99	1.9	0.0	18.3	1.0	23.6
14 Jun 99	2.8	0.0	17.2	0.0	31.0
14 Jul 99	2.2	0.0	18.2	0.0	30.9
24 Sept 99	1.6	0.0	18.2	0.0	28.0
26 Oct 99	1.7	0.0	18.3	1.0	33.3
16 Dec 99	1.0	0.0	19.3	0.0	18.5
10 Jan 00	1.2	0.0	19.1	0.0	22.8
21 Mar 00	1.2	0.0	19.5	0.0	26.2
15 Apr 00	2.4	0.0	16.6	0.0	28.3
21 Jun 00	1.8	0.0	27.9	0.0	-
25 Jul 00	1.5	0.0	17.2	0.0	30.6
23 Sep 00	1.8	0.0	18.5	0.0	30.7
24 Oct 00	2.5	0.0	17.0	0.0	27.7
26 Dec 00	0.2	0.0	19.4	0.0	24.5
16 Feb 01	0.3	0.0	19.3	0.0	23.5
14 Apr 01	1.3	0.0	17.7	0.0	26.2

Table 7.5 LFG Monitoring Data for Monitoring Well D120

Date	CO2 % (v/v)	CH4 % (v/v)	O2 % (v/v)	Pressure (mbar)	Temperature (^oC)
9 Feb 98	8.4	0.1	8.9	5.0	19.6
10 Mar 98	12.0	16.7	6.5	1.0	16.9
14 May 98	11.0	0.9	0.0	0.0	26.2
28 May 98	1.8	0.3	16.5	1.0	25.1
26 Jun 98	0.0	0.0	20.4	0.0	29.7
10 Jul 98	0.0	0.0	20.3	0.0	29.7
23 Jul 98	2.4	0.0	18.1	1.0	33.8
6-7 Aug 98	0.0	0.0	20.4	0.0	29.8
19 Aug 98	0.0	0.0	20.3	0.0	31.7
11 Sep 98	0.0	0.0	20.4	0.0	29.7
24 Sep 98	3.1	0.0	16.2	2.0	32.6
27 Feb 99	0.0	0.0	20.0	0.0	24.8
22 Apr 99	1.3	0.0	19.0	0.0	24.9
21 May 99	4.9	0.0	15.1	3.0	26.3
14 Jun 99	0.2	0.0	19.9	---	---
14 Jul 99	0.3	0.0	19.6	1.0	32.8
12 Aug 99	8.2	0.0	5.6	8.0	26.8
24 Sep 99	2.1	0.0	17.2	1.0	28.1
26 Oct 99	6.7	0.0	11.6	1.0	31.0
21 Nov 99	3.5	0.0	16.6	2.0	24.1
16 Dec 99	0.2	0.0	20.1	0.0	20.3
10 Jan 00	0.0	0.0	20.0	0.0	23.0
16 Feb 00	0.1	0.0	19.8	0.0	16.6
21 Mar 00	12.7	0.0	4.3	0.0	26.5
15 Apr 00	9.2	0.0	1.9	0.0	25.3
9 May 00	0.6	0.0	19.2	0.0	25.5
21 Jun 00	3.8	0.0	27.5	0.0	-
25 Jul 00	0.8	0.0	18.7	0.0	29.8
15 Aug 00	0.3	0.0	19.4	0.0	31.7
23 Sep 00	0.3	0.0	20.1	0.0	28.3
24 Oct 00	8.4	0.0	4.0	0.0	27.6
6 Nov 00	3.6	0.0	17.1	0.0	24.0
22 Jan 01	2.8	0.0	18.8	0.0	22.2
2 Mar 01	1.2	0.0	18.4	0.0	24.8

Table 7.6 LFG Monitoring Data for Monitoring Well D126

Date	CO2 % (v/v)	CH4 % (v/v)	O2 % (v/v)	Pressure (mbar)	Temperature (^oC)
10 Mar 98	9.1	0.0	6.8	1.0	16.3
28 May 98	8.0	0.0	8.4	2.0	26.0
19 Jun 98	0.0	0.0	20.3	0.0	30.8
26 Jun 98	0.0	0.0	20.3	0.0	30.7
10 Jul 98	6.2	0.0	7.7	5.0	29.3
23 Jul 98	5.4	0.0	18.6	2.0	33.8
6-7 Aug 98	3.4	0.0	15.8	1.0	29.8
19 Aug 98	5.2	0.0	8.7	4.0	30.3
11 Sep 98	4.0	0.0	15.3	0.0	30.7
24 Sep 98	4.8	0.0	12.7	2.0	33.7
14 Oct 98	4.0	0.0	13.6	2.0	29.6
29 Oct 98	4.2	0.0	10.3	3.0	28.7
10 Nov 98	2.6	0.0	17.2	1.0	26.2
25 Nov 98	4.2	0.0	15.7	0.0	24.8
12 Dec 98	9.2	0.0	8.1	1.0	22.4
23 Dec 98	8.9	0.0	9.8	1.0	20.6
27 Feb 99	3.4	0.0	13.4	0.0	25.8
21 May 99	5.2	0.0	13.8	4.0	26.6
12 Aug 99	8.8	0.0	10.8	0.0	26.6
21 Nov 99	5.0	0.0	13.7	2.0	22.9
16 Feb 00	4.0	0.0	15.9	0.0	16.9
9 May 00	7.7	0.0	11.9	0.0	24.6
15 Aug 00	0.0	0.0	19.7	0.0	28.8
6 Nov 00	3.4	0.0	15.6	0.0	23.4
23 Jan 01	0.1	0.0	22.5	0.0	20.4
2 Mar 01	6.6	0.0	13.0	0.0	21.8

Table 7.7 LFG Monitoring Data for Monitoring Well D130

Date	CO2 % (v/v)	CH4 % (v/v)	O2 % (v/v)	Pressure (mbar)	Temperature (^oC)
9 Feb 98	2.2	0.0	18.4	2.0	20.9
20 Apr 98	3.0	0.0	17.5	0.0	23.7
28 May 98	2.7	0.0	16.8	2.0	27.5
19 Jun 98	0.0	0.0	20.3	1.0	28.9
26 Jun 98	0.0	0.0	20.0	0.0	29.0
10 Jul 98	2.6	0.0	16.9	1.0	30.4
23 Jul 98	2.1	0.0	18.2	1.0	32.1
6-7 Aug 98	0.0	0.0	20.4	0.0	29.6
19 Aug 98	3.6	0.0	14.9	2.0	32.4
11 Sep 98	0.0	0.0	20.3	0.0	28.9
24 Sep 98	2.1	0.0	18.2	0.0	32.0
14 Oct 98	0.2	0.0	19.8	1.0	28.4
29 Oct 98	2.8	0.0	12.4	2.0	30.6
10 Nov 98	3.8	0.0	11.4	2.0	26.4
25 Nov 98	3.1	0.0	15.8	1.0	25.2
12 Dec 98	5.6	0.0	14.3	1.0	22.2
23 Dec 98	4.9	0.0	15.2	0.0	22.3
27 Feb 99	3.2	0.0	14.8	1.0	27.6
21 May 99	1.3	0.0	19.1	4.0	27.0
12 Aug 99	0.2	0.0	19.9	0.0	27.3
9 May 00	0.1	0.0	19.6	2.0	27.7
15 Aug 00	0.0	0.0	19.8	0.0	32.4
6 Nov 00	0.2	0.0	19.8	0.0	25.3
23 Jan 01	0.8	0.0	21.7	0.0	22.1
2 Mar 01	0.3	0.0	18.9	0.0	25.6

Remark:

D130R is used to replace D130 which was decommissioned since June 2000. D130R is located about 30m away from the original location of D130.

Table 7.8 LFG Monitoring Data for Monitoring Well D157

Date	CO2 % (v/v)	CH4 % (v/v)	O2 % (v/v)	Pressure (mbar)	Temperature (^oC)
9 Feb 98	2.2	0.0	17.9	4.0	17.3
10 Mar 98	2.9	0.0	17.3	1.0	17.0
28 May 98	3.4	0.0	15.8	2.0	25.4
19 Jun 98	1.4	0.0	19.2	0.0	29.2
26 Jun 98	3.4	0.0	15.8	2.0	28.9
10 Jul 98	2.3	0.0	18.2	1.0	29.4
23 Jul 98	4.2	0.0	12.1	3.0	32.6
6-7 Aug 98	3.2	0.0	15.8	1.0	29.6
19 Aug 98	2.3	0.0	18.2	1.0	30.4
11 Sep 98	3.4	0.0	15.8	2.0	28.9
24 Sep 98	4.2	0.0	13.8	1.0	32.9
14 Oct 98	3.6	0.0	14.2	3.0	30.4
29 Oct 98	2.8	0.0	17.1	2.0	30.4
10 Nov 98	2.2	0.0	14.8	1.0	27.0
25 Nov 98	2.8	0.0	17.3	1.0	25.6
12 Dec 98	0.3	0.0	20.0	0.0	23.2
23 Dec 98	0.2	0.0	20.2	0.0	22.3
27 Feb 99	2.4	0.0	17.6	2.0	23.8
22 Apr 99	4.2	0.0	14.7	0.0	24.8
14 Jun 99	3.7	0.0	14.2	---	---
14 Jul 99	3.8	0.0	15.4	0.0	33.1
24 Sep 99	0.4	0.0	19.9	0.0	28.1
26 Oct 99	2.5	0.0	16.8	1.0	26.0
21 Nov 99	4.5	0.0	15.7	2.0	23.8
16 Dec 99	2.0	0.0	18.0	- 1.0	19.1
10 Jan 00	0.2	0.0	19.8	0.0	19.6
16 Feb 00	2.8	0.0	17.7	2.0	17.8
21 Mar 00	2.6	0.0	17.4	0.0	23.2
15 Apr 00	2.5	0.0	15.8	1.0	21.7
21 Jun 00	1.8	0.0	25.6	- 1.0	-
25 Jul 00	3.1	0.0	11.7	0.0	27.4
23 Sep 00	0.6	0.0	19.9	0.0	27.5
24 Oct 00	4.9	0.0	13.8	0.0	29.1
26 Dec 00	3.8	0.0	16.2	0.0	20.9
16 Feb 01	4.3	0.0	15.7	0.0	19.8
14 Apr 01	4.7	0.0	14.7	0.0	22.7

Table 7.9 LFG Monitoring Data for Monitoring Well 1DG1

Date	CO2 % (v/v)	CH4 % (v/v)	O2 % (v/v)	Pressure (mbar)	Temperature (^oC)
31 Aug 98	5.4	0.6	7.2	2.0	31.4
10 Sep 98	6.2	0.0	5.4	0.0	31.5
23 Sep 98	5.9	0.0	5.6	5.0	34.6
13 Oct 98	7.0	0.0	4.8	3.0	32.4
30 Oct 98	5.4	0.0	9.6	5.0	30.8
12 Nov 98	4.5	0.0	15.8	2.0	25.8
26 Nov 98	5.4	0.0	8.7	0.0	24.8
12 Dec 98	4.9	0.0	12.1	2.0	23.6
21 Dec 98	3.4	0.0	14.9	1.0	26.6
12 Jan 99	4.7	0.0	10.1	1.0	24.2
20 Apr 99	3.9	0.0	14.4	5.0	29.3
10 Jun 99	7.2	0.0	12.1	0.0	24.6
12 Jul 99	5.2	0.0	13.1	2.0	---
24 Sep 99	4.0	0.0	12.6	0.0	25.8
25 Oct 99	2.0	0.0	15	2.0	28.3
14 Dec 99	1.2	0.0	18.4	0.0	19.4
12 Jan 00	0.8	0.0	18.5	0.0	20.1
21 Mar 00	2.6	0.0	14.7	0.0	24.1
18 Apr 00	5.1	0.0	10.3	0.0	22.5
16 Jun 00	3.9	0.0	29.1	0.0	-
26 Jul 00	6.9	0.0	6.2	0.0	34.9
23 Sep 00	0.9	0.0	19.2	0.0	29.1
18 Oct 00	1.4	0.0	19.4	0.0	24.3
26 Dec 00	0.5	0.0	19.9	0.0	18.9
9 Feb 01	0.3	0.0	18.7	0.0	15.4
11 Apr 01	3.5	0.0	12.0	0.0	19.1

Table 7.10 LFG Monitoring Data for Monitoring Well 1DG2

Date	CO2 % (v/v)	CH4 % (v/v)	O2 % (v/v)	Pressure (mbar)	Temperature (^oC)
31 Aug 98	10.7	1.9	0.8	1.0	31.2
10 Sep 98	12.7	12.4	0.2	0.0	32.0
23 Sep 98	9.3	5.9	3.9	5.0	33.2
13 Oct 98	13.1	12.2	0.2	5.0	33.2
30 Oct 98	13.8	6.1	0.1	6.0	30.0
12 Nov 98	16.0	1.6	0.2	4.0	29.2
26 Nov 98	7.0	11.2	0.3	2.0	26.2
12 Dec 98	10.3	0.0	6.0	1.0	23.5
21 Dec 98	12.3	0.0	4.9	0.0	25.7
26 Feb 99	6.2	0.0	13.1	4.0	29.3
21 May 99	8.3	0.0	9.3	3.0	26.2
12 Aug 99	12.7	0.0	4.1	0.0	27.0
18 Nov 99	5.1	0.0	14.0	0.0	24.6
13 Feb 00	4.8	0.0	13.8	0.0	24.8
9 May 00	1.9	0.0	16.0	0.0	26.6
17 Aug 00	1.2	0.0	15.5	0.0	30.2
6 Nov 00	3.3	0.0	15.4	0.0	23.6
17 Jan 01	3.6	0.0	18.4	0.0	16.4
2 Mar 01	3.4	0.0	14.1	0.0	24.3

Table 7.11 LFG Monitoring Data for Monitoring Well 1DG3

Date	CO2 % (v/v)	CH4 % (v/v)	O2 % (v/v)	Pressure (mbar)	Temperature (^oC)
31 Aug 98	8.0	0.0	0.2	1.0	34.8
10 Sep 98	10.8	18.0	0.6	1.0	32.3
23 Sep 98	8.2	1.1	0.2	5.0	34.4
13 Oct 98	1.4	12.0	2.8	3.0	32.4
30 Oct 98	3.4	13.2	1.7	5.0	30.8
12 Nov 98	14.2	0.5	0.1	3.0	27.6
26 Nov 98	12.1	2.1	1.0	2.0	26.4
12 Dec 98	7.4	0.0	10.8	1.0	22.4
21 Dec 98	6.1	0.0	11.5	0.0	27.1
12 Jan 99	4.2	12.6	1.2	1.0	24.4
20 Apr 99	5.2	0.0	11.4	6.0	31.6
10 Jun 99	9.1	0.0	0.4	0.0	24.0
12 Jul 99	11.1	0.4	0.6	2.0	33.4
24 Sep 99	8.0	0.0	2.5	1.0	27.2
25 Oct 99	10.8	0.7	0.3	2.0	28.6
14 Dec 99	4.1	0.0	15.2	0.0	19.4
11 Jan 00	3.4	0.0	15.3	0.0	19.4
21 Mar 00	6.4	0.0	10.5	0.0	26.2
22 Apr 00	5.1	0.0	8.4	0.0	34.8
16 Jun 00	6.6	0.0	28.4	0.0	-
26 Jul 00	9.1	0.0	5.9	0.0	35.9
23 Sep 00	7.5	0.0	12.3	0.0	27.3
18 Oct 00	4.7	0.0	16.9	0.0	23.9
6 Dec 00	6.4	0.0	12.7	0.0	23.2
9 Feb 01	5.2	0.0	13.4	0.0	15.2
11 Apr 01	0.0	0.0	19.7	0.0	19.2

Table 7.12 LFG Monitoring Data for Monitoring Well 1DG4

Date	CO2 % (v/v)	CH4 % (v/v)	O2 % (v/v)	Pressure (mbar)	Temperature (^oC)
31 Aug 98	9.5	8.7	17.2	0.0	32.3
10 Sep 98	9.8	0.6	7.2	0.0	32.5
23 Sep 98	13.6	2.8	0.1	5.0	34.0
13 Oct 98	0.4	10.4	6.2	3.0	31.8
30 Oct 98	1.1	11.2	4.4	4.0	30.6
12 Nov 98	2.0	10.8	0.8	3.0	27.4
26 Nov 98	8.7	1.3	8.8	2.0	25.4
12 Dec 98	9.6	0.0	10.1	2.0	20.4
21 Dec 98	8.4	0.0	10.8	2.0	25.0
12 Jan 99	1.4	12.2	3.6	1.0	24.8
26 Feb 99	6.2	0.0	13.9	5.0	29.3
20 Apr 99	7.8	0.0	12.2	1.0	28.5
21 May 99	13.3	0.0	5.3	3.0	25.8
12 Jul 99	14.8	0.0	2.3	0.0	33.8
12 Aug 99	4.8	0.0	1.2	0.0	26.8
25 Oct 99	11.5	0.0	9.3	2.0	30.5
18 Nov 99	9.5	0.0	12.1	0.0	22.0
11 Jan 00	5.2	0.0	15.0	0.0	19.3
13 Feb 00	0.4	0.0	19.3	0.0	24.6
19 Apr 00	8.2	0.0	10.8	0.0	29.5
9 May 00	8.5	0.0	4.8	0.0	26.3
26 Jul 00	12.9	0.0	6.6	0.0	34.2
17 Aug 00	0.0	0.0	18.8	0.0	28.2
9 Oct 00	3.5	0.0	17.3	0.0	32.9
6 Nov 00	8.8	0.0	12.2	0.0	22.7
17 Jan 01	6.7	0.0	16.2	0.0	15.7
2 Mar 01	4.7	0.0	14.8	0.0	22.9

Table 7.13 LFG Monitoring Data for Monitoring Well 1DG5

Date	CO2 % (v/v)	CH4 % (v/v)	O2 % (v/v)	Pressure (mbar)	Temperature (^oC)
31 Aug 98	10.1	0.8	1.4	0.0	30.8
10 Sep 98	10.5	1.2	1.0	0.0	32.4
23 Sep 98	7.3	0.0	4.9	6.0	33.8
13 Oct 98	1.6	11.4	1.2	3.0	32.6
30 Oct 98	2.3	13.8	1.0	4.0	31.0
12 Nov 98	12.3	0.0	1.1	2.0	26.6
26 Nov 98	11.2	1.4	2.1	3.0	25.2
12 Dec 98	16.0	0.0	1.1	1.0	22.7
21 Dec 98	11.9	0.0	6.6	3.0	26.4
12 Jan 99	3.4	13.5	0.8	1.0	24.5
10 Jun 99	5.0	0.3	0.4	0.0	24.7
12 Jul 99	7.3	0.1	0.9	0.0	31.5
24 Sep 99	4.4	0.0	0.7	0.0	28.4
25 Oct 99	7.9	0.0	1.3	2.0	2.0
14 Dec 99	10.8	0.0	5.9	0.0	19.3
11 Jan 00	13.4	0.0	2.1	-1.0	18.4
21 Mar 00	12.6	0.0	2.4	0.0	25.7
22 Apr 00	6.9	0.0	3.5	0.0	33.5
16 Jun 00	6.6	0.0	29.8	0.0	-
26 Jul 00	3.9	0.0	0.9	0.0	38.6
23 Sep 00	2.9	0.0	3.0	0.0	30.5
18 Oct 00	3.1	0.0	4.3	0.0	24.7
26 Dec 00	4.9	0.0	6.2	0.0	19.3
9 Feb 01	7.5	0.0	3.6	0.0	14.9
11 Apr 01	8.6	0.0	1.5	0.0	18.9

Table 7.14 LFG Monitoring Data for Monitoring Well 1DG6

Date	CO2 % (v/v)	CH4 % (v/v)	O2 % (v/v)	Pressure (mbar)	Temperature (^oC)
31 Aug 98	1.2	2.3	0.6	0.0	31.4
10 Sep 98	0.6	1.8	2.3	1.0	33.0
23 Sep 98	1.3	0.9	0.2	6.0	35.7
13 Oct 98	2.8	1.8	3.2	4.0	32.8
30 Oct 98	2.4	2.2	4.1	4.0	31.2
12 Nov 98	3.1	0.0	0.1	3.0	27.4
26 Nov 98	4.2	2.3	3.2	2.0	25.4
12 Dec 98	3.4	0.0	1.1	1.0	26.2
21 Dec 98	3.1	0.0	0.4	3.0	28.3
26 Feb 99	4.1	0.0	2.8	6.0	31.6
20 Apr 99	5.0	0.0	0.7	2.0	21.9
21 May 99	5.2	0.0	0.1	3.0	26.2
12 Aug 99	5.5	0.0	0.8	1.0	26.6
18 Nov 99	6.6	0.0	3.2	0.0	19.1
13 Feb 00	4.9	0.0	3.4	0.0	25.1
9 May 00	4.2	0.0	8.9	0.0	29.7
17 Aug 00	5.8	0.0	6.2	0.0	31.9
6 Nov 00	6.2	0.0	6.4	0.0	23.3
17 Jan 01	6.0	0.0	3.1	0.0	18.6
2 Mar 01	6.2	0.0	7.6	0.0	26.9

Migration Pathways

Natural Pathways

7.5.5 The presence of natural underground cavities is directly related to the prevailing local geology. The key concern is the presence of fault lines and lineaments that LFG may exploit as a preferential migratory pathway. Since the local geology of Roads D1 and D10 is fill composed of natural earth overlying marine mud, there will be no significant fault lines persisting between TKO Stage I and Road D1 or Road D10. Therefore, the natural pathways are considered Moderately Short/ Direct.

7.5.6 Cross-sections between TKO Stage I and Road D1, and between TKO Stage I and Road D10 are depicted in Figure 7.5 and Figure 7.6 respectively. Figure 7.6 also depicts the depths of monitoring wells 1DG1 to 1DG6, as provided by EPD.

Man-made Pathways

7.5.7 There is no stormwater drain or sewer constructed directly between TKO Stage I and Road D1, and between TKO Stage 1 and Road D10.

7.5.8 From a layout plan of utilities around the landfill, it is found that there are no existing utilities branching off to Road D1 or Road D10. The existing utilities follow the corridor described by Wan Po Road, and these cross the critical migratory pathway at right angle.

Targets

7.5.9 The construction of Roads D1 and D10 will mainly involve temporary excavation and road pavement. The level of risk for the excavations is considered Medium. In the case of Road D10, bored piling will be carried out for the foundation of the section across the Eastern Drainage Channel. However, since the piling will be carried out within the Eastern Drainage Channel, which will act as migration barrier for LFG, the bored area is not considered a sensitive target for LFG migration.

7.5.10 Some utility manholes and inspection chambers, ducts or other confined spaces are anticipated to be present along Road D1 and/or Road D10. Since access to these confined spaces will be restricted to authorised persons who have awareness of LFG hazard, the risk level for these targets is considered Medium.

Summary of qualitative hazard assessment

7.5.11 Based upon the available information, a source-pathway-target analysis has been undertaken for the different combination of source, pathways and targets discussed above, and is presented in Table 7.15.

7.5.12 According to Landfill Gas Hazard Assessment – Guidance Note, for the purpose of categorising a subject site, the category is based on the highest level of risk nominated for any of the potential impacts identified. However, if four or more different impacts arise in a particular risk category, then the overall risk classification may be considered to be one category higher.

7.5.13 In this assessment, the overall risk level for both the concerned section of Road D1 located within the Consultation Zone of TKO Stage I Landfill and for the whole of Road D10 is ‘high’. As such, engineering and detection measures are required to protect the concerned section of Road D1 and the whole of Road D10.

Table 7.15 Summary of Qualitative Source-Pathway-Target Analysis

Source	Pathways	Targets	Risk
It has been estimated that LFG generation rate peaked in 1988. The available monitoring data from July 2000 to July 2001 indicated that the methane levels at all the existing monitoring wells are very low but the carbon dioxide levels at some monitoring wells are significant (> 5%). A comprehensive restoration works for TKO Stage I have been completed. These include active extraction of LFG for flaring and electricity generation, passive venting of LFG across landfill site, leachate collection and treatment, and surface and subsurface drainage systems. These works enable the safe use of TKO Stage I for public amenity purposes and entail the construction of comprehensive gas containment, collection and disposal infrastructure. ( Major Source)	Road D1 – Natural Pathways Road D1 will situate on reclaimed land. No known lateral fault lines persistent between TKO Stage I and Road D1. Eastern Drainage Channel will act as barrier of LFG migration. Closest distance from Road D1 is about 150 m. (Moderately Short / Direct)	Utility manholes, inspection chambers, ducts, etc. (Medium sensitivity)	Medium
		Temporary excavation during construction. (Medium sensitivity)	Medium
	Road D1 – Man-made Pathways (i.e. Utilities) No utilities directly link the landfill and Road D1. Utility companies would take LFG in the subsurface into account in the design, construction and maintenance of their works. (Long / Indirect)	Utility manholes, inspection chambers, ducts, etc. (Medium sensitivity)	Medium
		Temporary excavation during construction. (Medium sensitivity)	Medium
	Road D10 – Natural Pathways Road D10 will situate on reclaimed land. No known lateral fault lines persistent between TKO Stage I and Road D10. Closest distance from Road D10 is about 50 m. (Moderately Short / Direct)	Utility manholes, inspection chambers, ducts, etc. (Medium sensitivity)	Medium
		Temporary excavation during construction. (Medium sensitivity)	Medium
	Road D10 – Man-made Pathways (i.e. Utilities) No utilities directly link the landfill and Road D10. Utility companies would take LFG in the subsurface into account in the design, construction and maintenance of their works. Voids around any service ducts or cables within conduits would be filled with gas resistant mastic. (Moderately Short / Direct)	Utility manholes, inspection chambers, ducts, etc. (Medium sensitivity)	Medium
		Temporary excavation during construction. (Medium sensitivity)	Medium

7.5.14 In the event that there are substantial changes in the alignments of Roads D1, D8 and D10, a review of the qualitative source-pathway-target analysis as presented in Table 7.15 is considered necessary.

7.6 Mitigation Measures

7.6.1 Generally, higher risk categories will warrant the use of more active control measures, or more sophisticated combinations of mitigation measures. As suggested in Landfill Gas Hazard Assessment - Guidance Note, the measures required for development of high risk are use of semi-active or enhanced passive gas controls, or detection in some situations.

Utilities Protection Measures

7.6.2 Prior to and during development or construction, utility company would be advised of the possible presence of LFG in the subsurface and this will be taken into account in the design, construction and maintenance of their works.

7.6.3 It will be specified in the future contracts that the void around any service ducts, pipes or cables etc. within conduits would be filled with gas resistant mastic. All ducts, manholes, and chambers, either specific to the proposed developments or to utility services shall be sealed off from the ground to prevent gas entry and provided with vented covers to allow any gas that enters to dissipate harmlessly to atmosphere. Procedures shall be established for monitoring any such confined spaces before entry in accordance with the requirements of the Factories and Industrial Undertaking (Confined Spaces) Regulation, where applicable.

Construction Measures

7.6.4 During site formation and construction works, portable gas detectors shall be used to regularly check the levels of methane, carbon dioxide and oxygen in depressions, trenches, and other excavations. The monitoring shall be undertaken at the beginning of each half working day (i.e. morning and afternoon) and for the entire period the excavation remains open. If elevated LFG level is measured, ventilation of such excavations shall take place and sources of ignition shall be kept away from areas where build-up of gas is possible. These measures shall be laid down clearly and followed rigidly regarding trenching and excavation, and creation of confined spaces at, near to or below ground level. These measures must be incorporated into Contract Documentation.

7.7 Monitoring Requirements

Monitoring of Utilities

7.7.1 Prior to entry, the atmosphere within utility manholes or chambers shall be checked for methane, carbon dioxide, and oxygen. It is recommended that the pre-entry monitoring shall be undertaken, where applicable, in accordance with the requirements of the Factories and Industrial Undertaking (Confined Spaces) Regulation. Routine monitoring shall also be conducted regularly at the manholes and chambers on a regular basis. The frequency of the routine monitoring is recommended to be biweekly during the initial stage. The “initial stage” is considered as the “initial stage” of the operational phase of the project. However, there is no government’s regulation/guideline on the duration of such “initial stage” for monitoring. As such, it is recommended that the “initial stage” refer to the first 3 months of the operational phase. The frequency shall be subsequently reviewed on a quarterly basis through discussion with EPD, based on all the available monitoring data for methane, carbon dioxide and oxygen (including that of pre-entry monitoring).

7.8 Conclusions

7.8.1 Although the methane levels recorded in the period from July 2000 to July 2001 at the monitoring wells located near the boundary of TKO Stage I Landfill were very low, significant levels (> 5%) of carbon dioxide were recorded in some wells. As such, the source of LFG at TKO Stage I Landfill is considered as ‘major’.

7.8.2 There are no known fault lines between TKO Stage I and Road D10. There are no existing utilities branching off to Road D1 or Road D10. The Eastern Drainage Channel situated between the landfill and Road D1 would act as a barrier of landfill gas migration to this road. The migration pathways for LFG are considered moderately short /direct for both Road D1 and Road D10.

7.8.3 The common targets of LFG migration at both Road D10 and the concerned section of Road D1 are confined space such as utility manholes, inspection chambers or ducts, and the temporary excavations created during the construction phase. Since these areas are not accessible to public and restricted to trained personnel, the risk level of these targets are considered medium.

7.8.4 The overall LFG risk posed by TKO Stage I Landfill to both the concerned section of Road D1 and to the whole of Road D10 is assessed to be ‘high’. As suggested in the EPD’s Guidance Note, some engineering and detection measures should be provided to safeguard the concerned section of Road D1 ant the whole of Road D10.

7.8.5 Mitigation measures regarding utility protection and construction, and monitoring of utilities are recommended in order to ensure that they are safe from LFG. When the detailed designs of the proposed developments are available, the necessary protection measures for individual developments within the Consultation Zone will be confirmed.

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[1] Scott Wilson Kirkpatrick (in association with Aspinwall & Company) (Oct 1994) Agreement CE 11/92 Restoration of Tseung Kwan O Landfills – Final Report.