2                                            Contamination Assessment background Information

This Section presents the summary of the results of the contamination site investigation conducted.  The full details of the methodology used during the soil and groundwater sampling work, details of field observations such as visual observations made during the investigation programme, analytical results from soil and ground water sample analyses are provided in the CAR, Annex A.

2.1                                      Site Investigation Programme

The Site Investigation (SI) was conducted during 14 to 19 November 2005, as described in the CAR and consisted of drilling of six boreholes (BH-1 to 6), installation groundwater sampling wells where groundwater was encountered (BH-1 to 6), soil and groundwater sampling, laboratory sample analysis and field QA/QC programme.  The locations of the boreholes installed in the SI are presented in Figure 2.1a.

The SI programme was designed and carried out in accordance with the EPD’s Practice Notes for Professional Persons: Contaminated Land Assessment and Remediation ProPECC PN 3/94 and the Guidance Notes for Investigation and Remediation of Contaminated Sites of Petrol Stations, Boatyards, and Car Repair/Dismantling Workshops, 1999.

2.1.1                                Field Observations

The underground soil profiles observed during the exploration were recorded using boring logs (see CAR).  No obvious sign of hydrocarbon staining, discoloration or odour was observed in any underlying soil sample and/or groundwater sample during the borehole drilling and soil and groundwater sampling.  No free oil product was observed on the groundwater surface.

2.1.2                                Soil Sample Results

A soil sample taken from BH3, near the No. 5 oil tank, at depth 0.8 m below ground level (m bgl) contained total TPH at concentration of (2,745 mg/kg) (sum of light diesel (C10-C14) (78 mg/kg), heavy diesel (C15-C28) (1,800 mg/kg) and heavy oil (C29-C36) (945 mg/kg)) exceeding the Dutch B level for mineral oil (1,000 mg/kg) and indicated potential TPH contamination.  The concentration was below the Dutch C value for mineral oil of 5,000 mg/kg.  No soil sample was collected at deeper level at this location as the underlying geology comprised of granite rock from 1.1 meter below ground level (m bgl) to the end of drilling at 7.1 m bgl.  TPH was also detected at BH4 at 1.0m depth (201 mg/kg), the concentration was within the Dutch B level for mineral oil.

No exceedance to Dutch B values were detected for parameters other than TPH in other parameters analysed at BH3, and for all parameters analysed at BH1 to BH6.

2.1.3                                Groundwater

The static groundwater levels measured at boreholes during the Site investigation for this project were between 4.3 to 4.8 m bgl.  Based on the groundwater water levels measured and the local surface hydrology, the shallow groundwater in the Site area is anticipated to flow in a generally southern or south-westerly direction.

The total TPH concentrations detected in the groundwater samples collected from wells BH4 (421 mg/L) and BH6 (381 mg/L) (ie total of heavy diesel and heavy oils) exceeded the EPD Dutch B value of 200 mg/L but did not exceed the Dutch C Value of 600 mg/L for mineral oil.  The total TPH concentrations detected in the groundwater sample from wells BH1 (1,185 mg/L), BH3 (1,424 mg/L) and BH5 (706 mg/L) exceeded the EPD Dutch C value of 600 mg/L for mineral oil.

No free-floating oil product was observed on the groundwater surface.

BTEX and PAHs listed in the Dutch List were not detected in the groundwater samples collected.

2.2                                      Assessment of Land Contamination Extent

TPH in Soil at BH3

Concentrations of TPH (light diesel (C10-C14), heavy diesel (C15-C28) and heavy oil (C29-C36)) in soil materials at the BH3 at 0.8 m bgl exceeded the Dutch B value but was below the Dutch C value.  At this location, the soil stratum ended at 1.1 m bgl where bedrock was encountered.  TPH was also detected at BH4 but was within the Dutch B value.

No. 5 oil tank was located above the ground and had been provided with a secondary containment system for potential oil spills or leakages.  No spills or leakages of oil have been reported for this tank, and no visual evidence of spills or leakage was observed during the Site investigation.  The potential source of the TPH contamination at location BH3 and BH4 was not confirmed at this stage.

The vertical transport of TPH in the soil material, if any, is anticipated to cease where the head rocks are located.

As no TPH was detected in soil at other sampling locations, the TPH detected at the two sampling locations, therefore, was considered to be localized.  Based on the finding of the site investigation and the extent of the excavation works, it is estimated that about 600 m3 of contaminated excavated soil ([1]) will be generated at the No. 5 oil tanks.  The extent of the horizontal transport of contamination shall be further investigated during the demolition of the tanks and sump as described in Section 3.  No contaminated excavated soil is anticipated to be generated at the No.4 oil tank or the oil separator sump.

TPH in Groundwater

TPH (heavy diesel and heavy oils) contamination at above Dutch B and C levels has been reported in the groundwater samples (BH1, BH3, BH4, BH5 and BH6).  The higher TPH concentrations were detected in the samples from wells BH1 and BH3 and were primarily contributed by heavy diesel fraction. 

As both oil tanks were located above the ground and have been provided with secondary containment systems.  No oil spills and leakages have been reported or observed during the Site investigation for both tanks.  The oil separation sump, was built with waterproof concrete.  No spills or leakages have been reported for the oil separation sump.  The source of TPH contaminations in the groundwater of the Site area could not be confirmed at this stage.

As groundwater is not used as a potable water supply source in Hong Kong and taking into consideration the absence of the free-floating oil products and the relatively low concentrations of TPH detected in the groundwater, no potential adverse impacts are anticipated from the demolition of the tanks and sump.

2.3                                      Evaluation of Construction Methods for FGD Plant

2.3.1                                Tanks and Oil/Water Separator Sump Demolition

The construction of the proposed FGD plants for Generation Units L4 and L5 will involve demolishing of the 2 existing 250 m3 No.4 and No. 5 Light Oil Tanks and the associated fixtures/appendages/ foundations/bund walls and the in-ground oil separator to provide space for the proposed installations. 

 

The demolition works will take about 5 months and are scheduled to commence in April 2006.

 

A top-down approach will be adopted for the demolition of the facilities.

 

Tanks, foundations, other concrete structures and oil separator sump will be washed/decontaminated prior to demolition rendering the tanks and sump structures cleaned.  This will also enable the materials to be disposed of as non-contaminated construction wastes.

A total of about 29 tonnes of scrap steel will be produced from the demolition of the oil tanks and the associated fixtures/ appendages such as pipeline, spiral stair and catwalk attached to the oil tanks.  The steel sheet of the circular roof and sidewall will be cut into small panels (about 1.5m x 1.5m) for easy transportation.  All the scrap steel will be transported off-site by barges for recycling.

A total of 660 m3 uncontaminated reinforced concrete (after applying a bulking factor of 1.4) will be generated from demolition of the oil tanks foundation, oil separator and bund walls.  The reinforced concrete is not contaminated and will be separated from other waste to avoid contamination.  The reinforced concrete (public fill) will be reused on-site for reclamation of Lamma Power Station Extension or sent to public filling facilities /other reclamation site for reuse.

2.3.2                                Excavated Materials

Non-Contaminated Excavated Materials

Excavation works would be required for the construction of the foundations of FGD absorber units (booster fans, gas-gas heaters, gas ducts supports) and the shallow foundation of the switchgear and equipment building.  A total of 2,300 m3 (after applying a bulking factor of 1.2) of excavated soil materials (mainly soft materials, ie sand and clay) will be generated from the demolition of the No. 4 and No.5 Oil Tanks and oil/water separation sump areas and excavation for construction of foundations for new facilities.  This volume does not include the volume of the existing in-ground tank foundations and sump structures which were included as part of the demolition wastes in the Section 2.3.1. 

In addition, approximately 560 m3 of soil and rock materials will be generated from the bore pile excavation.  Pre-bored socketted H-pile/Miround oni-pile piling method was adopted over an alternative driven pile method due to the fact that the construction will occur amongst existing power generating facilities sensitive to vibrations.  The bore pile method will reduce the vibration impacts to the facilities.  In addition, the geological investigation of the Site showed the underlying soil strata to contain significant amount of boulders which also will affect driven piling activities.

After excavating the in-ground oil separator, the area will be backfilled with the excavated soil (about 190 m3) to original ground level.  About 2,670 m3 ([2]) of surplus excavation materials will therefore be generated.  The surplus excavated materials (public fill) will be reused on-site for reclamation of Lamma Power Station Extension or sent to public filling facilities/other reclamation site for reuse.

Contaminated Excavated Materials

Excavation works would be required at the No.5 Light Oil Tank area for the construction of the foundations of a FGD absorber unit.  A total of 600 m3 (after applying a bulking factor 1.2) of excavated soil materials (including soft soil materials, ie sand and clay and bore pile excavations) will be generated.  This volume does not take into account existing in-ground No. 5 tank foundation structures, which were already included as part of the demolition wastes in the Section 2.3.1.

The contaminated soil will be remediated on-site in accordance with the EPD’s Guidance Notes for Investigation and Remediation of Contaminated Sites of Petrol filling Stations, Boatyards, and Car/Repair/Dismantling Workshops, May 1999.  Proposed strategy for the excavation, testing, and remediation works is presented in the following Section 3.



([1])  The volume of the excavated contaminated soil at No. 5 oil tank was estimated assuming the contaminated area to be at base of the tank plus 2m each side (18x18 m2) to 1.5m below ground level.  The calculation includes soil around the foundations, 0.5m of soil underneath the foundations, up to 3m of bore piles and pile caps in the area, plus bulking factors of 1.2 was applied for excavated soil.  The calculation does not include the tank and bund wall foundations which are not anticipated to be contaminated and will be demolished and handled in a similar manner as other tank structures and disposed of as fill materials.

([2])  The volume included volumes of all non contaminated excavated soil materials for the construction of foundations and piling works proposed in the project area, excluding existing foundations and other concrete structures.