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.
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.
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.
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.
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.
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 (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.
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.
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.
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.