Following the
completion of the land contamination site investigation at the Site, the
following remediation programs are proposed.
The scope of work
for soil remediation shall include:
No. 4 and 5 Light
Oil Tanks and the oil separation sump will be demolished and soil materials
around and underlying the tank and sump will be excavated during the retrofit
programme.
·
For the
remediation of light petroleum hydrocarbon contaminated soil around the No.5
Light Oil Tank (TPH detected at BH3 0.8 m bgl above Dutch B level), the
following steps are proposed.
·
Cleaning of
the tank prior to demolition. Oily
water and sludge collected from the cleaning shall be collected and disposed of as chemical waste
at Government chemical waste treatment facility.
·
Only
licensed waste contractors shall be used to collect and transport any chemical
waste. The necessary waste
disposal permits will be obtained, as required, from the appropriate
authorities, in accordance with the Waste Disposal Ordinance (Cap 354C) and
Waste Disposal (Chemical Waste) (General) Regulation (Cap 35), as required.
·
Excavation
of the No. 5 Light Oil Tank and bund wall foundations (parcel 18x18 m2)
and underlying soil strata from 0 to 1.5 m bgl or to the rock head for
temporary stockpile. The proposed
initial excavated parcel at the No.5 Light Oil Tank is shown in
Figure 3.1a.
·
Stockpile
shall be contained (eg soil will be stored on a liner sheet and covered with
HDPE sheets).
·
Conduct verification
sampling for TPH contamination in soil around the excavated parcel. The soil samples shall be obtained at
the north, west, south and east boundaries of the parcel at 0.5 and 1.5 m bgl;
and in the middle of the parcel at the bottom of the excavation. Soil samples shall be taken using
excavator and hand tools.
·
If the
analytical results of the verification samples are below the Dutch B levels for
TPHs, the removal of the contaminated material shall be considered completed.
·
If the
analytical results of the verification samples exceed the Dutch B levels,
additional soil shall be excavated either laterally or vertically, depending on
the analytical results and the verification sampling and analysis processes
repeated until all confirmation samples are below the relevant action
levels. The excavation will be
limited to the Site area only.
·
The
excavated contaminated soil will be remediated to remove volatile and
semi-volatile fractions of TPH and bio-remediate the contaminated soil
respectively 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. The treatment will be carried out either at the Site or at
the Lamma Power Station Extension area.
·
Remediation
of hydrocarbon impacted soils can be conducted through the use of enhanced
bioremediation in a ‘biopile’. The
soil treatment is accomplished by stimulating naturally occurring
hydrocarbon-degrading bacteria in the soils by providing optimal conditions for
the bacteria within the biopile.
The method for construction of a typical biopile treatment cell is
presented below and diagrams of a typical biopile cell are presented in
Figures
3.1b and c ([1]).
A method statement for the on-site treatment of contaminated soil will
be prepared and submitted to EPD prior to the commencement of the treatment
work.
·
Periodic
sampling and analysis of the soil materials during the treatment shall be
carried out to monitor the progress of the treatment. The number, frequency and testing methods for the treatment
monitoring will be specified in detail in the treatment method statement.
·
Where the
treated soil showed TPH concentrations below the Dutch B value, the treatment
is considered completed and the soil can be used as fill materials on-site, at
the Lamma Extension area.
·
Vehicles containing any excavated
materials shall be suitably covered to limit potential dust emissions or
contaminated wastewater run-off, and truck bodies and tailgates shall be sealed
to prevent any discharge during transport or during wet conditions.
·
Records of
the quantities of wastes generated and disposed of will be maintained.
·
Excavated
area will be backfilled with clean filled materials and the surface reinstated.
·
A
decontamination verification report shall be prepared upon completion of the
decontamination works.
Groundwater was
found to be contaminated in the groundwater samples collected from the wells in
the No. 4 Light Oil Tank and Oil Separation Sump. These included the groundwater from BH1 with the total TPH
at concentration 1,185 µg/L, from BH3 with the total TPH at concentration 1,424
µg/L, BH4 with total TPH at concentration 421 µg/L, BH5 with total TPH at
concentration 706 µg/L and BH6 with total TPH at concentration 381 µg/L. As such, it is proposed to sample and
test the soil materials on all bottoms and all sides of the No.4 Light Oil Tank
and oil separation sump after demolition and excavation to identify potential
TPH contamination. The following
steps are proposed.
·
Cleaning of
tanks and sumps prior to demolition.
Oily water and sludge collected from the cleaning shall be treated at
the on-site wastewater treatment facility. Oily water and sludge collected from the cleaning shall be
collected and disposed of as chemical waste at Government chemical
waste treatment facility.
·
Only
licensed waste contractors shall be used to collect and transport any chemical
waste. The necessary waste
disposal permits will be obtained, as required, from the appropriate
authorities, in accordance with the Waste Disposal Ordinance (Cap 354C) and
Waste Disposal (Chemical Waste) (General) Regulation (Cap 35), as required.
·
Excavation
of the No. 4 Light Oil Tank and bund wall foundations (parcel 18x18 m2)
and excavation of oil separation sump structures (parcel 12x9 m2 to
the sump depth of about 1.65 m).
The proposed initial excavated parcels at the No.4 Light Oil Tank and
oil separation sump are shown in
Figure 3.1a.
·
Conduct verification
sampling for TPH contamination in soil around the excavated parcels. The soil samples shall be obtained at
the north, west, south and east boundaries of the parcels at 0.5 and 1.5 m bgl
for the No. 4 tank; at the north, west, south and east boundaries of the parcels
at 0.5 and 1.65 m bgl for the sump; and in the middle of each of the parcels at
the bottom of the excavation. Soil
samples shall be taken using excavator and hand tools.
·
Stockpiling,
sampling, testing, further excavation and disposal of soil materials shall
follow the same procedures as described in the Section 3.1.1.
As groundwater is
not used for either domestic or industrial purposes at the Site and in the
adjacent areas, remediation of TPH detected in the groundwater of the Site is
not considered necessary. No
groundwater pumping is anticipated during the construction of the foundation of
the project as the proposed excavation during the demolition and construction
are at levels well above the measured groundwater table at the Site. However, if the groundwater is
encountered during the retrofit programme, the groundwater abstracted or
collected should be recharged back to the Site.
The remediation
work will involve tank cleaning, oil tank and oil separation demolition, soil
excavation, soil stockpiling, and soil sampling. The tanks will be cleaned by an explosive-proof, manual
operated steam or water cleaner then by manual wiping. In order to minimise the contacts of
the workers with the contaminated materials and to ensure safe work
environments during the remediation works, the following control measures are
proposed.
·
Prior to
commence any remediation work, a health and safety risk assessment should be
performed for the remediation work to identify potential work related hazards
and prepare appropriate control measures.
·
Appropriate
personal Protective Equipment (PPE) such as safety hat, chemical protective
gloves, masks (for both dust and vapour), eye goggles, protective clothing and
protective footwear shall be provided to staff who would be involved in the
tank cleaning and contaminated area (BH3) remediation works. No works should be allowed without the
suitable PPE.
·
Workers
shall inspect and check their PPE before, during and after use. In cases where
any of the PPE is impaired. The
worker shall stop work immediately and inform their supervisor. The worker shall not be allowed to
re-start his work until the impaired PPE is replaced.
·
As the oil
tank is defined as a confined space, appropriate control measures and
procedures for entering a confined space should be provided and followed when
entering the tanks during the remediation.
·
Workers
shall always maintain basic hygiene standard (e.g. hand wash before leaving the
contaminated work area). Workers
shall also be responsible for cleaning and storing their own PPE in a secure
place before leaving the site.
·
Eating,
drinking and smoking must be strictly prohibited within the contaminated site
area.
·
The
designated site management representatives must be informed if any workers feel
uncomfortable physically or mentally during the remediation works. All workers shall leave the work areas
and the work shall be temporarily suspended until the reason for the
uncomfortable feeling has been identified.
·
The works
shall be stopped or discontinued when any typhoon signal number 3; yellow, red
or black storm signals are hoisted.
All stockpile materials (if any) shall be covered immediately by
tarpaulin or other similar protective and waterproof materials.
·
The use of
bulk earth-moving excavator equipment would minimise construction workers'
potential contact with contaminated materials.
The base of the treatment cell is constructed by placing a liner (such as
tarpaulin) on a flat graded surface.
Bunds such as hay bales or concrete blocks are then placed along the
perimeter of the liner to create a containment berm for the treatment
cell. Perforated collection pipes
are then installed on top of the base liner to collect excess moisture from the
treatment cell (ie, leachate). The
leachate collection pipes are covered with pea gravel (or coarse sand) and a
geotextile liner. Soil requiring
treatment is then placed on top of the geotextile liner in lifts of typically 2
to 3 feet. A nutrient mixture used
to increase hydrocarbon-degrading bacteria is sprayed on the impacted soils
while they are being placed into the treatment cell.
After each lift of soil is placed in the cell, slotted ventilation pipes and
soil vapour monitoring points are installed on top of the lift. The ventilation pipes are used to
supply oxygen to the bacteria in the soil and the vapour monitoring points are
used to monitor carbon dioxide in the treatment cell (carbon dioxide is one of
the primary by-products of the hydrocarbon biodegradation process). Alternating additional lifts of soil
and piping are then installed to the top of the cell. Once the cell is complete, it is covered with a liner that
is anchored to the ground.
The ventilation pipes are connected to a blower system to create airflow
through the cell. As necessary,
the air discharge from the blower can be connected to a treatment system to
reduce emissions to the atmosphere.
Monitoring of the cell is conducted through measurement of carbon
dioxide concentrations in vapours within the cell and hydrocarbon
concentrations in air extracted from the treatment cell. As remediation progresses, both carbon
dioxide and hydrocarbon concentrations will decrease until successful reduction
of the chemical concentrations in the soil has been achieved. Confirmation samples are then collected
from the formerly impacted soils to confirm the remedial goals have been
reached.