Technical Memorandum
Annex 14
ANNEX 14 : GUIDELINES FOR ASSESSMENT OF WATER POLLUTION
1. General
1.1 The annex describes the commonly adopted approaches and methodologies
for assessment of water pollution arising from designated projects. The
methodologies may vary from case to case, depending on the nature of the
water quality issues and the latest development in methods and techniques.
The assessment shall be quantitative wherever possible.
2. Aquatic System subject to Water Pollution Impact
2.1 In identifying and evaluating water pollution impacts of the aquatic
environment, the following aspects shall be considered:
- water quality as characterized by:
- physical and chemical features such as temperature, salinity,
conductivity, pH, colour, suspended solid, floatable, turbidity,
oil and grease, and organic material concentration measured by TOC,
COD or BOD,
- eutrophication related factors measured by dissolved oxygen,
nutrients and chlorophyll-a,
- harmful or toxic substances including ammonia, heavy metals,
PCB, PAH, pesticides, and radionuclides, and
- pathogenic micro-organisms and viruses indicators e.g. E.coli;
- Hydrology including factors concerning currents,
tidal flows, drainage, erosion, sediment deposition and other physical
phenomena;
- Bottom sediments characterized in terms of physical,
chemical and microbiological properties and constituents, including
parameters such as particle size, pH, organic contents, nutrients, sulphide,
and toxic substances such as heavy metals, pesticides and antifouling
paints; and
- Ecology including flora and fauna composing of
bacteria, phytoplankton, zooplankton, benthic organisms, coral, shellfish,
fish, mangroves, wetland and other aquatic biota.
3. Beneficial Uses Sensitive to Water Pollution
3.1 Existing or potential beneficial uses that are sensitive to water
pollution shall include, but not be limited to:
- areas of ecological or conservation values including marine conservation
areas, existing or gazetted proposed marine parks and marine reserves,
sites of special scientific interest (SSSI), existing or gazetted proposed
country parks and special areas, wetlands, mangroves and important freshwater
habitats;
- areas for abstraction of water for potable water supply;
- water abstraction for irrigation and aquaculture;
- fish spawning grounds, fish culture zones, shellfish harvesting/culture
site and brackish/freshwater fish ponds;
- beaches and other recreational areas;
- water abstraction for cooling, flushing and other industrial purposes;
- areas for navigation/shipping including typhoon shelters, marinas
and boat parks.
4. Assessment Approach
4.1 Assessment shall rely on the concept of assimilative capacity
of the receiving water body and water quality objectives. Assimilative
capacity will vary with the characteristics of each site and with the
type and number of discharges or activities or affected beneficial uses.
Quantification of the assimilative capacity of the receiving environment
shall take into account physical processes, as well as all chemical, biochemical
and biological processes. Sensitive receivers based on beneficial uses
shall be identified and the water quality impact shall be assessed with
reference to the Water Quality Objectives or criteria covered in Annex
6. Assimilative capacity of a water body is regarded as exceeded if the
water quality objectives (WQOs) for the most sensitive target of the beneficial
uses to be protected for that water body are exceeded.
4.2 In evaluating water pollution impacts, both point and non-point sources
of water pollutants shall be considered. Non-point pollutants refer to
those substances which can be introduced into the receiving water body
as a result of urban or rural runoff. Point sources are related to specific
discharges from municipal or industrial facilities.
5. Assessment Methodology
5.1 Assessment methodology shall be site- and activity-specific. Assessment
framework shall include the following elements:
Identification of Impact-causing Factors
5.2 It involves the identification and characterization of the impact-causing
factors associated with a project. Information shall be based on specific
features of the project, including coastline and river modifications,
construction activities such as dredging and dumping, quality and quantity
of wastewater and thermal discharges, changes in land-use and drainage,
oil and chemical spills, maritime wastes, waste disposal facilities and
leachates, and non-point pollution sources. Emphasis shall be placed on
activities or pollutants that will result in nutrient enrichment leading
to eutrophication or structural changes in biological community, bathometry
change, reduction in flushing or assimilative capacities, loss or modification
of aquatic habitats, and threat to ecological and human health from exposure
to toxic substances, pathogens and biotoxins.
Determination of the Impact Boundary
5.3 An essential first step in assessing the impact of an activity on
the water body is the determination of the impact boundaries. The impacted
area can be divided into the near-field and far-field. The near-field
includes the initial dilution and the mixing zones and is basically determined
by physical processes such as the hydrodynamic processes. The far-field
is more difficult to determine and depends on water transport, biological
processes, geochemical processes, and physio-chemical processes. Estimating
the impact area has to be carried out at the early stage of the assessment
but may have to be revised in the light of information that emerges during
the assessment process.
Baseline Study
5.4 It involves the description of the existing quality and quantity
(as appropriate) of the receiving aquatic environment, with emphasis on
the quality parameters related to the water pollutants arising from an
activity and the parameters relating to the affected beneficial uses.
Field surveys shall be carried out to supplement existing information
in situation when existing data are out dated or insufficient. Baseline
study involves the development of a survey and sampling programme which
shall cover aspects of meteorological, geological, hydrological factors,
physio-chemical characteristics, biology and beneficial uses. Sampling
programmes to provide information for environmental assessments must be
carefully designed to meet identified objectives and shall be directed
to potential problems. The baseline study must consider changes that may
arise from seasonal and climatic changes, other natural changes such as
sedimentation and ecological succession, and impacts from other current
or proposed development in the area.
Impact Prediction and Assessment
5.5 Assessment shall make use of the scientific knowledge of near-field
and far-field transport and dispersion of pollutants coupled with modelling
and measurements obtained from the baseline study. Both construction and
operation (or decommissioning as appropriate) aspects of the project shall
be considered. Assessment shall be based on quantitative techniques which
can range from the use of simple mass balance approaches to sophisticated
computer models. In situations involving various degrees of complexity
and uncertainty, numerical or probabilistic modelling approaches are useful.
It shall be noted that the use of models requires acquisition of good
comprehensive baseline and monitoring data. Models to be selected must
be well proven and be satisfactorily calibrated and verified with field
data. The modelling capabilities and approach shall meet the relevant
government requirements being in force.
5.6 Assessment of biological effects shall include the organismic as
well as community or ecosystem level. Factors governing the availability
and accumulation and transformation of pollutants shall be considered.
5.7 The predictions will provide information which can used as the basis
for determining whether the aquatic resources and beneficial uses are
at risk, or that the assimilative capacity will be exceeded as a result
of implementation of the project.
5.8 While impact prediction can be assisted by the uses of checklists,
matrices, flow-charts and networks analyses, they do not assess the nature,
magnitude or significance of the impacts.
Mitigation Measures
5.9 Mitigation shall aim to avoid, reverse, minimize, or compensate for
an impact. Consideration shall also be given to opportunity to enhance
existing conditions. The principles shall be to prevent rather than to
rectify and to eliminate environmental damage at source. The approach
shall be to minimize the risk of harm to human health and the ecosystem,
to minimize the risk of impairment to the beneficial uses, to prevent
pollution at source and to apply the most suitable technical solutions
to prevent and rectify pollution problems.
Monitoring
5.10 Monitoring is generally conducted to gather information about compliance
with regulations and licence requirements, model verification, and trends.
Monitoring is required whenever there is uncertainty about the level,
extent or duration of impacts, or the effectiveness of proposed mitigation
measures. Monitoring provides the information for the validation process
and the feedback needed for verifying the predictions and improving the
monitoring programme as well as to justify any later changes to a project.
6. Activity/Project Specific Guidelines
Waste Discharges
6.1 Waste discharges shall be pretreated to levels sufficient to protect
the sewerage system downstream and the receiving water. The near-field
and far-field effects shall be addressed by quantitative modelling techniques.
Model for predicting the physical, chemical and biological processes which
determine the transport and fate of pollutants associated with outfalls
shall include initial dilution, effects of stratification, advection towards
shore, coliform die-off, dissolved oxygen depletion, dissolution of metals,
particles settling, biochemical conversion and bioaccumulation of trace
contaminants.
6.2 Discharges into inland waters where there are little base flows to
provide sufficient dilution and dispersion are normally required to be
treated to secondary level as a minimum. However, secondary treatment
may result in high levels of dissolved nutrients in a form which may stimulate
algal blooms and further treatment to remove nutrients may be required.
Secondary or tertiary sewage treatment plants are complex and are often
faulty due to operation, repair and maintenance problems. On-site treatment
systems are therefore discouraged and connection to public sewers leading
to a municipal treatment system is always the preferred solution. Discharge
into public sewerage systems must not overload the hydraulic capacities
nor contain substances that will cause damage to the sewerage systems.
6.3 Pollutants of major concerns include floatables, pathogens, particulates,
toxic substances and nutrients. The predictability of the fate of toxic
substances in the receiving water body are uncertain and source control
is the only feasible means of control. Toxic substances that may interfere
with or pass through the treatment processes must be eliminated or controlled
at source. Wastes that contain pathogens shall be discharged a sufficient
distance from shellfish harvesting, mariculture, beaches and other water-contact
areas. The use of disinfection shall be carefully evaluated as it can
result in increase in effluent toxicity and has its own adverse effects
on the marine environment. If disinfection by chlorination is unavoidable,
de-chlorination facilities shall be provided.
6.4 On-site treatment and disposal facilities shall include adequate
and appropriate stand-by and other provisions to prevent and minimise
breaking down of the facilities, to facilitate rapid repair and to avoid
by pass of waste discharge. By-pass outfall designed to cope with an emergent
situation shall be located away from any sensitive receivers as far as
possible.
6.5 For collection of waste discharge to a public sewer which is the
generally preferred approach, impacts on the downstream public sewerage,
sewage treatment and disposal facilities shall be assessed as follows:
- The assessment shall cover all sewage collection, treatment and disposal
facilities affected by the project. The actual extent will depend on
the quality and quantity of wastewater discharged, the capacity of the
sewerage systems, and the assimilative capacity and water quality objectives
of the receiving water bodies.
- The assessment shall take into account wastewater discharges into
the same sewerage systems under consideration from all existing sources,
and committed and planned developments to be implemented within the
same time frame of the project.
- Prediction, assessment and evaluation of impacts shall be based on
the established principles and guidelines available in Hong Kong.
- The capacity of the sewage treatment and disposal facilities and
sensitivity of the receiving water bodies may limit the quality and
quantity of wastewater that could be discharged into a sewerage system
and this may render connection of the project to the nearest sewerage
system not acceptable. In such circumstances, connection to sewerage
systems outside the catchment area where the project is located or on-site
treatment and disposal will be required.
- Wastewater discharges from the project shall not cause over-loading
or deterioration to the service conditions of any sewage collection,
treatment or disposal facility.
- The Director shall take advice from the Director of Drainage Services
on the scope and programme of all recommended mitigation if diversion
of flows or addition, alteration, disruption, or modification to any
existing public sewage collection, treatment or disposal facility is
involved.
Breakwaters, Reclamations and Other Works Involving Coastline
and Bathometry Modifications
6.6 Assessment shall focus on the impacts on overall reduction in assimilative
capacity of the affected flow channels, hydrology, sedimentology, and
water quality of the water body behind and outside the structures (e.g.
typhoon shelter) . Modelling shall be used to quantify these effects.
The structures shall be designed to avoid creation of water stagnation,
to prevent entry and accumulation of pollutants from waste discharges
and contaminated runoff in the poor flushing areas, and to prevent sediment
accumulation and contamination.
Dredging, Sand Filling, and Dumping
6.7 Simulation modelling can be used to determine the short-term as well
as the long-term fate of sediment. The size of the plume depends on type
of dredging equipment use, quantities of sediment suspended and hydrodynamic
conditions at the sites. The nature of the sediment is the first factor
to consider to predict sediment suspension. When toxic or harmful constituents
are present in the sediment, the chemical effects are important and shall
be addressed. Contaminants in the sediments shall be determined and analysed
by bulk sediment, elutriate and pore water tests. In some special circumstances,
assessment on the effects such as toxicity and bioaccumulation may be
necessary. The principle in managing contaminated sediments is to minimize
disturbance and isolate them from contact with the aquatic environment.
Dredging of contaminated sediments shall be avoided as far as possible
and a survey and sampling of contamination of bottom sediments shall be
undertake before dredging. Mud disposal proposal must include detailed
assessment of the characteristics of the sediment, objective comparison
of all alternatives for disposal and careful selection of site and disposal
methods, careful selection of dredging method and equipment, and in accordance
with the guidelines adopted by the contracting parties of the London Dumping
Convention.
Thermal Discharges
6.8 Assessment shall be based on mathematical model studies using plume
model to characterize the near-field and hydrodynamic and advection-diffusion
model to characterize the far-field to predict the extent of the impacted
area which can be defined by criteria based on temperature rise and the
residual of biocides used. Intake site shall avoid spawning grounds and
beach areas and shall be located away from polluted water. The combined
effects of thermal discharge and other discharges may result in a complexity
of plumes with the possibility of additive or synergistic effects of different
pollutants. Mitigation shall include heat reduction by cooling towers
or beneficial uses of the heat, installation of diffusers, and minimisation
of the use of anti-fouling agents by using only effective doses or by
the use of alternative means of control.
Toxic Substances
6.9 Toxic substances can be classified into five subcategories: a) nonmetallic
inorganic toxicants (e.g. ammonia, cyanide); b) heavy metals and submetallic
inorganic substances (e.g. mercury, cadmium); c) easily degradable organic
toxicants (e.g., volatile phenols, benzene); d) refractory organic substances
(e.g., DDT, PCB, PAH); and e) radionuclides. Techniques for assessing
potential human health risks involve critical pathways, specific activities
and mass balances approaches. All these approaches require detailed knowledge
of sources, transport, diffusion, flushing times, sinks, etc. to allow
calculation of probable concentrations to be expected in the system and
ultimately of human exposures. The effects of toxic substances on ecological
systems are more difficult to evaluate than human health effects. The
two techniques however involve the same fundamental principles. Basic
elements for consideration in ecological health risk assessment include
the types of stress, level of ecological organization, ecosystem type,
spatial and temporal scales at which the effect of concern occurs. Assessment
shall aim to quantify the routes, transformations, and sources of toxic
substances released to the aquatic environment, to determine the bioavailability
and bioaccumulation of the toxic substances and to determine the relationship
between exposure to toxic contaminants and effects.
6.10 The most effective and viable approach is to reduce at source the
amount of these substances entering the sewer or discharging to the environmental
waters. The four basic source control alternatives are pollution prevention,
pretreatment, recycle and reuse.
Non-point Pollution Sources and Stormwater Discharges
6.11 Non-point or diffuse sources include all inputs that are not point
sources. Assessment shall aim at identifying all the sources including
erosion from construction sites; runoff from urban areas; erosion from
agricultural lands, roads; runoff from livestock farms; runoff from land
contaminated by fertilizers, pesticides, and herbicides; and, atmospheric
deposition. Prediction of impacts can be made by diffuse pollution models
ranging from simple statistical routines and screening models to the more
sophisticated continuous models. These diffuse pollution models simulate
the generation and movement of water and its pollution content from the
sources to the points of discharge into the receiving water, and can interface
with receiving water quality models to assess the impact of non-point
source pollution on the aquatic environment.
6.12 The strategy to control non-point source pollution is to prevent
or minimize the potential of pollutants coming into contact with rainfall
or runoff. The most common source reduction measures include elimination
of expedient connections, prevention of illegal dumping of wastes, coverage
of chemical storage areas, prevention and containment of spills, minimization
of chemical applications, catch basin cleaning, erosion control, and landuse
control. Devices designed to control pollution in a drainage system include,
minimization of directly-connected impervious areas, provision of swales,
filter strips, infiltration basins and trenches, detention facilities,
and artificial wetlands.
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