The Town Planning Board has recently sought comments on
three Draft Outline Zoning Plans for areas in “country park enclaves”, namely:
1. Hoi Ha - Draft Hoi Ha Outline Zoning Plan No. S/NE-HH/1;
2. Pak Lap - Draft Pak Lap Outline Zoning Plan No. S/SK-PL/1;
3. So Lo Pun - Draft So Lo Pun Outline Zoning Plan No. S/NE-SLP/1.
In terms of dealing with sewage issues, there is no existing
public sewer or planned sewerage project in any of these enclaves and at
present, each house is typically served with its own on-site septic tanks and
soakaway (STS) system. Any increase in residential developments would require
additional facilities, including each property having their own individual STS
system and these would need to comply with relevant standards and regulations,
such as EPD’s Practice Note for Professional Person (ProPECC) 5/93. According
to the relevant EPD standards and regulations (Guidance Notes on Discharges
from Village Houses1), this means:
1. Dispersing untreated wastewater into the
surrounding soil and relying on the soil to remove polluting material from the
wastewater;
2. Polluting material being removed from the wastewater only
after it has travelled a long distance in the soil; and
3. An STS system is not feasible in areas with a high
ground water table.
This is accepted policy. According to the Minutes of 1032nd Meeting of the Town Planning Board held on
26.4.20133, at Para 82,
the Chairman of the Town Planning Board stated:
“ … on sewerage concern, the use of septic tanks for sewage treatment
purpose in Small House developments was common and acceptable”2
Unfortunately, this statement and the Government’s whole
approach to STS systems in these enclaves is wrong, for the following reasons:
Issue 1 – The
underlying geology in these enclaves will not support the use of STS systems
The use of STS systems for sewage treatment purpose in Small
House developments, as proposed in the Draft Outline Zoning Plans is not
acceptable, because the Town Planning Board have not been briefed about the Geological Constraints of allowing the
use of on-site septic tanks and soakaway (STS) systems in these enclaves.
The surface sediment is porous and highly permeable
The first problem is the superficial surface sediment in each
of these enclaves comprises porous and permeable sedimentary deposits, as
follows:
1. The surface sediment in Hoi Ha is a mixture of alluvium, beach
deposits and debris flow deposits.
2. The surface sediment in Pak Lap is a mixture of alluvium
and beach deposits.
3. The surface sediment in So Lo Pun is a mixture of
terraced alluvium, debris flow deposits and estuarine and intertidal deposits.
All of these deposits are porous and permeable and allow for
rapid drainage, so no matter how far the distance, adequate purification cannot
be achieved and because each of these enclaves is at the mouth
of a watercourse, this means all of the pollutants in the wastewater will
ultimately be deposited untreated into the sea.
See the Geological Survey maps below:
Hong Kong Geological Survey map (Sheet 8 – Sai Kung) - Geology at
Hoi Ha3
Key: Alluvium
(Qa), Beach Deposits (Qb) and Debris Flow Deposits (Qd)
Hong Kong Geological Survey map (Sheet 12- Clearwater Bay) - Geology
at Pak Lap4
Key: Alluvium (Qa) and Beach Deposits (Qb)
Hong Kong Geological Survey map (Sheet 3 – Sheung Shui) - Geology at
So Lo Pun5
Key: Alluvium (Qpa), Debris Flow Deposits (Qpd)
and Estuarine and Intertidal Deposits (Qi)
Issue 2 – The
underlying geology means these enclaves are susceptible to alluvial flooding
The second problem is that because of these deposits are
permeable and lie on top of impermeable deposits and because each of these
enclaves is on a watercourse, it is susceptible to alluvial flooding. Alluvial
(or permeable superficial deposits) flooding occurs where rivers or streams are
hydraulically connected to alluvial material because the river valley comprises
permeable deposits overlying impermeable rocks. See an illustration of how this
occurs in the diagram below:
Diagram illustrating how alluvial flooding occurs6
Evidence of Alluvial Flooding in the three enclaves
One of the most obvious signs of this in such areas is the
presence of surface flooding and illustrations provided in each of the Draft
Outline Zoning Plans shows evidence to suggest that alluvial flooding is taking
place in each of these enclaves. See the pictures below:
Figure 5c in TPB
Paper 9368 - Draft Hoi Ha Outline Zoning Plan
The water logging
in Photo 4, suggesting the presence of a high water table as a result of
Alluvial Flooding
Plan 12 in TPB
Paper 9420 – Draft Pak Lap Outline Zoning Plan
The presence of
“water ferns”, suggesting the presence of a high water table as a result of Alluvial
Flooding
Figure 5c in TPB
Paper 9331 - Draft So Lo Pun Outline Zoning Plan
Freshwater Marshes at the Central Part of So
Lo Pun. The water logging in Photo 10, suggesting the presence of a high water
table as a result of Alluvial Flooding
In such systems, the groundwater in the
alluvial deposits and the stream are hydraulically
connected, which means they are not separate, but are part of the same
interconnected system. The implications of this on allowing the use of STS
systems in each of these enclaves are considerable:
1. Polluting material will not be removed from the
wastewater, but will flow untreated directly through the underlying sedimentary
deposits into the stream;
2. On-site septic tanks and soakaway (STS) systems
will not function because of the high ground water table in each of these
enclaves; and
3. Buffer zones will not work because of the
hydraulic connectivity between the groundwater in the alluvial deposits and the
streams.
The implications for human health and the likely
impact on the environment are of even more concern
Issue
3 - Threat to human health
Outbreaks of groundwater borne pathogens
linked to STS systems in the USA7
According to investigations conducted in the
Northwest United States, outbreaks of groundwater borne viral pathogens linked
to Septic Tank Systems have been documented in several locations, with the
following significant findings:
1. More than 400 people were infected with
gastroenteritis related to contaminated groundwater in 1995;
2. A number of major outbreaks of
gastroenteritis have been attributed Norwalk-like virus in numerous states; and
3. Improperly constructed drainfields, high
water tables or inappropriate geological
settings can allow pathogenic bacteria and viruses to reach groundwater,
where they can survive for days and travel up to 30 metres.
These findings are significant, because they
relate to “viral outbreaks”. SARS and Bird Flu are both viral infections and
Hong Kong has had outbreaks of both of these diseases in recent years - H5N1 in
1997 and SARS in 2003. Moreover, the threat of a virus with future global pandemic
potential reaching, then spreading from Hong Kong has not subsided.
Specific health concerns in Hong Kong
In an article in the South China Morning Post on 15 August
201318 on an H7N9 bird flu outbreak in China,
the results of a study show: “The H7N9 bird flu that emerged on
the mainland in March may be spreading through human faeces”. In respect of these findings, Hong Kong microbiologist Yuen
Kwok-yung is quoted as saying: "This has important implications on the
infection control strategies for H7N9 virus infection, as the influenza virus
in stools may contaminate the surrounding environment".8
It should be noted, Hong Kong’s contingency plan for
pandemics is mainly geared towards standard control measures like personal and environmental hygiene, social
distancing and hospital infection control.
Issue 4 - Threats
to species and habitats of conservation
and ecological significance
Releasing untreated wastewater into the environment will
have devastating consequences9
It is widely recognized that conventional septic systems are
not adequate for removing nitrate, and only partly remove phosphorus, certain
pathogens, and certain other compounds, especially where soils or ground water
conditions are marginally suitable. Anything that is not removed by the soil
will end up in groundwater. It is important to note the two main nutrient
products of STS systems are nitrogen and phosphorous.
Nutrient enrichment of surface waters by nitrogen and
phosphorus is one of the leading causes of pollution of lakes, rivers, and
coastal bays in the United States. Nutrient enrichment can cause a host of
negative ecological effects on streams and lakes, including loss of water
clarity, proliferation of aquatic weeds, algae blooms, and drop-offs in
dissolved oxygen, a critical factor for fish and other aquatic life.
In terms of the three enclaves, Hoi Ha Wan is home to coral
communities and So Lo Pun is home to an Ecologically Important Stream (EIS)
and marshland habitats, both of which will be highly susceptible to any
increases in nitrogen and phosphorous levels in the groundwater, which because
of the local geology will not be filtered by STS systems as planned.
Suspended and bedded sediments (SABS)
In addition to nutrients, Septic Tanks also produce Suspended
and Bedded Sediments (SABS). SABS are defined by the US Environmental
Protection Agency (EPA) as particulate organic and inorganic matter that
suspend in or are carried by the water, and/or accumulate in a loose,
unconsolidated form on the bottom of natural water bodies10.
According to the Civil Engineering and Development
Department (CEDD) in Hong Kong:
“… human activities such as sewage discharge generate suspended sediments”11.
The effect SABS have on fish populations include:
1. Behavioral effects, such as inability to see prey or feed
normally;
2. Physiological effects, such as gill clogging; and
3. Effects due to sediment deposition, such as burial and
suffocation of eggs and larvae.
In terms of the three enclaves, Pak Lap Wan is home to a
healthy amphioxus community whilst in So Lo Pun the Ecologically Important
Stream (EIS) and its adjoining freshwater marsh support a healthy and natural
population of Oryzias curvinotus, all
of which will be highly susceptible to the effects of SABS.
How does this
relate to the enclaves
The full extent of the problem can be seen when the Proposed
Land Use map for each of the enclaves in the Draft Outline Zoning Plan is
overlain onto the Hong Kong Geological Survey Map of each of the enclaves, as
is done below:
Proposed Land Uses
– Figure 11 - Draft Hoi Ha OZP
Overlay of Proposed
Land Uses map on
Hong Kong
Geological Survey map of Geology at Hoi Ha
Land Use Proposal
– Plan 1 - Draft Pak Lap OZP
Overlay of Land Use
Proposal map on
Hong Kong
Geological Survey map of Geology at Pak Lap
Proposed Land Uses – Figure 12 - Draft So Lo Pun OZP
Overlay of Proposed
Land Uses map on
Hong Kong
Geological Survey map of Geology at So Lo Pun
Summary
To summarise this information:
1. On-septic tanks and soakaway pits (STS) systems will not
work in Hoi Ha, Pak Lap or So Lo Pun because the underlying geology will not
support their use;
2. Each of the proposed development areas in these three
enclaves is susceptible to alluvial flooding because of the underlying geology,
which even according to the governments own guidelines means STS systems cannot
be used in such areas;
3. Buffer zones will not separate the discharges from STS systems
from the streams, no matter how great the distance, because the groundwater
in the alluvial deposits are hydraulically connected to the water in
the stream, which means they are not separate, but are part of the same
interconnected system;
4. The use of STS systems in these
enclaves also poses an unacceptable health risk. One of the strategies for
preventing the spread of a global pandemic from Hong Kong is environmental hygiene,
something which using STS systems in these enclaves threatens. On this matter,
understanding the implications of the following information is critically
important - H7N9 bird flu may be spreading through
human faeces and this has important implications on the infection control
strategies for the virus, as the influenza virus in stools may contaminate the
surrounding environment;
5. The use of STS systems in these
enclaves also poses an unacceptable risk to the environment, as wastewater will
neither be filtered nor buffered as proposed in the government guidelines, with
potentially devastating consequences for the environment.
It should be noted, these issues
are not confined to these three enclaves. Similar issues are evident in Tai
Long Sai Wan and are likely to be present in most of the enclaves under threat
of development. This is not selective criticism, it is a straight forward
question of geography and the siting of human habitation.
Conclusion
In short, the government needs to
find an alternative solution to the use of on-septic tanks and soakaway
pits (STS) systems in Hoi Ha, Pak Lap and So Lo Pun and any other enclaves
which are situated in similar circumstances.
References
1See EPD Document “Guidance
Notes on Discharges from Village Houses”:
2See para 82 of the Minutes of 1032nd Meeting of the Town Planning Board held on
26.4.2013:
3Hong Kong Geological Survey map. Sheet 8 – Sai
Kung.
4Hong Kong Geological Survey map. Sheet 12 –
Clear water Bay.
5Hong Kong Geological Survey map. Sheet 3 –
Sheung Shui.
6Mckenzie,A.A, Bloomfield, J.P, Hulbert, A and Rutter H.K, 2010. Confidence and Groundwater Flood
Susceptibility Mapping. [online]
British Geological Survey. Available at:
ncg.nuim.ie/gisruk/materials/proceedings/PDF/6B2.pdf
ncg.nuim.ie/gisruk/materials/proceedings/PDF/6B2.pdf
7Septic
System Impact of Surface Waters. 2005. Tri-State Water Quality Council.
8H7N9
bird flu may be spread through human faeces, research suggests. [online]. SCMP. 15 August 2013
Available at:
9Septic
System Impact on Surface Waters – A review for the Inland Northwest. 2005. [online] Tri-State
Water Quality Council. Available at:
10What
are Suspended and Bedded Sediments (SABS)? [online] 2003. US
Environmental Protection Agency. Available at: http://water.epa.gov/scitech/datait/tools/warsss/sabs.cfm
11Suspended
Sediment in Hong Kong Waters.GEO Report No. 106.[online] By S. Parry.CEDD
HKSAR Government. November 2000. Available at: http://www.cedd.gov.hk/eng/publications/geo_reports/doc/er106.pdf
Note
Hong Kong Geological Survey maps are available from the Maps
Publication Centre, Survey and Mapping Office, 23/F, North Point Government
Offices, 333 Java Road, North Point, Hong Kong.
