Septic tanks and soakaway pits (STS) systems will not work in country park enclaves

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 1032^nd  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”:

http://www.epd.gov.hk/epd/english/environmentinhk/water/guide_ref/files/guide_wpc_dv.pdf

²See para 82 of the Minutes of 1032^nd  Meeting of the Town Planning Board held on 26.4.2013:

http://www.info.gov.hk/tpb/en/meetings/TPB/Minutes/m1032tpb_e.pdf

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

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:

www.scmp.com/news/hong-kong/article/1296797/h7n9-bird-flu-may-be-spread-through-human-faeces-research-suggests

⁹Septic System Impact on Surface Waters – A review for the Inland Northwest. 2005. [online] Tri-State Water Quality Council. Available at:

watercenter.montana.edu/training/decisions/landuse/docs/septic_system_impacts.pdf

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.