Geology will not support the use of On-site septic tanks and (STS) soakaway system at Pak Lap

According to the Minutes of 1032^nd  Meeting of the Town Planning Board held on 26.4.2013, at Para 82, it is stated:

“The Chairman said that on sewerage concern, the use of septic tanks for sewage treatment purpose in Small House developments was common and acceptable”

THIS IS WRONG, 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 Pak Lap or any of the other Country Park enclaves.

Proposals in Draft OZP for treatment of sewage and wastewater.

According to the Draft OZP, there is no existing sewer or planned public sewer for the area, 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 their own individual STS system, which need to comply with relevant standards and regulations, such as EPD’s Practice Note for Professional Person (ProPECC) 5/93.

EPD standards and regulations

According to the relevant EPD standards and regulations, this involves dispersing untreated wastewater into the surrounding soil and relying on the soil to remove the polluting material from the wastewater and adequate purification can only be achieved after the wastewater has travelled a fairly long distance through the ground. The relevant standards and regulations also note, pollution would result if a STS is located too near to a beach or a stream. 

Details of planned zonings for Pak Lap can be found in the Draft Pak Lap Outline Zoning Plan at the following link:

http://www.info.gov.hk/tpb/en/papers/TPB/1044-tpb_9420.pdf

Underlying geology of Pak Lap

The problem with this is the underlying surface sediment in Pak Lap comprises porous and highly permeable sedimentary deposits, which are a mixture of alluvium (Qa) and beach deposits (Qb). Such deposits allow for rapid drainage, so no matter how far the distance, interstices in these deposits means adequate purification cannot be achieved before the wastewater discharges into the amphioxus communities in Pak Lap Wan.

Amphioxus communities in Pak Lap Wan

In 2003 - 2004 surveys were conducted by City University of Hong Kong in Pak Lap Wan, to determine the extent of distribution of amphioxus as well as their abundance, habitat preference and community structure. Results showed the presence of amphioxus in shallow water depth between 5 and 23 m with high population abundance being found at some specific locations in Pak Lap Wan.

Sediment analysis indicated that amphioxus was only confined to sediments containing a high percentage of sand and granule, with a median diameter, low organic content and low moisture content. Further research on the habitat requirements of amphioxus was conducted by monitoring the sediment and water quality at one site in Pak Lap Wan from November 2003 to October 2004 and the results indicate that seawater at the amphioxus habitats had two essential characteristics:

1. Oceanic;

2. Low content of suspended solids.

Such findings were consistent with results obtained from laboratory observations, namely that amphioxus could not survive at low salinity and high suspended solids in the water could induce physical damage on the animal’s oral cirri. From this, it was concluded that clear oceanic water combined with sand sediment with low organic content are the most important habitat requirements for amphioxus.

See results of study at the following link:

http://dspace.cityu.edu.hk/handle/2031/4884

Zonings proposed in Draft Pak Lap Outline Zoning Plan (S/SK-PL/1)

Geological Survey Map of Pak Lap village area

Legend of Geological Survey map

Overlay of Draft Pak Lap Outline Zoning Plan on Geological Survey Map of Pak Lap village area

Comment

The Town Planning Board has NOT BEEN PROPERLY BRIEFED on the Geological Contraints of allowing Small House type development in Pak Lap.

Because of this, the Draft Pak Lap Outline Zoning Plan (S/SK-PL/1) is an INVALID PLANNING DOCUMENT.

The underlying geology will not support the use of on-site septic tanks and soakaway (STS) systems to disperse untreated wastewater into the surrounding soil in Pak Lap. Pollution will not be filtered as proposed, and the discharge of untreated wastewater into the environment will lead to irreparable damage being caused to the ecologically sensitive amphioxus communities in Pak Lap Wan.