Hong Kong Countryside Watch: Underlying geology will not support the use of on-site septic tanks and soakaway (STS) systems in Hoi Ha, posing a threat to the coral communities in Hoi Ha Wan Marine Park

Introduction

This is a comment on Draft Hoi Ha Outline Zoning (OZP) Plan No. S/NE-HH/1.

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.

Underlying geology of Hoi Ha Village

The problem with this is the underlying surface sediment in Hoi Ha village comprises porous and highly permeable sedimentary deposits, which are a mixture of alluvium (Qa), beach deposits (Qb) and debris flow deposits (Qd). 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 reaches the sea, and because Hoi Ha village is at the mouth of a watershed, this means all of the pollutants in the wastewater will ultimately be deposited in Hoi Ha Wan.

See the maps at the bottom of the page..

Threat to coral communities in Hoi Ha Wan

Coral communities are sensitive receivers and the coral communities in Hoi Ha Wan lie downstream from Hoi Ha village. The watershed feeds into these communities. Coral communities thrive in nutrient-poor habitats by containing many species whose complex food chains recycle essential nutrients with great efficiency, making them especially sensitive to any process that disrupts recycling. For example, if wastewater is not properly treated nutrients such as nitrogen and phosphorous in the waste can stimulate prolific growth of algae, which overgrows corals, or form algal mats which prevents new polyps from settling and establishing themselves. Worst still, some chemicals in the wastewater could simply kill the more sensitive species of coral in the bay.

Threats to coral communities in Hoi Ha Wan from pollution and sediment specifically highlighted

Hoi Ha Wan is an enclosed bay. When it was designated as an SSSI, one of the stated reasons was because critically the bay was not affected by pollution and the only identified hazard to the reef was that future changes to land use may cause water pollution or sedimentation. When the Marine Park was designated, pollution was specifically identified as a threat to the coral communities, but water quality in the park was good because there was no large water disposal nearby. It was also noted, the government’s approach to water treatment was not treatment, but disposal, yet this is what is now being planned.

Comment

Given the short distance between Hoi Ha village and the sea and because of the porous and permeable nature of the underlying surface sediment in the area, what is certain is the STS system will not remove pollutants from the wastewater as envisaged. This untreated, nutrient rich mix will then feed into Hoi Ha Wan and over time this will result in the degradation or destruction of the coral communities.

It is important to note, two of the worst nutrient threats to coral communities are excess nitrogen and phosphorous, whilst two of the main nutrient products of STS systems are nitrogen and phosphorous.

Summary

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 Hoi Ha. 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 coral communities in Hoi Ha Wan.

Geological Survey Map of Hoi Ha Village