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Woolies site too wet for effluent, say locals

Wastewater engineer Duncan Dey and Garry Scott undertake soil testing on the Station Street site. Photo Jeff Dawson.

The determination of Woolworths S96 application for its proposed supermarket in Mullumbimby may well hinge on the technical assessment as to the adequacy of the site to support onsite sewage management, according to local activists. A crucial factor in that assessment is the ability of the site’s receiving soil to contain the nutrients, pathogens and the hydraulic load emanating from the treatment system, says Mullum wastewater engineer Duncan Dey.

Saying that Byron Shire Council, Woolworths consultant BRW or the NSW Department of Planning appear not to have undertaken soil testing, Mr Dey and colleague Garry Scott did so last Thursday. Mr Dey drilled a 75mm hole to a depth of over one metre in the centre of the area that appears, from plans submitted to NSW Planning, to be Woolworths’ latest proposed sewage disposal area. 

Dey and Scott noted that vegetation on most of the Station Street site and within what appears to be the Woolworths disposal area includes a considerable number of sedges. Samples of two of these sedges were taken to local wetland ecologist Jo Green for identification. 

One type was Cyperus polystachyos and the other likely to be Baumea sp. Both are indicative of swamp conditions and of soils that are for considerable times 100% saturated, according to Dey and Scott 

‘The clay is riddled with ferrous staining know as striations, rust marks caused by moisture lying in the soil matrix for long periods without moving – the moisture corrodes the iron content of the soil,’ Mr Dey told The Echo. ‘Ferrous staining indicates long-term saturation, due to low soil permeability.  It was even noted in the 200mm of topsoil. 

‘When the investigation hole was complete, water rose in it to 50mm below the soil surface. The plants, the soil and the watertable all say the same thing: it’s a location that is often 100% saturated for long periods. 

‘The original assessment of the soil and suitability for onsite sewage management undertaken for Mallams appears to have made no comment on the level of the watertable, on the striations or on the plants indicative of saturated soils. It may be that the abnormal long period of drought that the north coast was experiencing at the time and the low watertable presented a more encouraging picture, however even in times of drought it would be expected an experienced observer would still have noted the indicative plants and rust stains.’

Mr Dey said that, after decades of undertaking site assessments for onsite sewage systems, he had never had a client ask him to investigate a site in 100% saturated soil.

‘No normal person is so ludicrous as to propose an onsite disposal area in a saturated heavy clay soil that is totally inadequate for such purposes. No authority in its right mind would allow onsite wastewater disposal within such soil.’