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Environmental Impact Assessments

On Site Effluent Disposal

Residential development often has a major impact on the flora and fauna of nearby bushland. One of the major problems is that of polluting chemicals and nutrients which flow off lawns and gardens in stormwater run-off and into drainage lines and eventually the creeks of adjacent bushland areas. A further impact is caused by concrete and mortar, which cause changes to the acidity (pH) of natural soils.


Many species of native vegetation are unable to cope with increases in nutrients such as nitrogen or phosphorus, or changes in the water table. Changes in pH also affect the availability of soil nutrients. Aquatic animals, ranging from micro-organisms and small invertebrates to tadpoles, often cannot tolerate the detergents, heavy metals, siltation and garden chemicals that enter the water ways downstream of a house site. Often this chemical pollution results in the death of sensitive native plants and animals and the invasion of nutrient-loving weeds. Many animals, such as platypus, which may not be directly affected by the pollutants, are deprived of their natural habitats and food sources and can also be displaced.


Runoff water may travel very slowly through the soil, or quickly as overland flow in stormwater. The chemical load may therefore travel quickly or slowly. The concentration of pollutants is affected by the rate of water flow and the dilution effects of stormwater. The total quantity of pollutants in the environment however, is not affected by dilution or flow rate.


Some soil types adsorb chemicals and release them slowly over time. This process of adsorption is primarily due to the presence of clay particles which are capable of retaining chemicals. Adsorption is a weak chemical bond formed between two chemical particles. Sandy soils are not as effective at capturing chemicals as clay soils because they are unable to form these soil-chemical complexes. Clay particles in soil initially adsorb excess nutrients in subsoil flow but, with continual input of effluent water, all the clay particles can become saturated and may no longer act as a nutrient sponge. Nutrients which enter saturated soil will not be adsorbed but either flow through to creeks or remain free in the soil.


Shallow, skeletal sandy soils do not provide the absorption capabilities required by on site effluent disposal systems. Any absorption system used on a site such as this will result in increased nutrient loads and moisture content within the soil. Such conditions can result in drastic and harmful changes to the vegetative community of the development site including the loss of native plant species and the infestation of weeds. Any changes in vegetation often will also affect the fauna of the area.


Runoff can also have a direct effect on frog species and other aquatic animals in the area. Flora and fauna located in lower parts of the catchment area may also be affected gradually as it receives contaminated runoff.


Some types of vegetation are particularly sensitive to such pollution, for example hanging swamps and sandstone heath communities. It is possible to protect sensitive vegetation units to some extent by providing a natural vegetation buffer zone between the sensitive vegetation and a house site. Urban runoff (fertilisers, detergents, heavy metals) carried in groundwater will require at least a 50 to 100 metre buffer zone and an absorption strip of vegetation and soil to slow and buffer impact on sensitive vegetation units. An artificially constructed wetland could be provided downslope of any house and lawn areas to polish surface effluent.


Commercial effluent disposal systems use various means to remove polluting nutrients from the water before it is released into the surrounding environment. One method is to add a chemical called ferric alum to the sewage tanks. This captures nitrogen and phosphorus, and settles to the bottom of the tank. This resulting sludge needs to be pumped out annually for the system to continue to work effectively. Another nutrient absorbing process uses “red earth”, a by-product of bauxite mining or iron ore blast furnaces. In this method, effluent water passes through the red earth, where the nutrients attach to the earth particles. This system also requires physical removal, but at longer time intervals, of ten or more years.


On site effluent disposal systems usually disperse the effluent water onto lawns. The good work of nutrient removal done by the effluent system is often undone when home owners add artificial fertiliser to the lawn to make it grow well.


A commitment to ongoing weed control may also be required. Weeds may be spread into the bush by flowing water, but also by wind, dumped garden waste and fruit-eating birds.


Dense planting of nutrient absorbing local native vegetation may help buffer the impact of excess nutrient and water flow. Such species also provide a fire retardant vegetation barrier, because of their high leaf water content and low flammable oil content. Some examples of these plants are listed below for the Sydney sandstone soils. Trial and error will show which ones work for a particular site. For other soils and localities, a local native plant supplier will be able to recommend suitable local plants which are available through the nursery trade.


Examples of plants for an effluent irrigation area include:


Grey Gum Eucalyptus punctata
Swamp Mahogany Eucalyptus robusta
Cedar Wattle Acacia elata
Lemon-scented Myrtle Backhousia citriodora
Grey Myrtle Backhousia myrtifolia
Red Bottlebrush Callistemon citrinus
Broad-leaved Paperbark Melaleuca quinquenervia
Brush Cherry Syzygium luehmannii
Native Fig Ficus rubiginosa


We are able to recommend a colleague who specialises in effluent disposal assessments and design.

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