Risk Category and Constraints of Property (from Risk Maps and LCA Rating) Basic Site and Soil Assessment Land Capability Assessment Standard Designs and Sizing Tables Detailed System Design Cumulative Risk Assessment and/or Pollutant Modelling Residential Development Applications Low on-site wastewater risk X X Moderate on-site wastewater risk X X High on-site wastewater risk X X X Fails to comply with setback distances X X <4,000 m2 X X X Within a potable drinking water catchment or bore field X X X Commercial or Light Industrial Development Applications Low on-site wastewater risk X X Moderate on-site wastewater risk X X High on-site wastewater risk X X X Fails to comply with setback distances X X X The proposed built out areas will exceed 60% of the total land area X X X Within a potable drinking water catchment or bore field X X X Table 3.2: Assessment Framework for Unsewered Development for Subdivision Developments Risk Category and Constraints of Property and Proposed Subdivision Lots (from Risk Maps and LCA Rating) Land Capability Assessment Standard Designs and Sizing Tables Detailed System Design Basic Lot Configuration Detailed Lot Configuration with Land Application Cumulative Risk Assessment and/or Pollutant Modelling Residential Subdivision Development Applications
Low on-site wastewater risk X X X Moderate on-site wastewater risk. X X X High on-site wastewater risk X X X X Any lots that fail to comply with setback distances X X X Any lots <4,000 m2 X X X X Any lots within a potable drinking water catchment or bore field X X X X Commercial or Light Industrial Subdivision Development Applications Low on-site wastewater risk X X Moderate on-site wastewater risk X X X High on-site wastewater risk X X X X Any lots that fail to comply with setback distances X X X X Any lots <4,000 m2 X X X X Any lots within a potable drinking water catchment or bore field X X X X 3.1.2 Detailed System Design A detailed system design will entail a full justification for the selection and sizing of all on-site system components. It should incorporate a mean monthly water balance for land application systems (an example water balance is provided in the model site report and in the Standard Designs supplements to the Code). Details on the configuration and basic engineering design of proposed land application systems should be provided to guide installers and regulators toward the desired outcome. A detailed system design would be required as part of a LCA for single lots and subdivisions with a High risk category. 3.1.3 Basic Lot Configuration A basic lot configuration should be submitted with residential and commercial/light industrial subdivision applications on lots classified as Low risk. A basic lot configuration is a surveyed site plan showing the following features: All proposed lots within the subdivision (including the size of each lot)
Electrical Conductivity (salinity) Electrical conductivity is used to infer the salinity of the soil and its potential impact on plant growth. The EC. of the saturated extract (ECe) is calculated by first measuring the electrical conductivity of 1:5 soil in water suspensions and using appropriate multiplier factors to convert EC (1:5) to ECe. It can be measured with a hand-held EC meter. Moderate (range) to high (range) salinity typically inhibits or limits growth of many plant species, thereby reducing plant uptake of nutrients and increasing the risk of erosion on the land application area. An EC test should be carried out on a soil sample from each horizon from each test pit excavated during the soil assessment. This test can be carried out using a hand held meter, or in a soil testing laboratory. Sodicity (exchangeable sodium percentage The proportion of sodium on the cation exchange sites reported as a percentage of exchangeable cations. Levels above 6% may cause soil structural problems and reduced permeability. ESP should be carried out if there is any evidence of dispersion, slaking, or structural decline in the soils on or near the potential land application area/s. This test is carried out in a soil testing laboratory. Phosphorus adsorption (mg/kg) Phosphorus adsorption (P-sorption) is the capacity of soils to ‘take up’ or immobilise phosphorus, which is present in various forms in wastewater and treated effluent. Sandy soils typically have low capacity and do not necessarily need to be tested if a conservatively low (100 mg/kg) rate is assumed for any nutrient balances undertaken. Clay soils and soils high in iron and/or aluminium often have high (up to 450 mg/kg and greater). Sites where a nutrient balance is to be carried out, e.g. sites where risks of phosphorus export should be minimised, or where the soil is intended to be used as a phosphorus removal mechanism. There are a range of tests, carried out in a soil testing laboratory. sorption sorption sorption
and vegetation. The nutrient balance used here is based on the simplistic DLG (1998) methodology, but improves this by more accurately accounting for natural nutrient cycles and processes. It acknowledges that a proportion of nitrogen will be retained in the soil through processes such as ammonification (the conversion of organic nitrogen to ammonia) and a certain amount will be lost by denitrification, microbial attack and volatilisation (Patterson, 2003, Geary & Gardner 1996). Patterson (2002) estimates that these processes may account for up to 40% of total nitrogen loss from soil. In this case, a more conservative estimate of 20% is adopted for the nitrogen loss due to soil processes. Table 4 presents the key data used and results of the water and nutrient balances. Table 4: Data Used and Results of the Water and Nutrient Balance Modelling DATA Parameter Units Value Comments Average effluent load L/day 700 Max anticipated from 3 bedroom dwelling on each lot (assuming no pool backwash water) Precipitation mm/month Girraween mean From BoM Data Drill (Girraween) Pan Evaporation mm/month Middle Point mean From BoM station (Middle Point) Retained rainfall unitless 1 Proportion of rainfall that remains and infiltrates the soil; i.e. 1 assumes nil rainfall will run off and is the most conservative approach Crop Factor unitless 0.7 – 0.8 Conservative annual value for turf grasses – adjusted for season Design percolation rate (DPR) mm/week 25 Nominated rate for Category 4 soil, based on DIR values given in AS/NZS 1547:2012. Effluent total nitrogen concentration mg/L 50 Conservative target effluent quality for typical biological filter systems. Nitrogen lost to soil processes (denitrification and volatilisation) annual 20 Patterson (2002) Effluent total phosphorus concentration mg/L 12 Conservative target effluent quality for typical biological filter systems. Soil phosphorus sorption (P-sorption) capacity mg/kg 300 Estimated P-sorption capacity for the sandy clay loam topsoil, based on known P-sorption values for similar soil types elsewhere in the NT. Nitrogen uptake rate by plants kg/Ha/yr 200 Approximately half the rate expected of effluent irrigated pasture grasses (NSW Agriculture, 1997).
Bulk density Has a conservative bulk density factor (≤ 1.3g/cm3) been used for calculations in the absence of site-specific testing? Soil pH Has each parameter been analysed by a suitable laboratory using approved methods? Have the results of analysis been interpreted correctly? Are the results realistic in light of known soil properties of the area (if applicable?) Soil Electrical conductivity Soil Sodicity (ESP) Soil P-sorb capacity Overall LCA Rating Do you have sufficient information to accurately determine land capability for on-site wastewater management? On-site Wastewater System Selection and Design Component / Process Assessment Criteria Yes / No Justification Outcome (e.g. Proceed, Further information, Refuse) Model LCA Reference Treatment Components Are the proposed treatment components appropriate for the site based on the chosen land application system and the LCA? Sections 4 and 5 Are there other, more suitable options that should be considered?