Development of a GIS Based Decision Support Toolset to Assess the Feasibility of on-Site Wastewater Treatment and Disposal Options in Low Permeability Subsoils.

The domestic wastewater of over one third of the population in Ireland is treated by on-site wastewater treatment systems. For single houses in areas with no main drainage on-site systems typically consist of septic tanks followed by a percolation area. However, where the subsoil permeability is not sufficient to take the effluent load, surface ponding and runoff of pollutants to surface waters may occur. This represents a serious health risk and can contribute to eutrophication in nutrient sensitive water bodies. With large areas being covered by low permeability tills, Ireland needs to consider alternative treatment and disposal options to be able to allow further development in these areas and to deal with polluting legacy sites.

Therefore the aim of this research was to investigate alternative wastewater treatment and disposal options for rural housing in these areas and to develop a web based GIS (Geographic Information Systems) decision support toolset for Local Authority planners and managers to evaluate alternative strategies on the basis of both cost-benefit and environmental sustainability principles. Considered alternative on-site disposal options are pressurized distribution systems (i.e. low pressure pipe or drip distribution), sealed basin evapotranspiration systems as well as cesspools with regular emptying and disposal at a central wastewater treatment plant. Where the impermeable soil layer is shallow enough to be excavated the discharge of treated effluent through an imported media filter into more permeable subsoil or bedrock was also considered. Furthermore, the toolset assesses the possibility to connect houses to the nearest existing sewer network or the feasibility of clustering together several houses that could be served by a decentralized treatment plant with a consented discharge to a water course. Solutions were developed for various scenarios, incorporating geospatial datasets of human settlements, the physical environment comprising geology, land cover, hydrology, and infrastructure such as transportation and utility networks. To assess the feasibility of each solution, capital and operational costs as well as operational sustainability are calculated within the model.