Traditional on-site wastewater treatment systems have proven to be unsuitable in areas of low permeability subsoils, representing a risk to human health and the environment. 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. The paper describes the development and structure of a geographic information system (GIS)-based decision support toolset to evaluate possible alternative strategies for these sites. The programme takes as its initial input the location of an existing house located in an area of low permeability subsoils. Through a series of interconnected GIS geoprocesses the model outputs appropriate solutions for a site, ranking them in terms of environmental sustainability and cost. However, the final decisions are still dependent on on-site constraints so that each solution is accompanied by an alert message that provides additional information for the user to refine the output list according to the available local site-specific information.

The work presented in this paper highlights the development of a web based intelligent Geographical Information System (GIS) spatial decision support tool. Engineered over the Amazon cloud computing environment, the GIS system has been designed to enable local authorities in Ireland to identify solutions for the treatment of domestic waste water in areas of low permeability subsoil. The data used in the geo-spatial modelling includes the XY coordinates of existing treatment systems, topography, soils and subsoil, and road networks. Spatial modelling is performed through ESRI's ArcMap and published as a web service on ArcGIS Server 10.1. This decision support system is made available to the remote user over Amazon's EC2 as a GIS based web service, through Representational State Transfer (REST) architecture. The tool will assist local authorities to better manage domestic wastewater in low permeability areas, thereby reducing the likelihood of pollution of local surface waters. Wexford is the test area.

The domestic wastewater of over one third of the population in Ireland is treated by on-site treatment systems. However, if there is insufficient permeability and/or subsoil depth to take the effluent load, surface ponding may occur resulting in a risk of effluent discharge and runoff of pollutants to surface water. The aim of this project is to create a decision support tool to identify solutions and better manage on-site wastewater in such areas. Geospatial modelling is being developed for testing in four counties: Wexford, Limerick, Leitrim and Sligo. Solutions are being developed for various scenarios, incorporating human settlements, the physical environment comprising geology, land cover, hydrology and meteorology, and infrastructure such as transportation and utility networks. Initial outputs indicate that onsite wastewater treatment systems are aligned with road networks, forming ribbon developments, and are frequently clustered. Modelling at this stage suggests that such is the number and distribution of onsite systems in low permeability areas that they pose a high risk of surface water pollution.