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.