GIS-based site identification for the land application of wastewater: Christchurch City, New Zealand

Land application of wastewater can have the effect of treating effluent as well as disposing of it. In addition, nutrients and irrigation water can be recycled for the use in agriculture and forestry. Therefore, land application is an economical means of water reuse and effluent disposal. However, if inappropriately planned and managed, it can have negative impacts on the environment. Site selection is particularly crucial when planning a wastewater land application scheme. An appropriate site selection ensures the technical and economic feasibility of such a scheme and helps reduce environmental impacts and risks. In addition, social and economic aspects need to be considered for the site selection. The present study aimed at identifying potential sites for the land application of the effluent of the Christchurch City Wastewater Treatment Plant. The analysis was carried out using a GIS, which enabled an efficient combination of all factors influencing the suitability of a site, and included the area of Christchurch City and its three adjacent districts; Waimakariri, Selwyn and Banks Peninsula. To ensure technical feasibility, environmental sustainability, social acceptability and economic viability a number of factors had to be considered for the site selection. These include the following: Slope, climate, soil type, soil pH, soil depth, land use, distance to groundwater, distance to surface waters, and distance to dwellings. Raster layers of these factors were overlaid in the GIS. This resulted in the elimination of all areas that are unsuitable for land application and the identification of candidate areas. A digital layer was created showing the candidate areas in the study area. Four potential sites were identified from the candidate areas to highlight the relative suitability between the sites. A rating system using numeric values was used to assess and screen the candidate areas and the potential sites. The rating was based on five of the above mentioned suitability factors and the distance between site and plant.