A Catchment Management Tool (CMT) is being developed for the Irish Environmental Protection Agency (EPA) that will allow River Basin Managers to evaluate the environmental risk from organic and inorganic nutrient loadings due to various land uses and human activities. The CMT was built using Open Source GIS (Geographical Information System) software, to facilitate development and widen the research user-base of the tool. The user interface of the CMT is designed to be flexible to allow local knowledge to be included in the system as well as the possibility to try what-if scenarios in relation to environmental assessment. Contaminant loadings are calculated from publicly available data and hydrogeologically susceptible areas (HSAs) are formed by combining soil and geological GIS layers. Further combinations of Loadings and HSAs allow Critical Source Areas to be delineated that identify areas contributing significant amounts of a given contaminant to any selected water body.