05 - I NVESTIGATING WATER FLOW AND CONTAMINANT PATHWAYS TO RIVERS USING CHEMICAL HYDROGRAPH SEPARATION

Diffuse contaminants originating on the land surface can make their way into rivers via a number of different pathways, including overland flow, interflow, shallow groundwater flow and deep groundwater flow. Identification of the key pathway(s) delivering contaminants to a receptor, for example a river, is of considerable importance for implementing successful water management strategies. Traditionally, physical hydrograph separation methods, such as master recession curve analysis and digital filtering methods, have been used to determine the proportion of flow that contributes to streams via the different pathways. However these methods have significant limitations. Chemical separation of stream event hydrographs can be used to supplement these more traditional approaches to reduce uncertainties and provide more robust conceptual models. The underlying premise is that groundwater with a relatively long residence time has a much higher mineral content (e.g. silica) than rainfall, while water derived principally from overland or near surface flow may be higher in other constituents, such as organic carbon, than groundwater. If a distinct, stable chemical signature can be derived for each of the end members during a rainfall event, then the proportion of flow from each of the different pathways can be determined. This technique has been applied only rarely in Ireland thus far, but has been widely used in the UK and further afield. Various natural chemical tracers such as silica, chloride, dissolved organic carbon and other major ions, and contaminants (as artificial tracers) such as nitrate and phosphate, are being trialed in four Irish catchments as part of the ‘Pathways Project’, a project funded under the EPA STRIVE Programme to develop a national catchment management tool. The tool will enable practitioners to identify the critical source areas for diffuse contaminants, and the key pathways by which they travel to receptors.