Geophysical Assessment of Contamination from a Wastewater Treatment System in the Milltown Lake Catchment, Ireland

Residential on-site wastewater treatment systems (OSWTS), where contaminated wastewater discharges to the subsurface, act as the dominant means of domestic wastewater disposal in rural Ireland; septic tanks constitute the most common technology employed. The objective of this study was to determine the effectiveness of a number of non-invasive geophysical techniques, employed in conjunction with hydrogeological data, for characterising the three dimensional extent of a contaminant plume generated by septic tank effluent discharging to glacial-till subsoils at a test site within the Milltown Lake Catchment, Co. Monaghan, Republic of Ireland. It was found that the integrated use of three geophysical techniques, electromagnetics, electrical resistivity tomography and seismic refraction with existing hydrogeological and water quality data, significantly improved our understanding of the contaminant plume and associated subsurface contaminant pathways. The geophysical results, when combined with hydrogeological data suggest that the majority of wastewater contamination originating from the septic tank is being transported through a fractured zone of bedrock, far quicker than originally expected.