title: Geophysical and hydrogeological characterisation of the impacts of on-site wastewater treatment discharge to groundwater in a poorly productive bedrock aquifer.
abstract: Contaminants discharging from on-site wastewater treatment systems (OSWTSs) can impact groundwater quality, threatening human health and surface water ecosystems. Risk of negative impacts becomes elevated in areas of extreme vulnerability with high water tables, where thin unsaturated intervals limit vadose zone attenuation. A combined geophysical/hydrogeological investigation into the effects of an OSWTS, located over a poorly productive aquifer (PPA) with thin subsoil cover, aimed to characterise effluent impacts on groundwater. Groundwater, sampled from piezometers down-gradient of the OSWTS percolation area displayed spatially erratic, yet temporally consistent, contaminant distributions. Electrical resistivity tomography identified an area of gross groundwater contamination close to the percolation area and, when combined with seismic refraction and water quality data, indicated that infiltrating effluent reaching the water table discharged to a deeper more permeable zone of weathered shale resting on more competent bedrock. Subsurface structure, defined by geophysics, indicated that elevated chemical and microbiological contaminant levels encountered in groundwater samples collected from piezometers, down-gradient of sampling points with lower contaminant levels, corresponded to those locations where piezometers were screened close to the weathered shale/competent rock interface; those immediately up-gradient were too shallow to intercept this interval, and thus the more impacted zone of the contaminant plume. Intermittent occurrence of faecal indicator bacteria more than 100m down gradient of the percolation area suggested relatively short travel times. Study findings highlight the utility of geophysics as part of multidisciplinary investigations for OSWTS contaminant plume characterisation, while also demonstrating the capacity of effluent discharging to PPAs to impact groundwater quality at distance. Comparable geophysical responses observed in similar settings across Ireland suggest the phenomena observed in this study are more widespread than previously suspected.
authors: School of Planning, Architecture and Civil Engineering, Queen's University Belfast, David Keir Building, Stranmillis Road, Belfast BT9 5AG, Northern Ireland, United Kingdom.
authors: Shane
authors: S
authors: Donohue
authors: Department of Applied Sciences, Dundalk Institute of Technology, Dublin Road, Dundalk, Ireland.
authors: Valerie
authors: V
authors: McCarthy
authors: Department of Applied Sciences, Dundalk Institute of Technology, Dublin Road, Dundalk, Ireland.
authors: Patrick
authors: P
authors: Rafferty
authors: School of Planning, Architecture and Civil Engineering, Queen's University Belfast, David Keir Building, Stranmillis Road, Belfast BT9 5AG, Northern Ireland, United Kingdom.
authors: Alison
authors: A
authors: Orr
authors: School of Planning, Architecture and Civil Engineering, Queen's University Belfast, David Keir Building, Stranmillis Road, Belfast BT9 5AG, Northern Ireland, United Kingdom.
authors: Raymond
authors: R
authors: Flynn
mesh_terms: D004784:Environmental Monitoring; D060587:Groundwater / Q000737:chemistry*; D007494:Ireland; D014865:Waste Disposal, Fluid; D062065:Wastewater / Q000032:analysis*; D014872:Water Movements; D014874:Water Pollutants, Chemical
chemical_list: D062065:Wastewater; D014874:Water Pollutants, Chemical
keywords: Coliforms; Geophysics; Groundwater quality; Poorly productive aquifer; Wastewater