Conclusions Effective treatment of effluent generated by on site wastewater
Effective treatment of effluent generated by on-site wastewater treatment systems (OSWTS) is essential for the protection of groundwater and surface water quality. This combined geophysical/hydrogeological investigation, carried out to assess of the impact of an OSWTS located over a thin layer of glacial till subsoil resting on a poorly productive greywacke/shale bedrock aquifer, has demonstrated that microbiologically contaminated effluent has the capacity to cause gross contamination of groundwater in the immediate vicinity of the percolation area. At the Drumaliss Test Site this Ro 31-8220 occurred in a hydrogeological setting where the role of attenuation in the vadose zone proved negligible. The multidisciplinary approach adopted in this study provided a means for investigating impacts of OSWTS effluent to groundwater both close to the percolation and at distance.
The contributions made by seismic profiling and ERT in this investigation demonstrate the value of geophysics as an integral element of a site characterisation programme aimed at improving understanding of the impacts of OSWTS pollution on groundwater water quality in areas underlain by poorly productive aquifers. Observations of comparable geophysical characteristics in Lower Palaeozoic shales elsewhere in Ireland suggest that the deposits investigated in this study are widespread. This is corroborated by observations made by soil scientists working in areas underlain by Ordovician and Silurian shales across the northern part of Ireland, covered by Drumlins, who noted that weathered shale forms the standard overburden material over intact shale bedrock (M. Breathnach, Soil Survey of Ireland (Ret.), pers comm). Consequently the phenomena observed in this study are suspected to be more widespread than previously anticipated.