An investigation into the effects of the density of on-site wastewater treatment systems on Irish aquifer

Abstract

Groundwater is an important resource in Ireland and given that over 25% of all water supplies are currently provided directly from groundwater abstractions, it is critical to protect this resource from contamination. Most streams and rivers in Ireland also receive baseflow from groundwater and the requirement under the European Water Framework Directive (WFD) is to achieve good status in these waters by 2015. There are nearly half a million on-site wastewater treatment systems across the country and given that the treated wastewater from these systems ultimately discharges to groundwater, any contaminants not treated or attenuated in the subsoil can migrate under natural gradients toward points of exposure for receptors of concern, e.g. humans and drinking water supplies or sensitive surface waters. Ireland’s Environmental Protection Agency (EPA) has published recommendations aimed at defining subsoil conditions that will provide an acceptable level of treatment for on-site wastewater in order to protect such groundwater resources from contamination. However, there are currently no guidelines on what the maximum density of on-site wastewater treatment systems should be specific to the groundwater protection plans that are in place across the country. Given that these systems are usually constructed at rural developments that are clustered in nature this is an issue of some importance with respect to groundwater quality. The lack of guidance has led to an inconsistent planning policy with no national strategy in this area. The focus of this thesis is a 3-year study to quantify the effects of the density of on-site wastewater treatment systems on groundwater quality at different groundwater vulnerability areas across the country.