Enhanced understanding of coastal karst aquifer system through monitoring of source-specific contamination occurrences at intertidal springs

Lowland coastal karst aquifers in the west of Ireland are considered to be highly vulnerable to
pollution and impacted by multiple contamination sources on land (in particular, diffuse rural sources
from agriculture and on-site domestic wastewater effluent) as well as by saltwater intrusion near the
coast. Most of these aquifers are exceptionally complex which makes their protection and
management extremely challenging. Human wastewater effluent from on-site wastewater treatment
systems (DWTSs) is identified as a significant threat to groundwater quality in such lowland Irish
karst environments, since approximately one-third of the population in Ireland is relying on these
systems for the treatment of domestic wastewater. However, it is difficult to distinguish between
agricultural pollution and DWTS effluent using only traditional water quality parameters or any single
environmental tracing method. Hence, the impact of contaminants from DWTSs on groundwater
quality must be assessed using a source-specific multiple-tracer approach. Such an approach allows
the development and application of effective management strategies on the catchment scale.

Kinvara Bay catchment in the southern part of County Galway has been selected for this study due
to a large number of DWTSs located above a well-developed shallow epikarst and extensively
karstified Carboniferous limestone bedrock across the largest part of the catchment. The karst
conduit network also receives significant quantity of relatively fast allogenic runoff from Devonian
Old Red Sandstone bedrock hills in its eastern part. The catchment has been the subject of many
studies over the years with respect to groundwater flooding, ecohydrology of turloughs and
submarine groundwater discharge into the bay. Previous studies have estimated discharge and the
nutrient loading from the two main springs into the Kinvara Bay using hydrological modelling; one
of these springs being the outlet of a major karst conduit network (known to be the output of the
allogenic recharge from the hills), while the other spring predominantly discharges water from more
diffuse/autogenic sources.

Monthly hydrochemical and water quality parameter monitoring has been carried out over the
course of a year, with concurrent application of microbial and chemical wastewater contaminant
fingerprinting techniques. Such techniques include fluorescent whitening compounds,
fluorescencebased investigations of organic matter with parallel factor (PARAFAC) analysis, specific
anion ratio signatures, flow cytometry and faecal sterol and stanol profiles and ratios. The observed
variations over time in concentrations of hydrochemical parameters and targeted contaminants
specific to onsite wastewater sources at two springs were contrasted. The results, which have been
compared to the known locations and densities of DWTSs in the area, have highlighted the
previously suggested complexity of groundwater flow and distinctive conduit pathways within the
same karst aquifer system. Such information can contribute to improved future catchment
management policy to minimise the effect of contaminants on karst groundwater resources.