Understanding the impacts of human wastewater effluent pollution on karst springs using chemical contamination fingerprinting techniques

Groundwater from karst aquifers is a major source of drinking water worldwide. These complex aquifer systems are exceptionally vulnerable to pollution and may be impacted by multiple contamination sources. Consequently, water contaminated with pollutants, such as microbial and chemical, from different sources can reach water sources used for human supplies (i.e. karst springs, boreholes, and wells that are being used for domestic purposes and/or irrigation).

In rural and suburban areas, human wastewater effluent (from on-site domestic wastewater treatment systems - DWTSs) and agricultural sources are generally considered among the most significant threats to groundwater quality. This is particularly of concern in Ireland given that more than one third of the population (>500,000 homes) use DWTSs. However, significant knowledge gaps exist with respect to linking contaminants with the origins of pollution and quantifying different pollution impacts on groundwater quality in karst environments.

The domestic wastewater is primarily discharged from toilets, washing machines, showers, and dishwashers, therefore, a wide range of contaminants (including source-specific contaminants) eventually reach the environment even after on-site wastewater treatment processes. We evaluated a range of chemical contamination fingerprinting techniques in terms of their ability to determine human wastewater pollution impacts on karst aquifers. Springs provide appropriate natural locations for monitoring pollutant concentrations in karst aquifer systems as they provide an integrated picture of contaminant transport through a karst conduit network, compared to wells and boreholes which are not necessarily directly connected to the most transmissive parts of the aquifer. Hence, nine separate karst springs in the West of Ireland (of varying catchment sizes) were studied and monitored over a 14-month period.

The results demonstrate how fluorescent whitening compounds (FWCs; well-known indicators of human contamination since their origin is mostly from laundry detergents), microplastic particles, and faecal sterols and stanols can be used together to cover different detectability chances, and provide useful information about DWTSs pollution impacts on karst springs. This study also provides an important benchmark for microplastic contamination in low-lying karst aquifer systems. Furthermore, a link between changes in FWCs signals and microplastic concentration changes in karst groundwater has been found, which indicates that the majority of microplastic particles originated from human wastewater sources. Unsurprisingly, the highest detection rates of FWCs and high concentrations of microplastic particles were found in karst catchments with very high densities of DWTSs and high percentages of DWTSs in the catchment that are within 200 m of at least one karst feature (such as swallow hole), indicating a direct pathway into the underlying aquifer. Moreover, the results suggest that while total sterol content in collected groundwater samples was generally low, faecal sterols and stanols can still be used as chemical faecal markers at karst springs.