Origin of microplastic particles in karst aquifer systems

Karst aquifers are extremely complex to understand and are generally highly vulnerable to pollution as a result of predominantly rapid recharge of water from the surface and strong aquifer heterogeneity. In Ireland, low-lying karst catchments exhibit widespread surface water – groundwater interactions which makes these aquifers very susceptible to direct contamination from surface water ingress. These aquifers are usually impacted by multiple contamination sources (e.g. domestic wastewater effluent, agricultural, industrial sources, etc.) which makes their protection and management challenging. Human wastewater effluent from on-site domestic wastewater treatment systems is identified as significant threat to groundwater quality in such lowland Irish karst environments since approximately one-third of the population in Ireland relies on decentralized wastewater treatment systems for the treatment of domestic wastewater. The domestic wastewater is primarily discharged from toilets, washing machines, showers and dishwashers; thus a wide range of contaminants eventually reach the environment even after appropriate on-site wastewater treatment processes. Microplastic particles are found with other solid materials in the wastewater effluent principally due to household washing and cleaning processes. Investigations of microplastic contamination of aquifers are still extremely rare despite the fact that microplastic occurrences (and related ecological concerns) have been well studied in marine and other aquatic environments in recent years. Toxicological and health concerns with microplastic presence in groundwater used or potentially used for human water supply are a result of ability of microplastic particles to absorb a number of other pollutants (e.g. persistent organic pollutants), while the wider ecological and environmental concerns are mainly related to springs – points at which water flows from an aquifer to the land surface affecting also surface water quality and ultimately contributing to microplastic pollution in the oceans. In this study, five karst springs in the West of Ireland have been sampled over five consecutive months for quantification and identification of microplastic particles using Fourier-transform infrared spectroscopy (FTIR), and over 14 months for the analysis of fluorescent whitening compounds (FWCs; well-known indicators of human wastewater contamination due to the fact that their origin is mostly from laundry detergents) as well as for the analysis of faecal sterol and stanol profiles and their ratios. The results show a very strong and significant correlation between microplastic particle counts and fluorescent whitening compound signals at several springs, which helps to understand the contribution of household-derived contaminants to this environmental problem. Furthermore, analyses of faecal sterols and stanols have been used to additionally examine and/or confirm the potential origin of microplastic pollution in karst aquifers. Our results indicate that this faecal source tracking method can be useful in karst aquifer systems despite the fact that concentrations of sterols and stanols of interest were usually low which makes the interpretation of results challenging.