Investigation of microplastic particles in rural karst groundwater systems and their links with on-site domestic wastewater effluent

Groundwater from karst aquifers, through springs and wells, is a major source of drinking water for one quarter of the world’s population. These aquifers are exceptionally vulnerable to pollution as a result of predominantly rapid recharge of water from the surface and strong aquifer heterogeneity. Groundwater pollution is a complex problem that can be associated with a variety of sources, but human wastewater effluent and diffuse agricultural sources are generally considered among the most significant threats to groundwater quality worldwide. This is particularly true in rural and suburban areas where the primary wastewater treatment options for communities without access to centralized wastewater treatment facilities are on-site domestic wastewater treatment systems (DWTSs). 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 on-site wastewater treatment processes. Microplastic particles, as contaminants of emerging concern, are found with other solid materials in the wastewater effluent principally due to household washing and cleaning processes. While microplastics occurrences and related ecological concerns have been well studied in marine and other aquatic environments in the past, investigations in groundwater systems are extremely rare. Toxicological and health concerns with microplastics presence in groundwater used or potentially used for human water supply are a result of ability of microplastic particles to absorb persistent organic pollutants (POPs), while the wider ecological and environmental concerns are related to springs – points at which water flows from an aquifer to the land surface affecting surface water quality and ultimately contributing to microplastic pollution in the oceans. In this study, a number of karst springs in the west of Ireland have been sampled over time for quantification and identification of microplastic particles using Fourier-transform infrared spectroscopy (FTIR) and for the analysis of fluorescent whitening compounds (FWCs; well-known indicators of human contamination since their origin is mostly from laundry detergents). Our results show a significant correlation between microplastic particle counts and detected FWCs signals at different springs, which helps to understand the contribution of household-derived contaminants to this environmental problem.