Out of sight out of mind in the Plastisphere?

Plastics have been an integral part of human society since the 1950’s. Their unsustainable use, poor waste management and durability raise environmental and public health concerns. Our understanding of the fate and transport of microplastics (MPs) in rivers and lakes is improving rapidly. However, few studies have taken place in groundwater even though they represent the largest reservoir of liquid fresh water on earth and their connectivity with the surface means they are often vulnerable to pollutants from surface freshwaters and the land, including plastics. The increasing evidence of MP presence in soils and groundwaters sparks serious concerns that we are at risk of polluting highly valuable and vulnerable water resources and also risk creating a pollution legacy for future generations.

To date, little data on subsurface contamination with MPs is available and their long-term impact on subsurface ecosystems remains largely unknown. Here we present the initial results of two studies designed to redress this imbalance. Our first project aims to identify pathways of microplastic uptake into groundwater food webs (comprising biofilms, microcrustacea and macrofauna), and determine how microplastic properties affect microplastic uptake. Initially we performed controlled feeding experiments to assess rates of microplastic ingestion and the impacts on a model aquatic protist, Tetrahymena pyriformis demonstrating the potential for an aquatic ciliate to uptake microplastic across a gradient of environmental concentrations. The ingestion of microplastic beads by Tetrahymena pyriformis exhibited a positive but saturating relationship with increasing environmental concentrations. This aligns with a Type II functional response and suggests that T. pyriformis actively uptakes microplastic even where relatively rare in the environment, but that significant handling time limits a further increase in the ingestion rate of microplastics. Furthermore, the growth rate of T. pyriformis decreased with increasing microplastic concentration suggesting that the ingestion of microplastic has a negative effect on reproduction rates and fitness.

Our second project is a global groundwater sampling effort undertaken by the scientific community. To date, using a specially designed protocol, 30 collaborators from 22 countries ranging from Australia through Europe to the United States have provided 250 samples for microplastic analysis. These samples were collected from 75 different sites, which included springs, wells and open boreholes as well as caves. At each sampling site, about 300 L of groundwater were collected and filtered on-site through a series of metal sieves (123 and 25 µm mesh size). Filtrates and meshes were then collected in glass vials and stored for further analysis.

Sample processing included organic matter digestion over several days using hydrogen peroxide, density separation in glass separation units using zinc chloride, and staining with Nile Red dye, while MP characterisation and polymer identification were carried out using Raman spectroscopy. First results indicate a higher presence of fragments than fibres and of MPs between 25-123 µm than >123 µm. We hope that our findings will provide an insight into global levels of groundwater MP contamination and begin to demonstrate the impact of these MP on groundwater ecosystems.