Microplastic in the environment – Analyses of the Warnow catchment

Background and Objective

To date, microplastic research is mainly restricted to oceans. Studies are missing that elucidate the connections between contaminations across multiple ecosystems. Plastic tarps used in agriculture, or microplastic entering the field with application of sewage sludge and fermentation residues, can accumulate in the soil and erode into surface waters.

In the exemplary Warnow river catchment (Mecklenburg-Vorpommern),  this collaborative project funded by the Federal Mistry of Education and Research will identify microplastic sources and sinks on land and in surface waters. The use of models will help quantify relevant pathways into the open Baltic. In a sub-project, the Thünen Institute approaches the aim to estimate spatial distributions of microplastics on fields and field margins and to identify pollution hot spots. Eventually these results will be used to infer potential mitigation measures.

Approach

In a first step, a land use analysis identified areas or regions that are contaminated with microplastic due to their historic land use. Regulative reporting data, statistic data and expert information was used, amongst others. Microplastic concentrations in sewage sludge and compost were derived from literature data. The modelling approach provides the expected microplastic concentrations in soils originating from agricultural use only. Other sources, such as improper disposal (littering) and tyre wear, are not considered in this model.

The spatially explicit results were fed into a hydrogeological model that quantified the pathways into surface waters.

The work in MicroCatch_Balt was conducted in direct cooperation with the partner project PLAWES.

Our Research Questions

What are the relevant sources and sinks of microplastic associated with agriculture?

Do non-agricultural sources, such as plastic litter in road side ditches, play a role?

How can input pathways be characterized, e.g., do lakes act as a sink?

Results

According to our initial estimates, a total of 213 t of microplastics were introduced into agricultural soils in the Warnow catchment area from 1960 to 2016 through sewage sludge, 52 t through compost, and 7 t through mulch and cover films. Especially in the case of sewage sludge, the spatial variability of the inputs is very high. In comparison, the results of the catchment areas of the Warnow and the Weser show clear differences. In the rural Warnow catchment, the stronger influence of sewage sludge stands out, while compost and films are more important in the Weser catchment. Sewage sludge is more widely applied in the Warnow catchment due to the light, sandy soils and is partly imported from other federal states. Plastic-intensive special crops play only a minor role.