# Spatial Connections between Microplastics and Heavy Metal Pollution within Floodplain Soils
## 5 Conclusions  
Based on our geospatial research approach and with regard to the study objectives, we were able to demonstrate the presence and spatial distribution of heavy metals and plastic particles in floodplain soils. Furthermore, we were able to detect spatial correlations between both contaminants in the studied floodplain soilscapes. Those spatial correlations can be expressed by a comparable variability in concentrations across the catchment and in an accumulation in upper soils (0-50 cm). Furthermore, macro- and coarse microplastic particles were also found to contain heavy metals of adsorbed or additive origins. The spatial correlations are indicative of similar pathways of release, transport, and deposition of both contaminants in floodplain depositional systems. In terms of the evaluation and assessment of the spatial relationships found, however, there is a lack of comparative data. Therefore, more quantitative studies considering plastic concentrations in soils and floodplain systems should be conducted using comparable and standardized methods. In particular, improvements can be made in the analysis of heavy metals or plastic particles from the environment. In contrast to the method used here with a nearly complete digestion of the particles, different digestion protocols should be used in the future. These offer the advantage that, depending on the strength of the digest used, a distinction between adsorbed and additive metals in plastic particles and, if necessary, different adsorption rates can be documented in order to establish a better correlation with metal contents in the surrounding soil matrix. More research is needed to understand the chemical and physical relationships between plastic particles and heavy metals, as well as to assess the environmental impacts and hazard potential of plastics and microplastics. As with \"recognised\" pollutants, only a scientific basis can provide a foundation for the introduction of limit values based on laws and management strategies. Finally, we encourage further and deeper research at this interface between terrestrial and aquatic ecosystems, whose soil landscape is subject to dynamic processes. In particular, with regard to the presence and spatial relationships of plastics and heavy metals, these could also be present for other pollutants (e.g., organic pollutants). Further research should, therefore, initially focus on the following points:  
* Quantitative spatial assessment of plastic and heavy metal contaminations in different floodplains and river systems. * Consideration of heavy metals in different chemical bonds and analysis of adsorbed and additive metals in order to draw conclusions about the sources and interactions. * Examination of contaminants in both soil water (pore water) and groundwater in order to better understand their mobility and possible associated risks. **Supplementary Materials:** The following are available online at [[https://www.mdpi.com/article/10.3390/app12020595/s1](https://www.mdpi.com/article/10.3390/app12020595/s1)]([https://www.mdpi.com/article/10.3390/app12020595/s1](https://www.mdpi.com/article/10.3390/app12020595/s1)), Supplementary Material: Connections between microplastics and heavy metal pollution within floodplain soils, Table S1: Mesoplastic and coarse microplastic particles for metal detection, Figure S1: Grain size distribution for soil profiles of site ELM. a: Profile ELM-1; b: Profile ELM-2, Figure S2: Grain size distribution for soil profiles of site ROT. a: Profile ROT-1; b: Profile ROT-2, Figure S3: Grain size distribution for soil profiles of site STD. a: Profile STD-1; b: Profile STD-2; c: Profile STD-3; d: Profile STD-4, Figure S4: Grain size distribution for soil profiles of site LIM. a: Profile LIM-1; b: Profile LIM-2; c: Profile LIM-3; d: Profile LIM-4, Figure S5: Heavy metal and metalloid As concentrations (mg kg-1) from floodplain soils samples (\(n=112\)) ordered by descending mean concentration. **Author Contributions:** Conceptualization, C.J.W. ; methodology, C.J.W. and J.H.