# Spatial Connections between Microplastics and Heavy Metal Pollution within Floodplain Soils
## 4 Discussion  
### Spatial Correlations  
Macroplastics and microplastics, as well as heavy metals, are present in floodplain soils of the entire Lahn River catchment. Furthermore, based on the conducted geospatial sampling approach, the results indicate spatial relationships between both contaminants. Various studies have determined the presence of partwise increased metal concentrations in the floodplain soils, pore water, and water, as well as suspended particulate matter of the Lahn River (72,73). In general, heavy metal loads and their spatial distribution are comparable to the results of previous studies conducted in the Lahn River catchment (48,49). In the case of the metals Cu, Pb, and Zn evaluated by [PERSON] (2008, 2012) for near-channel positions, the concentration ranges correspond to the data presented here for those three metals (49). The increase in concentrations described in these studies for the course of the river could not be traced here, neither for proximal sites (near-channel) nor for the entire transects. Regarding the total plastic concentrations, a regional comparison was not possible due to the lack of case studies to date. In comparison to microplastic concentrations in soils, documented for typosils under agricultural usage, the contents found here are comparatively low (14,15,17,68). For example, [PERSON] et al. (2018) reported average concentrations of 70.0 p kg\({}^{-1}\) (>0.02 mm plastics) and [PERSON] et al. (2019) documented a range of 0.6-10.4 p g\({}^{-1}\). The microplastic concentrations reported for Swiss floodplain topsols (average: 5 mg kg\({}^{-1}\)) are hard to compare with our results due to the different units applied (18). The average concentrations of microplastics documented for the Inde River catchment (North Rhine-Westphalia, Germany), ranging from 25.4 p kg\({}^{-1}\) up to 47.9 p kg\({}^{-1}\), are comparable to the results of [PERSON] et al. (2021) and show that the mean contents in the floodplains of the Lahn are significantly lower and only maximum contents reach the mean range in the Inde river (19). However, it is important to note that due to the different sampling and analysis methods, only limited comparability can be ensured here. This also applies to the investigated (micro-)plastic size classes, as many studies, unlike ours, also examine particles < 300 \(\upmu\)m, which can explain the increased concentrations. In contrast to the problems for the comparison of plastic concentrations, the spatial distribution of plastic concentrations in floodplain soils is comparable to the studies of [PERSON] and [PERSON] (2018) and [PERSON] et al. (2021). Both studies documented an increase in plastic concentrations with stream length or, rather, a correlation between plastic concentrations and population density within the catchments. With reference to this lateral distribution over the catchment, the Ptot concentrations also show an increase along the length of the river, with the exception that higher contents (mean and maximum) were already present in the upper reaches (Figure 3). No direct impact of the urban centers along the Lahn River (the cities of Marburg, Giessen, Wetzlar, and Limburg) was found. However, a general increase in Ptot levels may suggest an increased occurrence of potential sources along the river. The same applies to heavy metal contents, even if these show a higher variability at the catchment level, probably caused by the different environmental sources (e.g., geogenic activity, mining) (5). In addition to the correlations in variability at the lateral catchment level, spatial correlations occur primarily at the vertical level (depth distribution in the floodplain soils). The clear separation with higher contents in the upper soil layers (0-50 cm) and significant correlations between Ptot and metal contents illustrate these spatial relationships.