# Spatial Connections between Microplastics and Heavy Metal Pollution within Floodplain Soils
## 3 Results
### Spatial Relationships between Plastic and Metal Concentrations
#### 3.2.2 Heavy Metal Concentrations  
The presence of heavy metals in soils, and especially floodplain soils, due to the delivery and sedimentation of eroded soil material from the entire catchment is widely recognized. The analyzed heavy metals as well as the metalloid As were detected in all samples, regardless of site position or sampling depth. In general, the following metals are present, in decreasing order based on their average values: Zn (51.39 mg/kg) \(>\) Ni (29.05 mg/kg) \(>\) Cr (28.00 mg/kg) \(>\) Pb (22.20 mg/kg) \(>\) Cu (15.56 mg/kg) \(>\) V (15.40 mg/kg) \(>\) Co (10.18 mg/kg) \(>\) As (6.24 mg/kg) \(>\) Cd (0.25 mg/kg) (Figure S5). Those concentrations are comparable to the concentrations of Cu, Pb, and Zn in previous studies conducted by [PERSON] (2012; 2015; 2019). The spatial differences in the contents of the individual metals are very heterogeneous (Table A1). Compared to mean global concentrations in topsoil [70], the maximum values of Cr, Ni, Cu, As, Cd, and Pb as well as the top 25% of the concentrations (third quartile) from Ni, As, Cd, and Pb exceed the comparison values  
Figure 4: Depth distribution of plastic and heavy metal concentrations in floodplain soils. (**a**): With mean pollution load indices (Ef and PLI) based on geochemical background values; (**b**): With mean pollution load indices based on risk assessments (CSI and RI). Color separation according to proximal or distal location in the floodplain. Sizes of the dots depend on the plastic concentration (Plot) standardized to the maximum content.  
(Table A1). All maximum values as well as the mean values of Cr, Co, Ni, Cu, and Cd also exceed the geogenic background levels [61]. The legal precautionary levels for loamy soil, according to the German Federal Soil Protection Ordinance (BBodSchV, 1998), are only exceeded by eight samples for Cr, Ni, Cu, and Cd (Table A1) [66].  
Whereas the Enrichment factor (Ef) and the Contamination security index (CSI) indicate a moderate enrichment of heavy metals with low-to-moderate severity of contamination, the Pollution load index (PLI) and the Ecological risk index (RI) show a deteriorate-ion of soil quality as well as a strong ecological risk for 38.1% and 33.9%, respectively, of all samples (Table A1).  
Regarding the spatial distribution of heavy metal contamination on the catchment scale and based on the indices that are calculated with geogenic background contents (Ef and PLI), the average and maximum loads show only minor differences with the course of the river (Figure 3). Sampling sites or points with clear outliers do not appear. Against this background, it is possible to assume that the Lahn floodplains are contaminated with heavy metals across the catchment area, with these being of partly geogenic and partly anthropogenic origin.  
At the sampling site level, for both the location within the floodplain cross transect and the vertical distribution, a division comparable to the spatial distribution of the Plot content is detectable. The highest average as well as maximum values of all calculated indices are always reached at the near channel sites (proximal floodplain), except site STD (Table A1). Regarding the vertical distribution, a clear decrease in the indices' values, most clearly for Ef and RI, and for proximal as well as distal floodplain locations, can be observed for the upper 50 cm of floodplain soils (Figure 4). In addition, the values of CSI and PLI show a decrease over depth, but this is less explicit than the other calculated pollution indices. Even if there are outliers in deeper soil layers depending on the location, the content of heavy metals decreases significantly (\(p\leq 0.01\)) for all pollution indices in the lower (>50 cm) soil layers.