# Spatial Connections between Microplastics and Heavy Metal Pollution within Floodplain Soils
## 4 Discussion
### Risk Assessment  
The main difference between plastic particles and heavy metals in terms of their environmental risk is that comparatively little is known about the actual risk posed by plastic contamination. In general, it should be clear that plastics, in contrast to metals, do not have any benefit or use within the environment, as they are anthropogenic foreign objects in soils, sediments, or water. Metals, on the other hand, are partly essential (e.g., Fe, Co, Cu, Ni) for all organisms [4]. However, excessive concentrations in combination with toxic metals (e.g., As, Cd) lead to a wide variety of potential risks for ecosystems, organisms, soil functions, and ultimately human health [4]. Common practices for risk assessments of heavy metal pollution in soils are based on different exposure pathways, exposure times, and potential doses as well as on national legislation values considering those factors [4; 65; 66]. The early state of plastic research, especially in soils, has so far not resulted in limit values or other legislation on maximum levels of plastic in soils [66; 70]. Likewise, long-term monitoring of heavy metals in soils often provides geochemical background values for comparison [70]. These cannot be determined in the case of plastic contamination, as the geogenic background content of plastic in soils must be zero due to its purely anthropogenic origin. Against this background, the results of the present study show that in floodplain soils, a low-to-moderate risk can be expected solely by heavy metal enrichment. Legal precautionary levels are exceeded only very singularly by contents of Cr, Cu, Ni, and Cd, whereby no spatial hot spots can be identified (Table 1). The calculated pollution indices demonstrate a low-to-moderate enrichment or severity (Ef, CSI), but a general deterioration of soil quality (PLI) and a strong ecological risk (RI). Even if the calculated indices are limited by the introduced comparative values, the data basis (341 samples) of the regional background values in floodplain soils can be assessed as comparatively good. Furthermore, floodplain soil properties indicate a rather low mobility, since both the high contents of organic material and the widespread hydromorphic conditions (ground-water influence) promote the binding of the metals to humans complexes as well as to Fe-Mn oxides [2]. As the floodplains are mostly used for agriculture, these risks exist for the soil-crop impact pathways, but also for the soil-groundwater pathway due to heavy metal enrichment up to a depth of 2 m [65; 66]. However, as the levels are low in contrast to the excessive contamination found in the direct surroundings of industrial plants and military sites, or in anthropogenic and urban soils in general, the basic environmental risk from metals alone can be assessed to be low [4; 60]. Remembering the restrictions on assessing the risk of plastic contamination, it can only be argued that the widespread presence of plastic particles in floodplain soils down to a depth of two meters and the accumulation in upper soil layers form the potential for possible risks. The variety of possible risks was discussed in a wide range of reviews [7; 26; 60]. However, for the area of floodplain soils, the following contamination pathways should be pointed out in particular: As in the case of heavy metals, microplastics, especially those of small particle size, can be taken up by plants and organisms, which can ultimately lead to their uptake into the human food chain [24; 25; 79]. Evidence of microplastics in human excrement or even in placentas suggests that consequential risks from plastic itself (foreign bodies) or additives are possible [79]. Furthermore, floodplains are important ecosystems and, in some cases, important agricultural areas due to their fertile soils. Therefore, all risks that affect soil fertility or soil functions (e.g., influence on soil aggregates, C-dynamics) should not be neglected, even if they cannot yet be conclusively assessed [26; 27; 28].