# Spatial Connections between Microplastics and Heavy Metal Pollution within Floodplain Soils
## 4 Discussion
### Pollution Sources and Interactions  
Assuming the same sources and deposition conditions for both contaminants in floodplain soils, the question arises as to whether this only applies to the river and floodplain area, or also to the wider catchment area, and whether and how these contaminants interact. In contrast to plastics, heavy metals are a natural component of river and floodplain sediments, expressed by the natural and thus geogenic background contents [40; 70]. In case of the Lahn River catchment, the regional geology, which includes rocks of the Rhenish Slate Mountains (Devonian and Carboniferous), Triassic sandstones, and Tertiary volcanic rocks, provides a high spatial variation of geogenic metal contents [61; 74]. Both contaminants, metals and plastic, have the characteristic that they can be introduced anywhere within the catchment area. In contrast to heavy metals, which have been entering the environment for a long time mainly through human use, whether through mining or industry, the situation is different for plastics [75]. Plastics, as polymers, were first developed by humans in the last 100 to 120 years, whereby global plastic production first began in the 1950s [7; 75]. Thus, plastic particles, regardless of their input pathway, have only been able to enter the environment and, thus, river systems and floodplains for about 70 years. The steady and exponential growth of global plastic production makes it likely that large quantities of plastic have only recently been released into the environment [76]. The short time span in which plastic can enter the environment is also expressed in the accumulation of plastic particles in the upper floodplain soil layers and has already been discussed by [PERSON] et al. (2021). Since the findings demonstrate that this accumulation also partially applies to heavy metals, it may suggest that the current sources of metals and plastics could be identical. With respect to the environmental and landscape properties of the Lahn River catchment sources, such as urban areas (and especially urban wastewater) [77], traffic (e.g., tire abrasion: plastic; brake abrasion: heavy metals) [71] or agriculture [37; 38] are able to release both groups of contaminants into the environment and the fluvial system of the Lahn River. In addition to the environmental sources described in Section 2, heavy metals can also originate from the plastic particles themselves [34; 57]. The results show that all metals analyzed were present in the investigated macroplastic and coarse microplastic particles. Previous research in the aquatic as well as in the terrestrial environment has shown both phenomena--the adsorption of metals from the surrounding environmental medium and the release of additive metals [29; 33; 37; 57]. Despite the unmanageable number of additives in plastic production, some of the notable concentrations from the results could perhaps originate from such additives. The comparatively high contents of Zn can be attributed to inorganic flame retardants or slip agents, for example [34]. In addition, the highest total contents of metal load within plastic particles was detectable within two CSM (chlorosulfonated polyethylene) particles, a polymer type that obtains its special properties through vulcanization with metal oxides (e.g., lead(II) oxide or zinc oxide) [78]. A statement about the adsorption on plastic particles is not possible due to the given methodological limitations based on digestion method. The presence of heavy metals in the surrounding soil matrix from which the plastic particles were isolated would make adsorption processes possible. Previous studies have shown that adsorption is metal-specific, concentration-dependent, and promoted by degraded particle surfaces [35; 38]. The accumulation of plastic held in the upper soil layers in combination with mostly degraded or highly weathered plastic particles would thus promote adsorption of the metals, but also promote their release through degradation of the particles [35].