## 1 Introduction  
Floodplains are intermediary areas between a river and the surrounding landscape. They are formed and dominated by fluvial processes and have been the preferred setting for human settlement and development since ancient times. Floodplains can also function as sinks for materials transported into and through them by different processes. In particular, soils in the floodplains can store matter, such as carbon, salts, or metals [1, 2]. Since many floodplains are used for food production [3, 4], metal contamination of soils can be detrimental to human health [5, 6]. Studies on metals such as cadmium (Cd), chromium (Cr), nickel (Ni), lead (Pb) and zinc (Zn) have shown that they affect soil biota and can accumulate in plants growing in the soils, potentially entering the human food chain [7, 8, 9]. Once metals enter the soil, they may migrate along the soil column and into the groundwater, thus affecting the environment [10, 11]. Furthermore, soils in floodplains that have functioned as sinks for long periods can become sources of metals when conditions (such as pH) or processes (such as erosion and deposition) change [12, 13, 14]. Under these changed conditions, significant amounts of metals may be released back into the streams and cause adverse effects to aquatic and connected terrestrial ecosystems [15]. Therefore, understanding the distribution of metals in floodplain soils and their controlling factors is essential. Metals in floodplain soils are not distributed homogeneously, but rather show spatial patterns in their distribution [16; 17]. These spatial patterns result from transport, deposition, and accumulation processes that depend on environmental factors [18; 19]. Factors that affect metal concentrations in soils are geology, land cover, hydrology, terrain, and emission sources [20; 21; 22]. As these factors are not homogeneously distributed along a floodplain but vary with location, so do their effects on metal accumulation in floodplain soils [23; 24; 25; 26]. Knowledge of these patterns can be used to identify areas for specific use cases, remediation measures, or scientific studies on the effects of metal concentrations. Much research has been undertaken to find and describe the patterns of metal accumulation and the factors that cause them. For example, bank morphology affected heavy metal accumulation in soils along rivers [27]. The spatial distribution patterns of metals in floodplain soils were found to depend on flooding and sedimentation frequency [28]. This accumulation in the floodplain soils was controlled by the emission from the parent material of soil formation [29; 30] and affected by human activity and land cover patterns [31; 32]. While these studies researched metal patterns and the processes that lead to them, they focused on the distribution patterns of individual metals in soils, not or only seldom on the co-occurrence of different metals in soils, i.e., multi-metal distribution patterns. Depending on the processes that lead to these metals' release, transport, and accumulation, the spatial distribution of several metals may show similar patterns. Finding these similarities between different metals and linking them with the underlying processes may allow for several improvements, such as inferring information about less commonly sampled metals from more commonly sampled metals or from existing datasets, or increasing the efficiency of combined soil quality improvement measures. Additionally, identifying the specific combination of metals in certain locations may improve the association of the metals with their most probable sources, as different combinations of metals may point to different emission and transport pathways. Furthermore, most of the existing studies focus on a relatively small selection of metals; these are commonly a subset of Cd, copper (Cu), iron (Fe), Pb, magnesium (Mg), manganese (Mn), Ni, or Zn.