## 3 Results
### R.F. Stump Analysis  
The PDP offers a way of investigating the behavior of the probabilities along a gradient of the respective MIV. However, they generally do not facilitate the interpretation of critical decision values that affect the classification result of R.F. To find these critical decision values, the stump analysis was performed. The results of this analysis allow evaluating the values at which the entire dataset is split into a lower or a higher probability of metal pollution. Table 4 shows the results of the stump analysis. In meta-groups C1 and C2, the F.G. Geology has an effect via lithological K\({}_{2}\)O that differs between the metals. In Al and V, a value for K\({}_{2}\)O \(\leq\) 2.1 results in a classification as L.V., while in Co and Mn, the same value results in a classification as H.V. Lower elevations cause lower As values in C1, and all classifications below the given NDVI and EVI values are H.V. in Al and As in C1 and for EVI in Sn in C3. In C2 runoff plays a role for all three metals. In Co and Mn, the  
Figure 6: PDP plots for group C3. The \(x\)-axis represents a MIV, the \(y\)-axis represents the associated probability of a L.V. (lower value) classification. Each line represents the relationship for a single metal. The plots show: (**a**) Impervious area (percentage of impervious surface area in 2001), (**b**) PG dam total years since construction (sum of the years since construction for all dams of type Gravity), (**c**) RE dam average catchment area (average area contributing to dams of type Earth), (**d**) ER dam total storage (total storage of all dams of type Rockfill), (**e**) EVI majority value (most common EVI value), (**f**) Impervious area near rivers (percentage of impervious surfaces in a buffer 100 m around rivers), (**g**) NDVI majority value (most common NDVI value) and (**h**) Woody wetlands. “EVI” = Enhanced Vegetation Index, “NDVI” = Normalized Difference Vegetation Index. Lines are displayed after smoothing with a polynomial regression for display purposes. classification below the given threshold is H.V. For V, the classification below the threshold is L.V. Runoff below a certain threshold in C2 and C4 is strongly associated with H.V. for Co, Mn, Cr, and Hg. Lower precipitation and air temperature values in C4 are associated with H.V. for Cr. In C3, dams play a prominent role, especially the variables describing the average basin size of dams, such as RE dam avg. basin area. In Pb, Sn, and Zn most of these variables lead to a L.V. classification below the threshold. Only the average basin area of ERTC dams in Pb results in a H.V. classification below the threshold. Land Cover in C3 shows a pattern in which woody wetlands below a certain threshold produce a H.V. classification for Cd, Sn, and Zn. The relationship between the metal concentrations in the river sediments and the factors is individually different for most metals. Figure 7: PDP plots for group C4. The \(x\)-axis represents a MIV, the \(y\)-axis represents the associated probability of a L.V. (lower value) classification. Each line represents the relationship for a single metal. The plots show: (**a**) Precipitation 1999 (mean annual precipitation in 1999), (**b**) Runoff 2000 (mean annual runoff in 2000), (**c**) Precipitation 1998, (**d**) Runoff 1996, (**e**) Air temperature 1996 (mean annual air temperature in 1996), (**f**) Precipitation 1996, (**g**) Precipitation 2000 and (**h**) Runoff 1998. Lines are displayed after smoothing with a polynomial regression for display purposes. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Group** & **Metal** & **MIV 1** & **MIV 2** & **MIV 3** & **MIV 4** & **MIV 5** \\ \hline C1 & Al & Lith, K\({}_{0}\)O \(\leq\) 21: L.V. & Lith, sulfur \(\leq\) 0.03: H.V. & Mean EVI \(\leq\) 3911.9: H.V. & Median EVI \(\leq\) 3806.5: H.V. & Mean NDVI \(\leq\) 5885.6: H.V. \\  & As & Mean EVI \(\leq\) 39119. H.V. & Median NDUV \(\leq\) 7835.3: H.V. & Median EVI \(\leq\) 3307.0: H.V. & Max. elevation \(\leq\) 625.4 L.V. & Min. Elevation \(\leq\) 506.8 L.V.