# Spatial Mobility of U and Th in a U-enriched Area (Central Portugal)
## 3 Results and Discussion
### Contamination Indexes  
Several international methods for the evaluation of potentially toxic elements in stream sediments and soils have been described in different study cases, such as contamination factor, contamination degree, enrichment factor, geoaccumulation index, and potential ecological risk index (e.g., [25; 26; 27; 28]). Some of them were applied to assess the enrichment of stream sediments located under the influence of mine activities [25; 29; 30]. The degree of pollution in mine dumps and sediments was classified using the geoaccumulation index, and most of them were classified as moderately to extremely polluted, particularly upon receiving the influence of dump materials. Stream sediments were polluted relatively to As (35.8 mg/kg), W (4.76 mg/kg), U (454.04 mg/kg), and Th (32.02 mg/kg), and they had high electrical conductivity (154.20 uS/cm). The contamination factor of each element is the ratio of the element content to the reference value of the element [29]. The considered reference values were the median of background concentrations for stream sediments, collected upstream of the mine influence of the Canto Lagar area (Table 2). The range and median values of contamination factor to mine dumps and sediments for the studied elements are indicated in Table 3.  
Practically all the selected potential toxic elements from the mine dump exhibited a degree of contamination classified as low to moderate. Otherwise, for As and W the contamination degree was classified as high, while U had a very high contamination. The mobility of elements from mine dumps to stream sediments was low and presented a low to moderate contamination (Table 3). Figure 2: Box plots for physicochemical parameters, trace elements, and REE concentrations in stream sediments, soils, and mine dumps from Canto Lagar region. The symbol \(\bullet\) represents values of minimum (lower end), maximum (upper end) and outlier (intermediate) concentrations. The average values of contamination factor for stream sediments varied in the following descending order: U \(>\) W \(>\) As \(>\) Th \(>\) Fe \(>\) Zn \(>\) Pb \(=\) Mn \(>\) Cr \(>\) Ba for mine dumps, and Zn \(>\) W \(>\) Pb \(>\) Ba \(=\) Cr \(>\) Fe \(>\) Th \(>\) U \(=\) Mn for stream sediments. The sum of the contamination factor was reclassified as a contamination degree considering the total of variables (Table 3). In the Canto Lagar mine area, the contamination degree varied from 8.7 up to 54, indicating a low to very high contamination degree, particularly in the mine dump sediments (Table 3). Throughout the analyzed samples, about 50% presented a low contamination factor for Mn (77%), Ba (77%), Cu (83%), Zn (60%), and Pb (60%), moderate contamination for Cr (60%) and Th (60%), and very high contamination for U (50%; Figure 3). According to the obtained results, most of the selected potential toxic elements from mine dumps exhibited a contamination degree varying from low to moderate. Moreover, As and W presented a high contamination degree, while U presented a very high contamination degree. This situation is primarily associated with the Canto Lagar uranium mineralizations.