## 4 Discussion  
Based on the results, using geographical information system and data sampling on-site provided a deeper understanding of how flooding events affect the accumulation of Mn in soils near the two rivers. The degree of Mn contamination in floodplain soils at the two rivers was also measured and mapped using \(I_{geo}\), and areas with the highest contamination indices were identified. In areas with heavy Mn contamination, the Mn concentration in floodplain soils reached 5.9% by weight (\(\sim\)59,000 mg/kg), an amount higher than that found naturally in soils (40-900 mg/kg) [69] and exceeds permissible limits for agricultural use [70].  
These maps could guide relevant national and local government agencies in planning appropriate mitigation and rehabilitation plans to limit human exposure to highly contaminated areas along the Mogprog and Boac rivers.  
A moderate positive correlation was observed between flood height and Mn content of soils, suggesting that more extensive flooding promotes Mn contamination of floodplain soils in the two rivers. The most likely explanation for this correlation is the mobilization of Mn-bearing silt and sediments from the abandoned mine pits and TSFs and their transport and re-deposition in downstream areas during flooding events. This is also consistent with the moderate negative correlation between pH and Mn content in soils; that is, Mn contamination originated from geologic materials with more acidic pH, like those found in the abandoned mine pits and TSFs (Figure 6).  
The most likely carriers of Mn during these flooding events are Fe-oxyhydroxides, which are not only more stable than Mn-oxides (Figure 8) but also more abundant in the soils. These deductions are consistent with the moderate correlation between Fe and Mn in floodplain soils (r = 0.542) and are supported by the Mn geo-accumulation map shown in Figure 7. Fe-oxyhydroxides like ferrihydrite can sequester Mn from solutions via co-precipitation and adsorption reactions, both of which are well-known processes in environmental studies [71]. It is also interesting to note the strong negative correlation between pH and Mn concentration in surface water, a relationship attributed to the redissolution of Mn-bearing minerals in soils like Mn(OH)\({}_{3}\) and Hausmannite under acidic conditions, as explained in Figure 8b. Curve number, on the other hand, is a dimensionless number directly correlated to the permeability of the soil that pertains to its capability to absorb water from the surface. In the case of the Boac and Mogprog rivers, where surface water carries Mn, this plays a vital role in how the concentration of this element increases in soil. Frequent flooding in the area provides a course for Mn to enter the soil and increase its concentration to a point above permissible limits and background concentration. An increase of concentration above its background concentration then promotes an increase in the geoaccumulation level, which is what has happened in the Boac and Mogprog rivers over time. There are some areas, such as the USA, where a community does not frequently experience flooding, but after one major flooding event, a significant increase of heavy metal concentration was observed in their area [27].