## 5 Conclusions  
The objective of this study was to investigate the effects of flooding on the accumulation of Mn in the floodplain soils of the Boac and Mogpog rivers. The Inverse Distance Weighting method was used as an interpolation technique to aid in the spatial analysis of the area. Mn concentration in surface water ranged from 0.004-3.33 mg/L with an average of 0.273 mg/L. The average Mn concentration of surface water in the area is still within the standards (0.4 mg/L). However, some surface water samples recorded Mn concentration beyond the reference guideline limits of the USEPA and the International Labour Organization. Mn concentration in soil ranged from 8584-59,206 mg/kg, above the WHO allowable limit of 4000 mg/kg. Marinduque, like other areas in the Luzon Island and other parts of the country, experiences extreme typhoon events that cause floods in the area, especially in the Boac and Mogpog rivers. We discovered that 40% of the total area considered in the study is subject to high hazards, experiencing flood heights of above 1.5 m based on the available Lidar data. Correlation analysis and geochemical modeling identified flood height to have a moderate positive correlation with Mn accumulation in the soil, while pH had a moderate negative correlation. This indicates that flooding carries more Mn-bearing materials from the abandoned mine pit and TSFs to floodplains downstream. The correlation result of pH emphasized the effect of acidic mine drainage from the abandoned mine site. These contribute to higher geo-accumulation of Mn with time. Finally, a strong positive-negative correlation between pH and Mn concentrations in surface water, a relationship to be expected because Mn-bearing precipitates like Mn(OH)\({}_{3}\) and Hausmannite become more soluble under acidic conditions. Contamination levels of Mn in floodplain soils of the two rivers were identified using Muller's geo-accumulation index. The abandoned open pit and TSFs for the last two decades have moderately contaminated 77.5% of the total area (1 \(<I_{geo}\leq\) 2) while those close to the abandoned open pit and TSFs were moderately to heavily contaminated (2 \(<I_{geo}\leq\) 3). These results suggest that intervention, remediation, and rehabilitation strategies should be carried out by the national and local governments immediately, espe  
Figure 8: (**a**) Log activity–pH diagram of Fe-H\({}_{2}\)O System created using the Geochemist’s Workbench® in equilibrium with atmospheric O\({}_{2}\) (Fugacity = 0.2), and (**b**) Log activity–pH diagram of Mn-H\({}_{2}\)O system created using the Geochemist’s Workbench® in equilibrium with atmospheric O\({}_{2}\) (Fugacity = 0.2). The dotted and dashed lines represent the maximum and minimum values measured for Mn concentrations and pH in surface water on-site, respectively. cially around the abandoned mine pit and TSF. This could limit the further spread of heavy metal contaminants downstream of the Mogpog and Boac rivers. Conceptualization, D.B.S. ; methodology, D.B.S. and C.E.F.M. ; software, C.E.F.M. ; validation, J.J.M.M., P.M.N. and K.L.M.d.J. ; formal analysis, P.M.N., C.E.F.M. and C.B.T., investigation, D.B.S., P.M.N., J.J.M.M. and C.E.F.M. ; resources, D.B.S. ; data curation, D.B.S., C.E.F.M. and J.J.M.M. ; writing--original draft preparation, D.B.S., C.B.T. and C.E.F.M. ; writing--review and editing, D.B.S., C.B.T. and K.L.M.d.J. ; visualization, C.E.F.M. and C.B.T. ; supervision, D.B.S. ; project administration, D.B.S. ; funding acquisition, D.B.S. All authors have read and agreed to the published version of the manuscript. This research was funded by the Department of Science and Technology-Philippe Council for Health Research and Development (DOST-PCHRD), D-HIVE 4B Capital Research Project. All data are contained in the manuscript. This is to acknowledge the support-in-kind of Mapua University, Marinduque Local Government Units, University of the Philippines LiDAR Portal for Archiving and Distribution, Marinduque State College, and the National Mapping and Resource Information Authority.