## 1 Introduction
Acid mine drainage (AMD) is formed when sulfide-rich minerals in the TSFs, waste rock dumps, and open-pits are exposed to the atmosphere with a sufficient amount of oxygen and water [11; 12; 13]. [PERSON] and coworkers [14; 15; 16], for example, reported that the elevated heavy metals concentrations of the Boac and Mogpog rivers could be attributed to mine drainage from the abandoned Maguilaquila TSF located upstream of both rivers. More recently, a previous study of the authors [17] detected eight heavy metals such as chromium (Cr), manganese (Mn), iron (Fe), lead (Pb), cadmium (Cd), zinc (Zn), nickel (Ni), and copper (Cu) exceeding the allowable environmental limits for soils. Among them, Mn had the highest measured concentration at 68,620 mg/kg, a 65-fold increase from 1998 measurements (\(\sim\)1060 mg/kg) [14; 17]. Manganese is naturally occurring in soils and is necessary for the normal nutrition and development of plants. At high concentrations, however, it is toxic to plants and humans [18; 19]. Excessive Mn exposure of humans causes damage to the central nervous system exhibiting similar symptoms as lead (Pb) poisoning [20; 21]. Manganese is also a problematic contaminant because of its high solubility, similar to Cu and Zn in the pH range of surface and ground waters (i.e., pH 6-9) [22], which makes it highly bioavailable. Bio-accumulation of Mn in food crops, for example, has been reported in Marinduque with rice samples harvested in Boac and Mogpog containing around 16.0 and 7.12 mg/kg of Mn [23]. Previous studies have also reported elevated Mn concentrations in the island not only in sediments [13] and soils [16] but also in crustaceans and tilapia [24], string beans, sweet potatoes, eggplant, and bitter melon [25], and corals [14]. United States Environmental Protection Agency (USEPA) stated that humans could only consume 0.14 mg/kg-day of Mn to avoid toxicity. Environmental challenges related to Mn contamination will also become critical due to the clean energy transition and decarbonization because Mn, together with lithium and cobalt, is an essential metal used in rechargeable batteries essential to renewable energy systems and clean storage technologies [26]. Previous studies have shown that flooding is a major contributor to the increase of heavy metals content in soils [27; 28; 29; 30]. [PERSON] et al. [27] reported higher Mn levels in the floodplain after a major flooding event caused by Hurricane Harvey. In Poland, [PERSON] et al. [28] noted that river flooding events facilitated the migration of heavy metals from sewage treatment plants, landfill leachates, and industrial areas into arable soils. In areas with historic mining operations, soil contamination is exacerbated during flooding events because of overflows from upstream mining areas [29]. [PERSON] et al. [30] recently reviewed how flooding affects the mobility of heavy metals in floodplain soils and noted five key parameters: (i) soil redox potential, (ii) pH, (iii) dissolved organic matter (DOM), (iv) Fe and Mn hydroxides, and (v) sulfate and metal sulfides. Another factor promoting heavy metal accumulation in soils in flooded areas is the soil infiltration capacity. [PERSON] et al. [31] demonstrated the significant relationship between the permeability of the soil and heavy metal accumulation by developing a centrifuge infiltration setup to quantify metal mobility in soils. Also, the curve number was the parameter used in the study to represent permeability. This is a dimensionless number that indicates runoff response on land-based soil type and land use. These previous studies suggest that the accumulation of Mn in the soils near the Boac and Mogpog rivers is likely promoted by frequent typhoon-induced flooding events. One way to determine the extent of soil contamination is to calculate pollution indices by comparing heavy metal contents of the study area to a reference or \"background\" site.