## 1 Introduction  
Mining has been an important economic activity in the Philippines, which started centuries ago. With a lack of environmental regulations until recent times, old mining operations have led to many abandoned mine sites, and consequently resulted in high heavy metal concentrations in soils and sediments in the vicinity. In the Philippines, there are more than 20 mine sites that are classified as inactive and abandoned, one of which is the Palawan Quicksilver Mines, Inc. (PQMI) in Puerto Princesa City, Palawan [1]. This former mine site is considered as one of the Hg waste hot spots of the world, because the Hg pollution is affecting the marine environment and the people living nearby [2]. Most of the mercury mining areas around the world have been investigated in relation to their Hg pollution in their local ecosystems, which pose potential risks to humans living nearby. In the past decades, risk assessments on soils and sediments contaminated with Hg from the mining activities have been studied in Hg mining sites in Spain [3, 4], China [5, 6], New Zealand [7], United States of America [8], and others. Similar patterns of high to extremely high (46,000 mg kg\({}^{-1}\)) Hg concentrations in mine waste calcines and runoff sediments were reported from these Hg mining sites, which pose potential contamination on soil, water, and air in the area [7]. In the Philippines, a naturally occurring deposit of Hg can be found only in Puerto Princesa City on the island of Palawan. It mostly occurs in cinnabar (HgS) ones hosted by opalite bodies deposited along fractures and weak zones in the bedrock [9; 10]. From 1953 to 1976, PQMI mined the Hg deposit in the village areas of Sta. Lourdes and produced around 2900 metric tons of mercury [11]. Today, the abandoned open-pit mine is filled with water, and transformed into a pit lake located about 3 km west from Honda Bay coast (Figure 1). During the mining operations of PQMI, around 2,000,000 metric tons of cinnabar mine waste calines were produced [11]. Part of those mine waste calines were used to construct a 600 m by 50 m and 3 m thick wharf at the nearby Honda Bay to serve as port for mining operations from 1953 to 1976. The wharf is now used as port for tourist and fishing boats. The rest of the mine waste calines were stocked northeast near the pit lake shore (Figure 2), which was bare and had little vegetation until now. For the past four decades, remediation work has not been carried out at this site. Today, the abandoned Hg mine area is in the list of abandoned and inactive mines in the Philippines which pose a high risk to human health and the environment, and which require rehabilitation [1]. Mercury is a highly toxic element that is found both naturally, and as an artificially introduced contaminant, in the environment. Mercury mine sites remain as significant sources of Hg pollution in the environment, due to the unconverted HgS ore's left in the mine waste calines. The calines contain soluble Hg minerals such as montroydite, eglestonite, corderoite, and elemental Hg. Leaching of these calines, which has been reported from several abandoned Hg mines, becomes a significant source of Hg if released into the environment [7; 11; 12; 13; 14]. These anthropogenic activities can create widespread Hg contamination in soils of abandoned mine areas [15], while bare and unvegetated soils can produce dust with particulate-bound Hg during summer, that may be inhaled by the residents. Exposed HgS and mine waste calines pose a very serious health problem for the residents, as the absorbed Hg from HgS and calines is mainly accumulated in the kidneys, resembling the disposition pattern of inorganic Hg [16]. Inorganic Hg found in HgS and calines, may be moved along with the sediments and through water runoff. It can undergo methylation by microorganisms and convert it to methylmercury (MeHg) [17]. Methylmercury may bioaccumulate, and concentrate in the marine and freshwater food chain that might end up in human body [17; 18].