Total Mercury in Soils and Sediments in the Vicinity of Abandoned Mercury Mine Area in Puerto Princesa City, Philippines

Abstract

The abandoned mercury (Hg) mine area in Puerto Princesa City, Palawan was included in the list of abandoned and inactive mines in the Philippines which pose a high risk to human health and the environment, and require rehabilitation. The mine site, operated by Palawan Quicksilver Mines, Inc. (PQMI) from 1953 to 1976, is located approximately 3 km inland from Honda Bay coast and within the catchment of the Tagburos River, which is a local fishery and recreational area. In this study, total Hg levels in soils and sediments were measured to assess the possible release of Hg from the site into the surrounding natural environment. Results showed that total Hg (THg) concentrations found in soils (0.04–67.5 mg kg⁻¹), mine waste calcines (52.7–924.2 mg kg⁻¹), river sediments (1.8–119 mg kg⁻¹), and marine sediments (0.04–12.7 mg kg⁻¹) were elevated compared to the global background of 0.045–0.16 mg kg⁻¹. The high concentrations of Hg in soils and river sediments were influenced by the different pathways for the release of Hg from its mine operations; while Hg in marine sediments was caused by the erosion of mine waste calcines near the pit lake, and calcines used to construct a wharf at the nearby Honda Bay. Mine wastes represent the largest source of Hg contamination in the area, due to the low efficiency of the recovery process during calcination. This work corresponds with the Minamata Convention on Mercury on the updated environmental assessments of abandoned Hg mines as potential source sites of mercury contamination.

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) ores 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 calcines were produced [11]. Part of those mine waste calcines 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 calcines 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 ores left in the mine waste calcines. The calcines contain soluble Hg minerals such as montroydite, eglestonite, corderoite, and elemental Hg. Leaching of these calcines, 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 calcines pose a very serious health problem for the residents, as the absorbed Hg from HgS and calcines is mainly accumulated in the kidneys, resembling the disposition pattern of inorganic Hg [16]. Inorganic Hg found in HgS and calcines, 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]. Excess concentrations of Hg in the human body can cause serious health threats, like damage to the central nervous system, and is detrimental to fetal development [17,18,19].

Studies conducted in the abandoned PQMI mine area since 1994 show that the population in the affected areas have relatively high Hg concentrations in their hair and blood samples [20,21,22]. In 1994, health reports revealed significant elevation of blood Hg levels exceeding the previous recommended exposure level of 20 ppb in 12 out of the 43 residents examined, after their complaints of unusual symptoms (e.g., miscarriages, tooth loss, muscle weakness, paralysis, anemia, tremors, etc.) [21]. Recent reports from the Department of Health (DOH) and Department of Environment and Natural Resources (DENR) say that 33–40% of the 10,000 combined Tagburos and Sta. Lourdes residents have ‘chronic mercury poisoning’, from the exposure to mine tailings and ingestion of marine products with high Hg content [23]. However, a recent survey of the surface water quality of different water bodies in the vicinity of the PQMI mined area showed undetected Hg concentrations in the pit lake and river, and low Hg concentrations in coastal water near the wharf (0.001 mg L⁻¹) and in hot spring water (≤0.0004 mg L⁻¹) [1]. The very low Hg concentrations measured in the surface water suggest that there is low solubility and bioavailability of inorganic Hg from HgS, and mine waste calcines. For the past 20 years after the last known study on Hg contamination in the area, Hg from HgS and calcines might be carried to the river by erosion and surface runoff, and eventually deposited in Honda Bay where they become a source of contamination and enter the food chain.

The purpose of this study is to document the current status of Hg pollution in different sites surrounding the abandoned PQMI mine area, describe how legacy mining still affects the area, and how it has evolved since the last studies performed in the 1990s. To evaluate the potential Hg contamination in this area, Hg concentrations were measured in the PQMI pit lake, Tagburos River, Honda Bay, Honda Bay Wharf, mine waste calcines, soils (including agricultural soils) in the vicinity, and other rivers which are not tributaries of the Tagburos River. Assessments on the Hg concentration in fish and the extent of human exposure to Hg were not included in the study. The output of this study will serve as guide to the rehabilitation prioritization of the abandoned PQMI mine area. This study will also conform with the requirements of the Minamata Convention on Mercury, where Philippines is one of the signatories, and give updates on the current environmental conditions of possible point sources of Hg, such as abandoned Hg mining sites.

2. Materials and Methods

Analyses of total Hg concentrations in different soils and sediments in the vicinity of the abandoned PQMI mine area were conducted to determine the current Hg concentrations in different sub-areas, especially in Tagburos River and Honda Bay. Characterization studies on the Hg contamination in this area is very limited, with the last known published article in 1996 [21]. In 1999, sedimentation rate in Honda Bay was estimated at 1 cm yr⁻¹ based on the ²¹⁰Pb dating of core samples collected from Honda Bay [10]. With a wider scope of sampling area considered in this study, including other rivers and mountainous terrain nearby, assessment of the extent of Hg contamination in the abandoned mine area can be drawn.

2.1. Study Area and Sampling Sites

The study area, Puerto Princesa City, is located in the midsection of Palawan Island. It is bound to the east by the Sulu Sea, and to the west by the South China Sea (Figure 1). The area in Puerto Princesa City with reported human Hg risk encompasses two villages: Sta. Lourdes and Tagburos. The study site, located around 14 km north of Puerto Princesa City center (118°43′ N, 09°50′ E), is approximately 3 km inland from the Honda Bay coast, and situated within the catchment of the Tagburos River, which is a local fishery and tourist recreational area. In this area, a naturally occurring deposit of Hg ore (cinnabar), which is hosted by the Tagburos Opalite formation, can be found. Cinnabar was mined from 1953 to 1976, and the open pit mine site was naturally filled with water, forming a lake.

2.2. Sampling

A total of 102 soil and sediment samples were collected from the study area, between September 2018 to October 2019. Soil samples were collected from the vicinity of former mining sites; while surface sediments were collected from the bottom of the pit lake, Honda Bay, Tagburos River, and other rivers not traversing the pit lake in the area. Mine waste calcine samples were collected from the stockpile near the pit lake and from the scattered calcines situated 500 m northeast of the pit lake. All samples were classified into seven groups: PQMI pit lake, Tagburos River, Honda Bay, Honda Bay Wharf, mine waste calcines, soils (including agricultural soils) in the vicinity, and other rivers which are not tributaries of Tagburos River. Samples were collected using a plastic trowel, from the top 10 cm of the soil surface and surface sediment of rivers. Sediments from Honda Bay and the pit lake were collected using a sediment grab sampler (Wildco Petite Ponar^® Grab Sampler, Yulee, FL, USA), collecting the top 5–10 cm of surface sediments. All collected soil and sediment samples were placed in polyethylene resealable plastic and transported to the laboratory at the Philippine Nuclear Research Institute in Manila.

2.3. Total Mercury Analysis

Analyses of THg in the soil and sediment samples were performed at the laboratory of the Philippine Nuclear Research Institute. Samples were prepared by air-drying and sieving up to US Standard 230 mesh (63 µm), to separate the silt-clay (<63 µm) size particles from the sand fraction. Only the <63 µm fractions were analyzed for THg concentrations. Total Hg analyses in the soil and sediment samples were determined using a direct mercury analyzer (Milestone DMA-80 evo, Sorisole, BG, Italy) following the U.S. EPA Method 7473 (mercury in solids and solutions by thermal decomposition, amalgamation, and atomic absorption spectrometry). This Hg analyzer is capable of determining THg concentrations in solid, liquid, and gas samples. Soil samples were weighed (0.10 g) and placed in a nickel boat without any pre-treatment. The boat is then introduced to the direct mercury analyzer where it is initially dried at up to 250 °C, and then thermally decomposed at 900 °C in a continuous flow of oxygen. The THg content is determined using atomic absorption spectrometry at 254 nm. All reported THg concentrations of soil and sediments analyzed were the average of three replicates. The THg analysis results from DMA-80 were validated using certified reference material for stream sediments (GSD-1) prepared by the Institute of Geophysical and Geochemical Exploration of China, with a mean (S.D.) THg value of 0.018 (0.004) mg kg⁻¹. The mean (S.D.) value of the analyzed results using DMA-80 was 0.019 (0.002) mg kg⁻¹, which is in good agreement with the certified value.

3. Results and Discussion

3.1. THg in Mine Waste Calcines and Honda Bay Wharf

From all the surface sediments collected in the vicinity of the abandoned mining site, there are two noticeable hotspots of very high Hg concentration relative to their surroundings (Figure 3). These are the mine waste calcines near the pit lake, and the calcine used in the construction of Honda Bay wharf.

Mine waste calcines were found to be stocked northeast of the abandoned open pit lake (Figure 2). The mine waste calcines have poor sorting, angular grains, and can reach around 6 to 8 m in thickness. The calcine exhibits a reddish to orangey brown color due to the weathered iron bearing minerals and residual cinnabar. It mostly contains crushed bedrock material hosting the Hg deposit, along with few grains of silica from the opalite, which brought on the mineralization.

In this study, the measured THg in mine waste calcines have concentrations ranging from 52.7–924.2 mg kg⁻¹. These calcines were stocked at the shore of the lake and scattered up to 500 m northeast of the lake. Total Hg concentrations found in the mine waste calcines are largely similar to the concentrations measured in the same study area in 2002–2003, but this time, a higher concentration (924.2 mg kg⁻¹) was measured at one sampling point. A possible cause of the high THg concentration in mine waste calcines, is the inefficient and incomplete calcination or roasting of cinnabar ores. The range of THg concentrations in mine waste calcines measured in this study were generally within the range of THg concentrations on calcines from China, New Zealand, and the USA, as presented in

Table 1. Total Hg concentrations in soils and mine waste calcines collected from selected Hg mines worldwide.

Location	n	Total Hg (mg kg−1)
Palawan Hg mine, Philippines (This study)
Soils	31	0.04–67.5
Mine waste calcines	7	52.7–924.2
Palawan Hg mine, Philippines [11,22]
Soils	21	0.012–168.72
Mine waste calcines	11	3.68–660
Wuchuan Hg mines, Guizhou, China [5]
Soils	*	0.33–320
Mine waste calcines	*	79–710
East Guizhou Hg mines, China [6]
Soils	*	5.1–790
Mine waste calcines	*	5.7–4400
West-central Nevada Hg mines, USA [8]
Mine waste calcines	36	1.9–2000
Puhipuhi Hg mine, Northland, New Zealand [7]
Soils	4	35.3–1486
Almaden Hg mine, Spain [15]
Soils	6	13–64
Usagre Hg mine, Spain [4]
Soils	26	0.2–311
La Soterraña Hg mine, Northern Spain [3]
Soils	56	1.73–502
Branalamosa Hg mine, Northern Spain [3]
Soils	28	0.5–895
Maramuñiz Hg mine, Northern Spain [3]
Soils	23	2.0–577
Global background for pristine soils [25]		0.045–0.16

* number of samples was not provided in the literature.

.

Total Hg concentrations measured in soil of Honda Bay wharf and stockpiled mine wastes exhibit similar ranges of concentration. Samples collected in Honda Bay wharf display the same physical characteristics as the calcines stockpiled near the pit lake. This confirms that the soil materials used in the construction of the wharf were the mine waste calcines from PQMI mine site. High THg concentration (731.6 mg kg⁻¹) was also measured in the calcines stocked near the wharf. It was higher than the estimated THg concentration (570 mg kg⁻¹) of all the calcine materials used in Honda Bay wharf in 1996 [24]. Sediment samples collected 10 m north of the wharf shore have Hg concentrations of >100 mg kg⁻¹, which is in agreement with the concentration reported by Benoit and co-workers [24] in 1996. As stated in previous studies [20,21,24], Honda Bay wharf is considered one of the possible sources of Hg contamination in Honda Bay. The removal of Honda Bay wharf as well as the dredging of contaminated tailings and sediments in Honda Bay, were suggested in the study of Williams and co-workers in 1996 [21], but these recommendations were not followed. Today, the inhabitants of the villages located in the vicinity of the abandoned mine area including Honda Bay, which is home to around 10,000 inhabitants, are exposed to potentially serious health issues due to Hg contamination.

3.2. THg in Sediment in Tagburos River and Pit Lake

Tagburos River, which is the main tributary in the abandoned mine area, has measured THg concentrations in the sediments ranging from 1.8–119 mg kg⁻¹. Sediments found in the river were influenced by the geology of the surrounding area, as the exposed soils with Hg can be transferred into the river and eventually deposited in Honda Bay. High Hg concentration in the river sediments suggest that exposed mine waste calcines with elevated Hg concentration in the abandoned mine area are being carried to the river by erosion and surface runoff, and are eventually deposited in Honda Bay where they become a source of Hg contamination and enter the food chain. Rehabilitation programs, such as revegetation, are recommended in this abandoned mine area, to avoid the erosion of soil particles with high Hg concentrations going to the river and depositing in Honda Bay.

High THg concentration measured in the pit lake sediments ranged from 37.5–454.1 mg kg⁻¹. The highest concentration was measured near the exposed bedrock on the lake shore, as shown in the inset of Figure 3. These values are within the range of the measured THg in pit lake sediments (6.9–400 mg kg⁻¹) in 2003 [11]. The high THg concentration at the lake bottom may be due to naturally occurring cinnabar in the bedrock, and deposition of eroded calcine stocked near the lakeshore.

3.3. THg in Soils

Natural concentrations of Hg in soil are dependent on the source parent rock’s mercury concentration, which becomes concentrated through the processes of weathering and natural erosion. At the vicinity of abandoned mine area, measured THg concentrations (0.04–67.5 mg kg⁻¹) are 70 times higher than the global background for pristine soils (

Table 2. Mean total Hg concentration of different soil and sediment samples collected from different areas.

Site Location	n	Mean (Range) Concentration (mg kg−1)	S.D. (mg kg−1)	Global Background (mg kg−1) [25]
Pit lake	11	124.5 (37.5–454.1)	123.1	0.045–0.16
Tagburos River	14	36.7 (1.8–119)	41.4	0.045–0.16
Honda Bay	28	3.5 (0.04–12.7)	3.6	0.045–0.16
Honda Bay wharf	6	357.3 (102–731.6)	245.5	-
Mine waste calcines	7	231.4 (52.7–924.2)	309.8	-
Other rivers	5	10.5 (5.3–11)	3.7	0.045–0.16
Soils	31	7.4 (0.04–67.5)	13.8	0.045–0.16

). The soil THg concentrations measured in this study were lower than the range of concentrations (0.01–168.7 mg kg⁻¹) analyzed in the same area in 2002–2003. Though the lowest Hg concentration measured was 0.04 mg kg⁻¹, corresponding to the background concentration; the other parts of the area might have been influenced by the movement of Hg-laden soil particles from the stocked mine waste calcines and other mine debris scattered in the abandoned mine area during operations. With consideration of the elevated sedimentation rate at Honda Bay (1 cm/year) [10], annual soil erosion to the bay via Tagburos River is likely a major source of Hg contamination for downstream ecosystems. Another human exposure pathway of Hg in the area is the dust coming from the bare soil during summer, which produces particulate-bound Hg that may be inhaled by the residents in the area.

3.4. THg in Honda Bay Sediment

For the past four decades after the abandonment of the Hg mine operations by PQMI, there were very limited studies and monitoring conducted to determine the Hg concentrations in Honda Bay marine sediments. In the present study, analytical results of the THg concentrations in sediments collected from Honda Bay ranged from 0.04–12.7 mg kg⁻¹. This range of data shows a greater range of THg concentration, compared to the previous measurements conducted by different researchers in the same area some 20 years ago [20,21,24]. In the study by Benoit and co-workers in 1994 [21], the estimated Hg concentration in Honda Bay sediments (0.003–3 mg kg⁻¹) came from the wharf, which transported seaward or along the coast; while in this study, the identified possible sources of Hg contamination in Honda Bay marine sediments are the wharf (up to 12.7 mg kg⁻¹, 250 m from the wharf) and Tagburos River (up to 9.6 mg kg⁻¹ in river mouth). The present THg concentration in the marine sediment also higher than the global Hg background concentration for sediment. Furthermore, the values gathered in the present study are in the same range of sediment THg concentration in the dredged area of Minamata Bay in 2006, as shown in Figure 4 [26]. Finally, such an elevated range of Hg concentrations at Honda Bay could also be biomethylated inside the Bay, becoming a significant source of methylmercury for both the marine and freshwater food chains that might be consumed by local inhabitants.

3.5. THg of Sediment from Other Rivers

Collection of sediment samples were even extended to other rivers that flow through the abandoned mine area, to determine the trend of Hg concentration surrounding the study area. The measured THg concentrations across several rivers in the area ranged from 5.3–11 mg kg⁻¹. These values are more than 60 times higher than the global background, indicating Hg accumulation in the river caused by the erosion of the soil in the area with a naturally higher Hg background concentration. In comparison, sediments in Tagburos River show significantly higher Hg concentration than the other rivers since it traverses the abandoned mine site and the stockpiled calcines. Mercury may be moved along with the sediments and through water runoff, thereby increasing Hg values in the sediment deposited in Tagburos River.

4. Conclusions

In order to determine the pollution status and availability of Hg from the abandoned Hg mine in Puerto Princesa City, this study investigated the soil and sediment samples collected from the surroundings of the abandoned PQMI mine area. Results of the Hg analyses of these samples followed similar contamination patterns from other reported Hg mine sites worldwide. Mercury concentration in mine waste calcines ranged from 52.7–924.2 mg kg⁻¹, of which some values were higher than those analyzed in 2003. Mercury levels measured in Honda Bay wharf were found to be similar in Hg concentration and characteristic with the mine waste calcines, confirming that the construction of wharf utilized the calcines from the PQMI mine site. Sediment samples obtained from Tagburos River and other tributaries, Honda Bay, and soils also exhibited Hg values that exceeded the global background, due to naturally high Hg concentrations in the mineralized area, and from the erosion and deposition of mine waste calcines into the surrounding areas. A greater range of Hg concentrations were measured at Honda Bay compared to the previous measurements conducted by several researchers in the same area some 20 years ago. These increased Hg concentrations in Honda Bay sediments may undergo methylation and enter the food chain, which was observed in Hg mining sites around the world [3,7,12]. This study concludes that one of the pathways of the reported Hg contamination in the area is caused by sediment transport from two point sources: (1) the calcines stocked at the shore of the pit lake and nearby area with scattered calcine, and (2) Honda Bay wharf. Sediment transport from the first point source was facilitated by Tagburos River and its tributaries, while the second point source was due to coastal erosion. Further studies are needed to evaluate the methylmercury levels in biota and particle-bound Hg in the air to evaluate health issues. Mining rehabilitation activities, such as revegetation to reduce soil erosion and soil particle suspension, need to be implemented based on the results of this study.