surface soils, brown rice grain (Jasmine rice #105) and Neem flowers of terrestrial habitats were in the ..... ment of Commerce, National Institute of Standards and.
Journal of Environmental Science and Health Part A (2007) 42, 1081–1093 C Taylor & Francis Group, LLC Copyright ISSN: 1093-4529 (Print); 1532-4117 (Online) DOI: 10.1080/10934520701418573
Mercury emission and distribution: Potential environmental risks at a small-scale gold mining operation, Phichit Province, Thailand PORANEE PATARANAWAT1 , PREEDA PARKPIAN2 , CHONGRAK POLPRASERT2 , R. D. DELAUNE3 , and A. JUGSUJINDA3∗ 1
Post-Graduate Education in Environmental Toxicology, Technology and Management, Inter-University Program between Chulabhorn Research Institute, Asian Institute of Technology, and Mahidol University 2 School of Environment, Resources and Development, Asian Institute of Technology, Thailand 3 Wetland Biogeochemistry Institute, School of Coast and Environment, Louisiana State University, Baton Rouge, Louisiana, USA
Mercury (Hg) contamination was assessed in environment near an amalgamation gold recovery operation located at a small scale mining operation (Phanom Pha) in Phichit Province, Thailand. Total mercury (THg) concentrations was determined in water, sediment, bivalves in the aquatic environment and as dry deposition or atmospheric fallout on surface soil and leaves of Neem tree (Azadirachta indica Juss. var. siamensis Valeton) near the mining operation. THg in surface soil, Neem flowers (edible part) and rice grain in surrounding terrestrial habitat and with distance from the mining area were also evaluated for possible contamination. Potential environmental risks were evaluated using the hazard quotient equation. Hg analyses conducted in the aquatic habitat showed that THg in water, sediment and bivalves (Scabies cripata Gould) ranged from 0.4 to 4 µg L−1 , 96 to 402 µg kg−1 dry weight (dw) and 15 to 584 µg kg−1 wet weight (ww), respectively. High concentrations of THg in water, sediment and bivalves were observed in the receiving stream near the mining operation which was located near the Khao Chet Luk Reservoir. Whereas the THg concentration in water, sediment and bivalves from monitoring stations outside the gold mining operation (upstream and downstream), were considerably lower with the values of 0.4–0.8 µg L−1 , 96–140 µg kg−1 dw and 88–658 µg kg−1 dw, respectively. The elevated concentration of Hg found in the sediment near the mining operation was consistent with Hg accumulation measured in bivalves. The elevated Hg levels found in living bivalves collected from highly contaminated sites suggested that the sediment bound Hg was bioavailable. THg in surface soils, brown rice grain (Jasmine rice #105) and Neem flowers of terrestrial habitats were in the range of 16 to 180 µg kg−1 dw, 190 to 300 µg kg−1 dw, and 622 to 2150 µg kg−1 dw, respectively. Elevated concentrations of mercury were found in Neem flowers with the concentration greater than 600 µg kg−1 ww, which exceeds the maximum permissible concentration reported for biota tissue (500 µg kg−1 ww). An evaluation of air and soil pollution near the mining operations showed high concentrations of THg in dry deposit from atmospheric fallout (139 µg m−2 d−1 ), and in surface soil (10,564 µg kg−1 dw) at station near where open burning of gold ore extracts using the amalgamation process occurred. High or elevated concentration of THg (1172–1301 µg kg−1 dw) in leaves of Neem tree was also measured near the mining operations. A survey of Hg in surface soil showed elevated Hg concentrations near the site which corresponded to the elevated THg concentration in dry deposition. These results suggested that atmospheric fallout is a major source of Hg to the area surrounding the mining or gold ore extraction. Results also suggest that Hg emitted into the air (estimated to be 60–150 g d−1 ) from the gold mining activities (over the past 10 years) contaminated air, the aquatic environment, surface soil and biota in the area surrounding the gold mining operation. Keywords: Small scale gold mining, amalgamation, Hg contamination in soil, water, air and biota, aquatic habitat, Hg accumulation.
Introduction The Hg-based amalgamation process; is currently used in the extraction of gold from secondary ore at a small-scale Address correspondence to A. Jugsujinda, Wetland Biogeochemistry Institute, School of the Coast and Environment, Louisiana State University, Baton Rouge, LA 70803-7511; E-mail: ajugsv@ lsu.edu Received January 24, 2007.
gold mining in Phanom Pha district, Phichit Province, Thailand. Final recovery of fine gold particles extracted is conducted through heating or burning of the amalgam which results in Hg emissions to the atmosphere. Gold in the past was removed from stream beds by panning, which relied solely on gravity separation. Later, the Hg amalgamation process was used to extract gold from rocky ores following crushing into smaller particles. The amalgamation process used in mercury recovery is conducted at many stream sites in the study area.[1] Although this method is not used
1082 or allowed for gold recovery in many countries around the world—today it is still used in Phichit Province, Thailand.[2] For every g of gold produced, 1.2–2 g Hg is lost to the environment. In many cases neither the area impacted nor the effect of Hg dispersion to the environment is known.[3,4] High Hg concentrations (0.1–5 µg m−3 ) were measured in the atmosphere of Almaden village, Spain and the THg flux into the atmosphere was estimated to range from 600 to 1200 g h−1 , even with considerable distance from the main emission sources (5–10 km) high air Hg levels (0.05– 0.1 µg m−3 ) were observed.[5] Hg uptake by plant grown on surface soils have been studied and the distribution of Hg in the vegetation were evaluated.[6,7] Tarras-Wahlberg et al.[8] reported that elevated levels of metal contaminants in aquatic organisms affected by small-scale gold mining activities were generally not present in water-soluble form but were associated with sediment. However, elevated levels found in biota showed the metals to be bioavailable. To date no studies have been conducted to evaluate the environmental impact and potential hazards related to Hg contamination in the ecosystem surrounding mining operations in Phanomm Pha district, Thailand. In Phichit Province, any Hg which enters the atmosphere can undergoes a variety of chemical transformations and with time is deposited on land surfaces attached to particulate matter. Once on the surface environment, Hg can migrate through watersheds and eventually enter receiving water bodies (e.g., wetlands and lakes). Hg moving through watersheds is subject to a myriad of chemical transformations. These transformations are often biologically mediated. The most important of these biological transformations is the generation of methyl-Hg (MeHg).[9] MeHg is a highly toxic form of Hg, and it is easily assimilated by microorganisms and planktonic organisms which form the base of the food chains of lakes. Through the processes of bioaccumulation and biomagnification, minute concentrations of MeHg can be passed up food chains, increasing concentration to elevated level in organisms at the top of the aquatic food web.[10] Hg released into the atmosphere is deposited onto terrestrial surfaces mainly as Hg(II), from either direct deposition of emitted Hg(II) or from conversion of emitted elemental Hg0 to Hg(II) through ozone-mediated oxidation. Significant amounts of the mercury emitted into atmosphere can travel great distances. Once deposited, Hg binds tightly to soil components. Some of the deposited Hg(II) can revolatilize and be released back to the atmosphere as Hg0 . Soil or sediment Hg(II) under anaerobic condition may also be methylated to form MeHg; mercury may remain in the soil or be transported throughout the watershed via runoff and leaching process.[11] With the exception of isolated cases of known point sources, the ultimate source of Hg entering most aquatic ecosystems is atmosphere deposition, primarily associated with rainfall. Atmospheric deposition contains the three principal forms of Hg, although the majority is as inorganic
Pataranawat et al. Hg (Hg2+ , ionic Hg). Once in surface water, Hg enters a complex cycle in which one form which can be converted to another. It can enter the sediments by particle settling, later be released back to the water column by diffusion or column resuspension. The concentration of dissolved organic carbon (DOC) and pH have a strong effect on the ultimate fate of Hg in an ecosystem. For the same species of fish taken from the same region, increasing the acidity of the water (decreasing pH) and/or the DOC content generally results in higher mercury levels in fish. Many scientists currently believe that higher acidity and DOC levels enhance the mobility of Hg in the environment, thus making it more likely to enter the food chain.[12] Hg entering the aquatic environment can accumulate in bivalves or other aquatic organisms of the receiving stream. Hg in the environment can be converted from inorganic forms to the more toxic methylmercury which can be biomagnified MeHg.[13−15] MeHg in aquatic organisms can move up in the food chain and the main pathway of Hg exposure into humans resulting from fish consumption.[16] Thus, exchanges of Hg among the abiotic and biotic environment compartments are of great interest. It has been reported that fresh water bivalves (Unio pictorum) can be used as a bio-monitoring tool for quantifying Hg contamination in the Almaden area in Spain.[17−19] A typical concentration of Hg in aquatic invertebrates from uncontaminated areas is generally less than 100 µg kg−1 ww.[20,21] By comparison tissue concentrations over 1000 µg kg−1 ww have been reported for area with anthropogenic point sources of Hg.[22] A fresh-water mussel, Unio elongates, was used as a measure of Hg accumulation in a long-term study involving annual testing of the natural and mining-induced Hg contamination of the Plagia River in Spain.[16] The objective of this study was to evaluate Hg pollution in the environment in vicinity of a gold recovery operation in Pichit Province Thailand and to evaluate the potential human health and environmental risk near the site.
Materials and methods Study area It was thought that a significant portion of Hg loss to the environment or emitted to the atmosphere over the years directly or indirectly entered the aquatic environment (mainly the Klong Dai Nam Khun and Klong Sa Luang canals) that runs east-west along the mining area. The study of mercury pollution in the aquatic environment is of great interest due to the possibility of Hg transformation and accumulation in the system. The small scale gold mining of Phanom Pha Hill may be a source of Hg pollution into the environment because the roasting processes were carried out at the mining site. The sampling protocol was designed around discharge path and likely impacted area.
Mercury emission in thailand
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Fig. 1. Map of sampling stations: aquatic habitat; A1-A13 (), terrestrial habitat; T1-T4 () and hazardous waste at workplace; W1-W5 (•).
The main water flow path (A1-A10) runs east-west along the study area, which lies on a tropical, semiarid environment (Fig. 1). Water flow in the aquatic environment ranged from little to none during late summer (April to June), while during the wet season (July to October) flood events occurred. The area under investigation is a section of the Dai Nam Khun Canal and Sa Laung Canal drainage basin, which extends from the upper Dai Nam Khun Canal downstream to the Khao Chet Luk Reservoir which emptied into the Sa Laung Canal. The mean average rainfall for a 12year period (1993–2004) in Phichit Province is about 1276 mm with daily maximum rainfall of 141 mm. Southwest (May-October) and northeast (November-February) monsoon occurs in this region. Thirteen stations (A1 to A13) were selected as representative sites for aquatic habitat sampling. A1 to A10 were located in the main aquatic area likely impacted by the mining operation, whereas A12, A13 were in other water bodies near the workplace area. A12 is a reservoir used in the dry season and A13 is a separatory ditch that used in gold ore separation process. A4 and A5 are located in the receiving
reservoir representing an inlet and outlet, respectively. Five stations (A3, A4, A5, A12 and A13) were chosen based on their location relative to known Hg-contaminated sites near the workplace where open burning occurred. Of the remaining seven sampling stations, two were located upstream (A1 to A2) and five downstream (A6 to A10). Three monitoring stations (A1, A10 and A11) were selected to represent the background/control areas for determining Hg levels in the sediment outside the mining operation. Water samples for all stations were not available during the dry season. When no water was present only the sediment samples were collected. The control site was located 5 to 6 kilometers from the workplace. Stations W2 to W4 were selected in order to investigate Hg contamination resulting from Hg emission near the workplace (dry deposition, surface soil and Neem leaves). Background level of Hg concentration at site with distance was determined using stations W1 and W5, which were upwind and downwind sites, during the monsoon seasons. Four stations (T1 to T4) were selected as the representative sampling sites for evaluating mercury levels at terrestrial
1084 sites. Station T2 was near the mining operation or work place whereas stations T1, T3 and T4 were distance sites or remote from potential Hg source.
Sampling and characterization of surface waters Selected water quality parameters were measured both in the field and in the water samples taken to the laboratory. The parameters measured in the field including temperature, and conductivity using a Salinity-ConductivityTemperature (S-C-T) meter. Dissolved oxygen (DO) was measured using a dissolved oxygen meter. The pH on site was measured with a portable pH meter. Concentration of suspended particulate matter (SPM) was measured using aliquot water sample collected on pre-weighed TTFE membranes (polytetralfluoroethylene membrane) as nucleopore membrane of 0.4 µm pore size.[28,29] Dissolved organic carbon (DOC) was measured using a total organic carbon analyzer; TOC-VCSN (SHIMADZU) following the 5310B, High Temperature Combustion.[23] Water samples were collected at 10 locations, 8 samples represent at the predominant aquatic track (A2 to A9) and 2 additions were from the area near Hg contaminated sites (A12 to A13). Duplicate set of water samples were collected at midstream depth during wet season sampling (June, 2004) using a non-metallic convertible water sampler (Kremerer water sampler). The concentration of particulate Hg concentration in the water samples was assessed using standard reference materials. All water samples were analyzed in the laboratory within 30 days following collection. Water samples for dissolved Hg were preserved with 20% (w/v) K2 Cr2 O7 solution; prepared in 1+1 HNO3, and kept in glass containers at 4◦ C. Concentration of Hg in water samples were determined against a set of Hg standards.
Sediment sampling and characterization Composite samples of sediment were collected at ten sampling stations representing the same site from which the water samples were collected (A2 to A9 and A12 to A13) plus an additional 3 sites (A1, A10 and A11) used for the determination of background level of Hg in the sediment outside the study areas. Each sediment sample was scooped from the surface 5 cm of the sediment only when a sediment core could not be used. All sample bags were preserved in ice and brought back to the laboratory. Sediment was airdried (under shade), crushed and sieved to remove particles greater than 2 mm. Over 90% of contaminates reported are present in the particles smaller than 2 mm.[24] The samples were ground with an agate mortar to a particulate size of 63 µm, and retained for analysis. Since sediments are less affected by seasonal variations they were collected only during the 2004 dry season.[25] Processed sediment samples were kept in plastic bags and labeled until chemical analysis.
Pataranawat et al. Measurement of dry sediment pH was carried out using deionized distilled water with a 1:1 soil : solution ratio. The pH of the solution was measured using a combination of glass electrode and pH meter. Cation exchange capacity (CEC) was measured by replacing exchangeable cations with 1 Nammonium acetate pH 7.0.[26] Organic carbon was determined by Walkley & Black method.[27] Organic carbon converted to organic matter (OM) was reported as percentage. The foregoing physico–chemical characteristics of sediment were used in evaluating the relationship between sediment Hg concentrations and selected sediment properties. Sampling of surface soil Composite samples of surface soil (0–5 cm) were collected at various stations representing a total of nine samples (W1 to W5 and T1 to T4). Four of the samples (W2, W3, W4 and T2) were near the workplace area and the another 5 were taken with incremental distances from the workplace. Samples were prepared using the same method as described previously for the sediment samples. Sampling of dry deposit A set of glass containers containing distilled water was used to collect dry deposition of atmospheric particulates at 1.50 m height above ground at five stations (W1 to W5 for a period of 7 d) around the workplace. Concentrations of THg in samples were measured in both filtrated water and particulate matter remained on the TTFE Nucleopore membrane of 0.4 µm pore size. Sampling of biota Bivalves, rice grain, leaves and flowers of Neem Tree were collected in order to determine the bioaccumulation of Hg in various biota. A representative sub-sample (20 g) was removed following grinding. Samples were kept in the freezer prior to chemically analysis. Mature leaves of Neem tree (Azadirachta indica Juss. var. siamensis Valeton) were collected from 5 stations, W1 to W5, which surrounded the gold recovery area including sites at distance receiving less atmospheric deposition. The outer portion of leaves (not including the mid rib) were striped and used for analysis. Fresh leaves and flowers were ground separately. Grains of Jasmine rice # 105 (Oryza sativa L.) were collected at harvest (December) from four stations (T1 to T4) surrounding terrestrial habitat. Grains were polished once as brown rice prior to be blended 15 seconds and a sub sample collected for Hg analysis. Approximately 20 bivalves (Scabies cripata Gould) with a medium size of 3–4 cm in length were collected from five locations (A1, A5, A6 and A10, A11). Samples were collected by hand only in the 2004 dry season. The population was limited and difficult to locate in some locations.
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Mercury emission in thailand After sampling, the bivalves were placed in plastic bags and kept in a refrigerator prior to dissection. Soft tissues of bivalves were removed from the shell and then cut in sections, homogenized samples kept in polyethylene bag and frozen until analysis. THg analysis and determination All samples were digested for THg analysis by the method used at the Wetland Biogeochemistry Institute, Louisiana State University.[28,29] The Hg concentration was determined by Atomic Absorption using CV-AAS (Cold Vapor– Atomic Absorption Spectrophotometry; Varian SpectrAA 600) after NaBH4 (Sodium borohydride) reduction for total Hg analysis. The detection limit was 0.001 µg L−1 . Laboratory quality control The accuracy of the analytical procedure was calibrated using 3 replicated samples of standard reference material (SRM 2704 for sediment, SRM 2709 and SRM 2710 for soil and SRM 1515 for plant leaves) from the U.S. Department of Commerce, National Institute of Standards and Technology (NIST) and 4 samples of blanks. The method detection limit (MDL) was calculated and used as a tool for verification of all Hg analyses. The application of t-test for comparison showed that the certified and calculated values did not differ significantly (significance of 95%). Potential environmental risks A quantitative screening of risk using the hazard quotient (HQ) approach, which compares estimates of ecotoxicity to exposure, was used to estimate risk. Background concentration in the study area (about 4-6 km upstream and downstream from the area of concern) and the criteria/standard developed for other countries were used in establishing the ecotoxicity value for calculation of potential risk. An HQ value greater than 1 would indicate a state of risk.[11]
Results and discussion Laboratory quality control The analytical procedure used in Hg determination calibrated against standard reference materials showed values close to certified values (Table 1). Method detection limit (MDL) was 0.6 µg Hg L−1 with the percentage recovery ranging from 96.59–99.32%. Site characterization and Hg contamination Based on a study conducted by S.P.S. Consulting Service Co., Ltd in 2000,[1] soil texture in the project area consisted of sand 34.3%, silt 23.8% and clay 41.8%. The pH was 6.28.
Table 1. Analysis of standard reference materials for Hg ( n = 10) SRM
Certified value Measured value Recovery (µg g−1 dw) (µg g−1 dw) %
SRM 2704 1.40–1.54 1.41–1.51 (sediment) (1.47 ± 0.07) (1.46 ± 0.05) SMR2709 1.316–1.484 1.31–1.43 (San Joaquin Soil) (1.40 ± 0.08) (1.37 ± 0.06 ) SRM 2710 27–37 28.86–34.94 (Montana Soil) (32 ± 5) (31.9 ± 3.04) SMR 1515 0.040–0.048 0.041–0.045 (Apple leaves) (0.044 ± 0.004) (0.043 ± 0.002)
99.32 99.14 98.44 96.59
The soil contained high level of nitrogen in the form of organic matter (3.98%), high potassium (118 mg kg−1 ), and low phosphorus (0.15 mg kg−1 ) levels. Based on the characteristics the soil in this area was a heavy clay texture with moderate levels of nutrient except for phosphorus. Such soil type could play a significant role in the geochemistry and movement of Hg and other pollutant in the studied area. S.P.S. Consulting Service Co. Ltd.[1] reported slightly elevated Hg concentration in water, 4 µg L−1 at Khao Chet Luk Reservoir (station W4), exceeding surface-water standard for Hg (2 µg L−1 ). In addition, the Hg in the water samples from the water bodies at the project area: Khao Chet Luk Village, near station W5 (3 µg L−1 ); and Khao Chet Luk Reservoir, station A5 (4 µg L−1 ) were slightly exceeded surface-water standard for Hg (2 µg L−1 ). Corresponding higher concentration of Hg (4 µg L−1 ) has been observed in the ground water at Noen Phuang Village, about 1.7 km south of the contaminated site. The analysis followed standard methods for the examination of water and wastewater, 1998.[1] The processing of primary gold quartz veins and supergene gold mineralization in Eastern and Southern Africa use between 1.2–1.5 g of Hg for every 1 g of gold produced. Reported gravimetric material flow analyses show that 70– 80% of the Hg is lost to the atmosphere during processing, 20–30% are lost to tailings, soils, stream sediments and water.[4] It has been reported that the vast majority of Hg lost to the environment in California was from placer-gold mines with 10% to 30% loss per season resulting in highly contaminated sediments near mine sites.[30] Based on the above, for this Thailand study site we estimated that approximately 1 g of Hg would be released into the environment for each gram of gold recovered at the mining site. This estimate was applied to both the dry (April, 2004) and wet season (June, 2004) since the same group of workers processed the gold extraction. Since the amalgamation process was in operation for six d per week (Monday thru Saturday), approximately 60–150 g of gold ore would be processed daily as a result we estimated approximately the same amount of Hg (60–150 g) would have been emitted or lost (loss in the form of emission
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Table 2. Water characteristics in wet season Station
Station description
pH pH
Depth (m)
Temperature (◦ C)
Conductivity (µs/cm)
DOC (mg L−1 ) (nucleopore)
SPM (gm L−1 )
A-2 A-3 A-4 A-5 A-6 A-7 A-8 A-9 A-12 A-13
Upstream Upstream Reservoir Reservoir Downstream Downstream Downstream Downstream Pond Separatory ditch
6.6 6.9 6.9 6.8 7.2 6.7 6.7 6.9 7.6 7.7
0.5 0.5 5.2 5.4 1.5 1.2 2.5 1.5 2.0 2.8
31.5 30.8 30.1 32.0 35.7 30.7 31.9 32.6 35.0 35.1
67.2 84.5 69.2 66.8 52.5 55.5 50.1 55.8 229.0 185.0
5.36 4.99 4.81 5.33 4.41 4.04 5.26 5.82 3.04 4.12
0.67 0.73 0.76 0.76 0.70 0.68 1.43 1.10 0.04 0.14
∗
A3, A4, A5 and A12, A13 = workplace area.
and other losses such as wastewater disposal into the environment). Hg released into the air would be in vapor form. To a smaller extent some would enter into water bodies and soil near the site. This is of major concern because of the potential long-term impact to the habitat and human health near the gold recovery operation and surrounding area. Aquatic habitat Surface water . Water flow was observed only in the wet season (June, 2004) with little water flow in the dry season. Measured parameters of water were in the normal range (Table 2). The average concentrations of suspended particulate matter were very low even in wet season with the value of 0.67–1.43 gm L−1 . The concentration of dissolved organic carbon (DOC) among stations monitored were relative low (3.04 to 5.82 mg L−1 ) with little variations.
The THg measured was also very low not exceeding the standard for freshwater (2 µg L−1 ) except in the receiving reservoir (Khao Chet Luk; A4 and A5) when values of 3.22 and 4.19 µg L−1 (1.10 and 0.91 µg L−1 for filtered/DHg), respectively were measured. At stations A12 and A13 Hg concentrations are slightly elevated due to heavy mining activities and amalgams processing in the area. By comparison reported Hg contamination in the area near the gold mines in the Pocone area, Mato Grosso, Brazil was very low with a concentration range between 0.018 and 0.16 µg L−1 (filtered/ dissolved) compared to 0.03 ± 0.009 µg L−1 at a reference site.[31] The Hg concentrations with distance from the workplace (A3 to A5), and downstream (A6 to A10) or upstream (A1 to A2) is shown in Table 3. Two major water quality parameters (OC and pH) that could affect the Hg concentration in the water did not vary significantly among the stations studied. Thus the elevated
Table 3. THg concentrations in various media of aquatic habitat THg Concentration Water column ( µg L−1 ) Stations
Station description
A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 A11 A12 A13
Upstream Upstream Upstream Reservoir Reservoir Downstream Downstream Downstream Downstream Downstream Another track Pond Separatory ditch
Sediment (µg kg−1 dw)
DHg + PHg*
DHg
Bivalves (µg kg−1 dw)
126.7 139.9 204.9 156.6 401.9 131.6 96.4 140.3 129.5 127.2 98.8 298.4 293.8
— 0.79 0.82 3.22 4.19 1.10 0.58 0.74 0.43 — — 1.52 1.55
— 0.43 0.20 1.10 0.91 0.62 0.03 0.23 0.12 — — 0.79 3.85
87.5 — — — 3650 1054 — — — 658 542 — —
∗ A3, A4, A5 and A12, A13 = workplace area. DHg = dissolved Hg (filtered), PHg = particulate Hg.
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Mercury emission in thailand Hg concentration measured in the water column would be contributed to Hg from emitted sources which showed a slight decrease with the distance from the workplace. The transformation of Hg0 (g-gas) to Hg(II) (aq-aqeous) and Hg(II) (p-particulate) in turbid water is a possible mechanism by which anthropogenic sources of Hg0 to air can result in Hg deposition to land and water. This could explain the results obtained in this study. It has been suggested that direct wet deposition occurred in the wet season sampling. Anthropogenic inputs of Hg(II) is the predominated form of input back into the water column.[32] So in the dry season atmospheric fallout is likely the main mechanism of Hg deposit to the aquatic environment, whereas in the wet season wet deposition of Hg would be the predominated mechanism.
Fig. 2. THg in water column (µg L−1 ) and sediment (µg kg−1 dw) along the main aquatic track
Sediment Characteristics of sediment collected in dry season (January, 2005) in the main aquatic environment (A2 to A9) and two additional water bodies sampled in the project area (A12 to A13) are shown in Table 4. The soil characteristics, as defined by the Soil Conservation Service (SCS) and U.S. Environmental Protection Agency (USEPA)[25] were used to classify the sediment which showed low organic matter of sediment (less than 2%) and low cation exchange capacity (CEC) (less than 12 meq 100 g−1 ) were observed in sediment in all stations monitored. The clay content varied from 31.63–55.50 %. Lower clay content (23.76%) was found in the separatory ditch (A13) where sand from the mineral ore was disposed into the water bodies following the gold separating process. High content of clay (more than 40%) was found only in the Khao Chet Luk reservoir (A4, inlet) and at Ban Khao Chet Luk (A7) with the value of 55.50 and 45.49%, respectively, reflecting indigenous parent materials. The sediment texture at this site ranged from coarse texture to heavy texture (Table 4). Heavy clay texture is preferable in terms of binding Hg and preventing leaching or release back to the surface water following Hg deposition into the sediment. Based on
sediment texture (high clay), this aquatic environment containing high clay content in sediment would be at less risk in terms of Hg remobilization. As expected a slight increase in Hg concentration (value as high as 4.19 µg L−1 ) was found in the water from the receiving reservoir, particularly at the outlet (A5) (Table 3 and Fig. 2). THg concentrations of superficial sediments was significantly higher at station A5 (402 µg kg−1 dw). The concentration in sediment in the other 4 stations near the workplace (A3 to A4 and A12 to A13) ranged from 160 to 298 µg kg−1 dw. Three stations in which sediment THg concentrations in sediment were considerably high were A5, A11 and A12. The values were 402, 298 and 294 µg kg−1 dw (Table 3). Elevated levels at the three stations could be explained by slow flow as compared with ongoing mining activities. Particulate matter containing Hg settled out and accumulated. In these areas no relationship was found between pH, OM or % clay content and Hg concentration in the sediment. A comparison between Hg concentration in the water and sediments along the main aquatic sampling locations is shown in Figure 2. The lower Hg content in stations A1,
Table 4. Sediment characteristics in dry season Station
Station description
pH
OM (%)
CEC (meg/100 g)
Texture
Sand (%)
Silt (%)
Clay (%)
A-2 A-3 A-4 A-5 A-6 A-7 A-8 A-9 A-12 A-13
Upstream Upstream Reservoir Reservoir Downstream Downstream Downstream Downstream Pond Separatory ditch
5.5 8.6 5.0 5.9 5.0 5.9 6.8 5.2 8.1 6.5
1.25 0.30 1.17 0.63 0.72 0.91 0.52 0.06 1.07 1.23
12.03 11.48 17.95 13.75 10.63 7.18 12.45 9.23 7.00 15.80
Sandy loam Sandy clay loam Clay Sandy clay Sandy clay loam Clay Sandy clay Clay loam Sandy clay loam Clay loam
49.72 51.94 30.00 51.64 45.72 16.15 46.36 44.52 59.86 41.79
14.57 16.43 14.50 8.94 20.65 38.36 14.08 18.06 16.38 20.57
35.71 31.63 55.50 39.42 33.63 45.49 39.56 37.42 23.76 37.64
∗
A3, A4, A5 and A12, A13 = workplace area.
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A2 and A7 thru A10 (including A11), as shown in Table 3, was consistent with the lack of known Hg sources input in the area. Therefore, these sites were used to represent background level in sediment (control) with the value around 174 µg kg−1 dw for this area. Reported investigations in areas near gold mines in the Pocone area, Mato Grosso, Brazil,[31] showed elevated Hg concentration in the range between 23 to 198 µg kg−1 dw, which is lower than reported in this study area. However, in our study THg concentrations in the water column were very low even in area of potentially pollution (A4 and A5). Likewise, the Hg in sediment in the aquatic environment may not be easily remobilized back to the water column as a result of the strong association with the heavy clay texture of the sediment. Because of the THg in water column seemed in part to come directly from the deposition from the open burning area resulted in the high elevated concentration in the area.[32] Bivalves Bivalves were not available for collection at all sampling locations. Fortunately, bivalves were available for detecting THg accumulation in areas of concern (A5,A6, A10, A1 and A11). Both upstream and downstream stations were used as a control for detecting background Hg in the tissues of mussel not impacted by Hg pollution. The results (Table 5) showed that total Hg in bivalves from the highly polluted station (A5) was highly elevated with value of 584 µg kg−1 ww as compared with values in bivalves collected from both upstream and downstream sites (A1 and A10) and a site at the location with distance mining activities (A11) where the Hg concentration ranged between 15– 105 µg kg−1 ww. The results also confirmed that the greater the distance between the station and the point source, the less the Hg concentration in the bivalves. Likewise, the THg concentration in bivalves was slightly correlated with those in sediment. The bivalves found in the area containing elevated Hg levels were Scabies crispata Gould which are consumed by local population as a food source. The measured high mercury in tissue would suggest that the mussel living in the reservoir (A5) may not be safe for human consumption (Fig. 3). Table 5. THg in bivalves and sediment
Station A1 A5 A6 A10 A11 ∗
Station description Upstream Reservoir Downstream Downstream Another track
Bivalves
Sediment µg kg−1 ww µg kg−1 dw (µg kg−1 dw) 15 584 169 105 98
A5 = reservoir near workplace area.
88 3650 1055 658 542
127 402 132 127 99
Fig. 3. THg in sediment (µg kg−1 dw), bivalves (µg kg−1 ww) and Neem Flowers (µg kg−1 ww).
Hg forms are preferentially associated with suspended particles in water or sediments. Hg methylation by naturally occurring benthic microbes, however, can greatly increased exposure because MeHg formed from methylation of Hg is more readily absorbed by living organisms. The proportion of MeHg in surface waters coming from soils via runoff varies substantially. It is still unclear how Hg/MeHg in runoff enters the food chain, but a significant correlation has been observed in Sweden between the catchment/ lake ratio and the levels of Hg in fish. This suggests that the catchment contribution to Hg/MeHg in this reservoir might be of importance for bioaccumulation of Hg in bivalves.[33] Hg accumulation in fish, invertebrates (bivalves), mammals, and aquatic plants and the concentration tends to increase with increasing trophic level (biomagnification). Although inorganic Hg is the dominant form of Hg in the environment it can be methylated relatively quickly. MeHg produccd is bioaccumulated by organisms. The percentage of MeHg, as compared to THg, also increases with age in both fish and invertebrates.[34] However, elevated Hg level found in the bivalves collected from the contaminated site (A5) suggested that some of sediment-bound Hg fractions were bioavailable. Results suggest that accumulation of Hg in sediments at the site could as a result of bioaccumulation lead to a reduction in aquatic biodiversity in this mining area. In regard to the high elevated mercury concentration in the water column, sediment and bivalves in the reservoir
1089
Mercury emission in thailand Table 6. THg concentration in Jasmine rice and surface soil THg concentration
Station T1 T2 T3 T4
Station description Remote area Work place Remote area Remote area
Jasmine rice # 105
Surface Soil µg kg−1 ww µg kg−1 dw (µg kg−1 dw) 185 268 172 222
206 298 191 247
178 181 93 16
near the mining site, result would suggest that the reservoir acts as a source of not only inorganic, sediment-based mercury, but likely also bioavailable MeHg. Terrestrial habitat (surface soil, Jasmine rice and neem flower) THg concentrations in grain of Jasmine rice # 105 (dominant rice species grown in the study area) and surface paddy soil which exhibited some elevated concentrations in Hg at station T2 (rice field in front of Khao Chet Luk reservoir) is shown in Table 6. High or elevated concentration of Hg in surface soil was also found in station T1. The source is likely from Hg deposition from rainfall associated with the southwest monsoon (May-October) which occurred during sample collection. The low Hg content of surface soil at the distance stations T3 and T4, which were not in prevailing wind direction from point source, supports low or no Hg contamination. Therefore, these sites could be used to represent background level in surface soil in this area (less than 100 µg kg−1 dw) and were close to background values reported by Parkpian, et al.[35] THg in Jasmine rice #105 varied from 172–268 µg kg−1 ww and was not correlated with THg concentration in surface soil (Table 6 and Fig. 4). Thus one possible
route for uptake THg by rice plant which could be via stomata exposure of leaves directly from mercury from the atmosphere.[36] Likewise, exposure might not be depended upon the concentration of THg but to elemental Hg (Hg0 ), the most common form of Hg in the atmosphere with an average residence time in the atmosphere of about 1 year.[37] Another possible source is methyl mercury in rice soils. Paddy rice soils with reducing soil conditions are also suitable for mercury methylation, which could also explain higher THg in rice. The processes through which monovalent and divalent Hg are converted to the elemental form are enhanced by sunlight. This would suggest that when there is strong light penetration into water bodies, the monovalent and divalent forms of Hg associated with the organic matter abundant in wetlands and some lakes may be preferentially converted to readily released elemental Hg. Less bioavailable MeHg that easily enters food chains would be less likely to form.[39] This could also be a possible reason why there were high or elevated concentration in the rice grain among the study stations. Samples of Neem flowers were collected only in selected locations along the main aquatic sampling track. Fortunately Nemm flowers were available at locations near the area of contamination and at site distance from the sites (background sites). The result suggested that there were much higher or elevated levels of Hg in Neem flowers in the area near the source of Hg emission (Table 7 and Fig. 3). Result also suggested that the THg concentration in Neem flowers that grow along the aquatic sampling site was slightly correlated with those in sediment at the same location. The availability of soil THg to plants is generally low, with a tendency for Hg accumulation in the roots, indicating that the roots may serve as a barrier to Hg uptake limiting translocation to the above ground biomass. Hg concentration in above ground parts of terrestrial plants appears to largely depend on foliar uptake of Hg0 volatilized from the soil. The lower foliar Hg content is attributed to the lower mean Hg air concentrations.[32,36] However, the elevated concentration of Hg in both Neem flowers and sediment at the same location near the Hg source (A5 and A6) showed that the Hg that accumulated might be from the atmosphere deposition of Hg0 released to the atmosphere from the workplace.
Table 7. THg concentration in Neem flowers
Station
Fig. 4. THg in surface soil (µg kg−1 dw) and Jasmine rice (µg kg−1 ww).
A6 A10 A11
Neem flowers
Station description
µg kg−1 ww
µg kg−1 dw
Downstream Downstream Another track
602 174 208
2151 622 742
1090
Pataranawat et al.
Table 8. THg in dry deposit for 24 hours, surface soil and Neem leaves at workplace and remote area
Station W1 W2 W3 W4 W5 ∗
Neem leaves
Station description
Dry deposit (µg m−2 day−1 )
Surface soil (µg kg−1 dw)
µg kg−1 ww
µg kg−1 dw
Remote area Work place area Work place area Work place area Remote area
32 24 139 37 69
38 144 10,564 417 632
464 651 586 599 548
967 1,301 1,172 1,222 1,095
W1, W5 = 3 km away from the workplace, down- and upwind.
Hg hazardous waste at workplace (dry deposition of atmospheric fallout, surface soils, Neem leaves) THg concentration in dry deposition was highest at the station W3 located at the site of open burning stoves where the amalgamation process for gold ore extraction is located. The Hg used for amalgamation (7–8 h of operation per d) would represent approximately 60 to 150 g during the study (April, 2004). Cumulatively, the estimate anthropogenic Hg released annually into the atmosphere would be approximately 30–40 kg in Phanom Pha gold mining. Likewise, parallel high concentrations of THg on surface soil (10,564 µg kg−1 ) was found at this site (W3) (Table 8 and Fig. 5). This suggests that Hg vapour released to atmosphere were deposited back on to the soil surface near the burning stoves. In regards to the tailing pile near the workplace, elevated Hg concentrations were found in surface soil near this area. Lateral and vertical dispersion of Hg lost to soils and stream sediments is very limited (laterally
