Small-scale Gold Mining in the Ambalanga Catchment, · PDF file · 2017-12-12Small-scale Gold Mining in the Ambalanga Catchment, ... Philex Mining Corporation, Pasig City, Philippines

INTERNATIONAL JOURNAL OF ENVIRONMENTAL SCIENCES Volume 2, No 2, 2011

© Copyright 2010 All rights reserved Integrated Publishing Association

Research article ISSN 0976 – 4402

Received on September, 2011 Published on November 2011 1048

Small-scale Gold Mining in the Ambalanga Catchment, Philippines: Its

Control on Mercury Methylation in Stream Sediments Corpus, T.J.

1, David, C.P.

1, Murao, S.

2, Maglambayan, V.

3

1- National Institute of Geological Sciences, University of the Philippines, Diliman, Quezon

City, Philippines

2- Institute for Geo-Resources and Environment, National Institute of Advanced Industrial

Science and Technology, Tsukuba City, Japan

3- Exploration Division, Philex Mining Corporation, Pasig City, Philippines

[email protected]

doi:10.6088/ijes.00202020062

ABSTRACT

Elevated mercury (Hg) concentrations have been found in the Ambalanga Catchment,

Benguet, where small-scale gold mining (SSM) is the main source of livelihood. To

determine the distribution of total mercury (THg) and methylmercury (MeHg), and the

possible physical controls on Hg methylation in the watershed, stream sediment sampling in

the eight subbasins of the Ambalanga Catchment was conducted on two periods: low-flow

(LF) stage in November 2001 and high-flow (HF) stage in July 2003. Results revealed high

THg concentrations in three subbasins (Acupan, Dalicno, and Sangilo) in the 2001 period and

six subbasins (Acupan, Dalicno, Sangilo, Surong, Gold Creek, and Upper Ambalanga) in

2003. The increase in contaminated subbasins is attributed to the increase in river discharge

which resulted in increased erosion of contaminated sediments. Significantly high

concentrations of MeHg were found in two subbasins (Acupan and Lucbuban) in 2001 and

four subbasins (Acupan, Dalicno, Sangilo, and Upper Ambalanga) in 2003. The high MeHg

may be driven by the enhancement of the methylation process in areas with large amounts of

fine-grained sediments that accumulated in lower hydrologic gradient (and ensuing increased

organic activity) close to and at some distance downstream from the point sources.

Keywords: Methylmercury, Total mercury, Background concentration, Methylation,

Channel morphology, Benguet

1. Introduction

Small-scale mining (SSM) in many Third World countries employs Hg amalgamation to

recover gold from river sediments, soil, alluvium, and lode ores (Lacerda, 1997; Lin et al.,

1997; Miguel, 2000; Avocat et al., 2001; Stamenkovic et al., 2004). Hg amalgamation has

been one of the main sources of Hg pollution in the aquatic environment (Lacerda, 1997;

Akagi et al., 2000). Transport of Hg in the stream is primarily in a particulate (solid) phase

which accounts for 70-90 % of total Hg transported in streams impacted by mine tailings

(Rytuba, 2000). Both Hg and MeHg are adsorbed onto iron oxy-hydroxide substrate and clay

particles or colloids (Rytuba, 2000; Miguel, 2000).

MeHg is the most toxic form of Hg commonly found in the environment (Enger and Smith,

1998; Winch et al., 2008). It is the form of Hg that is of greatest concern for the environment

and human health, as it is highly toxic and bioconcentrates up the food web, eventually

accumulating in fish, top predators, and humans (Wolfe et al., 1998; USEPA, 2001b). When

ingested in sufficient amounts by humans, Hg causes damage to the brain, kidney, and

mailto:[email protected]

Small-scale Gold Mining in the Ambalanga Catchment, Philippines: Its Control on Mercury Methylation in

Stream Sediments

Corpus, T.J., David, C.P., Murao, S., Maglambayan, V.

International Journal of Environmental Sciences Volume 2 No.2, 2011 1049

nervous system, and in extreme cases, death (USEPA, 1994; Lacerda, 1997; Enger and Smith,

1998; Cortes-Maramba et al., 2006). Hg methylation is primarily a result of anaerobic

microbial activity, which is typically enhanced in sediments with high organic matter

(Compeau and Bartha, 1985; Nevado et al., 2009). Stamenkovic et al. (2004) reported that

both pond/wetland and channel sites exhibited high potential for Hg methylation, for higher

rates would be expected at sites with slower water. Seasonal changes in sediment organic

content and the fraction of KOH-extractable THg may be important in controlling net Hg (II)-

methylation rates (Flanders et al., 2010). They also observed that the highest sediment MeHg

concentrations were in habitats with large amounts of fine-grained sediment driven by lower

hydrologic gradient.

Since the 1970s, artisanal miners in the Ambalanga Watershed have been employing Hg

amalgamation to extract gold from its ore, and later dumping the Hg-contaminated mine

tailings directly into the Ambalanga River. SSM in the Ambalanga Catchment presents

potential risks to the local community and the environment because of the unregulated

amalgamation practice and discharging of mine tailings into the river system. Floresca et al.

(1995) estimated the tonnage and gold grades of mine tailings produced by the small-scale

gold miners (averaging 430 tonnes/month at a grade range of 3.45 to 12.07 g/t Au). Mining

practices, gold-recovery processes, and problems encountered by small-scale miners in the

area have been documented by Caballero (1996), Cabria et al. (2002), Maglambayan and

Murao (2002), and Baluda (2002). Mercury contamination is cited as one of the pressing

environmental problems in these small-scale operations.

2. Materials and Method

2.1 Sample Collection

Sampling of stream sediments was conducted on two occasions: in November 2001 and July

2003. Sampling sites are on second- and third-order tributaries that drain mineralized areas

with known SSM activities and are located downstream of the gold-processing plants. A

control site not influenced by SSM in the Lucbuban Subbasin was also selected to establish

the Hg background level in the area. Lucbuban was selected since the catchment is barren of

gold mineralization and has no SSM and gold-processing history.

Thirty-seven sediment samples were collected comprising 17 sites in the November 2001

sampling period and 20 in July 2003 (Figure 1). Stream sediments were gathered from low-

velocity river channels where fine sediments accumulate. Fresh hand gloves were worn and

replaced at every sample site to avoid possible contamination. Polypropylene bottles (125-ml)

served to contain the composited sediments samples hand-grabbed from five randomly-

selected points. Fine fractions (minus 63 µm) were later collected by wet-sieving through a

0.063 mm nylon-mesh sieve by washing with deionized water. After air-drying, the sample

fractions were pulverized using porcelain mortar and pestle.

2.2 Analytical Procedure

All analytical procedures were carried out at laboratories in the U.S. and in Canada. Frontier

Geosciences Inc. in Seattle conducted the analysis for THg and MeHg concentrations for the

2001 samples. The sediments were digested using cold aqua regia. Analysis was done by

SnCl2 reduction, dual amalgamation, and cold vapor atomic fluorescence spectrometre (CV-

AFS) detection using modified EPA method 1631 (USEPA, 1999). Meanwhile, the MeHg in


Stream Sediments



sediments was prepared by bromide/methyl extraction and analyzed by aqueous phase

ethylation, isothermal GC separation and CV-AFS detection using modified EPA method

1630 (USEPA, 2001a).

Figure 1: Sampling location map showing the eight subbasins of the Ambalanga Catchment.

Two laboratories in Canada performed analyses in the July 2003 sediment samples. Total Hg

on fine sediment fractions was examined by ACME Analytical Laboratories Ltd. in

Vancouver, while the MeHg concentrations by Norwest Labs in Surrey. At ACME, the

samples were digested with 6 ml of HCl- HNO3-H2O (1:1:1) at 95°C for 1 hour and the

solutions were diluted to 20 ml. The THg was extracted with methyl isobutyl ketone (MIBK)

followed by ICP-MS technique.

At the Norwest Labs, samples for MeHg analysis were extracted into methylene chloride to

avoid possible methylation artifact effects followed by flameless atomic absorption

spectrometry method modified from Kennedy and Crock (1987). Samples for THg in

sediments were digested with 45% NaOH and L-cysteine and were reduced by SnCl2. CdCl2

is added to the SnCl2 (known as the Magos’ reagent) followed by cold vapor atomic

absorption spectrometry using a 5100 Perkin-Elmer instrument.


Stream Sediments



3. Results and Discussion

3.1 Hg Concentrations in Stream Sediments - November 2001

Hg distribution in sediments is widely variable in 2001 (0.005-1.86 ng/g MeHg and 1.18-

2,600 ng/g THg). Among the eight subbasins studied, Lucbuban was selected as a control site

(MAU1-E) for background concentration. Showing the highest Hg concentration among the

four sites represents the background concentration (42 ng/g) in the area. This is consistent

with the global background range in sediments at 10-50 ng/g (Gustin et al., 1994). In the

2001 period, three subbasins (Acupan, Dalicno, and Sangilo) revealed high concentrations for

THg and two subbasins (Acupan and Lucbuban) for MeHg (Figure 2). In general, high

concentrations for both THg and MeHg are coincident, although the absolute MeHg values

are comparatively low. Although MeHg levels are invariably low, reported values of >0.10

ng/g MeHg have been considered anomalous. In subbasins with elevated THg concentrations,

the background has been exceeded by a factor of 4 to 86.

Figure 2: High THg levels in Acupan, Dalicno, and Sangilo Subbasins and MeHg peaks

at Acupan and Lucbuban in 2001. Note that concentrations are in log scale.

In this study, Hg concentrations in sediments are grouped into four classes: 1) high MeHg-

high THg, 2) low MeHg-high THg, 3) high MeHg-low THg, and 4) low MeHg-low THg

(Figure 3). The first class is demonstrated only in the Acupan Subbasin, showing the highest

concentrations for both MeHg (0.18-1.86 ng/g) and THg (618-2600 ng/g) from four sites

(BAT-1E to BAT-4E). The high Hg concentrations are one order of magnitude above the

background level. The highest THg level (BAT-1E) was collected downstream of the Acupan

SSM area. Another elevated concentration (BAT-4E) in the Acupan Subbasin is evident

(Figure 2). This may have been distributed from the contaminated sediments coming from the

Dalicno Subbasin and the Upper Ambalanga Subbasin through a diversion tunnel located

upstream of and not far from BAT-4E. In terms of milling production, contribution from

Acupan accounts for 35%, assaying at 9.29 g/t Au in the Acupan proper, and with the highest

number of ball mills (40%) in the study area (Floresca et al., 1995).

Two subbasins (Dalicno and Sangilo) reflected low MeHg and high THg levels. THg

concentrations are considered significant, which are 30 and 7 times above background

concentration, respectively. Floresca et al. (1995) reported ten ball mills (11%) in Dalicno


Stream Sediments



with 3% share in milling production, and with moderately high assays of the mine tailings

(6.75 g/t Au). However, no recorded processing and production history was obtained for

Sangilo. This suggests that the source of the high Hg concentrations (7-times above local

background level) comes from SSM activity, by the release of mine tailings into the river

system during ore-processing. Also, the depressed MeHg concentrations could be a function

of the low organic content and probably the short distance (<1 kilometre) from the source

area to the sampling site. This may have hindered the methylation process to its full extent.

Likewise, moderately-elevated concentrations are manifested in two subbasins (Surong at

SUR-1E and MAN-1E, and Gold Creek) and at site AMB-1E. The moderate THg

concentrations could be attributed to the fact that most of the ore feed is from the peripheral,

low- to medium- grade quartz-calcite veins. For another, the tailings are mainly dumped into

small ponds located at some distance from the river system. What is at risk is the likelihood

of contaminating the groundwater adjacent to processing plants.

Figure 3: Elevated MeHg and THg levels in Acupan, Dalicno, and Sangilo Subbasins

on both sampling events.

As shown in Figure 2, there is a general downstream decreasing trend in concentrations of

THg along Batuang (Acupan) - Ambalanga River, starting from BAT-1E to BAT-4E and

dropping drastically at AMB-1E. In comparison, the MeHg concentrations indicated a

general inverse trend, increasing downstream, although of comparatively low values, over the

THg concentrations, and decreasing abruptly at AMB-1E.

Relatively high MeHg concentrations (0.16 ng/g at site PIT-1E and 0.17 ng/g at TUK-1E)

with corresponding low THg levels are also observed (Figure 2). Site PIT-1E is located in

the headwaters of the Acupan Subbasin, upstream of ore-processing plants. The elevated

MeHg concentration at PIT-1E is probably due to the Hg associated with the occurrence of


Stream Sediments



gold veins (although no processing plants were observed) in the vicinity that was enhanced

by the probable higher availability of organic matter. Site TUK-1E in the Lucbuban

Subbasin is positioned immediately downstream at the confluence of Tukok and Sapuan

Creeks, both exhibiting very low levels of MeHg and THg. What could have caused the

relatively high MeHg in TUK-1E could then be the unusual abundance of fine-grained

sediments and organic matter that reacted with natural Hg concentrations, causing

methylation under abnormal conditions.

Figure 4: MeHg and THg concentrations in 2003 are high at Acupan, Dalicno, Sangilo,

and Upper Ambalanga Subbasins and moderate at Surong and Gold Creek.

3.2 Hg Concentrations in Stream Sediments - July 2003

Concentrations of Hg in sediments increased significantly during the 2003 sampling episode

relative to 2001. Four subbasins showed elevated concentrations for MeHg and six for THg,

with their respective peaks coincident with one another (Figure 4). Of these, Hg levels in

three subbasins (Acupan, Dalicno, and Sangilo) are significant. These drain the center of the

Acupan gold deposit, while the other three (Surong, Gold Creek, and Upper Ambalanga)

have moderate Hg levels, peripheral to or at some distance from Acupan, Atok Big Wedge,

and Kelly gold deposits, respectively.

The distribution of Hg concentrations during the HF stage is classified into three: both high

for MeHg and THg, low MeHg but high THg concentrations, and both low for MeHg and

THg (Figure 3). High concentrations for both MeHg (150-1000 ng/g) and THg (161-3600

ng/g) are demonstrated in four subbasins (Acupan, Dalicno, Sangilo, and Upper Ambalanga)

and at stations AMB1-E and AMB2-E. THg levels exceeded the background concentration

by a factor of 4 to 86. Except for those which registered below detection limit (<40 ng/g),

MeHg concentrations are greater than background concentration by 4 to 24 times.

Similarly, a consistent decreasing trend in THg concentrations along the Batuang River

(Acupan Subbasin) - lower Ambalanga River System is observed (BAT1-E at 3,600 ng/g to

AMB2-E at 1065 ng/g) (Figure 5). The decrease in THg levels at AMB1-E and AMB2-E

does not follow the trend as observed in the BAT series samples. This shows that the source


Stream Sediments



of Hg is not only from Acupan but also from other subbasins upstream, although their

contributions are not as high as those coming from the Acupan Subbasin.

Figure 5: A consistent decreasing downstream trend for THg and general decreasing

levels for MeHg in 2003 along Batuang (Acupan) - lower Ambalanga River.

The concentrations of MeHg and THg at the farthest station (AMB2-E) are still high,

considering that the site is 7 kilometres downstream of BAT1-E. Hg concentrations

downstream of AMB2-E are expected to decrease further as there is no current SSM activity

in the location. Based on the estimated average concentration gradient of 340 ng/g per

kilometre, THg concentration is estimated to reach its background concentration for another 3

kilometres downstream from AMB2-E, or 10 kilometres from BAT1-E, by way of

attenuation process.

Although the trend of MeHg concentrations is generally decreasing downstream, the

concentrations at BAT2-E and BAT3-E are slightly higher than that of BAT1-E (460 ng/g).

This is almost consistent with the trend during the 2001 sampling. The same development is

observed from BAT4-E to AMB1-E and AMB2-E. This suggests that, as the contaminated

sediments are released from the source site, the process of methylation starts to work.

The concentration increases downstream until it reaches a certain distance where a maximum

is achieved. Afterwards, it abruptly decreases. The higher the THg concentrations in the

sediments, the more active the methylation process will be. This, in turn, produces more

MeHg wherever there is an abundance of organic matter present in the sediments. Compeau

and Bartha (1985) suggested that Hg methylation is primarily a result of anaerobic microbial

activity, which is typically enhanced in sediments with high organic matter. Overall, the

background concentration has been exceeded by a factor of 72 (BAT1-E) and 25 (AMB2-E).

The consistent decreasing trend of the Hg concentration downstream suggests that the main

source of Hg is at BAT1-E, the center of SSM activity in the Acupan Subbasin. Much of the

contaminated tailings have been transported from the source area, particularly from the

Acupan Subbasin.

Hg concentrations in Dalicno (DAL1-E) reflected high THg (3193 ng/g) and MeHg (640

ng/g), elevated compared to the background level by 64- and 13-fold, respectively. The

Sangilo Subbasin revealed the same Hg distribution pattern as in the Dalicno Subbasin yet of


Stream Sediments



relatively lower Hg concentrations. Not surprisingly, low MeHg-high THg concentrations

define the Gold Creek Subbasin and Manganese Creek in the Surong Subbasin-the former

draining the Atok Big Wedge gold mine, and the latter the Sierra Oro Gold Mine which is

peripheral to the main Acupan deposit.

In both sampling periods, Hg levels in the Lucbuban Subbasin are of background range (0.01-

42 ng/g). The subbasin was selected as background control since the area does not contain

any processing plants, does not perform any SSM activities, has no gold mineralization, and

is underlain by unaltered and unmineralized Itogon quartz diorite.

Higher concentrations and wider distribution of Hg were more significant during the 2003

sampling than in the 2001. This can be explained with more Hg-contaminated mine tailings

that are dumped over the steep banks of the river adjacent to the processing plants. Additional

mine tailings, organic matter and other contaminated sediments under conditions of higher

discharge and turbidity, can be effectively eroded and transported downstream.

3.3 MeHg Concentrations in Stream Sediments

High MeHg concentrations have been observed on both sampling periods, particularly during

the HF event. The methylated Hg could be a function of THg, geology, river discharge,

sediment grain size, organic content, and channel morphology. In the three subbasins

(Acupan, Dalicno, and Sangilo) where MeHg are highest, MeHg coincides with the drainage

at the center of the gold-rich Acupan vein system. As can be expected, the most number of

ball mills are located here - hence higher ore milling and mine tailings production. Of the 94

ball mills recorded in the area, 48 of these (or 51%) are located in the three subbasins, which

account for about 39 % (or 167 tonnes) of the total tailings production per month.

In the 2003 period, four subbasins showed high MeHg concentrations and six subbasins with

elevated THg levels. Others reported MeHg levels below detection limit. MeHg levels are

positively correlated with THg as supported by its correlation analysis (R2 = 0.6599).

Because of higher volume of Hg-contaminated tailings in the sediments, there is more Hg

available during methylation process. For instance, Hg availability from tailings in the

Acupan Subbasin reported 3.84-12.4 g/t THg, reflecting a range of 8- to 253-fold above the

background, indicative of anthropogenic origin.

Seasonal variation indicates differences in discharge with 52.5 m3/s in the 2003 period

against 6.7 m3/s in 2001, or about 8-fold higher in 2003. The higher discharge during periods

of HF and flood events (e.g., 2003) would translate to a higher carrying capacity, effectively

eroding and transporting larger volumes of mine tailings downstream from SSM wastes

dumped along the river banks.

Rytuba (2000) suggested that MeHg concentrations are relatively low at the discharge point

but increases significantly in mine drainage as it flows through and reacts with calcines (mine

tailings). This trend is clearly illustrated at Acupan and Surong Catchments and at stations

AMB1-E to AMB2-E during the HF period (Figure 5). As an example, the initial MeHg level

at BAT1-E (460 ng/g) increases and peaks about 1 kilometre downstream at BAT2-E (1,000

ng/g). MeHg progressively drops in concentrations, until the background level at BAT4-E is

reached, which is about 3 kilometres downstream of BAT1-E. This suggests that methylation

process starts at the source area and continues downstream up to a certain distance, until the

maximum level of methylation has been achieved. In contrast, the highest THg level is shown


Stream Sediments



at the point source (BAT1-E at 3,600 ng/g), followed downstream by consistently decreasing

concentrations.

Channel morphology also plays a significant role. Stamenkovic et al. (2004) and Flanders et

al. (2010) reported that both pond/wetland and channel margins/sites exhibited high potential

for Hg methylation, for higher rates would be expected at sites with slower water or lower

hydrologic gradient. In many instances, sediment samples collected below the water line

were taken in sites with slow flow. It is where gentle to relatively flat river slopes can be

expected where water flow tends to slow down. This therefore serves as a temporary sink for

Hg and organic matter conducive to methylation activity.

4. Conclusion

Hg availability in sediments is largely drawn from anthropogenic Hg associated with

contaminated mine tailings. Sediment sampling on two periods (November 2001 and July

2003) revealed elevated MeHg and THg concentrations attributed to anthropogenic sources.

During the 2001 episode, two subbasins (Acupan and Lucbuban) showed high MeHg levels

and three subbasins (Acupan, Dalicno, and Sangilo) demonstrated elevated THg

concentrations. In the 2003 period, four catchments (Acupan, Dalicno, Sangilo, and Upper

Ambalanga) exhibited high MeHg levels and six subbasins (Acupan, Dalicno, Sangilo,

Surong’s Manganese Creek, Gold Creek, and Upper Ambalanga) showed high THg

concentrations. In general, four basins (Acupan, Dalicno, Sangilo, and Upper Ambalanga)

have high values for both MeHg and THg, and the two subbasins (Surong and Gold Creek)

contained moderate THg levels. On both sampling occasions, the local background level has

been exceeded by 4- to 86-fold.

A positive correlation between THg and MeHg is indicated as supported by its R2 value of

0.6599. This explains the higher MeHg concentrations driven by higher volume of Hg-

contaminated tailings transported during the HF period. Subbasins with high MeHg and THg

are centered on the Acupan gold-rich deposit, where the most number of ball mills are located

such that higher milling and mine tailings are produced. More elemental Hg is utilized, thus

more Hg availability for methylation. With the 8-fold higher discharge in HF over the LF

period, flashing out of more Hg-contaminated mine tailings dumped into river banks is

justified. Flanders et al. (2010) suggested that higher amounts of fine-grained sediments

building up in pools and channel margins have the highest MeHg levels which seem to favor

Hg methylation.

Overall, the methylation process can be explained by physical characteristics in the area such

as Hg availability from anthropogenic sources, discharge, sediment grain size, and channel

morphology which indirectly increase organic activity resulting to increased methylation of

available Hg.

Acknowledgement

This paper is part of the master’s thesis of the first author at the University of the Philippines.

This work could not have been done if not for the help of the National Institute of Advanced

Industrial Science and Technology, Tsukuba City, Japan; the Environmental Agency of

Japan; the University of the Philippines-National Institute of Geological Sciences especially

Rushurgent Working Group and Dr. Joselito Duyanen; the University of the Philippines

Diliman-Office of the Vice-Chancellor for Research and Development; and the Mines and


Stream Sediments



Geosciences Bureau-Cordillera Administrative Region, Baguio City for providing support

during the course of sampling. The management and staff of Benguet Corporation are

appreciated for their assistance and their permission to conduct sampling in the study area.

Dr. James Rytuba of the United States Geological Survey, Menlo Park California is thanked

for his sound advice and for facilitating the sample analysis in different U.S. laboratories. A

review by an anonymous reader is much appreciated.

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