BRITISH GEOLOGICAL SURVEY

TECHNICAL REPORT WC/95/72R Overseas Geology Series

Mercury contamination in artisanal gold mining areas of Eastern Mindanao, Philippines: A preliminary assessment. Martin Williams', Antonio Aposto12 & Juliet Miguef 1: British Geological Survey, Minerals and Geochemical Surveys Division, Keyworth, Nottingham, UK. 2: Mines and Geosciences Bureau, Land Survey Division, Quezon City, Philippines

A report prepared for the Overseas Development Administration under the ODABGS Technology Development and Research Programme, Project R6226.

ODA classification Subsector: Geoscience ZXeme: Identify and ameliorate minerals-related and other geochemical toxic hazards Project title: Mitigation of mining-related mercury pollution hazards. Project Reference: R6226

Bibliographic reference: Williams, T.M, Apostol A. and Miguel, J: Mercury contamination in ai-tismal gold mining areas of Eastern Mindanao, Philippines: A preliminary assessment.

Key words: gold mining, mercury, Philippines.

Cover illustration: Diwalwal gold-rush area, Davao del Norte, Mindanao , Philippines.

ABSTRACT

The use of mercury (Hg) by artisanal gold miners in SE Asia, sub-Saharan Africa and Latin America has increased significantly in the last decade, and may now exert an important influence on the total anthropogenic flux of contaminant mercury into the environment. A Technology Development and Research (TDR) programme to address the adverse impacts of mining-related Hg contamination was initiated by the British Geological Survey (BGS) in April 1995 under funding from the UK Government Overseas Development Administration (ODA), Engineering Division. The central aims are:-

(i) to provide a protocol for monitoring the sources, transport pathways and fates of Hg in artisanal gold mining regions. (ii) to assess the human and ecotoxicological impacts of Hg pollution in selected case- study areas. (iii) to formulate and promote improved methods of gold recovery within the small-scale mining sector. (iv) to make practical recommendations for legislative abatement of mining-related Hg pollution.

In July 1995 a pilot study of selected gold-rush areas in Eastern Mindanao was undertaken by BGS personnel in close liaison with the Philippines Mines and Geosciences Bureau (MGB). Preexisting data, collated by the MGB suggest that the Agusan River basin, the principal catchment in Eastern Mindanao, has received an influx of at least 140 t of Hg as a consequence of escalating artisanal mining activity during the 1980’s and early 1990’s. The principal objectives of the pilot survey were, therefore, to assess the general magnitude of Hg enhancement recorded in a range of sample media, to evaluate the jusMication for further, more detailed geochemical and ecotoxicological surveys, and to test a number of alternative field sampling methods.

Hydrochemical and geochemical sampling was undertaken in three gold-rush areas; the Gango/Guintosan Creek area near Cagayan de Oro, and the Mainit and Diwalwal localities in Davao del Norte. The last-named locality supports the largest artisanal mining community on Mindanao, currently numbering around 50,000 (having peaked at approximately 100,000 during the late 1980’s). At Gango and Diwalwal, the use of Hg for amalgamation is probably now in decline, due to the local development of CIP cyanidation facilities, operated on a cooperative basis.

Physico-chemical field measurements and multi-element ICP-ES and data for surface waters from the pilot-survey locations provided no indication of gross perturbation, with maximum conductivity values of 310 pS and aggregate heavy metal abundances generally 4 mg/l. An exception occurs in the immediate vicinity of Diwalwal settlement, where domestic waste (including sewage) influxes have induced surface water anoxia, enhanced concentrations of NO, (19.8 mgA) and significantly elevated heavy metal abundances (including Fe, Zn, Cu, MO and Ni). A high concentration of dissolved Na at this locality can be tentatively ascribed to the influence of CIP plant discharge.

XRF analyses of major oxides and a wide range of minor and trace elements (excluding Hg) in sediments from all pilot-survey areas produced geochemical signatures which clearly depict lithogeochemical variations (including base-metal mineralisation), but no widespread anthropogenic overprint.

Data showing the concentration of Hg in water, sediment and heavy mineral concentrates (HMC) at Gango, Mainit and Diwalwal were acquired by cold vapour atomic fluorescence spectrophotometry (CVAFS). Data for water from Gango and Guintosan Creek showed no systematic contamination, with only one anomalous value (2.84 pgA) recorded in outflow water from a CIP plant engaged in the reprocessing of tailings previously treated

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by amalgamation. Data for sediments from Gango highlight contaminant influences more clearly, values rising to more than two orders of magnitude above the regional background at a sampling station 200 m downstream of the principal adits and processing plant. Acute Hg contamination in the vicinity of the Diwalwal gold-rush area is indicated by the data acquired for all sample media. Surface water values of up to 2906 pg/l were recorded within the settlement area, declining by more than three orders of magnitude over a c. 10 km downstream distance. The HMC component of the drainage sediment was found to be the most enriched medium at the perceived contaminant source, declining by two orders of magnitude 10 km downstream. Concentrations in sediment decline more gradually with distance. with only a two-fold reduction evident (from 23 mgkg to 11 mgkg) 10 km from the source. This sample type may, therefore, provide the best available medium for systematic Hg dispersal mapping.

The maximum Hg values recorded in water and, to a lesser extent, sediment from parts of the Agusan River basin compare unfavourably with published data for large gold-rush areas in Amazonia A more detailed geochemical and ecotoxicological appraisal of the Hg hazard in this part of Mindanao is therefore recommended.

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1. INTRODUCTION

The adverse hwnan and ecotoxicological consequences of mercury (Hg) contamination in terrestrial and aquatic systems have been recognised since the 195O‘s, when the Minamata poisoning episode in Japan (arising from human exposure to methyl Hg in fish) triggered a tightening of legislative controls on mercury discharges across Europe, North America and parts of south-east Asia. Until recently, the anthropogenic contribution to the global mercury budget emanated predominantly from industrialised regions, notably from hydrocarbon combustion (c. 8,400 t/yr; Bowen, 1979), the chloro-alkali industry and a range of other manufacturing processes (c. 10,OOO Vyr; Kaiser and Tolg, 1980). During the past decade this situation has changed significantly due to the expansion of artisanal gold mining activities in many Pacific Rim, Latin American and sub-Saharan African nations. Mercury amalgamation, a virtually ubiquitous method of gold recovery with particular applicability for the beneficiation of alluvial Au, typically q u i r e s the use of 2 tons of Hg per ton of Au recovered. With gold production by small-scale miners worldwide currently of the order of thousands of tons per annum, the mobilisation of Hg through such activities may now exert a first-order control on the global pollutant Hg budget.

A Technology Development and Research (TDR) programme to address the adverse impacts of mining-related Hg contamination was initiated by the British Geological Survey in April 1995 under funding from the UK Government Overseas Development Administration (ODA), Engineering Division. The central aims are (i) to provide a protocol for monitoring the sources, transport pathways and fates of Hg in artisanal gold mining regions, (ii) to assess the human and ecotoxicological impacts of Hg pollution in selected case-study areas, (iii) to formulate and promote improved methods of gold recovery within the small-scale mining sector and (iv) to make practical recommendations for legislative abatement of mining-related Hg pollution.

In July 1995, a brief pilot survey was undertaken in three catchment areas of Mindanao, the southernmost island of the Philippines, to assess the suitability of the region as a focus for subsequent (more-detailed) case-study research. In this report, data for water, sediment and heavy mineral concentrate samples are presented, and their general implications discussed.

2: MERCURY GEOCHEMISTRY AND TOXICOLOGY

2.1 : Physical chemism Mercury is the third member of Group IIB of the periodic table, with an atomic weight of 200.59 and a density of 13.53 g/cm3. It has an unusually low melting point of -38.9”C and is distinguished from all other metals by its persistence in a liquid state under most surface-

3

environment conditions. Mercury possesses three oxidation states (MO - M+2) of which Hg” compounds and elemental Hg are most common.

Mercury is among the rarest of naturauy Occurring elements, with an average crustal abundance of 0.08 pg/g (Greenwood and Earnshaw, 1984). It is characteristically enriched in acid relative to mafk differentiates (average 0.08 pg/g in granites and 0.01 pg/g in basdts), with concentrations frequently 1-2 orders of magnitude higher in ar@aceous and organic-rich sedimentary facies (Wedepohl, 1978). Mercury is strongly chalcophile and is predominantly partitioned into cinnibar (HgS) or less abundant sulphide phases such as metacinnibar (Hg,Zn,Fe)(S,Se), corderite (Hg,S,Cl,) and livingstonite (HgSbJ,). The principal economic ore mineral is cinnibar, from which Hg is r ecoved by flotation and subsequent roasting of the crushed ore to yield HgO and ultimately free mercury.

2.2: Aaueous chemistry Instrumental constraints on the accurate determination of Hg in natural waters have, until recently, resulted in the over-estimation of natural background concentrations, and much of the published data acquired prior to the advent of cold-vapour atomic fluorescence (CVAFS) technology should be interpreted with caution. The typical concentration range for Hg in pristine freshwater systems is 5-50 ng/l (Fergusson, 1990), rising to 0.1-60 pgh in geothemal waters (Fergusson, 1990). Recently published values for mining-impacted drainage systems in Latin and North America range from 0.2-10.0 pg/l (Pfeiffer et al., 1989; 1991). At moderate or high redox potentials, inorganic Hg is predominantly stabilised as Hg(OH), at pH>6, while at lower pH speciation is variably dominated by HgOH’ or chloro-complexes (depending on the ambient concentration of chloride). Free Hg” forms a stable dissolved phase in acid waters at redox potentials of around 0.W. Free Hg and HgS phases are stabilised at redox potentials of <0.3V across a wide pH range.

Mercury forms a range of alkyllmethyl species which typically constitute 0 5 3 % of the total dissolved Hg load in natural waters. The principal agent involved in the methylation of Hg (and several other heavy metal cations) is methylcobalamin (MeCoB,,), which yields CH,’ or CH,‘ through reaction with a suitable reducing agent. The methylation process may be chemically or biotically mediated. The former usually involves transmethylation with suitable donors such as (CH,)Sn,or (CH,),Pb, while biotic methylation processes can be mediated by aerobic (e.g. Enterobacter aerogenes) or anaerobic (e.g. Clostidiurn) bacteria present in sediments, yeast compounds and animal intestines. Highest rates of methylation occur at relatively low redox potentials of +100 - -200 mV. The methyl species formed is strongly pH dependent, (CH,)Hg dominating under acid conditions and (CH,)ag prevailing in alkaline systems.

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2.3: Toxicitv and human exDosure Mercury is a potent toxin with no known beneficial role in human metabolic processes. It's toxicity is strongly species-dependent, generally increasing through the sequence:- phenyl Hg salts - Hg2+ salts - free Hg vapour - alkyl species. Of the methyl species, (CHJHg is considerably more toxic than dimethyl mercury. Adverse effects of Hg ingestion are evident with respect to both physiological and neurological processes, of which the degeneration of the central nervous system by (CHJHg is most pronounced.

Human exposure pathways vary considerably, with both occupational and dietary factors potentially significant. Data compiled by Fergusson (1990) indicate typical Hg concentration ranges of 5-20 ng/g in most grain crops and 1-40 pg/g in vegetables. Methyl Hg is prone to gross bioaccumulation in aquatic systems, with relative levels for lake water, macrophytes, algae and fish reported by Craig (1982) to be 1: 1OOO: 2800: 23200 respectively. The risk associated with consumption of shellfish and fish is thus disproportionately high, due to the typical presence of S O % total Hg in methyl form. The average daily intake of Hg derived from food (30 pg/d) is an order of magnitude greater than that derived from water (Fergusson, W O ) , for which a potable supply standard of 1 kg/l is applied internationally.

3: REGIONAL SETTING

3.1: PhvsiograDhy and rreology of Eastern Mindanao. The physiography of Eastern Mindanao is dominated by the "E-trending East Mindanao Ridge (Fig. l), a steep mountain range attaining an elevation of over 2400 m. The ridge lies within the east-facing Philippine arc system, with active subduction in the Philippine Trench approximately 100 km offshore. To the west, the Agusan-Davao Trough is characterised by low-lying, gently undulating terrain.

A generalised stratigraphy for Eastern Mindanao, based largely on UNDP (1987) mapping in the Surigao Gold District is outlined in Figure 2. A Mesozoic basement of epidote-chlorite schists is exposed only in the Agusan del Norte province of north-east Mindanao, and the Pujada Peninsula in the south-east. Ophiolitic suites of mid-Cretaceous age occur in the extreme south- east and north-east of the Island, notably at Dinagat (Sugigao del Norte) and Pujada. A well- established sedimentary and volcanic succession of late-Eocene to Pliocene age comprises (i) conglomerates and interbedded limestones of Eocene age overlying serpentinised ultrarnafks (termed the Madanlog Formation in Surigao), (ii) basaltic breccias, pillow lavas and minor flows interbedded with limestone sediments of Oligocene-Lower Miocene age (the Bacuag Formation

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' 126O DINAGAT

a f i I S F 4

I T ] ALLUVIUM

A

Figure 1: Simplified geological map of Mindanao, showing the location of field study areas (from Mitchell and Leach, 1991). 1 = Gango, 2 = Mainit, 3 = Diwalwal.

of Surigao and its local equivalents further south), (iii) a series of mid-late Miocene andesitic- basic clastics, (iv) coraline limestones of Upper MiocenePliocene age and (v) Pleistocene marine and terrestrial sediments and andesitic volcanics. Intrusions of andesite porphyry of probable mid-Miocene age are widespread in eastern Mindanao, notably within a 20 km wide belt which hosts virtually all of the island's epithermal gold mineralisation (Mitchell and Leach, 1991). The N-S trending Philippine Fault forms the primary structural feature within the region, passing through westernmost Surigao, southward along the western margin of the East Mindanao Ridge and then south-eastward to the Pujada Peninsula.

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PACO AND MAN I AYAO ANDESITE!

Hornblende Andosite and Andts i te Parph.

Marls, Mudstones

Andesi t ic To D a c i t i c Clost ics, Minor Lavas , Wackes

TUGUNAN F M 600111

L I M E S T O N E 300111

ANDESITE LAVAS 200 m P L I O C E N E

UPPER MIOCENE

MIDDLE OR EAR LY UP PE R M I O C E N E

-

TAMBIS BEDS SOm

M A B U H A Y CLASTI CS

500 m -

MOTH E R LOO E

1- TURB I D I TE S

BACUAG FM 1-5 km

MADANLOG FM Ikm

GREENSTONE, PHY LL I 'I ES

- 0 0 0 0 v v v o v o v o - - v - v -v v - v - - -

0

U - e 1 :A*:. n

- -

Biot i te and H b l Bi I Ander i te Flows. T u f f s ; Conglomerate, S i l t y Mudstone, Shel l Beds

UPPER O U A T E R N A R Y

1 UPPER PLIOCENE Massive Limestone

Basa l t i c Turb id i tes , I Shales, Debris Flows

Marl, Limestone LOWER MIOCENE

Conglomerate

Massive L imestone

Basa l t Brecc ias

C a l c i r u d i te Basa l t i c Wacke

Massive Lavas

P i l l o w Lavas

Limestone, Waclce, Basa l?

LOWER MIOCENE AND

UPPER OLIGOCENE

Wacke, C a l c i r u d i t e , Limestone, Shell Beds

Conqlomero t e

UPPER EOCENE

Ophiol i te 1 M I D - CRETACEOUS

Schists. P h y l l i t e Fol iated D i o r i t e

1 MESOZOIC

Figure 2: Schematic stratigraphy of eastern Mindanao (from Mitchell and Leach, 1991).

3.2: Gold mineralisation and mining Epithermal Au minerisation in Eastern Mindanao is geographically focused along an arcuate belt (Fig. 3) extending from the northern tip of the Surigao Peninsula to the northern margin of the h j a d a Ophiolite (Mitchell and Leach, 1991). Vein and stockwork systems account for all of the known ore at the majority of deposits and prospects, with the notable exceptions of the stratiform deposits at Placer, Siana and Tambis. The mineralised systems are intrinsically associated with andesitic clastic rocks and andesite porphyry intrusions, which characteristically show pervasive alteration adjacent to the veins for lateral distances of up to 100 m. Primary rock textures ate destroyed, and a pyritic greenish-grey appearance is evident. X-ray diffraction spectra have shown the altered pyritic rocks to typically consist of an illite-pyritequartz-chlorite assemblage, invariably without epidote. At many deposits, the zone of alteration described is fringed by an outer halo of propylitic alteration.

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Figure 3: Known epithermal gold deposits and prospects in eastern Mindanao (from Mitchell and Leach, 1991).

Mitchell and Leach (1991) document four currently operative gold mines, plus two additional epithermal orebodies and at least 35 lode prospects within Eastern Mindanao (Fig. 3). In Surigao District, mechanised mining was initated prior to World War Two, focusing mainly on the stratiform ore and associated vein and stockwork systems of Placer and Siana. The former has produced a total of c. 3 t of Au at grades of up to 100 g/t. Siana yielded 4.8 t of Au and 8 t of Ag

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during the period 1955-67 at a cutoff grade of 7.5 g/t Au. In 1984, existing reserves were estimated at 4.2 mt, grading 5.05 g/t. In Central District a new mechanised mining operation centred on the CO-0 orebody was opened during the 1980's, working a series of veins up to 5 m wide with an average grade of 9 g/t. In southeastern District, the Masara and Hijo mines have both been worked intermittently during the last 15 years.

During the 1980s eastern Mindanao was subject to a series of largely uncontrolled gold rushes by small scale miners, with substantial communities (totalling >lOO,OOO) centred on Diwalwal and Compostella. The poorly-controlled use of Hg for gold amalgamation in these areas has resulted in numerous poisoning incidents amongst miners, the first of which was reported from Tagum, Davao del Norte, in 1987 (11 injuries and 1 fatality). The Philippines Mines and Geosciences Bureau (MGB) has estimated that 140 tons of mercury was released into the Agusan River catchment during the period 1986-88 (the peak of the Mindanao gold rush), after which discharges declined slightly due to a decline in artisanal mining activity, coupled with the increased use of cyanidation technology.

3.3: Sites of current investipation. The pilot survey which fonns the basis of this report was focused on three localities:- Gango near Cagayan de Oro; and the Mainit and Diwalwal prospects in Davao del Norte (Fig. 1). At Gango, an ultramafk thrust sheet overlies a schist/phyllite sequence, with Au mineralisation occurring in narrow veins in the phyllites close to the contact with serpentinised peridotite (Fig.

4) -

In contrast to most other Au mineralisation in the Philippines, the auriferous veins are conspicuously deformed. This coupled with the lack of arc-volcanic lithologies in the Gango area supports a greater age for the mineralisation than that within the East Mindanao Gold Belt, and possibly a non-epithermal origin (Mitchell and Leach, 1991). Mining of placer deposits was initiated at Gango in 1960, but underground working of high-grade vein systems started as recently as 1988. A minor gold rush into the locality was witnessed at this time, and a community of c. 1400 was dependent on small-scale mining in 1990. Gold production in 1990 was estimated at 153 g/day, with 18 rod-mills operative for the processing of ore. Until the late 1980's, gold was amalgamated by the direct addition of Hg during milling, after which the residues were discarded. More recently, small-scale CIP cyanidation plant have been developed locally, and utilised on a cooperative basis (Fig. 5). On account of the low-recoveries attained by amalgamation, tailings previously treated in this manner (and hence containing Hg-residue) are now being widely reprocessed by CIP operators.

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Table 1: Practical CVAFS & ICP-AES detection limits for all elements determined in Mindanao waters (mg/l).

Hg Na K P B Ca Mn C O c u Pb Cd Si La

0.00001 0.0 15 0.057 0.002 0.018 0.02 1 0.001 0.009 0.007 0.035 0.004 0.007 0.013

Mg 0.030 A1 0.022 Sr 0.002 V 0.010 Cr 0.010 Fe 0.006 Ni 0.010 Zn 0.005 MO 0.009 Ba 0.002 Be 0.0002 Y 0.002

Blank water samples, made using distilled water (provided by the MGB, Manila) and field aciddreagents, were submitted in blind fashion to the BGS ICP-ES laboratory. W h e ~ appropriate, field sample data have been normalised against background interference recorded in the reagent blanks.

Sediment samples of approximately 100 g mass were collected by wet-screening active stream- bed detritus through a -100 BSI mesh (-150 pn) sieve, using a minimal amount of water to avoid the loss of fine silt and clay fractions. Samples were then sealed in securitainers to avoid evaporative losses. Heavy mineral concentrates were collected by screening around 2 kg of C2mm stream alluvium, and removing the lighter fractions in a conventional prospecting pan. Multi-element characterisation of both solid sample types was carried out by XRF analysis of 12g sub-samples following milling to e63 pm and the preparation of pressed pellets. Mercury analyses were carried out by CVAFS, using 1 g milled sub-samples digested at ~50°C in aqua regia.

4.2: General hydrochemical observations.

The fundamental hydrochemical characteristics of stream waters in each of the case-study areas

are indicated by the pH, Eh, conductivity, major cation and anion data presented in tables 2-3. A narrow, circum-neutral pH range was recorded at all sites (6.7 - 8.4), although systematically alkaline conditions prevail at and downstream of Diwalwal (DVO 4-6). All waters were found to

be moderately oxidizing, with the exception of the Diwalwal sites DVO 4 and DVO 5, in which the excessive turbidity and high organic loading from the human settlement has induced moderate to severe anoxia (Eh -20 - - 180). The low conductivity range recorded (170 - 3 18 pS) signifies a low total ionic strength in all drainage courses sampled.

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0 0

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Figure 9: Gango and Quintosan Creek sampling areas, near Cagayan de Oro.

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Figure 10: sam p 1 in g stations.

Mainit & Diwalwal gold rush areas, showing the location of

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Eh Temp Cond Ca Mg Na K so4 535 25 170 13.4 7.4 6.2 1.5 1.43 1.06

580 24 175 15.2 8.4 6.4 1.5 1.92 2.54 503 21 318 27.6 18.6 6.0 1.3 2.05 7.17

0.05 0.02 0.07 0.05

470 28 270 29.5 14.5 7.7 1.3 1.92 8.08

555 28 293 31.0 15.0 8.2 1.3 1.86 8.25

493 28 310 31.3 15.3 7.8 1.3 1.90 8.02

Table 2: Physico-chemical properties and major catiodanion data: - Cagayan de Or0 waters. Units:- Eh = mV, Temp. = C, Cond = pS, catiodanions = mg/l. Sample CDO 2 was analysed for Hg only. Samples CDO 1-4 are from Gango. Samples CDO 7-9 are from Guintosan Creek. Samples CDO5 & CM36 are reagent/distilled water blanks, analysed in conjunction with CDO field samples.

NO3 <0.1

~ 0 . 1 <0.1

<0.1 <0.1 <0.1 8.3

Samp. DVOl DVO2 DVO3 DVO4

Eh Temp Cond Ca Mg Na K C1 SO, NO, 554 27 227 28.0 5.7 8.5 1.5 1.48 33.7 <0.1 563 25 240 26.0 5.3 7.9 1.2 2.43 31.5 0.59

0.04 0.02 0.05 0.05

-180 23 309 22.7 1.8 43.0 2.8 9.77 33.3 19.8 -20 24 280 28.8 2.8 30.6 1.8 7.27 34.0 10.2 305 26.8 224 31.6 3.8 7.49 0.7 1.67 21.9 <0.1 .

The Cagayan de Or0 waters (Gango and Guintosan Creek) are Ca-dominated with typical Ca/Mg ratios of 2:l. Given the low concentrations of C1, SO, and NO, determined by IC analysis, HCO, must constitute the dominant anion. The Mainit and Diwalwal waters yield considerably higher Ca/Mg ratios (reflecting the greater influence of limestone and andesitic lithologies) and SO, is a major contributor to the total ionic balance. Human perturbation of the hydrochemical signature is evident at sites DVO 4-5, where Na is the dominant cation (probably from Na-rich effluent derived from CIP plant) and NO, concentrations are substantially elevated (max. 19.8 mg/l) due to the sewage influx to the surface drainage.

DVo6

Table 3: Physico-chemical properties and major catiodanion data:- Mainit & Diwalwal waters. Units:- Eh = mV, Temp. = C, Cond = pS, cationdanions = mg/l. Samples DVO 1-2 are from Mainit. Samples DVO 4-6 are from Diwalwal. Sample DV03 is a reagentldistilled water blank, analysed in conjunction with DVO field samples.

- 7.7 - 7.5 - - 8.4 - 8.1 8.3

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Data for minor and trace elements in the Cagayan de Oro, Mainit and Diwalwal sample suites are given in tables 4-5. Of the 22 analysed trace elements, data are not presented for Cd, V, Be, Li, Pb, Zr, CO, Y and La, all of which occurred at sub-detection limit concentrations throughout.

A contaminant influence in the ‘blank’ samples submitted with the CDO and DVO suites is evident, with anomalous concentrations of Mn (0.24-0.26 mgn), Si (11-13 mg/l) and Zn (36 - 45 pgll) in CDO 5 and DVO 3. Such trends indicate the leaching (probably during acid-washing) of metals from the plastic bottle in which the field supply of distilled water was dispatched by the MGB laboratory. In future. this Droblem can be avo’ ided bv us inP Nalgene HDPE containers only.

With the exception of waters from the Diwalwal area, the reported minor and trace element signatures suggest that samples are relatively pristine. The Diwalwal suite (notably DV04) shows significant enrichment of Fe (3.09 mg/l), Ba (0.238 mg/l), Zn (0.168 mg/l), Cu (0.168 mg/l in DVO 5) , Ni (0.068 mg/l) and MO (0.094 mgll). Given the strictly limited extent of polymetallic mineralisation at the site, these elements are assumed to be derived predominantly from human waste, including sewage. The solubility of these elements in conjunction with common inorganic ligands is low at the prevailing pH (S). The formation and influence of dissolved humic complexes is thus strongly inferred. Concentrations of all contaminant metals decline by an order of magnitude along the c. 10 km channel section between DV04 and DV06. A depression of dissolved Si is notable in the reducing waters of DV04 and DV05.

Table 4: Minor and trace element data: - Cagayan de Oro waters. Units mg/l. ND = not detectable. Sample CDO 2 was analysed for Hg only. Samples CDO 1-4 are from Gango. Samples CDO 7-9 are from Guintosan Creek. Samples CD05 & CD06 are reagent/dktilled water blanks, analysed in conjunction with CDO field samples.

I)

8 I)

I)

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Table 5: data:- Minor and trace element data - Mainit & Diwalwal waters. Units mg/l. Samples DVO 1-2 are from Mainit Samples DVO 4-6 ~IE from Diwalwal. Sample DVO3 is a reagentldistilled water blank, analysed in conjunction with DVO field samples.

'Site Fe Mn P Sr Ba Si Al Zn Cu MO Ni

DVOl 0.124 0.034 0.038 0.106 0.061 12.92 0.115 0.020 ND ND ND

DVO2 0.007 0.014 0.003 0.105 0.055 12.09 ND 0.021 ND ND ND

DV03 0.001 0.026 0.001 0.001 0.001 13.55 ND 0.036 ND ND ND

DVO4 3.091 0.006 0.045 0.147 0.238 4.91 ND 0.168 0.101 0.094 0.068

DV05 2.299 0.013 0.027 0.136 0.172 6.59 0.032 0.012 0.168 0.054 0.049

DV06 0.009 0.014 0.033 0.094 0.016 13.59 ND 0.007 ND ND ND

B Cr

ND 0.030

ND 0.014

ND 0.024

0.133 ND

0.079 0.019

0.018 0.018

. . 4.3: Sedimat c-sihoq.

Major oxide and trace element data for the CDO and DVO sediment suites are presented in Table 6. The lithogeochemical signatures of the pilot-survey catchments are faithfully reflected. Hence, the CDO suite is MgO-rich (3.1-6.9%) with high concentrations of V (rnax. 458 mg/kg), Ni (max. 555 mg/kg) and Cr (rnax. 2606 m@g) reflecting the presence of ultramafic lithologies. The Mainit samples (DV01-2) hold high concentrations of K 2 0 derived from andesite porphyry and related intrusive lithologies. The Diwalwal suite (DV04-6) are CaO-rich (rnax. 15.43%). Known mineralisation in the mid-reaches of the Bigaan River, further compounded by mining and processing activity, is indicated by anomalous values for Zn, As, Cd and Pb at station CD03. Porphyry-style mineralisation in the vicinity of Mainit is reflected by anomalous concentrations of Zn (max. 391 mgkg), Cu (max. 218 mg/kg) and Pb (rnax. 383 mg/kg). The Diwalwal samples DV04-5 display several significant features, notably extremely low concentrations of MnO and Fe20, (1% and <2% respectively), consistent with a high ambient organic loading and the generation of strongly anoxic conditions (under which both elements are reduced to mobile, low-valency states and leached from the substrate). An anomalous concentration of Ag (30 mgkg) may reflect the natural presence of Ag in the mineralised assemblage at the site, compounded by the production during ore processing of Ag amalgams.

4.4: Concentration and d istribution of mercury.

Data showing the concentration of Hg in water, sediment and heavy mineral concentrates (HMC) at Gango, Mainit and Diwalwal, plus a series of 'blank' samples (comprising distilled water, HNO, and K2Cr0,) and are given in Table 7, and the salient trends summarised below:- (i) Blank water data. * Mercury values for the blank samples CD05, CDO6 and DV03 in the range

19

> E a a @ E J a a

k' E cna a

m E u a a ~ 7 m r c ) m m m m c 7 0 m v v v v v v v v v v

O N ns U

Q E E a a Q E Cna a

aJ E

E -4

a, E

aJ L)

a n LJ a a

E+EBEE+E+E+E+EE u u u u u u u u u u 0 0 0 0 0 0 0 0 0 0

A a E (d m

20

0

0

0 0 0

0

Table 7: Mercury concentrations in filtered water, stream sediment and heavy mineral concentrate:. Cagayan de Oro, Mainit and Diwalwal sites. Water data are expressed in pg/l- Sediment data are expressed in mg/kg. Samples CDO 5-6 and DVO 3 are blank waters, analysed blind during the CVAFS sample run.

DV03 (Blank) 0.2 1 DVO4 2906.0 23.0 34.0 DVO5 1220.0 14.0 3.0 DVO6 0.64 11.0 0.42

0.18-0.26 pg/l are an order of magnitude higher than anticipated. Independent CVAFS analysis of the component reagents has indicated that the HNO, and K,CrO, used during the sampling campaign hold Hg at <OB05 pgh. The distilled water andor the acid-washed plastic container supplied by MGB for field storage is therefore considered the most likely contaminant source. This problem can best be averted in future by using pristine HDPE containers and batch-certifed water.

.. (U) Gang0 - & Gu intosan c reek da&: No systematic trend is evident in the Gango water suite (CDO1-4). The only strongly anomalous value (2.84 pg/l) was recorded in outflow water from a CIP plant at station CD02. Such a high dissolved Hg concentration can almost certainly be ascribed to the presence of Hg in the processing feed, much of which has previously been subjected to treatment by amalgamation. The chemistry of Au-Hg amalgams in cyanic solutions requires further investigation, but initial indications are that Hg is effectively mobilised following the formation of AuCN complexes from formerly amalgamated Au. Regulations and guidelines to control the Hg hazard associated with CIP discharges from plant treating tailings previously subjected to amalgamation processing may therefore be warranted. A control site, CDO1, situated on the Bigaan River upstream of the Gango alluvial and adit mine workings, yielded a value of 0.40 pg/l. This is significantly higher than stream-water values obtained for stations CD03 and CD04, located 200 m and 5 km respectively downstream of the active mining/mineral processing area. Data for Guintosan Creek samples CDO7-9 show no conspicuous mining impact, values of 0.14-0.20 pg/l only marginally above the (presumed) regional background.

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Mercury concentrations in sediments from the Gango ami fonn a coherent trend. A background value of 0.12 mg/kg was recorded in the Bigaan River upstream of the artisanal mining settlement (CDO1) rising by more than two orders of magnitude (with concomitant HMC enrichment) 200 m downstream of the active Gango adits and processing plant (CD02), and declining progressively to 1 mg/kg approximately 10 km downstream (CD04). Sediment Hg concentrations downstream of the Guintosan Creek mining settlements (CDO7-8) show so significant enhancement.

Jciii) Mainit data: Water samples from Mainit @VO1-2) show slight evidence of Hg enrichment (to 0.37 pgh), but additional data required to c o n f i i this. Sediment and HMC values lie within the regional background range.

{iv) Diwalwal data: Acute Hg contamination in the vicinity of the Diwalwal gold-rush area is evident from the data acquired for all sample media, and a coherent trend of declining Hg levels with increasing distance downstream from the source area is evident in all instances. Values for water lie in the range 1220-2906 pg/l at stations DV04-5, declining by more than three orders of magnitude over a c. 10 km downstream distance to DV06. The HMC was found to be the most enriched sample medium at the contaminant source (34 mgkg at DVW), declining by two orders of magnitude along the channel section to DV06. Concentrations in sediment appear to decline more gradually with distance, with only a two-fold reduction evident (from 23 mgkg to 11 mgkg) between sites DV04 and DV06.

CONCLUSIONS.

The principal objectives of the pilot survey described in this report were:- (i) to establish the general magnitude of Hg contamination in the Agusan River basin of eastern Mindanao, and to assess the justification for more detailed systematic studies; (ii) to evaluate the utility of a range of sample media for possible use in future surveys of mining-related Hg contamination.

On the basis of the limited data available, the magnitude of the contaminant Hg hazard in certain sectors of the Agusan River Basin, notably drainage emanating from Diwalwal, is considerable and warrants more detailed geochemical and ecotoxicological appraisal. The Hg concentrations recorded in Diwalwal water samples DV04 and DV05 are substantially higher than reported for gold-rush areas in many other areas of the world. For example, in the much publicised gold-rush areas of Rondonia, Brazil, Pfeiffer et al. (1989) have reported maximum values of only 8.6 pgh. The reported values for sediment at Diwalwal sites DVO4 and 5 (14-23 mgkg) are also abnormally high (>100 x background), but are more closely analogous to those published for gold-rush localities in Venezuela, Brazil and Ecuador (c. 20 mg/kg; e.g. Pfeiffer et al., 1989; 1991; Martinelli et al., 1988). The disproportionate impact of Hg contamination on surface water

22

quality at Diwalwal may be attributable to the high organic (sewage) loading to the catchment headwaters, and the presence of abundant dissolved humic complexes with which Hg2+ may readily bond. The formation of Hg-CN complexes in the drainage water may also be significant and warrants further investigation. Such trends suggest that future surveys of Hg contamination must incorporate methods for the collection of a wide range of parametem (including TIC, TOC, dissolved humics, Eh) which are likely to control the mobility, toxicity and ultimate fate of Hg.

The relative abundance of Hg in water, sediment and HMC shows little consistency and marked variations have been noted between pilot survey sites (Fig. 11). At Gango station CDO3, substantial contamination of sediment (25 mg/kg) is accompanied by a smaller, though pronounced enrichment in the HMC, but no signdicant response in surface water. In contrast, the response of the respective media to Hg pollution at Diwalwal station DV04 is disproportionately great in water (>10,0oO x background), followed by HMC (c. 3000 x background), with the peakhackground ratio in sediment substantially lower (c. 200) With increasing distance from the Diwalwal source (e.g. DVM), the relative impacts of Hg contamination on the HMC and water diminish, and sediment appears to show the greatest sensitivity to Hg contamination.

10000

1000 3

100

10

1

-1

. .01

+ Water

-+--HMc Sediment

CDO1 CDCB CDO4 CDO7 coo8 DVOl DVO2 DVO4 DVo5 DVO6

Sample

Figure 11: Relative response of different sampIe media to Hg contamination at Mindanao pilot-study sites (water -pg/l, sediment and HMC - mgkg)

23

The above trends support the conclusions that:- (i) the solubility of elemental Hg introduced by artisand miners is generally low, resulting in poor sensitivity of stream-waters to variations of inorganic Hg loading. Solubility is, however, dramatically increased by high ambient organic loadings. (ii) The HMC component of drainage sediments is an effective accumulator of contaminant Hg at, or close to gross pollution sources, probably through the formation of Au-Hg amalgams. These fractions have a high specific-gravity, and dispersion is thus strictly limited resulting in a relatively rapid downstream reduction of Hg content (as recorded between Diwalwal stations DVO4 - DVO5 and DV06). It is also possible that the retention of Hg in the HMC is associated wioth the presence of Au (i.e. a very low abundance component of the assemblage). Data generated from small HMC samples are therefore likely tobe poorly reproducable, with a high within-site sampling variance. (iii) The e150 jm fraction of sediments is enriched in Hg through sorption to physically mobile silt and clay particles, with the resaultant formation of long dispersion trains. Data for Hg in sediments are likely to be more re-producable (ie. to have lower within-site variance).

On the basis of this study, it is proposed that a comprehensive drainage survey of gold rush areas

in the Agusan River basin should be undertaken, to be executed by BGS and MGB personnel during the period November 1995 - March 1996. Stream sediments ~IE recommended as the principal dsampling medium, but it is envisaged that the database should ultimately include data for Hg in water, biota and possibly human hair or blood.

ACKNOWLEDGEMENTS

Thanks m extended to the Mines and Geosciences Bureau of the Philippines for the kind provision of logistic support and local expertise during field sampling. The work described in this report was carried out under funding from the UK Overseas Development Administration.

REFERENCES.

Bowen, J.H.M. 1979. Environmental Chemistry of the Elements. Academic Press. 333 pp.

Craig, P.J. 1982. Environmental aspects of organometallic chemistry. In Wilkinson, G., Stone, F.G.A. and Abel., E.W. (Eds) Comprehensive Organometallic Chemistry. 1982,2.

Fergusson, J.E. (1990) The Heavy Elements: Chemistry, Environmental Impact and Health Effects. Pergamon press.

Greenwood, N.N. and Earnshaw, A: 1984: Chemistry of the elements. Pergamon Press.

Kaiser, G and Tolg, G. 1980: Mercury. in Hutzinger, T.C. (Ed) Handbook of Environmental Chemistry, vol. 3, part A. Springer-Verlag.

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Martinelli, LA., Ferreira, J.R., Forsberg, B.R. and Victoria., R.L. 1988: Mercury contamination in the Amazon: a gold rush consequence. Ambio, 17, no, 4,252-254.

Mitchell, A.H.G. and Leach, T.M. 1991. Epithermal gold in the Philippines: Island Arc Metallogenesis, Geothermal Systems and Geology. Academic Press. 457 pp.

Pfeiffer, W.C., Lacerda, D.L., Malm, O., Souza, C.M.M., De Silva, E.G. and Bastos, W.R. 1989. Mercury concentrations in inland waters of gold mining areas in Rondonia, Brazil. Science of the Total Environment. 78,233-240.

Pfeiffer, W.C., Malm, O., Souza, C.M.M., Lacerda, D.L., De Silva, E.G. and Bastos, W.R. 1991. Mercury in the Madeira River ecosystem, Rondonia, Brazil. Forest Ecology and Management, 38,239-245.

Quiwa, N.D., Palaganas, U.M. and Pioquinto, W.P. 1987. Report on the geological survey and topographic mapping of Diwalwal Gold Rush area, Monkayo, Davao del Norte. Philippines Mines and Geoscience Bureau.

Wedepohl, 1978 1978. Handbook of Geochemistry. (Berlin-Heidelberg : Springer-Verlag).

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