Pollution by arsenic in a gold-mining district in Nova Scotia

Environmental Pollution(Series B) 4 (1982)109-117

POLLUTION BY ARSENIC IN A GOLD-MINING DISTRICT IN NOVA SCOTIA R. R. BROOKS,* J. E. FERGUSSON, J. HOLZBECHER, D. E. RYAN (~; H. F. ZHANG

Trace Analysis Research Centre, Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, Canada & J. M. DALE (~ B. FREEDMAN

Department of Biology, Dalhousie University, Halifax, Nova Scotia, Canada

ABSTRACT

Arsenic levels were determined in arseniferous goM tailings, waters, stream sediments, plants and aquatic organisms in the vicinity of a stream draining the Montague goM mining district near Halifax, Nova Scotia. Background levels @arsenic in Mitchell Brook, upstream from the former mining activities, were high (37ngml-1), although below the safety limit of 50ng ml-1 Levels did, however, decrease, due perhaps to ion exchange during passage of the water through a non-polluted swamp, but then increased to 190 ng ml - 1 as the stream passed through the arseniferous goM trailings of the abandoned Montague Gold Mine. Thereafter, levels decreased again and reached 46ng ml --1 at the outflow in Lake Charles. Arsenic levels in tailings were inordinately high and rangedfrom 0"38 % to 0"65 %, being highest in the downslope areas. In stream sediments (clay and quartz fractions), arsenic levels increased considerably in the vicinity of the mine and remained high for most of the remaining course of the stream. Arsenic levels in ashed twigs of alder Alnus rugosa growing on the stream banks correlated well with arsenic levels in water except where the latter became excessively high. The arsenic content of aquatic organisms correlated well with the arsenic content of water and rangedfrom 0"002 to 0"059 #g g- 1 (dry weight)for mayfly larvae, 0"002 to O"18 #g- ~ (dry weight)for caddisfly larvae and O"63 to 3.2 pg g - 1 (wet weight)for the banded killifish (minnow). * Present address: Department of Chemistry, Biochemistry and Biophysics, Massey University, Palmerston North, New Zealand.

109 Environ. Pollut. Ser. B. 0143-148X/82/0~)4-0109/$02.75 © Applied Science Publishers Ltd, England, 1982

Printed in Great Britain

110

R.R. BROOKSet al.

It is concluded that arsenic pollution from former gold mining activities in this area is appreciable and it is suggested that in future gold mining operations care should be taken to isolate arseniferous tailings from local drainage systems.

INTRODUCTION

Gold mining has been important in Nova Scotia for well over a century. Gold is confined to rocks of the Meguma Group of slates (Halifax Series) and greywackes (Goldenville Series) and is associated with arsenic, usually in the form of arsenopyrite. The arsenic is released from mine tailings after oxidation to the pentavalent state and is leached by percolating waters into wells and into the drainage systems of the local environment. At one time the Waverley area near Halifax was one of the most important goldproducing regions of Nova Scotia, and localised areas of old arseniferous gold tailings are to be found in the area. In 1976 a hospital patient from this area was found to be suffering from chronic arsenic poisoning. It was later established (Grantham & Jones, 1977) that the water in the well at the patient's home contained 5 #g ml-1 arsenic (compared with the Canadian drinking water safety standard (Anon., 1968) of only 0.05/agml-1). As a consequence of this, an Arsenic Task Force was set up to monitor arsenic levels in well waters. In the course of an investigation into 198 wells in the Waverley area (Grantham & Jones, 1977), thirtyfour (17 ~o) were found to contain arsenic levels above the safety standard. When consumers of water from contaminated wells were examined for clinical symptoms of arsenic poisoning (Hindmarsh et al., 1977), chronic toxicities were observed in twenty-seven of ninety-two persons examined. Apart from arsenic contamination as a result of gold mining activities, a substantial amount of arsenic can also be derived naturally from the generally high background levels in rocks of the Meguma Group (Boyle, 1966). All of the prior work on arsenic pollution in Nova Scotia has been centred on clinical studies and tests on well waters. We have therefore initiated a broader study encompassing analysis of stream waters, stream sediments, higher plants, railings and aquatic organisms in the Montague Gold Mine area near Waverley, Nova Scotia. The results of this study are presented in this paper.

TEST AREA

The Montague gold district is situated about 8km northeast of DartmouthHalifax, Nova Scotia. Commercial gold-mining activities began in 1863 and ended just after the turn of the century. During this time about 40,000 oz of gold were extracted from about 25,000 tons of ore. Most of the processed ore still remains as

ARSENIC POLLUTION IN NOVA SCOTIA GOLD MINING AREA

111

tailings tiistributed in a low-lying area occupying about 6 ha along the course of Mitchell Brook. The tailings contain up to 1% arsenic and are so toxic and xeric that they remain largely unvegetated, except for the common horsetail Equisetum arvense L. and soft rush Juncus effusus L. in wetter patches. In fact, it was in this area that Brooks et al. (1981a) showed that the reputation of Equisetum as a direct gold accumulator and indicator was false. Instead, this genus was shown to be merely highly tolerant of arsenic, accumulating up to 2500/~gg-1 arsenic in the ash. However, it could indirectly indicate the presence of gold where it is the only coloniser of toxic ground. The Montague Gold District lies between Lake Loon and Lake Charles (Fig. 1), which are connected by a small stream known as Mitchell Brook. This brook flows directly through the site of the abandoned gold mine and its highly arseniferous tailings. The stream, which is about 3 km long, rises in Lake Loon and outfalls into Lake Charles. All of the thirty sample sites (located about 100 m apart) were situated along this stream. Mitchell Brook has only a moderate fall between Lakes Loon and Charles, and for most of its course it is a sluggish stream passing through a number of swampy

?

4 5

6

/ Fig. 1.

Map of the Montague gold-mining district, Nova Scotia, showing sample sites.

112

R.R. BROOKSet al.

areas. It is only between sites 9 and 14 (Fig. 1) that there is a sufficient drop to produce a swift-running course with limited organic sediment.

MATERIALS AND METHODS

Sampling Water samples (50 ml) were collected at each sample site in sealed polyethylene bottles and were analysed immediately. Water samples were virtually free of particulate matter and were therefore unfiltered. Samples of stream sediments were collected at as many sites as possible and were stored in wide-mouthed polyethylene bottles. Samples were swirled gently with river water so that floating organic debris could be decanted off. Upon return to the laboratory, the sediments were shaken with distilled water and the supernatant clay fractions were filtered off, dried and stored. The residual quartz-rich sediment was similarly dried and stored. Two samples of tailings (designated A and B in Fig. 1) were dried and stored without pre-treatment. Samples of twigs of alder A lnus rugosa (Du Roi) Spreng. were obtained from most sample sites and were washed, dried and stored. Samples of mayfly and caddisfly larvae and banded killifish Fundulus diaphanus Le Sueur were collected from sites 15, 24 and 30 (mayfly larvae were also collected from site 1). As far as possible, fish of similar size were collected and stored in a refrigerator before further processing. In the case of the caddisfly larvae, only the insect was analysed, i.e. the cases were discarded. Sample preparation Dried sediment samples (0.5 g) were digested with 10 ml of aqua regia and 0.10 ml of bromine until the volume had been reduced to about 5 ml by gentle heating. The samples were then filtered, washed with distilled water and then diluted to 5t)ml. Further dilutions were performed where necessary. Samples of caddisfly and mayfly larvae were made into composites (ca. 0.1 g) for each sample site and were heated gently with concentrated nitric acid (2 ml) until the samples were dry. The residue was redissolved in 1 ml of redistilled 6M hydrochloric acid and gave a clear solution, thus indicating virtually complete digestion. Composite samples of banded killifish (ca. 10 g) were digested with 50 ml of a 4:1 nitric acid/perchloric acid mixture and fumed to dryness. The residue was redissolved in 5 ml of 2M nitric acid and a 0.5 ml aliquot was used for subsequent analysis. Water samples (40ml) were evaporated to about 2-5ml prior to arsenic determinations in order to concentrate the arsenic. This type of pre-concentration procedure has been recommended by Bunus et al. (1975). Tests on waters where the

ARSENIC POLLUTION

IN NOVA SCOTIA GOLD MINING AREA

l 13

original arsenic concentration was high enough to be determined directly showed that concentration of waters by evaporation gave a reliable value for the original concentration. Samples of alder twigs (ca. 20 g) were ashed in pyrex beakers over hot plates. Previous testing (Brooks et al., 1981 b) had shown that at 500 °C (the approximate temperature of ashing) there was little loss of arsenic. Ash samples (0.1 g) were dissolved in 10 ml of 2M hydrochloric acid prepared from redistilled acid. D e t e r m i n a t i o n o f arsenic with the graphite f u r n a c e

Arsenic was determined in stream waters and in acidic solutions of stream sediments, ashed alder twigs and aquatic insects by means of the procedure of Brooks et al. (1981 b). This procedure involves the use of a Perkin Elmer H G A 2200 heated graphite atomiser (graphite furnace) coupled to a Varian AA475 atomic absorption spectrometer with automatic background correction. Output was measured with a Fisher Recordall Series XY recorder. The graphite furnace was coated with tantalum carbide to improve precision and sensitivity. Standards were treated in the same manner as the samples. The instrumental conditions were: dry at 130°C for 30s; char at 650°C for 30s; atomise at 2400°C for 13s. Loss of arsenic during the charring stage was negligible (Brooks et al., 1981 b). Precision was of the order of 5 ~o at the 0.05 ~g m l - 1 level in the test solution and the limit of detection was about 0.1 ng arsenic. The accuracy was assessed by analysis of standard rocks (Table 1). Samples were quantified by the use of separate standards rather than by the method of additions. TABLE 1 ARSENIC CONCENTRATIONS IN AQUATIC ORGANISMS AND IN ARSENIFEROUS GOLD TAILINGS FROM THE MONTAGUE MINING DISTRICTs NOVA SCOTIA. DATA FOR STANDARD ROCKS ARE ALSO INCLUDED

Sample

Tailings Fundulus diaphanus (banded killifish)

Mayfly larva (order Ephemeroptera)

Caddisfly larva (order Trichoptera) Standard rocks MP-I SY-2 PCC-I

Site No.

Number in composite

Arsenic concentration (lagg - 1)

A

--

B

--

15 24 30 1 15 24 25 15 24 25 ---

--

3750 6500 3.2 1.3 0.63 0.002 0.059 0-002 0.026 0.018 0.002 0.007 7656 (7700) 13 (18)

--

--

0-05 (0.05)

Recommended values shown in parentheses (see text for references).

5 5 5 10 10 10 10 10 10 10

114

R.R. BROOKSet al.

Determination of arsenic by neutron activation analysis

Arsenic in mine tailings and in fish samples was determined by neutron activation analysis using a S L O W P O K E reactor with the following conditions: flux, 5 x 1011n c m - 2 s - 1 ; outer site; boron carbide shield; irradiation time; fish, 60 min; tailings, 5 min; decay time, 60 min; counting time, 30 rain. Counting was carried out with a Ge(Li) detector in conjunction with a TN-I1 Tracor Northern multichannel analyser.

RESULTS AND DISCUSSION

Data for arsenic concentrations in waters, stream sediments (clays and quartz-rich fractions) and plant ash are shown in Fig. 2. Values for tailings and aquatic organisms are shown in Table 1. This Table also contains values for three standard rocks (Abbey, 1980; Steger, 1980) in order to give an assessment of the accuracy of the analytical procedure. Arsenic in stream waters

The pattern for arsenic concentrations in stream waters of the Mitchell Brook was closely related to the influence of the Montague Gold Mine. At its origin at Lake Loon the brook contained a relatively high (37ngm1-1) background level of arsenic. Although high, this value was not unexpected because of the high arsenic background in rocks of the Meguma Group which surround the lake. For comparison, Klumpp & Peterson (1979) have reported up to 42 ng ml- 1 arsenic in the lower reaches of the arseniferous Carnon River in Cornwall, Great Britain. After leaving the lake, the waters of Mitchell Brook contained steadily less arsenic until the gold tailings at site 24 were approached. This loss of arsenic may have been due to an extensive swamp (sites 26 to 28) which might have allowed for the removal of arsenic by adsorption onto organic matter. As the brook passed through the tailings area and into another swamp, the level increased from 12ngm1-1 to 140ngm1-1, although this high value was only reached beyond the railings area and was found in an extensive swamp (sites 14 to 20). This swamp was heavily saturated with arsenic and there were probably two factors which account for the high concentration in the waters. First, the sediments were so highly contaminated with arsenic that the normal ion exchange mechanism between sediments and waters resulted in an accumulation of arsenic in the waters instead of a reduction as might occur upstream. Secondly, the Mitchell Brook diverged into a multitude of channels in the swamp, which ultimately combined together again at site 14. This much greater area of water increases the amount of evaporation and could thereby increase the arsenic concentration. Between sites 9 and 14, the brook flowed along a steep course with little organic matter in the substrate, so that removal of arsenic was minimal and high concentrations persisted until the brook entered the organic-rich lower

ARSENIC

POLLUTION

IN NOVA SCOTIA GOLD

20 00(3]

I0

MINING

AREA

115

CLAYS

0001

II

~/C~

iI Ii I

I

I /

~,iooo

I I

QUARTZ SEDIMENT Z 0 or" Z w z 0

II~l

ASHEDALDER TWIGS

(.) IOC

z

w o3

IHEALTH DEPT LIMIT FORWATER WATER

a: 5c~

I0

? ~ 4 5 6 ? 8 9-101112 13 1415 1617 1819 202122 23 24 2~,262728 29 30 SAMPLING

SITES

o

,ooo

DISTANCE [m)

ooo

ooo

Arsenic levels in stream waters, stream sediments and ashed alder A lnus rugosa twigs adjacent to the Mitchell Brook, Montague gold-mining district, Nova Scotia. Except for waters (ng m l - a) all values are expressed in pg g - 1 . F i g . 2.

reaches, where arsenic levels fell gradually downstream from site 9, and entered Lake Charles at about 45 ng ml- 1. In spite of these relatively high arsenic levels, it was only between sites 8 and 18 that they exceeded the safety level (Anon., 1968) of 50 mg ml- ~.

Arsenic in tailings Two values for arsenic levels in tailings are shown in Table 1. The lower value of 3750 #g g-1 (0.38 %) was from the upper part of the tailings. Downslope and adjacent to the large swamp, the value increased to 6500 lagg-1, due no doubt to downward leaching of mobile, oxidised arsenic.

116

R.R. BROOKSet al.

Arsenic in stream sediments Relative values for arsenic in clay-rich and quartz-rich fractions of the stream sediments are shown in Fig. 2. Levels were almost identical at all sites except in the more heavily polluted zone between sites 13 and 24. In this zone, however, the clays were higher by a factor of 2 to 4 and reached a maximum of 1600/~g g-1 at site 24. These higher levels probably reflected the much greater ion exchange capacity of clays compared with quartz. Arsenic in ashed alder twigs Arsenic concentrations in ashed alder twigs were relatively constant at about 100 #g g - l , although there was some evidence for a reduction in levels down the course of the Mitchell Brook. There was a significant correlation, however, (0.05 > P > 0.01) with arsenic levels in water at the beginning (sites 21 to 30) and end (sites 1 to 7) of the brook, where arsenic concentrations in the water were not inordinately high. This is very clear from Fig. 2 and is not surprising since the plants chosen were all growing on the river bank and probably derived some of their water from Mitchell Brook. Arsenic in aquatic organisms The banded killifish (minnows), which are higher in the food chain than the mayfly or caddisfly larvae, had appreciably higher arsenic levels than did the aquatic insects. These concentrations in all of these organisms were also higher at site 15 than elsewhere. The highest value of 3.2/~g g - 1 (wet weight) is nevertheless far below the ca. 20#gg -~ reported by Kiumpp & Peterson (1979) for benthic marine organisms (including bivalves) in the estuary of the arseniferous Carnon River in Cornwall. However, the organisms reported by these authors were all benthic in habitat and none were pelagic fish. For the three composite samples of fish, there was a steady decrease in arsenic levels at sample sites upstream from the heavily polluted site and these concentrations seemed to correlate better with the arsenic content of the water than that of the sediments. The caddisfly and larvae also had the highest arsenic concentrations at the heavily polluted site 15. Elsewhere, values were much lower. It should be mentioned that absolute arsenic concentrations were, in all cases, lower by about two orders of magnitude than in the fish, and this may be related to food chain effects. As with the fish, arsenic levels in the aquatic insects followed those of the waters rather than those of the sediments.

GENERAL CONCLUSIONS There can be no doubt that the mining activities of nearly a century ago have caused extensive pollution in the immediate environment of the Montague gold-mining

ARSENIC POLLUTION IN NOVA SCOTIA GOLD MINING AREA

1 17

area. It is also certain t h a t this p o l l u t i o n will c o n t i n u e to exist far into the future as the arseniferous tailings progressively b e c o m e oxidised a n d slowly release m o b i l e arsenic to the e n v i r o n m e n t . In view o f the 100-fold increase in arsenic c o n c e n t r a t i o n s u p the lower end o f the f o o d chain, it w o u l d be desirable to a n a l y s e fish such as t r o u t , which are at the t o p o f this chain (such specimens were u n f o r t u n a t e l y n o t available for this study), a n d m a y be c o n s u m e d by h u m a n s . Despite the ability o f o r g a n i c m a t t e r to purify circulating waters by a d s o r p t i o n processes, this m e c h a n i s m clearly d o e s n o t o p e r a t e where p o l l u t i o n is extensive, as at this test site. It is therefore r e c o m m e n d e d t h a t future g o l d - m i n i n g activities in N o v a Scotia s h o u l d be so c o n d u c t e d t h a t arseniferous tailings d o n o t have access to local d r a i n a g e systems.

ACKNOWLEDGEMENTS T h e a u t h o r s gratefully a c k n o w l e d g e G r a n t s f r o m the N a t u r a l Sciences a n d Engineering Council. T h e y w o u l d also like to t h a n k O. Maessen for assistance with field work.

REFERENCES ABBEY,S.(1980). Studies on standard samples for use in the general analysis of silicate rocks and minerals, Part 6. Geol. Surv. Pap. Can., 80-14, 30pp. ANON. (1968). Canadian drinking water standards and objectives, Ottawa, Queen's Printer. BOYLE,R. W. (1966). Origin of gold and silver in gold deposits of the Meguma Series, Nova Scotia. Can. Mineralogist, 8, 662. BROOKS,R. R., HOLZBECItER,J. & RYAN,D. E. (1981a). Horsetails as indicators of gold mineralization. J. Geochem. Explor., 16, 21-6. BROOKS,R. R., RYAN,D. E. & ZHANG,H. F. (1981 b). The use of a tantalum-coated graphite furnace for the determination of arsenic by flameless atomic absorption spectrometry. A tmos. Spectroscop., 2, 161. BUNUS,F., DUMITRESCU,P. & BUL^CEANU,R. (1975). Analyticaldetermination of trace elements in well water. J. Radioanal. Chem., 27, 77-81. GRANTnAM,D. A. & JONES,J. F. (1977). Arsenic contamination of water wells in Nova Scotia, J. Am. War. Wks Ass., 69, 653-7. HINDMARSH, J. T., McLETcmE O. R., HEFFERNAN,P. M., HAYNE, O. A., ELLENBERGER,H. A., McCuRoY, R. F. & TnmaAux, H. T. (1977). Electromyographic abnormalities in chronic environmental arsenicalism. J. Anal. Toxicol., 1,270-6. KLUMPP,D. W. 8/: PETERSON,P. J. (1979). Arsenic and other trace elements in the waters and organisms of an estuary in S.W. England. Environ. Pollut., 9, ! 1-20. STEGER, H. F. (1980). Certified reference materials. Ottawa, C A N M E T Rep. 80-6E, 30pp.