~ Pergamon Wal. Sci. Tech. Vol. 33. No.6. pp. 2S5-261. 1996. Copynght ~ 1996 IA WQ. PublIshed by Elsevier Science Ltd

Printed In Great Britain. All rights reserved. 0273-1223/96 SIS 00 + 0-00

PH: S0273-1223(96)00294-6

SEASONAL VARIATION OF METAL CONTAMINATION OF RIVERINE SEDIMENTS BELOW A COPPER AND SULPHUR MINE IN SOUTH-EAST IRELAND

Claudia Herr and N. F. Gray

Environmental Sciences Unit. Trinity College, University of Dublin, Dublin Z, Ireland

ABSTRACT

The discharge of acid mine drainage (AMD) from the abandoned sulphur and copper mines at Avoca (S.B. Ireland) results in the formation of ochreous deposits on the river substrate. Sediment sampling was carried out intensively during June and August, 1994 and again during March and April, 1995 to investigate seasonal variation of Fe. Cu and Zn concentration in riverine sediments of the receiving water (Avoca River). Temporal and spatial variations indicate that. apart from pH. hydrological factors playa major role in metal accumulation of highly energetic river systems. Zinc adsorption and desorption of 'ochre' is chiefly regulated by pH. while Cu removal seems to be primarily by co-precipItation with iron. Basic considerations with regard to sampling techniques are discussed. Copyright © 1996 IAWQ. Published by Elsevier Science Ltd

KEYWORDS

Acid mine drainage; Riverine sediments; pH; hydrology; Zn; Cu; Fe.

INTRODUCTION

Weathering of sulphide ores exposed to the atmosphere in inactive mines and spoil releases large quantities of sulphuric acid and heavy metals, often in toxic concentrations. These abandoned mine workings pose a serious and widespread threat to the environment (Kelly, 1988). The Avoca mines have exploited the rich volcanic massive of sulphide deposits, principally chalcopyrite (CuFeS2)' sphalerite (ZnS), galena (PbS) and pyrite (FeS2)' for the production of sulphur and copper (McArdle et aI., 1994). The Avoca River divides the mines into two discrete areas, East and West Avoca. Acid mine drainage (AMD) is produced on both sides of the river and discharged by two major leachate streams. The Deep Adit drains the eastern side, and the Ballymurtagh Adit the western side, resulting in severe contamination by toxic metals as well as the formation of ochreous deposits on the substrate, and the elimination of both invertebrates and vertebrates in the river. The mean annual AMD discharge in 1993/94 for the two leachate streams was 1.16 and 1.10 m3/day respectively ranging from approximately 0.81 m3/day during low flow to 2.3 m3/day during high AMD discharge. While during low flow conditions the metal contamination is principally attributed to AMD discharge from these two leachate streams, surface runoff from spoil heaps during heavy and prolonged rainfall also contribute elevating metal levels in the river system. The mean annual AMD discharge of Fe, Zn and Cu for the total of the two main leachate streams was 461, III and 11 kg/day, respectively during

255

256 C. HERR and N. F. ORA Y

1993/94 (Stone, 1995). The aims of this study were to assess spatial and temporal variation with regard to the distribution of Fe, Cu and Zn in riverine sediments and to identify major processes that are taking place.

MATERIAL AND METHODS

Figure 1. Map showing locations of Avoca Mines and individual sampling sites along the Avoca River.

Sediment samples and surface precipitate samples were collected during summer 1994 on two occasions (26.6.94,4.8.94) from six stations and again in the following spring 1995 on the 24.3.95, 6.4.95 and 13.4.95 at four main stations along the Avoca River (Fig. 1). Additional surface samples were also collected in November 1994. However, results in this paper are only illustrated from four main stations. At each station three sub-site samples were collected. All samples were wet sieved in the field using river water and the < 1 mm fraction was retained. Samples were frozen within eight hours of collection until analysed. Samples were wet sieved through a set of stainless steel sieves using river water (Salomons, 1993) and the < 63 Jlm fraction retained for further analysis. Samples were dried at 101'C to constant weight. Total Zn, Cu, Fe and Cd in sediment samples were determined by nitric acid digestion followed by flame atomic adsorption spectrophotometry (AAS) using a Perkin Elmer® 3100 spectrophotometer. For the acid digestion a sub•sample of between 0.3 to 0.4g of dried and ground sample material was weighed into digestion tubes and digested in 10 ml of concentrated nitric acid (70%) for 2 hours on a Tecator® digestion block at 170'C. The tubes were acid rinsed with 20% nitric acid prior to digestion. When the acid in the tubes had been reduced to 1-2 ml the digested samples were filtered through Whatman No 1 filters, thoroughly rinsed and made up to 50 ml volume in volumetric flasks. Quality control on metal analysis was maintained by digestion of the

Seasonal variation of metal contamination of riverine sediments 257

certified reference material MESS-l (estuarine sediment) and BCSS-l (coastal marine sediment) as well as by including method blanks and spiked samples. Organic carbon was determined by loss on ignition (APHA, 1989). Stones of approximately 20 cm in diameter were collected at site 2 to analyse composition of surface precipitate during. Three stones of equivalent size were treated as one replicate sample. Three replicate samples were taken at each site. Iron precipitate was washed off the stones, dried at 10 1° C to constant weight and digested as mentioned above. Samples were analysed for Fe, Zn, Cu and Cd using flame AAS.

RESULTS ANO DISCUSSION

Spatial variation. With distance from AMO source, metals vary considerably as is shown in Table 1. Iron in the sediment increased significantly (p<O.OOl) at site 2, and decreased downstream showing no significant differences between sites. The slow gradual decrease of Fe concentrations may indicate that Fe is transported over long distances in the solute or suspended phase as Fe(III)hydroxide floc or colloids. Copper concentrations increased significantly (p<O.OI) at site 2 (mixing zone), and at site 4 (p<O.05). A significant positive correlation (p<O.OOI) was found between Cu and Fe indicating co-precipitation of Cu with Fe (Table 2) (Chapman et al., 1983). Zinc is chiefly regulated by pH, which is also indicated by the positive correlation (Table 2) and therefore accumulation only occurs at high pH as observed at site 4 where significant increase in Zn (p<O.OOI) corresponds with a significant increase in pH (p<O.OI) to a mean of 8.9. The minimum pH value that causes Cu(II) to precipitate as hydroxides is 7.2 and for other salts 5.3, for Zn the minimum limits are 8.4 and 7.0, respectively (Kelly, 1988).

Table 1. Mean concentration and standard deviation (SO) for Zn (Ilglg), Cu (J.lglg), Fe (%) and pH for both sampling periods (summer 1994 and spring 1995, combined)

n site I site 2 site 3 site 4

(rcfcJl.'1lcc site) (mixing .tonc) ufs F.F. dis F.F. distance from mlnlnll arell (400 m u/s) (25 km dis) (l1.5km dIs) (13.5 km dis)

Fe(%) 24 5.2 8.4 7.2 6.3 (SO) (1.1) (1.9) (0.7) (0.6) Cu (Jlg go) 24 83.3 633.8 484.7 674.2 (SO) (18.8) (133.6) (112.4) (190.9) Zn (Jlg go) 24 615.0 523.0 477.1 1519.7 (SO) (142.6) (96.4) (101.9) (8\0.6)

pH 14 6.8 5.8 6.4 8.9 (SO) (0.24) (0.23) (0.14) (1.25)

F.F. = Fertilizer factory

Table 2. Pearson Product Moment Correlation for Zn (fJ.glg), Cu (fJ.glg), Fe (%) and pH (n=45)

Cu (J.lg!g)

Fe (%)

pH

Zn (J.lg!g)

0.375* 00.106

0.515 .... •

Cu (jJ.g!g)

0.623· ....

00.052

Fe (%)

-0.421 .... •

level of significance: p<0.05*, p<O.OI .... p<O.OOI .... •

Temporal variation. The influence of hydrological factors with respect to Cu and Zn precipitation is also of importance. Lui et al. (1992) reported large amounts of Cu and Zn in sediments collected during the dry season. It was suggested that Cu and Zn concentration in the sediment increased as a result of deposition of Cu and Zn associated with particulate suspended matter. Comparison of Zn, Cu and Fe concentration

258 c. HERR and N. F. ORA Y

revealed that concentrations of all metals analysed were lower in spring compared with the previous summer (Table 3). Significant differences were detected for Zn at sites I and 4, for OJ at sites 2 and 4 and Zn at site 2. Feltz (1980) reported that the sediment load is dependent on the knowledge of water discharge, sedimentation processes and transport from the study area. The water discharge in the Avoca River is strongly dependent on meteorological events (i.e. rainfall, dry spells). This results in large discharge variation and a highly energetic river flow regime in the upland section of the river catchment. Therefore it is assumed that lower metal concentrations in sediments ( found in spring 1995) are due to resuspension of the sediment, lower metal concentrations in the soluble phase (i.e. higher dilution from river water) and decreased deposition rate. During the spring sampling period it was also found that Fe and Cu concentrations increased gradually over time, while Zn varied (Fig 2 a, b, c). In spring, with decreasing rainfall, the river discharge decreased rapidly, whereas the leachate streams being chiefly influenced by groundwater, responded slower to dry weather conditions. This resulted in a higher contaminant concentration in the soluble phase and a slow build up of Fe-hydroxide precipitate. Results obtained from the survey in the preceding summer did not show the same trend (Herr and Gray, 1995) which may be due to a relatively constant AMDlriver discharge ratio.

Table 3. Comparison of mean concentrations of Zn (l1g!g», Cu (~g!g» and Fe (%) in the bottom sediments for summer 1994 and spring 1995 sampling

Fe Fe sig. Cu Cu sig. Zn Zn sig. (%) (%) (~~g) (~~g) (~~g) (~~g)

site !summer) !s12rins~ !~ummcr~ !s12ring~ !summerl !sI!ingl 1 5.9 4.7 NS 87 82 NS 624 536 • 2 8.2 7.3 •• 690 555 •• 509 560 NS 3 7.6 7.0 NS 571 443 NS 542 442 NS 4 6.4 6.2 NS WOO 578 • 2713 1043 ••

level of significance: p<O.05"', p<O.OI*"'p<O.OOI """*, N.S not significant (t-test)

Cadmium was also analysed for both sampling periods (summer 1994 and spring 1995). However, during the summer sampling survey, no Cd was detected at the operated detection limit in any of the samples analysed, while in the following spring Cd concentrations were found ranging from 1.0 to 4.4 ~g!g with a standard deviation of 1.6 ~g!g. The reason for this seasonal discrepancy and the high Cd concentrations at the uncontaminated site (site 1) is not known and needs to be further investigated.

Organic matter content. Organic matter content of the sub-surface sediments was determined by loss on ignition. Highest organic matter content was recorded at site 2 in the summer (18%) (Herr and Gray, 1995) as well as the spring (15%), whereas concentrations at all other sites ranged between 10 and 13% organic matter. High organic matter concentrations at the site most affected by AMD (site 2) may indicate increased water toxicity inhibiting biodegradation of organic matter by micro-organisms.

Metals in suiface samples. Metal concentrations were determined in surface precipitate (ochre) which was washed off large stones and small boulders. This was however only carried out at site 2. Additional sampling was also carried out in November 1994 to investigate changes in metal composition over time. Mean metal results of the analysis including standard deviation (SO) for Zn, Cu, Fe and Cd are shown in Table 4. Zinc concentrations in November 1995 are nearly 3 times of the June 1994 and April 1995 value. Copper concentrations decreased slightly in April compared with the preceding date. Pearson product moment correlation was calculated for the individual parameters. A significant positive relationship was detected for Cu and Fe (p<O.OOl) indicating co-precipitation of Cu with Fe (Chapman et al., 1983). Zinc and Cd (p<O.Ol) were also significantly correlated (p<O.Ol) suggesting that at near neutral pH values Cd and Zn are adsorbed to a similar degree to ochres (Fuge et at., 1994). Further significant relationships were also observed for Fe and Cd (p<O.OOl), Cu and Cd (p<O.05) and Cu and Zn (p<O.05).

.MT Il-I-S

Seasonal variation of metal contamination of riverine sediments

8 Fe in < 63 I.llll fraction

7 6

~5 :t: ~4 Q.

t&. 3 2 1 0

2 3 4

sampling site

2413196 ,. . 16141115 13141115 • pH

(a)

600 Cu in < 63 I.llll fraction

500

tiD 400

~300 :I: Q.

::I C) 200

100

0 2 3 4

sampling site

24r.1IIl5 • ' 6141115 13J~5 • pH

(b)

1400 Zn in < 63 IlD1 fraction

10

1200 9 8

tiD 1000 7

~800 6 :I:

600 5 Q.

C 4 N

3 400

200 2 1

0 0 2 3 4

sampling site

24f.l19!1 . 161419[, 1:\14196 • pH

(C)

Figure 2. Mean concentrations of (a) Fe (%). (b) Cu (Ilg/g». and (c) Zn (Ilg/g» showing build up of Fe and Cu over time. while Zn was variable (spring survey) .

259

260 C. HERR and N. F. GRAY

Sediment sampling considerations for toxicity studies. To assess the environmental impact of AMD on sediments it is suggested that besides the determination of total metal content, information on sediment toxicity is also obtained. In a river system which is affected by AMD metal concentrations in a particular location may vary considerably as was shown by Herr and Gray (1995). Field observations prior to any sampling survey on general characteristics of water and sediment provide valuable information to evaluate appropriate sampling sites. The application of appropriate field sampling techniques is critical to adequately characterise a site, so that collection of high quality data i.e. accuracy of results (repeatability and reproducibility) and representative samples is assured. In riverine systems a gravelly type substrate (i.e. riffles) only should be considered. These substrates provide a variety of niches for freshwater organisms (Le. invertebrates, periphyton) and is important for fish spawning. Owing to continuos precipitation of Fe•hydroxides and changes in hydrological characteristics throughout the seasons, restricted areas of increased metal accumulation may occur introducing bias. The data on metal concentrations collected in the Avoca River so far indicates that seasonal variations is evident, however, further research may need to be carried out assessing a whole seasonal cycle to identify peak metal concentration.

Table 3. Mean concentrations for Zn (~glg), Cu (~glg» Cd (~glg» and Fe (%) from surface precipitate (ochre) taken at several occasions

N 29.6.94 27.11.94 6.4.95 13.4.95

Zn (~g g-l) 3 330.0 919.1 338.0 251.8 (SD) 102.7 133.6 106.9 39.3 Cu (~g g-l) 3 1184.2 1317.7 968.5 955.0 (SD) 19.2 114.7 161.2 138.6 Fe(%) 3 17.1 24.1 24.2 27.8 (SD) 8.0 1.7 3.9 5.8 Cd (~g g.t) 3 <0.01 <0.01 2.4 2.2 (SD) <0.01 <0.01 0.2 0.4

CONCLUSIONS

Temporal and spatial variation in the sub-surface sediment seems to be evident which can be related to a decreased AMD/river discharge ratio. It is therefore concluded that hydrological factors as well as pH playa major role with regard to metal accumulation in the sediment. Comparison of Zn, Cu and Fe concentrations of the two seasons (summer 1994 and spring 1995) indicated lower metal concentrations in the spring compared with the preceeding summer. Correlation between Cu and Fe in the surface precipitate indicated co-precipitation of Cu with Fe, while the correlation between Zn and Cd may indicate similar adsorption behaviour of these two metals.

ACKNOWLEDGEMENT

This work was funded by the European Union under EU Contract: EV5V CT93-0248 (Biorehabilitation of the acid mine drainage phenomenon by accelerated bioleaching of mine waste)

REFERENCES

APHA (1989) American Public Health Association. Standard Methods in the ExamiflQtion 01 Water and Waste Water. loint publication of American Public Heallh Association. American Water Works Association and Water Pollution Control Federation.

Chapman. B.M .• lones. D.R. and lung. R.F. (1983) Processes controlling metal ion attenuation in acid mine drainage streams. Geochem. et Cosmochim. Acta. 47, 1957-1973.

Seasonal variation of metal contamination of riverine sediments 261

Feltz. H.R (1980). Significance of bottom material data in evaluating water quality. In: Contaminants and Sediments Vol I (cd. RA. Baker). Ann Arbor. New York.

Fuge. R. Pearce. F.M .• Pearce. N.J.G. and Perkins, W.T. (1994). Acid Mine Drainage in Wales and influence of ochre precipitation on water chemistry. In: Environmental Geochemistry of Sulphide Oxidation. (ed. G.N. Alpers and D.W. Blowes). ACS. Washington.

Herr. C. and Gray. N.F. (1995). Environmental impacl of acid mine drainage on Ihe Avoca River: Metal fluxes in water and sediment. Part 11. Metal conlamination of riverine sediments. EU contract: EV5V-CI'93-0248. Trinity College. Dublin. Ireland.

Kelly. M. (1988) Mining and the Freshwater Environment. Elsevier. New York. Lui. J., Tang, H. and Muller. G. (1992). UNESCO MAB co-operalive ecological research project on heavy metal pollution and its

ecological effects. 1. Environ. Sci. (China) 4(3) 4-13. Mc Ardle. P .• Gallagher. V .• O·Connor. P. (1993) Field Workshop Guide. Avoca Mining District. Co. Wicklow. EU contract:

EV5V -CI'93-0248. Geological Survey of Ireland, Dublin. Salomons, W. (1993) Sediment Pollution in the EEC. Council ofthe European Commission, Brussels. Stone, J. (1995) personal communication.