metal contamination of riverine sediments below the avoca mines, south east ireland

Environmental Geochemistry and Health (1997), 19, 73±82

Metal contamination of riverine sediments below

the Avoca mines, south east Ireland

Claudia Herr and N.F. Gray

Environmental Sciences Unit, Trinity College, University of Dublin, Dublin 2, Ireland

The River Avoca is severely polluted by discharges of acid mine drainage (AMD) from the abandoned sulphur and

copper mines at Avoca. The riverine sediments were studied during a low flow period to establish the degree of

contamination and to identify the major processes affecting sediment metal concentrations. pH plays a major role in

the regulation of zinc adsorption and desorption in sediments, showing a significant correlation (p<0.001). The zinc

concentration in the sediment falls below background levels after the input of AMD. However, the metal precipitated

when the pH increased downstream of a fertiliser factory (pH>8.0), some 7 km below the mine. In contrast Cu and

Fe significantly increased (p<0.001) both in the sediment (0±30 mm depth) and the surface ochre immediately

below the mixing zone. Copper removal appears to be primarily by co-precipitation. Higher sediment enrichment

factors for all metals were obtained in the surface sediment layer (ochre) deposited on larger stones and in floc

material collected in sediment traps, compared with the subsurface sediment. Cadmium was not recorded in any of

the sediment collected at the detection limit used (0.01 �g gÿ1). Metal deposition in the sediments was found to be

spacially variable, so sub-sampling is required, although replicates show little variation. Results indicate that short

term variation in metal inputs is identified by sampling the surface layer only, whereas sampling of the subsurface

layer (<63 �m fraction) is more suitable for identifying long-term trends in sediment quality. The implication of

sediment analysis in assessing environmental impact is discussed.

Keywords: Acid mine drainage, riverine sediments, pH, Zn, Cu, Fe

Acid mine drainage (AMD) is produced as a resultof oxidation of iron sulphides occurring in ore andcoal deposits during the weathering process. Sul-phide oxidation and dissolution in open pit andunderground mining works, waste rock dumps andprocessing tailings is a complex biochemical pro-cess involving numerous hydrolysis and redoxreactions as well as microbial catalysis (Stummand Morgan, 1981; Ritcey, 1989; Doyle, 1990).Because of the uncontrolled and continuous re-lease of acid effluents, surface and ground waterscan become severely contaminated with heavyand/or toxic metallic ions in particular Cu, Zn,Pb, Cd and As. Sediments contaminated by AMDmay represent a significant problem with respectto residual contamination. The discharge of acidicwater containing toxic concentrations of a varietyof metals from abandoned mine workings pose aserious and widespread threat to the environment(Kelly, 1988).

The Avonbeg and Avonmore Rivers join to formthe Avoca River at the meetings of the watersapproximately 5 km north of Avoca Village(Figure 1). The Avoca River flows through a steepsided valley known as the Vale of Avoca, toWoodenbridge, 2.5 km downstream of Avocavillage, where the river meets its largest tributary,the Aughrim River. From there the river enters thesea at Arklow town, 7.5 km further downstream.

The Avoca mines have exploited the rich volcanicmassive sulphide deposits, principally chalcopyrite(CuFeS2), sphalerite (ZnS), galena (PbS) and pyr-ite (FeS2), for the production of sulphur andcopper (McArdle, 1994). They are situated in theVale of Avoca between the meetings of the watersand the village of Avoca itself (Figure 1). The riverdivides the mines into two discrete areas, East andWest Avoca. The mining area is principallydrained by two major leachate streams. The DeepAdit drains the eastern side and the BallymurtaghAdit the western side. These adits discharge AMDdirectly into the Avoca River resulting in severecontamination by toxic metals, and the eliminationof both invertebrates and vertebrates in the river.When AMD enters the river system, dissolvedmetal concentrations of Fe, Zn and Cu increasesignificantly to 1.5, 0.73 and 0.06 mg lÿ1 respec-tively (mean values May±October 1994), and thepH is reduced. A considerable quantity of metalsprecipitate from solution and become associatedwith the sediment and organic matter.

The aims of this study are to (i) measure totalconcentration of Fe, Zn and Cu and organicmatter content of the sediment, (ii) determinespatial and temporal variation with regard tometal loading of Fe, Zn and Cu and (iii) todetermine metal variability within sites and sub-sites.

0269-4042 # 1997 Chapman & Hall

Materials and methods

Collection and analysis of subsurface sediment

Samples were taken from both the sediment layer(0±30 mm) and from the deposited ochre materialon the river bed. The former is defined here as thesubsurface sample, while the latter is the surfacesample. Approximately 3 kg of the subsurfacesediment was collected with a plastic scoop at eachsite and sub-site from five locations along the riveron 29 June 1994 and 4 August 1994 during lowflow conditions. All samples were pre-sieved in thefield through 1 mm aperture mesh using riverwater. The <1mm fraction was retained and storedin 21 polyethylene bottles. To measure variabilityof metals in the sediment within sites, three sub-sites were chosen at sampling sites 1, 4 and 6. Sub-site samples were taken as a cross-transect whenconditions made it possible, otherwise they weretaken randomly from the river bank to the furthestreach in the middle of the river. To measure sub-site variation three samples were taken at eachsubsite for each of the five stations along the river,making nine samples in all. Samples were frozenwithin 8h of collection until analysed.

In the laboratory sediment samples were wetsieved through 63 �m mesh using river water(Salomons, 1993), and the <63 �m fraction col-lected and dried at 1018C to constant weight. TotalZn, Cu, Fe and Cd in sediment samples weredetermined by nitric acid digestion followed byflame atomic adsorption spectrophotometry(AAS) using a Perkin Elmer1 3100 AAS. Forthe acid digestion sub-samples of between 0.3±0.4g dry weight were ground and weighed intodigestion tubes and digested in 10 ml of concen-trated nitric acid (70%) for 2 h on a Tecator1

digestion block at 1708C. The tubes were acidrinsed with 20% nitric acid prior to digestion.When the acid in the tubes had been reduced to1±2 ml the digested samples were filtered throughWhatman No 1 filters, thoroughly rinsed andmade up to 50 ml volume in volumetric flasks.Variability within the digestion unit was deter-mined by digesting replicate samples (2 and 3replicates). Quality control on metal analysis wasmaintained by digestion of certified referencematerial, MESS-1 (estuarine sediment) andBCSS-1 (coastal marine sediment), as well as byincluding method blanks and spiked samples.Organic carbon and carbonate was determinedby loss-on-ignition (APHA, 1989). Approximately0.5 g was weighed into pre-dried crucibles andheated to 5508C for 3 h, cooled in a desiccatorand weighed to determine organic matter. Afterweighing, the samples were further heated to10008C for 3 h and weighed to determine carbo-nate content.

Collection and analysis of floc and surface samples

Floc samples were collected in several ways: (i)from pools using a wide bore syringe and collectedin 50 ml bottles; (ii) in 31 eU plastic sediment trapsburied in the river bed at sites 4 and 6 at weeklyintervals, and brought back to the laboratory in 21 eU polyethylene containers; (iii) and stones ofapproximately 20 cm in diameter were also col-lected at sites 1, 4 and 6 for the analysis of surfaceprecipitate composition. Three stones of equiva-lent size were treated as one replicate sample.Three replicate samples were taken at each site.Floc samples collected by syringe were filteredthrough 0.45 �m cellulose nitrate filters, driedand digested in the microwave with 5 ml of nitricacid (70%) and 5 ml de-ionised water for 70 min.Samples were subsequently made up to 25 ml involumetric flasks. All other floc samples weredried at 1018C to constant weight and digestedas before. Iron precipitate was washed off thestones manually, dried at 1018C and digested.Samples were analysed for Fe, Zn, Cu, and Cdusing flame AAS.

Results

Metals in sediments (<63�m fraction)

Mean results from subsurface sediment analysisfor Zn, Cu and Fe in the <63 �m sedimentfraction for samples from uncontaminated andcontaminated sites taken on 29 June 1994 and 4August 1994 are shown in Table 1. No Cd wasdetected in any of the sediment collected in theAvoca River. Results of metal analysis of Zn, Cuand Fe at individual sites are shown in Figure 2 a,b and c respectively. The Student t-test was appliedto estimate differences in metal concentrationbetween individual sites (Table 2).

It was apparent from the two sampling episodes(which were five weeks apart) that the metalsmeasured in the <63 �m fraction behaved in thesame way at each sampling site on both dates.Numbers of sub-site samples collected were re-duced from three to two on the second field surveyat all sites to reduce work load. The change innumbers of samples at each sub-site, however, didnot affect the overall pattern for metals found inthe sediment fraction. All metal concentrations inthe following text are referred to as mean values.

The mean river pH, taken from weekly observa-tions over an 11 week period (23.6.94±31.8.94),dropped from pH 6.8 at site 1 to pH 5.7 at site 4.The pH slowly recovered downstream. An increasein pH was observed at site 7 where the river issubjected to a considerable discharge of ammoniaeffluent from a fertiliser factory.

74 C. Herr and N.F. Gray

Zinc concentration in the subsurface sediment(Figure 2a) decreased significantly between site 1(662.2 �g gÿ1) and sites 4, 5 and 6 with values of533.5 (p<0.05), 433.6 (p<0.001) and 498.0(p<0.01) �g gÿ1 respectively, after AMD effluententered the river. The change in Zn concentrationwas accompanied with a change in pH. A signifi-cant increase (p<0.01) in Zn concentration (2713�g gÿ1) was observed at the site 7 where pH alsoincreased significantly (p<0.01) to 8.7. Zn concen-tration decreased at site 8 although the concentra-tion as not significantly different from site 7.

Copper in the sediment, however, behaved some-what differently (Figure 2b). A significant increase(p<0.001) in Cu concentrations between the back-ground concentrations at site 1 (84.0 �g gÿ1) andsite 4 (681.3 �g gÿ1) was observed at site 4 wherecomplete mixing of AMD and river water hadtaken place. Another significant increase(p<0.05) occurred between sites 6 and 7 (84.0and 891.0 �g gÿ1, respectively) where pH increasedto 8.7. At site 8 the Cu concentration (430.4 �ggÿ1) compared with site 7 decreased, although not

significantly (p>0.05).

The only significant change (p<0.001) for Feoccurred between the background value at site 1(5.5%) and below the mixing zone at site 4 (9.0%).Fe levels after site 4 gradually decreased down-stream, however no significant changes were de-tected (Figure 2c).

Organic carbon and carbonate content

Loss on ignition at 5008C and 10008C gives anapproximate measure of organic content and totalcarbonate content respectively. Table 3 showsresults from organic carbon and carbonate contentanalysis by loss on ignition in the sediment <63�m fraction for samples taken on 29 June 1994.The results show that organic carbon levels arehighest at site 4. The organic matter content issignificantly reduced after the Avoca-Aughrimriver confluence at site 6. Analysis of variance(ANOVA) also shows significant differences(p<0.001) in organic matter (%) between sites.Results for carbonate contents expressed asCaCO3 (%) show that after the confluence, carbo-nate in the sediment increased to 10.22% at site 6.

Table 1 Mean concentration and standard deviation (inparentheses) of metals in uncontaminated and contami-nated sediment. Per cent recovery of metals from certifiedreference material using the same digestion procedure isalso given.

Zn (�ggÿ1)

Cu (�ggÿ1)

Fe (�ggÿ1)

Meanconcentration ofuncontaminatedsite

662.2(146)

84.0(22)

5.5(1.4)

Meanconcentration forall contaminatedsites

834(741)

593(173)

7.7(1.7)

Certified referencematerial

Percentage recovery

MESS-1 86.3 76.7 86.4BCSS-1 78.9 78.4 87.9

Table 2 Significant differences between sites for Zn, Cu and Fe in the subsurface sediments (0±30 mm).

site 1 site 4 site 5 site 6 site 7

Zn Cu Fe Zn Cu Fe Zn Cu Fe Zn Cu Fe Zn Cu Fe

site 4 ** *** ***site 5 *** *** *** NS NS NSsite 6 ** *** *** NS ** ** NS NS NSsite 7 * ** NS ** NS ** ** * NS ** * NSsite 8 * ** NS * NS * * NS * * * *** NS NS NS

Levels of significance is given: p<0.05*, p<0.01**, p<0.001***, NS not significant

Figure 1 Map showing location of Avoca River and samplingsites.

Contamination of riverine sediments below the Avoca mines 75

However, significant differences (ANOVA) werenot detected between sites.

Metals on surface samples

To compare metal concentration of the surfacelayer with that of the subsurface layer (<63 �mfraction), stones were taken from the river bed,washed and the recovered sediment digested. Thesurface scraping at site 1 consisted mainly oforganic matter, as there was no iron precipitatevisible. Table 4 compares results for metal con-centrations obtained from surface sampling withthe subsurface sediment. Zinc concentration in the<63 �m fraction at site 4 are higher (509.4 �g gÿ1)compared with the surface fraction (330.0 �g gÿ1),however not significantly (p>0.05). This observa-tion is reversed at site 6 where the subsurfacesediment is significantly (p<0.05) lower (542.2 �ggÿ1) compared with the surface fraction (966.6 �ggÿ1). Both Cu and Fe concentrations are higher atsite 4 and site 6 (1184.2 and 1878.4 �g gÿ1, and17.1 and 11.4%, respectively) in the surface frac-tion compared with the subsurface fraction (690.5and 570.9 �g gÿ1, and 8.3 and 7.5%, respectively).

However only Cu showed significant differences(p<0.001) at site 4, whereas at site 6 Cu and Fewere significantly different (p<0.01 and p<0.05,respectively). Metal concentrations at the control

Table 3 Mean concentration (%) and standard deviation(SD) of organic matter and carbonate content in subsur-face sediment (<63 �m fraction).

Organicmatter

Carbonatecontent

n Mean SD Mean SD

site 1 9 13.3 3.3 7.16 0.95site 4 9 21.6 5.4 8.18 1.58site 5 3 18.2 6.3 8.38 0.63site 6 9 10.3 2.0 10.22 5.45site 7 3 12.6 0.7 8.43 1.24site 8 3 9.7 1.2 6.92 0.32

Table 4 Comparison of mean concentrations of Zn (�g gÿ1), Cu (�g gÿ1) and Fe (%) in the subsurface sediment andsurface samples collected on 29 June 1994.

n surface samples n subsurface

Mean SD Mean SD

Zn (�g gÿ1)site 1 3 655.8 81.9 9 624.5 104.0site 4 3 330.0 103.0 9 509.4 173.9site 6 3 966.6 47.9 9 542.2 118.1

Cu (�g gÿ1)site 1 3 55.2 34.2 9 87.2 28.2site 4 3 1184.2 19.5 9 690.5 177.0site 6 3 1878.4 144.0 9 570.9 128.2

Fe (%)site 1 3 4.5 1.7 9 5.9 1.7site 4 3 17.1 8.3 9 8.2 0.7site 6 3 11.4 1.4 9 7.6 0.8

Figure 2 Mean concentrations of (a) Zn (�g gÿ1), (b) Cu(�g gÿ1) and (c) Fe (%) in the bottom sediments collectedon 26 June 1994 and 4 August 1994, including pH (& =29.6.94; & = 4.8.94; & = mean; Ð&Ð = pH).


site 1 do not show any great variation and nosignificant differences between the two samplingmethods.

Comparison of surface sampling from two sam-pling intervals at site 4 showed a significantincrease of Zn from 330.0 to 919.1 �g gÿ1(p<0.01), whereas Cu and Fe are not significantlydifferent. However, because of the limited numberof sampling surveys these results are not conclu-sive (Table 5).

Floc samples

Results of metal analysis from floc material col-lected in sediment traps at site 4 and site 6 areshown in Table 6. Organic matter content of thefloc material was approximately 50%. Zinc con-centrations are relatively low (592.8 and 911.2 �ggÿ1) compared with Cu (808.3 and 1879.0 �g gÿ1).Zinc and Cu increase significantly (p<0.01 andp<0.001, respectively) from site 4 to 6, whereasthe Fe concentration decreases only slightly(p>0.05).

In areas of stagnant water or extremely slowflowing water, floc material accumulates. This flocis easily scoured away when flow increases or whensubstrate is physically disturbed. Zinc is relativelylow at site 4 (539 �g gÿ1) and increased signifi-cantly (p<0.01) at site 6 (1940 �g gÿ1) nearly four-fold. Iron and Cu are approximately the same atthe two sites and showed no significant increase.Mean standard deviations (SD) are much higherfor floc analysed from pools (513 and 1213 �g gÿ1for Zn, 971 and 524 �g gÿ1 for Cu) compared withthe mean SD for floc collected in the sedimenttraps (94.7 and 63.6 �g gÿ1 for Zn, 86.8 and 474.0�g gÿ1 for Cu).

Metal variability in bottom sediment

Sub-sites variation studies were carried out atthree individual sampling sites (1, 4 and 6) on 29June 1994 and the results are shown inFigure 3a,b,c. Due to depth of the river sediment

samples could only be taken from a straight cross-transect at site 6. At sites 1 and 4 samples werecollected from the middle of the river bed andapproximately 2±3m from the river bank.

ANOVA showed significant differences for Zn(p<0.05), Cu (p<0.01) and Fe (p<0.001) concen-trations between sub-sites and also significantdifferences in metal concentration between sites(p<0.001). ANOVA was applied to test machinevariability of digested replicate samples on thedigestion block. There was no significant differ-ence for Fe, Cu and Zn between replicates,whether two (p=0.983, p=0.966, p=0.814, re-spectively) or three (p=0.854, p=0.996,p=0.996) replicate samples were digested.

Discussion

Spatial variation of Zn, Cu and Fe

The main discharge of dissolved Cu and Zn occursvia the two main adits (Ballymurtagh and DeepAdit). The mean monthly discharge for Fe, Zn andCu was 312, 117 and 7.2 kg dÿ1 respectively, withmaximum discharge in May (645, 210 and 21.7 kgdÿ1) and minimum discharge in October (169, 69and 1.5 kg dÿ1). The mass input of Fe, Zn and Cuin both adits decreased over the sampling period

Table 5 Comparison of mean concentrations for Zn (�ggÿ1), Cu (�g gÿ1) and Fe (%) from river surface samplestaken on 29 June 1994 and 27 November 1994.

n Zn ** Cu NS Fe NS

site 4 (29 June 1994) 3 330.0 1184.2 17.1(SD) (102.7) (19.2) (8.3)site 4 (27 November1994)

3 919.1 893.2 24.1

(SD) (133.6) (114.7) (1.7)

Levels of significance is given: p<0.05*, p<0.01**, p<0.001***,NS not significant (t-test)

Figure 3 Metal concentration in sediment showing sub-site/replicate variation for (a) Zn (�g gÿ1), (b) Cu (�g gÿ1),(c)Fe (%) at sites 1, 4 and 6 (replicates:& = 1;& = 2;&= 3).


(Gray, 1995).

The study showed that Zn concentrations in thesubsurface sediment (<63 �m fraction) at site 4had not increased, in fact they had decreasedslightly (Figure 2a) which would suggest that Znis leached from the sediment rather than accumu-lated. Zn only increased significantly at site 7(p<0.01) which was due to a significant increase(p<0.001) in pH in the surface water. This sug-gested that Zn may have come out of solution atthis site and adsorbed onto the sediment. It wasalso found that Zn in the surface layer was lowerat site 4 compared with sites 1 and 6 (Table 4). ThepH at site 4 was generally below 6 and increasedslowly downstream. The Zn concentration in thesediment was strongly correlated with pH(p<0.001), and Zn levels can be estimated usingthe regression equation Zn = log 1.04 + 0.27 pH.Thus it was concluded that pH is the key variableregulating Zn adsorption and desorption.

Sorption behaviour of metals onto sediments isstrongly influenced by pH and is also dependenton clay type. For example, Zn is less adsorbed tothe clay mineral kaolinite than to montmorilloniteand illite (Farrah and Pickering, 1977). Florenceand Batley (1980) found that Zn concentrationsare chiefly controlled by adsorption rather thanprecipitation which is strongly affected by pH,with virtually no adsorption at pH 6 and increas-ing adsorption at higher pH values. Acidic waterprevents the formation of Zn(OH)2 and thusprecipitation. It was suggested by Maes and Cre-mers (1975) that adsorption of Zn onto the claymineral Na-montmorillonite is nearly perfectlyreversible with low pH (approximately 98% re-covery at pH <6) where site occupancy is low orintermediate. The pH region where most of themetal ion is precipitated and sorbed is between 6.9and 7.9 (Pickering, 1980; Salomons and FoÈ rstner,

1984; Kelly 1988).

At site 4 the Cu and Fe concentration significantlyincreased (p<0.001) in the subsurface sedimentcompared with background levels at site 1 whichsuggested that these metals were deposited at thissite (Figure 2b,c). Fe decreased gradually down-stream showing no significant changes betweensites. The slow gradual decrease of Fe in thesediment may indicate that Fe is transported overlong distances in the solute phase. A significantcorrelation (p<0.001) between Fe and Cu in thesediment indicated co-precipitation of Cu withiron hydroxides. A significant increase (p<0.05)in Cu concentration occurred at site 7 which maybe due to co-precipitation at this elevated pH (pH8.7) below the factory. Although Stumm andMorgan (1981) found that Cu adsorption is regu-lated by pH, regression analysis showed no rela-tionship between mean pH and Cu in sediments ofthe Avoca River. Any trends associated with pHmay be hidden by other removal mechanismsaffecting Cu in the river.

The influence of hydrological factors with respectto Cu and Zn precipitation is also important. Luiet al. (1992) reported of large amounts of Cu andZn in sediments collected during the dry season. Itwas suggested that Cu and Zn concentrationincreased in the sediment as a result of depositionof Cu and Zn associated with particulate sus-pended matter. However, this is probably not thecase in the Avoca River where indications are thatthe Cu and Zn sediment concentrations are ele-vated in the winter during high river discharge.Comparison between two sampling events of thesurface sediment at site 4 showed large increases ofZn and Fe (Table 5). However, because of thesmall numbers of samples it cannot be concludedwhether this is due to variation in sampling orbecause of different hydrological and chemical

Table 6Mean concentrations and standard deviation (SD) of Zn (�g gÿ1), Cu (�g gÿ1) and Fe (%) from floc samplescollected in sediment traps and from pool areas.

Floc analysis from sediment traps Floc analysis from pools

n Mean SD p n Mean SD p

Zn (�g gÿ1)site 4 4 592.8 94.7 ** 12 539 513 **site 6 10 911.2 63.6 3 1940 1213

Cu (�g gÿ1)site 4 4 808.3 86.8 *** 11 1818 971 NSsite 6 10 1879.0 474.0 3 2099 524

Fe (%)site 4 4 9.8 1.7 NS 12 5.6 3.0 NSsite 6 10 7.7 1.0 3 4.6 2.0

Level of significance is given: p<0.05*, p<0.01**, p<0.001***, NS not significant (t-test)


regimes in the river.

Examination of organic matter content (Table 3)in the sediment showed highest levels at sites 4 and5 (21.6 and 18.2% respectively). An increasedgrowth of periphyton was noticed from visualexamination at sites 4, 5 and 6 on the substrateduring sampling in July and August. Althoughorganic contents in the sediment was lower at site6, almost 100% of the substrate was covered byperiphyton. However, periphyton consists to 95±98% of water and therefore an increased organicmatter content due to periphyton is stronglydependent on the amount collected in relation tothe sediment volume.

The low carbonate content of the sediment indi-cates that the acid inputs from mine drainage orthe production of acids from microbial activity areeffectively not neutralised by the sediment. Thesurface water of the Avoca River is soft (hardness<20 mg CaCO3) resulting in a low bufferingcapacity of the water and sediment. Highest car-bonate content is detected at site 6 (Table 3) due tothe input of the slightly harder water from theAughrim River, which is a major tributary of theAvoca River (Figure 1).

Comparison of the sediment metal concentrationsat site 1 with average geochemical backgroundlevels for shales and clays (Table 7), indicated thatZn levels were relatively high at the upstream site.In a second survey which included the AvonmoreRiver, it was found that Zn concentration was alsoelevated (745 �g gÿ1). This is due to 19th centuryZn±Pb mining operations in the Avonmore catch-

ment and resulting spoil heaps close to the river.Metals associated with the sediment phase, espe-cially the <63 �m fraction, are likely to be scouredduring high flow conditions. Therefore Zn fromthe Avonmore River may be an additional sourcefor elevated metal levels in the sediment at site 1.Dissolved Zn concentrations were found to bebelow detection limit of the instrument (0.01 mglÿ1) used for analysis and therefore dissolved Zn isnot likely to have an impact at site 1.

Comparison of Avoca River sediments with Lo AnRiver (China) sediments (Table 8) which areaffected by the Dexing Copper Mine, showedstronger sediment enrichment of Zn in the AvocaRiver, whereas the Lo An River sediments showedhigher Cu enrichment. This reflects the nature andquality of the ores mined at each site.

Metal accumulation in surface and subsurfacesediment

Fe levels in the subsurface sediment (<63 �mfraction) increased significantly (p<0.05) at site 4(Table 4). The increase in Fe concentration is dueto the formation of Fe(OH)3 which precipitatesfrom solution and which also has a large sorptioncapacity for heavy metals (Chapman et al., 1983).However, from visual examination of the site itwas expected that Fe concentration would havebeen even higher due to the rich ochre deposit.

Analysis of the orange surface layer from stones of20 cm in diameter and comparison of metal levelswith the <63 �m fraction showed that Fe and Cuconcentrations increased approximately two-fold

Table 7 Average background concentrations of some trace metals in shales and clays, river sediments and recent lakes.River sediments values are the average of 29 unpolluted surface river sediments from the Detroit River and western LakeErie (Canada). Avoca River values are the average of nine subsurface samples (<63 �m fraction).

Shales and clays(Turekian andWedepohl, 1961)

Recent lakes(FoÈ rstner andWittmann, 1983)

River sediments(Chester, 1988)

Avoca River(current study,upstream of mines)

Iron (%) 4.7 4.3 3.4 5.9Copper (�g gÿ1) 45 45 70 87Zinc (�g gÿ1) 95 118 263 625Cadmium (�g gÿ1) 0.3 ± 4 <0.01

Table 8 Average concentrations of river sediments contaminated by acid mine drainage.

Avoca River(Ireland)(current study, impactedsites)

Lo An River(China)(Ramezani, 1994)

Poyang Lake(China)(Ramezani, 1994)

Iron (%) 7.1 6.3 5.1Copper (�g gÿ1) 655 1191 122Zinc (�g gÿ1) 1187 509 250Cadmium (�g gÿ1) <0.01 1.2


at site 4, while Zn concentrations decreased(Table 4). This decrease in Zn confirms the earlierassumption that Zn is not deposited at site 4.Copper was enriched in the ochre layer by a factorof 26.3 compared with 15.3 in the subsurfacesediment (Table 9). This suggests that Cu ispreferentially co-precipitated with iron hydroxide(Chapman et al., 1983; Kondos et al., 1991), ratherthan adsorbed onto the subsurface sediment. Fugeet al. (1994) reported variable ochreous sedimentsranging in Fe content of 13.3±40%, and found thatCu was highly concentrated in the ochre andincreased with increasing pH. The Fe content inochreous sediments in the Avoca River rangedfrom 10±27%, while Cu concentrations rangedfrom 1163±2029 �g gÿ1. It has been proposed byBigham et al. (1990) that the degree of adsorptionof Zn, Cu and Cd is controlled by the nature andstructure of the ochreous precipitate which hasbeen found to vary with pH.

The current study showed that Fe is largelyconcentrated in the surface sediment showing asediment enrichment factor (SEF) of 4.0 at site 4(Table 9), but less concentrated in the subsurfacesediment (<63 �m fraction) with an SEF of 1.9.Highest Fe accumulation in the Lo An River wasfound to occur in the coarser sediment fractionconsisting of 1% clay, 45% silt and 54% sand. Itwas assumed that most of the Fe accumulated inthe silt and sand fraction (Ramezani, 1994) andtherefore was not primarily dependent on grainsize. Higher SEFs were obtained for Cu in thesurface layer and floc material compared with thesubsurface sediment (Table 9).

Metal concentration in floc material was close tolevels obtained in ochreous sediments (Table 4 and6). The floc material comprised of a significantportion of Fe and up to 50% of organic matter.Floc material was collected from the sedimenttraps at w) as these metals accumulate over time.Decrease in Fe concentration between the two floctypes may be due to decomposition of organicmatter resulting in Fe becoming associated withthe sediment. It is also interesting to note that Zn

concentrations in floc material are relatively low atsite 4 compared with site 6. This appears to be dueto the same reason as for surface and subsurfacesediments where the pH strongly influences ad-sorption.

Metal variability in sediment

Collection of representative samples and selectionof an appropriate sample location is of greatimportance in sediment analysis. FoÈ rstner andSalomons (1980) recommended the particle sizefraction <63 �m to be of most benefit becausemost metal is associated with this fraction.Although variability of the bed material cannotbe eliminated entirely due to the heterogeneousnature of the river, it can be reduced by analysingthe clay/silt fraction.

Ramezani (1994), working on river sediments fromthe Lo An River, showed that the concentration ofCu in the various grain size fractions <63 �m (20±63 �m, 6.3±20 �m, 2±6.3 �m and <2 �m) arereasonably consistent ranging from 687±817 �ggÿ1. A decrease was detected in the particle sizefraction >63 �m. In contrast the maximum con-centration of Zn (497 �g gÿ1) was found in the <20�m fractions, while concentrations in the fraction20±63 �m were significantly lower (92 �g gÿ1).

Studies were carried out at the three main sam-pling sites (1, 4 and 6) to test the metal variabilitybetween sampling sites, sub-sites and replicates(Figure 3). Comparison with typical geochemicalbackground concentrations of some trace metals inrocks (Table 7) suggest that metal levels for Zn areelevated at all upstream sites. Levels of Cu and Feat site 1 (sub-site 1) (124.3 �g gÿ1 and 8.1%respectively) are approximately twice as high aslevels at sub-sites 2 (67.9 �g gÿ1 and 4.4%,respectively) and 3 (61.5 �g gÿ1 and 4.4%, respec-tively). The sample at sub-site 1 was taken fromthe middle of the river, whereas samples 2 and 3were taken close to the left river bank. Thereforesub-site 1 would not represent average backgroundlevels at the upstream site and is not suitable for

Table 9 Calculated sediment enrichment factor (SEF) for Zn, Cu and Fe with recent lake sediments (Table 7).

Site No Subsurface sediment(<63 �m fraction)

Surface sediment (`ochre') Sediment traps (floc material)

Zn Cu Fe Zn Cu Fe Zn Cu Fe

1 5.3 1.9 1.4 5.6 1.2 1 ± ± ±4 4.3 15.3 1.9 2.8 26.3 4.0 5.0 18.0 2.35 3.8 12.6 1.7 ± ± ± ± ± ±6 4.6 12.7 1.8 8.4 41.7 2.6 7.7 41.8 1.87 23.0 22.2 1.5 ± ± ± ± ± ±8 14.7 9.9 1.4 ± ± ± ± ± ±


further sediment sampling.

ANOVA showed significant differences betweenthe mean of the sub-site and the mean of thereplicates for Zn (p<0.05), Cu (p<0.01) and Fe(p<0.05). This suggests that the collection ofreplicate samples at sub-sites is not necessary assignificant differences amongst replicates are notdetected. However, it is important to take severalsub-site samples to obtain a reasonably accurateestimate of metal concentrations within a site.Metals deposition seems to be uneven and variesconsiderably within a single sampling site locationas shown in Figure 3.

Environmental impact assessment

The conditions and processes within riverine sedi-ments contaminated by AMD vary considerablybetween the surface deposited layer and the sub-surface sediment (0±30mm), as well as spatially.The use of the <63 �m fraction provides a base fornormalising data from different types of sedi-ments. However, every attempt must be made tosample similar substrate types, and in riverinesystems these should be gravely type sedimentsonly. The sedimentation processes seen in AMDcontaminated rivers not only result in direct metaltoxicity, but also the destruction of the mostimportant habitat in such systems, i.e. gravelysubstrates. The substrate provides a variety ofniches for freshwater organisms (e.g. invertebrates,periphyton) and is important for fish spawning.Hydrous iron oxides will co-precipitate heavymetals to form a thick ochre layer over thesubstrate. In the most impacted areas this tendsto be so excessive that the substrate becomeswelded together by a compacted iron panning.Further downstream, only the larger stones be-come coated with ochre, with the smaller gravelymaterial discoloured, but with a variable burden ofassociated floc and fines which is constantly beingscoured away by the river flow inducing watermovement through and across the substrate.Therefore sampling should examine both the sur-face of stones which provides an important nichefor periphyton which is a major food resource forriverine invertebrates, and the smaller gravellytype substrate (subsurface sediment) in which upto 80% of the invertebrate fauna of fast flowingrivers are found. Ponds and still areas of riverwater close to the banks are often highly contami-nated with floc material which, as shown by thesediment trap data, is constantly moving down-stream affecting all sites. These flocs are oftenhighly contaminated with metals and can becomecompacted causing localised areas of substratepanning, especially when some areas dry out.The flocs should also be collected and analysed.Stony substrates such as those found in fastflowing rivers like the Avoca River are rarely

anoxic, with oxygen penetrating deeply into thesediment allowing invertebrates to live far belowthe sediment-water interface (Bretscho, 1991).

It is difficult to assess the environmental impact ofAMD on sediments as there are few thresholdtoxicity data available for freshwater animals orcommunities in metal contaminated sediments.Therefore physico-chemical sediment analysisshould be carried out in conjunction with biologi-cal assessment in the field at the time of sedimentcollection, as well as subsequent toxicity assess-ment using collected material in the laboratory(Nebeker et al., 1984).

Metals are constantly being deposited onto thesediment via a number of different processes.However, the majority of these are ephemeral,their deposition and contribution to the long termtoxicity of river sediments is governed by hydraulicconditions within the river. The metals associatedwith the deeper sediments (0±30 mm) provide along term monitor for the contamination fromwhich seasonal and annual trends can be derived.

Conclusions

pH is the key factor regulating Zn concentrationsin sediments of the Avoca River, with less Znaccumulation at lower pH. Cu seemed to havebeen concentrated in surface sediments (ochre)and also showed a tendency to be more highlyconcentrated in sediments with increased organicmatter content. However, removal of Cu fromsolution was primarily by co-precipitation withiron. Cadmium was not found in any of thesediments at the detection limit (<0.01 �g gÿ1).Short term variation in metal inputs were identi-fied by sampling the surface (ochre) layer only,whereas sampling of the subsurface sediment (0±30mm) (<63 �m fraction) is important for monitor-ing long-term trends in sediment quality. It isimportant to take several sub-site samples toobtain a reasonably accurate estimate of metalconcentrations within a site as metal depositionis variable within riverine sediments.

References

APHA. 1989. Standard Methods in the Examination ofWater and Waste Water, Joint publication ofAmerican Public Health Association, American WaterWorks Association and Water Pollution ControlFederation.

Bigham, J.M., Schwertmann, U., Carlson, L. and Murad,E. 1990. A poorly crystallised oxyhydroxyl-sulfate ofiron formed by bacterial oxidation of Fe(II) in acidmine waters. Geochemica et Cosmochimica Acta, 54,2743±2758.

Bretscho, G. 1991. Bed sediments, groundwater, streamlimnology. Verhandlungen der Internationalen Ver-einigung von Limnologen, 24, 1957±1960.

Chapman, B.M., Jones, D.R. and Jung, R.F. 1983.


Processes controlling metal ion attenuation in acidmine drainage streams. Geochemica et CosmochimicaActa, 47, 1957±1973.

Chester, R. 1988. The storage of metals in aquaticsediments. In: Strigel, G. (ed), Metals and metalloidsin the hydrosphere; impact through mining andindustry and prevention technology. UNESCO, Paris.

Doyle, F. 1990. Acid mine drainage from sulphide oredeposits. In: Gray, P.M.J. et al. (eds.), SulphideDeposits±Their Origin and Processing. The Institu-tion of Mining and Metallurgy, London, pp. 301±310.

Farrah, H. and Pickering, W.F. 1977. Influence of clay-solute interactions on aqueous heavy metal ion levels.Water, Air and Soil Pollution, 8, 189±197.

Florence, T.M. and Batley, G.E. 1980. Chemical spe-ciation in natural waters. CRC Critical Review inAnalytical Chemistry, August.

FoÈrstner, U. and Wittmann, G.T.W. 1983. Metal Pol-lution in the Aquatic Environment. Springer Verlag,New York.

Fuge, R., Pearce, F.M., Pearce, N.J.G. and Perkins, W.T.1994. Acid mine drainage in Wales and influence ofochre precipitation on water chemistry. In: Alpers,G.N. and Blowes, D.W. (eds.), Environmental Geo-chemistry of Sulfide Oxidation. ACS, Washington.

Gray, N.F. 1995. Main adit flow and metal dischargerates. Technical Report 13, Water Technology Re-search, Trinity College, University of Dublin, Ireland.

Kelly, M. 1988. Mining and the freshwater environ-ment. Elsevier, New York.

Kondos, P.D., MacDonald, R.J.C. and Zinck, J.M. 1991.Studies on the removal of heavy metals from acidicmineral effluents. Paper presented at the 2nd Inter-national Conference on the Abatement of AcidicDrainage. Montreal, Quebec, 16±18 September.

Lui, J., Tang, H. and Muller, G. 1992. UNESCO MABcooperative ecological research project on heavy

metal pollution and its ecological effects. Journal ofEnvironmental Science (China), 4(3), 4±13.

Maes, A. and Cremers, A. 1975. Cation exchangehysteresis in montmorillonite: a pH dependent effect.Soil Science, 119, 198±202.

McArdle, P. 1994. Evolution and preservation of vol-canogenic sulphides at Avoca, Southeast Ireland.Transaction of the Institute of Mineralogy andMetallurgie, 102, B149±B163.

Nebeker, A.V. et al. 1984. Biological methods fordetermining toxicity of contaminated freshwater se-diments to invertebrates. Environmental Toxicologyand Chemistry, 3, 617±630.

Pickering, W.F. 1980. Zinc interaction with soil andsediment components. In: Nrigau, J.O. (ed), Zinc inthe Environment. Part 1: Ecological Cycling, Wiley,London.

Ramezani, N. 1994. Heavy Metal Pollution of the LoAn River and the Poyang Lake, Jiangxi Province/China: Impact of the Dexing Copper Mine. Heidel-berger Geowissenschaftliche Abhandlungen Band 82Ruprecht-Karls-UniversitaÈt Heidelberg.

Ritcey, G.M. 1989. Tailings Management. Elsevier,New York.

Salomons, W. 1993. Sediment Pollution in the EEC.Council of the European Commission, Brussels.

Salomons, W. and FoÈrstner, U. 1984. Metals in theHydrocycle. Springer Verlag, New York.

Stumm, W. and Morgan, J.J. 1981. Aquatic Chemistry.Wiley, New York.

Turekian, K.K. and Wendepohl, K.H. 1961. Distributionof the elements in some major units of the earth'scrust. Bulletin of the Geological Society America, 72,175± 192.

[Ms No 396: received 9 March, 1995 and accepted afterrevision 19 September, 1995].


metal contamination of riverine sediments below the avoca mines, south east ireland

Documents