J ournal of Coastal Research 1150-1158 Fort Lauderdale, Florida Fall 1997
Hydrological Controls on Copper, Cadmium, Lead and Zinc Concentrations in an Anthropogenically Polluted Mangrove Ecosystem, Wynnum, Brisbane, Australia M. W. Clark, D. McConchie, P. Saenger and M. Pillswortht
Centre for Coastal Management Southern Cross University Lismore, New South Wales, Australia tSinclair Knight Merz, Pty. Ltd. Brisbane, Queensland, Australia ABSTRACT _
CLARK, M.W.;McCONCHIE, D.; SAENGER, P., and PILLSWORTH,1997. Hydrologicalcontrols on copper,cadmium, .tllllllll:. lead and zinc concentrations in an anthropogenically polluted mangrove ecosystem, Wynnum, Brisbane, Australia. .. - Journal of Coastal Research, 13(4), 1150-1158. Fort Lauderdale ADDITIONAL INDEX WORDS: Acid sulphate soils, contaminated sediments, mangrove protection, water pollution. INTRODUCTION (WALSH et al., 1979; THOMAS and EONG, 1984) to elevated heavy metal loads has been published, there are surprisingly Mangrove ecosystems can be found in all but the most few papers in the scientific literature (e.g., AUGUSTO et aZ., southerly estuaries and harbours in Australia where they 1990 and LACERDA et al., 1993) on the response of mangrove provide nursery and breeding grounds for many species of ecosystems to heavy metals. Furthermore, little is known marine fauna (SAENGER 1977; HUTCHINGS and SAEN et aZ., about how metal behaviour within mangrove sediments is GER, 1987). In Australia, recognition of the environmental affected by the trees and infauna. To investigate the behav and economic benefits of mangrove protection is reflected in iour of trace metals in mangrove ecosystems, a study of sev the legislative protection accorded to them by state govern eral sites, including the Wynnum site (Figure 1), was initi ments. However, despite this protection, many mangrove for ated by Saenger and McConchie in 1989. Data from this ests in Australia are polluted by metallic and non-metallic study for the Wynnum site were published by SAENGER et al. anthropogenic wastes. At some sites, effluent is discharged (1991), but further study was prompted by unusually dry directly into mangroves the sewage outfall just north of (e.g., weather in 1991, which gave an opportunity to evaluate pos the study site, Figures 1 and 2), at others mangroves act as sible seasonal influences on the distribution of metals in a buffer between refuse tips and open marine waters and, in mangrove sediments. many places, refuse is illegally dumped directly into the man In this paper we report on the seasonal variation in the grove forest. distribution of heavy metals in mangrove sediments at the The case for the protection of mangrove ecosystems as a Wynnum site (BRISBANE, Figures 1 and 2) where a domestic buffer between sources of metallic pollutants and nearby garbage tip is separated from Moreton Bay by a narrow (ca. aquatic ecosystems has been made previously (HARBISON, 200m wide) Avicennia dominated mangrove forest. The man 1986; SAENGERet al., 1991). Although some work on the re grove forest and the tip face are separated by a slight de action of mature mangrove plants (MONTGOMERY and PRICE, pression, roughly 50m wide, which is devoid of any macroflo 1979; PETERSON et al., 1979; NYE, 1990) and seedlings ra (Figure 3). The depression is characterised by highly re ducing muds (see Figure 10) which contain an abundance of 95034 received 24 March 1995; accepted in revision 28 May 1996. metallic and non-metallic refuse. The lack of vegetation in Polluted Mangro ve Syst ems in Australia 1151 o 6 Km Moreton Bay SouthernOcean N Figure 1. Map showing the locati on of the Wynnum lip study site (shaded area). The Wynnum sewag e outfall is near th e northern edge of the sha ded area. the depression is probably related to modification of the face north of the access roadway, through the unvegetated drainage system when the access road was constructed dur zone and the mangrove forest to Moreton Bay (Figure 3) are ing the late 1960's to early 1970's (CLARK, 1992), but within the same transects sampled by SAENGER et al. (1991). A third the depressed area mangrove die off may have been acceler transect was run from the tip face, south of the roadway, ated by changes in salinity or other anthropogenic changes through the mangrove forest toward Moreton Bay. Grab sam (SAENGER et al., 1991). Numerous small seeps ofleachate are ples were taken from two depths at each sampling site; one evident on the tip face and both the leachate and the break sample spanning the depth from 0-5 cm and the oth er cov down of metallic refuse dumped in the mangroves are poten ering the depth range from 30-35 cm. tial sources of heavy metals that can be adsorbed by the man All samples were oven dried at 65 °C for 3 days and then grove sediments. ground in a ring mill with a partially stabilised zirconia head. The climate of Moreton Bay is classified as Cfa on the Kop Although drying, even at this low temperature, may not be pen-Geiger system (KOEPPE and LONG, 1958; LINDACRE and the best method for preserving the original geochem ical con HOBBS, 1977; SAENGER et al., 1977) or as a humid subtropical dition of sediments (THOMSONet al., 1980; RAPIN et al., 1986 ), climatic region (GENTILLI, 1971; KOEPPE and LONG, 1958). alternative procedures such as anaerobic glove boxes and This climate is characterised by high humidity, cloud cover freeze-drying also hav e problems and were not available for and rainfall during the warmer summer months, with drier this study. conditions and a wide temperature range during winter. Be Sediments were digested by mixing about 1 g of sediment cause of these distinct changes between summer and winter with 10 ml of 70% nitric and allowing it to react for a period there may be seasonal effects on the water table and, as a of 15 minutes before he ating. At 50°C the first of three 3 ml result, there may be some changes in metal distribution. additions of 100 vol. hydrogen peroxide was made; after each addition of hydrogen peroxide the mixture was allowed to re METHODS act completely before addition of.the next 3 ml. Once all the Sampling was carried out in April 1991, along 3 line tran hydrogen peroxide had reacted, samples were cooled and fil sects (Figure 3). The two transects which run from the tip tered through a 0.45 urn membrane filter. The filtrate was Journal of Coastal Research, Vol. 13, No.4, 1997 1152 Clark et al. Moreton Bay !'i N t 1 Moreton Bay T2 o I Km Figure 2. Map showing detail of the Lytton and Wynnum ar ea of Bris bane and the location of the Wynnum North refuse tip . The boxed area delimits th e extent of th e study area. Shading repr esents ar eas of man grove forest. then evaporated to near dryness and made up to 10 ml with Milli-Q water. Copper, cadmium and lead were analysed by anodic stripping voltammetry, using a "PDV 2000" (Chem tronics, Western Australia) and the procedures of Me Figure 3. Detailed map of th e Wynnum site showing samplingtransects. CONCHIE et al. (1988); calibration was by the method of stan dard spike addition. Zinc was analysed by atomic adsorption spectroscopy using standard procedures. ital pH/mY meter. Eh was standardised against Zobell's so Because metal concentrations in a setting such as the Wyn lution (ZOBELL, 1946) with a standard potential of + 244 mV num site will be linked to leaching processes and redox con and pH was standardised using pH 4.0 and pH 7.0 buffers. ditions and thus to drainage and tidal flushing, a detailed Readings were taken by inserting the probe into the wet sed site survey was conducted. Different areas of the mangrove iment and allowing it to equilibrate for a period of about one forest also experience differing levels of tidal inundation; ar minute. eas close to Moreton bay are inundated twice daily, whereas Salinity of surface waters was measured using a Reichert areas near the salt marsh may only experience tidal inun Jung 10419 temperature compensated refractometer. Data dation a few times per month. During the dry season, infre collected from each of the 4 surveys was processed using Mac quently inundated areas have a higher salinity as evapora Gridzo ®3.1.3. tion concentrates dissolved salts before the next inundation, but during the wet season salinity in these areas is decreased RESULTS AND DISCUSSION by surface run off from nearby land. Height surveying was conducted using a D.C.L. Tw-20A Heavy metal concentration data for sediments from SAEN theodolite from an arbitrarily set reduced level. Peg 1 was GER et al. (1991) are presented in Figures 4 & 5 and heavy driven about mid way along the eastern edge of the northern metal data collected in April 1991 are shown in Figures 6, 7 tip cell and was assigned a reference height of zero metres. and 8. Data from the height, salinity, Eh and pH surveys are The Eh/pH survey was carried out in conjunction with the presented in Figures 9, 10, 11 & 12. height survey; Eh/pH readings could not be taken on the ac Data from the height survey (Figure 9) show that the to cess way and on the tip cell itselfbecause the sediments were pography is dominated by the tip cell, even though it was only dry . Eh and pH was measured with an Activon BJ513 com surveyed to a relative height of 2.0 m; the other topographic bination Eh/pH probe and an Activon 205D combination dig- feature is a small shell mound at the end of the access road . Journal of Coastal Research, Vol. 13, No.4, 1997 Polluted Mangrove System s in Australia 1153 1989Transect I Data A Transect I 1991 data at depth of 5cm 300 ...------, 150...------, ~ A. --0- Cuppm I\ 200 \ A, --0- OJ 100 I\ I ' ...... <>...... Ph ppm i \ ...... <>...... '\ i \, / \ Cd g, \I t '---0"-- Cdppm 0- \i \\ ~"" v.: --·-0---- Pb 100 ~ ----tJ---- Zn ppm , 50 \~ ----tJ---- Zn " O. '(}-----o·.'.(). ""'b------""6 <>.._.<> '-0 " -0 . o o o o o o o o o o '" ... o g Distance M Distance M Figure 4. Graphs of th e 1989 metal concentrations along transect 1 (af ter Saenger et al., 1991 ). B Transect I 1991 data at a depth of 30cm 600...----.,------, Two important geomorphological features are shown by th e ft map: 1) the water filled depression containing the dead man :\ 500 i \ groves (the depression has an ill defined seaward margin) : \ and : 2) the mangrove covered area, which slopes gently to ,: \, : \ 400 , . wards Moreton Bay, then steepens through a transition zone I\ from the mangroves to the mud-flats. The 100 mm contour i \ 300 interval, reveals changes in slope that are not always obvious I 4 --- -0···· Pb in the field. I \ 200 ; \ /l!\, ----tJ---- Zn The two major deficiencies with the contouring model for I \ I\ the map (Figure 9) are that the water filled depression at the I \ l \ " j \/ 100 I ' : \ end of the roadway, landward of the shelly mound is not fully b..J"l. ~ l\; ~ b------b. closed, whereas in the field it is closed and drains through a If,,,,, .-- ..":'.-~----'9- --.- t:l------_ narrow « 0.5 m) channel to the southeast. Secondly, the o o on o '"on Distance M 1989 Transect 2 Data Figure 6. Graphs of the 1991 metal concentrations along tr an sect 1. 400...------, Graph A presents data for sediment to a depth of 5 em, and gra ph B presents sediment dat a for the 25 to 30 ern depth interval. 300 --0- Cuppm area south of the roadway is depicted as a st eady slope from th e roadw ay to the coast, whereas it actually contains many 200 shallow depressions and tidal channels, which do not drain --·· 0 -" - Cdppm completely at low tide. ----tJ---- Zn ppm Eh data (Figure 10) reveal several distinct zones at the 100 Wynnum site. Areas of standing wat er contrast sha rply with areas that are well drained; where permanent water sta g nates, Eh falls sha rply. Redox potential is thought to be the prime control on the accumulation of metals in many sedi o o -e ments te.g., PEDERSON et al., 1989; SAENGER et al ., 1991; CLARK, 1992)_ Redox conditions may affect metals in sed i Distance M ments and soils either directly through a change in the oxi dation state of the met al or indirectly through a change in Figure 5. Graphs of the 1989 metal concentrati ons along transect 2 (af ter Saenger et al., 1991 ). the oxidation sta te of a ligand that can form complexes with the metal. Changes in redox conditions can also cause the J ourn al of Coastal Research, Vol. 13, No.4, 1997 1154 Clark et al. A Transect 2 1991 data at a depth of Scm A Transect 3 1991 data at a depth of Scm 125 -,,------, 200 ...",------, ft ,'.. 100 tJ:Alo 150 / \ A., ,\r \ --0-- OJ I' --0- OJ \. I\ J \ 75 \ I \ ...... ·0..·.. Cd J\ ·······0··· Cd \ I \ ll-----~ f \ )!s------tJ. l 100 ~ l\ a ~ ----0---- Pb \ : \,' ----0---- Pb \ I \./ SO \" / \ I t::i ----fl---- Zn \ " ----fl---- Zn ~"'.j ~.~, 50 ~:,~ 25 "'tF..--~ - -.~ ~ c> o o o o o o o ~ c> ~ o ~ o o N N ;:; :; -e Distance M Distance M B Transect 3 1991 data at a depth of 30cm B Transect 2 1991 data at a depth of 30cm 100 ,,------, ..--",-h._-••_.t>, 125 ...",------, ,/!l'" 75 100 ,ft. --0-- OJ ,/ \ / \ ...... 0.... Cd --0- OJ ., 75 ,.i \ ~ SO ----0---- Pb li.----~6~~~i:J.-~-- ..-s: \ ········0········ Cd ~ ----0---- Pb ----fl---- Zn 50 k""", 25 ~-.-o-----:jt:----o ----fl---- Zn 'll cr--O-y ~ 25 o o o o ~ o ~ o N ~ o o o o o g o N ~ -r Distance M DistanceM Figure 8. Graphs of the 1991 metal concentrations along transect 3. Graph A presents data for sediment to a depth of 5 em, and graph B Figure 7. Graphs of the 1991 metal concentrations along transect 2. presents sediment data for the 25 to 30 em depth interval. Graph A presents data for sediment to a depth of 5 em, and graph B presents sediment data for the 25 to 30 cm depth interval. idealised formula CHzO (J0RGENSEN, 1982; BERNER, 1984). decomposition of mineral species te.g., iron-oxides or -oxyhy The process of sulphide production in the presence of reduc droxides) that may adsorb metals. ible iron-oxyhydroxides in waterlogged settings can be rep At neutral pH two major reactions may occur when soil Eh resented by the reaction: falls below 120 mY; firstly, SO.2- can be microbially reduced 9CH + 480/- + 4Fe(OH)3 + 4Mz+ -,> te.g: Desulphiouibrio desulphuricans) to form S2- and the for zO 4Fe8.nH20 + 4MC03 + (l5-n)H + 5CO mation of metal sulphide solids te.g., pyrite) becomes possible zO z (the Fe8.nH will subsequently react further to form (SPOSITO and PAGE, 1985; BAULD, 1986) and, secondly, mi zO FeS ) crobial activity tends to degrade high molecular weight or z ganic materials to produce organic acids (SPOSITO and PAGE, where MZ+ represents any divalent cation (most commonly 1985). The overall process of sulphate reduction is represent Mg and Ca in marine settings). ed by the simple reaction: The two major stagnant-water areas (Figure 10) are con nected by a narrow band with low Eh values, which is inter 2CH + 80/ - -,> H + 2HC0 - 20 z8 3 preted to be a drainage line to the dead mangrove area that where sedimentary organic matter is represented by the the height survey was unable to locate clearly. This zone of Journal of Coastal Research, Vol. 13, No.4, 1997 Polluted Mangrove Systems in Australia 1155 ( 7,5m D Mangrove Forest D Mangrove Forest Figure 9. Contour plot of height da ta for the Wynnum site. The contour Figure 10. Contour plot of Eh da ta for th e Wynnum site. The contour int erva l is 300 mm except for the range of - 300 to - 900 mm where th e int erval is 150 mV. contour int erval is 100 mm. low Eh ru ns parallelto the roadway, then merges with pooled phides and the resulting production of small quantities of sul water at its eastern end. This zone is the most likely position phuric acid . Sulphide oxidation, when sulphidic sediments for the origi na l drainage line th at ran in a southerly direction dry out or become aerated, involves both Eh and pH cha nges, before construction of the access roa d. BLOOMFIELD and COULTER (1973) show th at sulphide oxi In well drained areas, such as the man grove flats to th e dation progresses by th e following reaction: south of th e roa dway and just eas t of th e southern tip cell, FeS + H + 7/20 ~ Fe2+ + S042- + H S0 Eh rises to + 250 mV and the sediment is well oxidise d (Sros 2 20 2 2 4 ITO and PAGE, 1985; CLARK, 1993). The only other majo r zone Large amounts of sulphuric acid are produced by this re that has a positive Eh is the man grove flats east of the dead actio n, lowerin g soil pH and potentially mobilising man y me man groves; the higher Eh in these areas is probably related ta llic species; (BLOOMFIELD and COULTER, 1973; VAN DAM to a low frequency of tida l flooding and a lack of fresh water and PONS, 1973; DENT, 1986). Non-ferrous metal sulphides input; the Eh rises where the loss of interstitial water, and may decompose by a simila r reaction. Another possibl e ex lowering of the effective water table allows greater aeration planation for the fall in pH may be the absence of the car of surficial sedime nts (HUTCHINGS an d SAENGER, 1987). bonatelbicarbonate buffer (pH 8.2) that is present in seawa The pH ma p (Figure 11) shows similar pattern s to those of ter; without the car bonate buffer, soil pH falls back to near the Eh map with areas of high Eh coinciding wit h areas of or slightly below neutral. low pH. The lowest pH (c. 4.5) corre sponds to the highest Eh To the south of the road way, where tidal cha nnels and (+ 250 mY) and occurs south of the roadway and east of the pooled water are common; the pH (Figure 11) of the pooled southern tip cell. The pH of water in the area of the dead water reflects the influence of the carbonatelbicarbonate buff mangroves ranges between 7.2 and 8.1, depending slightly on er, but high pH values recorded in some pools suggests local the position. Sediment pH falls in the mangrove forest as Eh bacterial ammonia production. If these ano malies are exclud rises, but rises agai n as the influence of tida l water increases ed th en there is a regional trend in pH with sediments in toward Moreto n Bay . The decrease in pH in the forest is regular contact with seawater having a pH close to that of linked to the rise in Eh an d reflects surficial oxidation of su l- sea water and sediments not in contact wit h seawater show a Journal of Coastal Research, Vol. 13, No.4, 1997 1156 Clark et al. 8.4 l!! l!! t t 75 m 75 m D Mangrove Forest I D Mangrove Fores t I Figure 11. Contour plot of pH data for the Wynnum site. The contour Figu re 12. Contour plot of sa linity data for th e Wyn nu m site . The con interval is 1.2 pH units. tour int erval is 15 ppk (parts per thousand). progressive decrease in soil pH to near of slightly below neu tion s for several metals showing anomalous highs in thei r tral as distance from th e sea increas es. profiles. These distributions are different from those origi The salin ity map (Figure 12) depicts areas that are influ nally found by SAENGER et al. (1991). SAENGER et al. (1991) enced by freshwater run-off, by regular tida l inundation, and found that met al concentrations in th e sediment fell with in by tidal inunda tion followed by eva poration. Sedimen ts reg creasing distanc e from the tip in a generally exponential ularly inundated by tida l waters have a soil salinity close to fas hion. The only transect to retain this trend is transect 1, th at of th e tid al water, whe reas eva poration produces local where met al concentrations decay expone ntially away from areas of high salinity an d freshwater run-off leads to areas the tip face at the 30 ern depth. of reduc ed sa linity. Transect 3 (Figure 8) show s that in th e area of high Eh a nd The area of mangrove forest that is affected by sulphide low pH , met als have been mobilised under acid sulphate con oxidation (high Eh , low pH; Figures 10 and 11) is also affect ditions and stripped from th e area. The movement of mobi ed by freshwater run-off, with salinity for the area falling to lise d metals down the hydraulic grad ient has produced a rise < 15 ppk (pa rts per thousand); the sa linity togethe r with the in metal concentration at about 150 m from the site for the the Eh/pH data , indicates that this site is well drained and surficial sediments and about 100 m for th e sediments at 30 experiences little or no tid al inundation. In contrast, the area cm depth. The trapping of metals migrating from th e zone of of dead mangroves shows increased salinity, commonly 1.5 acid sulphate soil is primarily controlled by the Eh gradient; times seawater , indi cating infrequent tid al inundati on and as Eh decrea ses towards Moreton Bay th er e is a correspond substantial evaporation. Further eas t, and to the south of the ing rise in met al accumulation. roadway, soil salinity shows th at the a rea is subject to reg When the data for the three tran sects (Figures 6, 7 & 8) ular tidal flush ing and little or no eva poration of pooled wa are viewed in relation to the Eh map (Figure 10), some of the ter. Th is interpretation is consisten t with other survey data tre nds become more obvious. The Eh map shows that tran for this area, which ind icate that th e area is wate r-logged and sect 1 runs parall el to the re duced Eh zone (along th e length anoxic, with a pH similar to seawate r. of th e access road ) which marks th e diffuse drainage line for Lateral distribution patterns for met als at the Wynnum the area of dead mangroves. In this low Eh zone, metals have site durin g the dry season are irregular, with the distribu- probably been immobilised as sulphides, thus preserving th e J ourn al of Coastal Research, Vol. 13, No.4, 1997 Polluted Mangrove Systems in Australia 1157 exponential character of the transect. On the sediment sur til geochemical conditions are encountered that allow re-pre face the trend is irregular and the data show that there are cipitation of the metals as insoluble sulphides. anomalous rises in metal concentration in or near depres Because the geochemical processes that influence metal ac sions which are characterised by low Eh values. Transect 3 cumulation in the environment are reversible it is important (Figure 8) has strongly oxidised sediments at one end and to realise that the environmental sink of today may become highly reduced sediments at the other, hence the curved met the pollutant source tomorrow. The breakdown in environ al concentration trend. On the other hand, transect 2 (Figure mental buffering capacity of the Wynnum mangroves be 7) shows an intermediate behaviour between transect 1 and tween 1989 and 1991 (z.e., the ability of the sediments to im 3 (Figures 6 & 8). This intermediate behaviour probably aris mobilise metals close to their source) shows that, while arti es because conditions are neither strongly oxidising nor ficial wetlands can be constructed and maintained for the strongly reducing. Thus, the breakdown of the exponential treatment oftip leachate, natural wetlands are neither chern distribution trends observed by SAENGER et al. (1991) reflects ically nor physically homogeneous and the pattern of hetero climatic conditions that have led to the drying of the sedi geneity must be expected to change over time. Because of this ment, oxidation of sulphides and the mobilisation of metal absence of temporal and geographic homogeneity, natural species. wetlands may not always form stable long term buffers be Whereas the present study shows the effect of extreme des tween anthropogenically polluted sites and adjacent environ sication on metal distribution within a mangrove ecosystem, mentally sensitive ecosystems. normal yearly, monthly, weekly, and daily variations may also affect metal distributions. Daily changes within the man ACKNOWLEDGEMENTS grove ecosystem may be related to diffusive processes and We would like to thank the Australian Research Council with the exception of cyclic changes in Eh and pH it is un (ARC) for funding provided to D. McConchie and P. Saenger likely that the effects of the changes will be detectable. Sea for the study of heavy metals in mangrove ecosystems. sonal variations on the other hand may have a significant Thanks are extended to Southern Cross University (formerly effect on metal distribution and will complicate predictions of UNE.NR) for laboratory and office space, and further thanks metal behaviour. Hence, in addition to absolute concentration are extended to Earth Watch (Australia) and T. Loder (Uni data, it is important to investigate all variation within man versity of New Hampshire) for additional funding for this grove ecosystems, that may affect metal distributions. In par study and for providing the happy-go lucky mud-wallowing ticular, consideration needs to be give to how environmental volunteers, who helped with the collection of survey data. data collected at one time may be affected by natural changes or changes caused by anthropogenic activity and planned de LITERATURE CITED velopment or environmental remediation work must antici pate and allow for the effects that geochemical changes (com AUGUSTO, C.; SILVA, R; LACEIWA. 1.0., and REZENIlE, C.E., 1990. Metal reservoirs in a red mangrove forest. Biotropica, 22 (4), 339 monly cyclic) have on sediments. 345 BAULD, J., 1986. Transformation of SUlphur species by phototrophic and chernotrophic microbes. In: The Importance ofChemical "Spe CONCLUSIONS ciation" in environmental Processes. 1M. BERNHARD, F.E. BRINCK MAN and P.J. SADLER, eds.) Berlin: Springer-Verlag, pp. 255-273. Data from SAENGER et al. (1991) and data collected in this BERNER, RA, 1984. Sedimentary pyrite formation: An update. 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