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Determination of lead, chromium and zinc in sediments from an urbanized river in Mauritius
Environment International 28 (2002) 315 – 324 www.elsevier.com/locate/envint
Determination of lead, chromium and zinc in sediments from an urbanized river in Mauritius Roshan T. Ramessur *, Toolseeram Ramjeawon Faculty of Science, University of Mauritius, Reduit, Mauritius Received 1 June 2002; accepted 7 June 2002
Abstract The mean concentration of Cr (105 F 30 mg kg 1), Zn (167 F 30 mg kg 1) and Pb (14 F 7 mg kg 1) in the sediments along St. Louis River situated in an urbanized and industrialized area in Mauritius were well below the limits of 600, 2500 and 700 mg kg 1 quoted for contaminated sediments adopted from the draft standards (24% clay and 10% organic matter by weight) from the Netherlands [Van Veen RJ, Stortelder PBM. Research on contaminated sediments in the Netherlands. In: Wolf K, Van de Brink WJ, Colon FJ, editors. Contaminated soil. Academic Publisher, 1998. p. 1263 – 1275.]. Industrial contamination appeared to undergo rapid dilution in the estuary as Cr had high levels near point sources from industries, but decreased rapidly in amount in the estuary possibly because of dilution by other sediments. The significant levels of Zn in sediments from upstream to the estuary suggest that the potential sources could be from the adjacent motorway and road runoff causing significant quantities to be trapped within the St. Louis River. Pb was two folds higher in the sediments in the estuary of St. Louis River compared to upstream and downstream indicating accumulation of Pb in estuarine sediments, which could be released continuously into the lagoon. The potential sources of Pb in sediments from upstream to the estuary were from the adjacent motorway and road runoff causing significant quantities to be trapped within the St. Louis River. Pb and Zn were significantly positively correlated in the sediments along St. Louis River indicating a common source for Pb and Zn. Significant negative correlations were also found for both Pb and Zn with dissolved oxygen in summer along St. Louis River which indicated that the presence of anoxic waters influenced the trapping of Zn and Pb in the sediment phase. This study has also highlighted that a phasedown of Pb in petrol is necessary and with the introduction of unleaded petrol and vehicles equipped with catalytic converters, studies on levels of Pd and Pt to provide baseline data need to be done in the near future and integrated in environmental development schemes and effective coastal zone management of small island states. D 2002 Elsevier Science Ltd. All rights reserved. Keywords: Trace metals; River; Estuarine sediments; Correlation
1. Introduction The sediment environment provides a long term record of the effects of anthropogenic discharges, such as industrial and domestic effluents and is an area where impacts often occur before effects are manifest in the water column. In contrast to the strong temporal and spatial variability in the aqueous concentrations of contaminants, sediments inteAbbreviations: AAS, Atomic Absorption Spectrometry; km2, square kilometres; mbar, millibars; mm, millimetres; ms 1, metres per second; mg l 1, milligrams per litre; nm, nonametres; ppm, parts per million (mg kg 1); ng ml 1, nanogrammes per millilitre (or ppb); jC, degrees Centigrade. * Corresponding author. Tel.: +230-454-1041; fax: +230-465-6928. E-mail address: [email protected] (R.T. Ramessur).
grate contaminant concentrations over time and that by focusing on sediments the number of samples needed in contamination assessment can be reduced (Breward et al., 1998). The accumulation of toxic and persistent substances in the coastal environment continuously increases owing to anthropogenic activities as the estuaries and their adjacent coasts are the focus for many economic activities. (Chiffoleau et al., 1994; Windom, 1992; Pereira et al., 1998). The observed presence of anoxic water significantly influences the trapping of Zn and Pb in the sediment phase and seem to be controlled by scavenging with iron sulphide according to various authors (Skei, 1983). However, Calmano et al. (1993) reported that pH and redox conditions controlled the mobilization of Zn and Pb from contaminated sediments in a study from Hamburg harbour. Rees et al. (1998)
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reported that large quantities of Pb and Zn, arising from mining related waste trapped in sediments within fluvial systems in the Trent and Ouse draining the Pennine orefields, remained high between source areas and the Humber estuary, whereas Cr which is associated with manufacturing industries had high levels near source cities, but decreased rapidly down the river systems because of dilution by other sediments. Fernandez et al. (1994) assessed heavy metal pollution in Jacarepagua basin, Rio de Janeiro and found that Pb in sediments outside areas of industrial releases and close to motorways were high and originated from the combustion of leaded petrol. Further, Wei and Morisson (1993) found that storm events led to significant changes in organic profile by resuspension/deposition of cleaner background sediments and removal of organic rich sediments which were being washed downstream in a small urban river in Goteborg and reported that interstitial water during periods of dry and light rain combined with storm resuspension played a key role in metal diffusion and distribution in the river. Owing to increasing urbanisation and industrialisation in the Eastern African area, river catchment areas are being subject to significant conflicts in uses with development. The threat of contamination of surface waters and surface sediments in Mauritius and deterioration in water quality by industrial wastes, in particular, metal pollution is only recent as compared to industrialized countries where problems of heavy metals from industrial wastes have been ascendant since the 1960s (Ramessur et al., 1998, 2001). The major areas for environmental and public concern in Mauritius are pollution of the surface water and underground waters by industrial and domestic effluents, untreated sewage, agricultural and storm-runoff. Sources of input of trace metals (in particular, Pb, Zn and Cr) in Mauritius in recent years include industrial effluents, urban and road runoff, sewage sludge, landfill leachates including scrap metal and solid waste leachates. The use of leaded fuel (200 million litres annually with an average Pb content of 0.4 g l 1) is potentially dangerous both as an atmospheric pollutant and through its introduction into foodchains via rain induced run-off from roads and measures are being taken for the introduction of unleaded petrol on the island (Anon., 1998). Mungur and Choong Kwet Yive (1998) indicated that vehicular emission was a major source of Pb in soil samples pollution ( > 700 mg kg 1) collected near the M1 motorway adjacent to the GRNW catchment area which has a daily traffic density of about 20 000 vehicles. Metal contaminants, which are stored in sediments may have a greater influence on biological uptake than those in waters, particularly in the case of benthic faunas as reported by Breward et al. (1998). In this paper, three trace metals (Cr, Zn and Pb) in both suspended solids and surface sediments and master variables along an axial transect along St. Louis River in the GRNW lower urbanised catchment area were assessed and any common linking in biogeochemical cycles investigated during the
wet and dry months during the period January– October 2000 using atomic absorption spectrometry with graphite furnace attachment. Correlation of some trace metals (Cr, Zn and Pb) in the surface sediments along the St. Louis River and some master variables were investigated and specific questions were formulated as below:
Are there any significant trace metal deposition (Cr, Zn and Pb) along St. Louis River? Are there any significant correlations between the trace metals in surface sediments and master variables along St. Louis River? Is there a common influential linking among Cr, Zn and Pb in surface sediments along St. Louis River?
2. Materials and methods 2.1. Study sites The St. Louis River has a catchment area of 14.5 km2 and is about 5 km long. The river runs adjacent to the Trunk Road M2 with both St. Louis Generating Power Station and Camp Chapelon, an urbanised area situated upstream whereas the river crosses the Port Louis St. Jean Road A1 downstream, Fig. 1. The St. Louis estuary indirectly receives wastewater discharges from road and urban runoff (Zn and Pb) and both domestic and industrial origins such as dye houses (Cr), battery manufacturers (Ni, Zn, Cd, Pb), soap and detergent industries, ethanol distilleries, chemical industries, paint manufacturers (Zn), food and beverages processors, galvanising and electroplating plants (Fe, Ni, Cu, Zn, Cr), downstream from industries situated midway adjacent to the St. Louis River in the Export Processing Zone in Plaine Lauzun. The average rainfall at a nearby meteorological station (Fort Williams, Port Louis) were 100 mm during summer 2000 and 25 mm during winter 2000. 2.2. Sediments Surface sediment and suspended particulate matter were collected from 10 stations (S1– S10) along an axial transect from upstream to the estuary of St. Louis River at fortnightly intervals from January to October 2000. A number of master variables (salinity, temperature, pH, D.O and SPM) were also determined along the river together with three trace metals (Cr, Zn and Pb) in both surface sediments and SPM using atomic absorption spectrometry with graphite furnace attachment. The establishment of stations was directed in a typical aquatic system in Mauritius which could be used as an example in a regional perspective. Sampling was done at a sufficiently high sampling density to ensure data integrity and also to maintain a sufficiently broad spatial coverage to include relatively uncontaminated
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Fig. 1. Map showing sampling stations along St. Louis River, Mauritius.
areas for reference purposes as recommended by Breward et al. (1998). The potential for a significant degree of localised distributional heterogeneity at each sampling incidence was considered and replicate sampling was done to obtain results within specified confidence limits.
2.3. Trace metals in suspended solids and surface sediments Replicate water samples were collected in 1000 ml polythene bottles along St. Louis River in the GRNW catchment area during the period January– October 2000,
Table 1 Draft standards for contaminated sediments, SPM and trace metal concentration in sediments in rivers and estuaries Targeta mg kg
1
Standarda mg kg
1
Limita mg kg
Chromium
100
125
600
Zinc
180
750
2500
Lead
50
125
700
Particulate Cr, mg kg 1 Particulate Zn, mg kg 1 Particulate Pb, mg kg 1 Suspended Particulate Matter mg l a b c d e
1
Van Veen and Stortelder (1998). Rees et al. (1998), rivers discharging in the Humber estuary. Fernandez et al. (1994), lagoon sediments. Vaithiyanathan et al. (1993), Cauvery Basin, India. Williams and Millward (1998), Humber estuary.
1
River
Estuary b
22 – 295 20 – 250d 75 – 1264b 15 – 230d 22 – 484b 5 – 60d 200 – 5600d 500 – 2000e 80 – 450d 5 – 200e
5.6 – 15c 62 – 470c 12 – 64c
500 – 1000e 1000 – 15 000e
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Table 2 Detection limits for the trace metals and recovery of SRM 1646a estuarine sediment using AAS Trace metal
Detection limit, mg kg 1
SRM 1646a certified, mg kg 1
SRM 1646a analysed, mg kg 1
% Recovery
Cr Zn Pb
2.3 2.2 1.2
40.9 F 1.9 48.9 F 1.6 11.7 F 1.2
25.6 F 1.5 40.5 F 2.2 8.7 F 1.6
62.8 82.8 74.4
transported in ice boxes and filtered using 0.45 Am Whatman cellulose nitrate membrane filter for trace metal determination in SPM. Surface sediments (0– 1 cm) were handcollected at 10 sampling stations along St. Louis River with the help of a polythene scoop. Sediment samples were homogenised, stored in plastic bags and frozen till analysis. 2.4. Analytical procedures: field measurements 2.4.1. Master variables Salinity was recorded in the different compartments along the St. Louis River using a Bellingham and Stanley field refractometer with a precision level of 0.2%. Replicate measurements of pH and temperature in situ the river were done using a Whatman pH portable meter A270. Replicate measurements of dissolved oxygen (D.O) were made in situ using a portable D.O meter Jenway Model 9071 consisting of a ‘Clark’ type polarographic oxygen electrode after calibration using a solution of 2% sodium sulphite. 2.4.2. Surface sediments Sediment samples were air dried for 3 days and oven dried at 70 jC for 1 day. After drying, the sediments were disaggregated and dry sieved using a 63-Am sieve. The dried sediment leaching were made in triplicate. A total of 0.5 g of the fine fraction was weighed into a teflon beaker cleaned with nitric acid (65%) and deionised water. Analar nitric acid (4 ml, 65%, BDH, UK) was added to remove most organic matter in the sample followed by 2 ml hydrofluoric acid and the mixture swirled until the vigorous
Table 3 Mean values of master variables along St. Louis River in summer 2000 (n = 8) Stations
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10
1
7.8 F 0.1 7.7 F 0.1 7.4 F 0.3 7.5 F 0.2 7.0 F 0.2 6.4 F 0.1 5.9 F 0.2 6.0 F 0.1 6.7 F 0.2 7.1 F 0.1
% Saturation
pH
Suspended particulate matter, mg l 1
Salinity, x
94.6 93.4 89.8 91.0 84.9 77.6 82.3 76.2 89.0 100.0
7.3 F 0.1 7.7 F 0.1 7.5 F 0.1 7.7 F 0.2 7.6 F 0.1 7.8 F 0.1 7.3 F 0.1 7.3 F 0 8.0 F 0 8.1 F 0.1
0.5 F 0.4 0.8 F 0.9 1.7 F 0.7 1.9 F 0.5 1.8 F 1.3 1.7 F 1.9 1.7 F 2 1.1 F 1.1 0.7 F 0.6 0.8 F 0.8
0 0 0 0 0 0 3 F 3.8 8.3 F 1.5 14.5 F 2.6 27.5 F 2.4
action stopped. All samples were digested in duplicate using a Milestone microwave digestion system. The samples were then left to cool and the contents filtered into 25 ml plastic flasks and the volume made up to the mark with deionised water. The extracted solution was then analysed in terms of total Cr, Zn and Pb, respectively. A blank consisting of only nitric acid (4 ml, 65%) and hydrofluoric acid (2 ml) and digested in the same way as the samples was also prepared. 2.4.3. Suspended solids Suspended solids were also determined for stations along the St. Louis River –estuary transect. Well mixed 200 ml replicate samples were filtered through weighed standard glass-fiber filters (0.45 Am) and the suspended particulate matter determined as the oven dry weight of particles retained on the replicate filters. All samples were digested in duplicate using a Milestone microwave digestion system after filters were oven dried at 60 jC overnight and dissolved in Analar nitric acid (4 ml, 65%) and hydrofluoric acid (2 ml). Twenty-five millilitres extracted solution was
Table 4 Mean values of master variables along St. Louis River in winter 2000 (n = 8) Stations
Fig. 2. Mean values for salinity, pH, D.O and SPM along St. Louis River, January – October 2000.
D.O, mg l
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10
D.O, mg l
1
7.9 F 0.1 7.8 F 0.1 8.8 F 0.8 8.8 F 0.8 7.9 F 0.5 7.7 F 0.3 6 F 0.1 6 F 0.1 7.6 F 0.4 7.2 F 0.6
% Saturation
pH
Suspended particulate matter, mg l 1
Salinity, x
92.3 91.2 100 100 92.3 90.0 70.9 73.4 94.4 98.2
7.4 F 0.1 7.8 F 0.1 7.8 F 0.3 7.8 F 0.3 7.6 F 0.2 7.8 F 0.1 7.4 F 0.1 7.4 F 0.1 8 F 0.1 8.1 F 0.1
0.5 F 0.1 0.6 F 0.1 1.2 F 0.2 1.5 F 0.3 1.4 F 0.5 0.6 F 0.1 0.7 F 0.1 0.4 F 0.1 0.4 F 0.1 0.3 F 0
0 0 0 0 0 0 2.5 F 1.4 7.7 F 4.8 14.5 F 9.1 30.7 F 2.1
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Table 5 Mean trace metal concentration in sediments and SPM along St. Louis River in summer 2000 (n = 8) 1
Stations
Cr, mg kg
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10
78.6 F 9.4 85.8 F 9.4 112.3 F 5 130.1 F 42.5 132.1 F 84.6 52.3 F 49.6 98.9 F 8.0 82.7 F 6.2 75.8 F 54.7 73.5 F 10.7
Zn, mg kg
1
134.6 F 8.6 143.3 F 18.3 145.4 F 21.6 154 F 22.4 163.2 F 22.9 173.9 F 39.2 177.1 F 41.2 170.2 F 21.6 179.2 F 60.2 170 F 30.4
Pb, mg kg
1
3.8 F 0.6 4.5 F 0.7 7.0 F 4.3 10 F 2.5 14.5 F 3.6 15.3 F 3.8 13.1 F 3.2 10.5 F 2 12.2 F 1.9 7.9 F 1.8
then analysed in terms of total Cr, Zn and Pb, respectively, using a Unicam 929 AAS with graphite furnace attachment. Blank filters digested in the same way as the samples were also prepared.
Particulate Cr, mg kg 1
Particulate Zn, mg kg 1
Particulate Pb, mg kg 1
3735 F 305 3800 F 140 4845 F 666 4055 F 455 4010 F 810 4825 F 910 3595 F 190 4070 F 230 3940 F 955 3930 F 700
1842 F 0.188 1796 F 126 2178 F 134 1988 F 60 2114 F 104 2328 F 144 1984 F 156 1802 F 186 1840 F 94 1908 F 70
239 F 29 270 F 30 299 F 28 328 F 57 270 F 90 480 F 60 510 F 58 540 F 57 270 F 29 240 F 28
aligned and the spectrometer optical system set up. For chromium, zinc and lead, 1000 ng ml-1 working solutions were prepared by 10 fold dilution of 10 mg l-1 stock solutions. The working solutions were further diluted 20– 100 folds in a 100-ml volumetric flask by adding distilled and deionised water with standards prepared in the range of 0 –20 ng ml 1.
2.4.4. Trace metal analysis in suspended particulate matter and surface sediments Cr, Zn and Pb were determined in the extracted solution from surface sediments and suspended particulate matter collected along the 10 stations during the period January – October 2000 using a UNICAM 929 Atomic Absorption Spectrometer (AAS) (Analytical Technology, 1993) with graphite furnace attachment. Standards used for calibration for trace metal determination in sediments were prepared from standard 1000 mg l 1 stock solution. The working solutions (10 mg l 1) of each Zn, Pb and Cr were prepared by diluting the stock solution in a 100 ml volumetric flask with deionised water. Standards were then prepared in the range of 1.0 –4.0 mg l 1 for Zn and Pb and 1.0 – 5.0 mg l 1 for Cr. Flame AAS used for trace metal analysis in surface sediments involved the use of a mixture of acetylene and nitrous oxide for Cr at a wavelength of 357.9 nm and an air-acetylene flame for Pb and Zn at a wavelength of 217.0 and 213.9 nm, respectively. The graphite furnace was used for determining particulate metals in the suspended solids. Cr, Zn and Pb hollow cathode lamps were installed and
2.5. Quality control Values for contaminated sediments adopted from the draft standards (24% clay and 10% organic matter by weight) from the Netherlands (Van Veen and Stortelder, 1998) and values for SPM in the Cauvery Basin, India and the Humber estuary (Vaithiyanathan et al., 1993; Williams and Millward, 1998) are shown in Table 1 as no international standards for sediment quality yet exist. The accuracy and precision of the method was evaluated using three replicate determination of National Institute of Science and Technology, (NIST) Standard Reference Material, SRM 1646a for estuarine sediments which yielded 25.6 F 1.8 mg kg 1 (Cr); 40.5 F 1.2 mg kg 1 (Zn); 8.7 F 1.3 mg kg 1 (Pb) compared to certified values of 40.9 F 1.9 mg kg 1 (Cr); 48.9 F 1.6 mg kg 1 (Zn) and 11.7 F 1.2 mg kg 1 (Pb), respectively (mean and standard deviation) as determined using ICP-MS by NIST with %
Table 6 Mean trace metal concentration in sediments and SPM along St. Louis River in winter 2000 (n = 12) 1
Stations
Cr, mg kg
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10
79.4 F 7.6 91.8 F 5 135.3 F 20.8 150.1 F 46.9 139.9 F 54.9 139.6 F 30.7 132.4 F 28.1 76.5 F 13.6 75.8 F 27.6 73.5 F 39.3
Zn, mg kg
1
123.3 F 29.8 128.4 F 34.2 171 F 90.5 103.6 F 48.9 182.6 F 73.8 183.8 F 106.2 162.3 F 89 205.8 F 72.8 238.4 F 89.5 204.1 F 71.9
Pb, mg kg 3.8 F 0.8 6.2 F 1.2 13.5 F 5.6 21.5 F 8.1 12.2 F 3.3 12.7 F 7.5 11.3 F 4.8 24.1 F 7.4 34.9 F 13.4 28.3 F 5.1
1
Particulate Cr, mg kg 1
Particulate Zn, mg kg 1
Particulate Pb, mg kg 1
3495 F 510 4155 F 160 4855 F 325 3745 F 450 3640 F 215 4305 F 235 3880 F 110 3660 F 265 3435 F 295 3245 F 250
1614 F 78 1702 F 200 2110 F 134 2254 F 156 2162 F 44 2110 F 128 1830 F 152 1708 F 124 1590 F 120 1472 F 92
209 F 29 241 F 31 328 F 28 359 F 58 300 F 30 510 F 30 540 F 60 570 F 30 330 F 120 240 F 120
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Fig. 3. Cr, Zn and Pb in sediments along St. Louis River in summer (s) and winter (w) 2000.
recovery as 62.8%, 82.8% and 74.4% for Cr, Zn and Pb, respectively. The main causes of losses could be attributed during the digestion procedure and could also include random and systematic sources of uncertainty during analysis using the atomic absorption spectrometer. Necessary corrections were made accordingly. The detection limits were 2.3, 2.2 and 1.2 mg kg 1 for Cr, Zn and Pb, respectively, as shown in Table 2. (The limits of detection were taken as 3r of three replicates of the procedural blank digested filter paper (3 standard deviation about the mean). The limits of detection were 1.5, 0.5 and 0.1 ng ml 1 for Cr, Zn and Pb using AAS with graphite furnace attachment.
3. Results and discussion 3.1. Master variables Mean values for master variables (D.O, pH, SPM and salinity) along St. Louis River are shown in Fig. 2 and Tables 3 and 4. Dissolved oxygen fell from 7 mg l 1 upstream St. Louis River to about 6 mg l 1 downstream and in the estuary. Salinity increased to 10 –15 g l 1 (S8 and S9) in the estuary and was about 25 – 30 g l 1 at S10. The mean suspended particulate matter in samples collected from an upstream-estuarine along St. Louis River was < 5.0 mg l 1 and was considered low compared to European
Fig. 5. Cr, Zn and Pb in sediments along stations S3 (Upstream), S6 (downstream) and S9 (estuarine) in St. Louis.
estuaries where values up to 400 mg l 1 in the turbidity maximum zone (TMZ) have been reported by Millward (1995). A TMZ for the uptake of dissolved constituents onto particles was however not observed along St. Louis River. SPM along St. Louis River was not high compared to suspended particulate matter in the high turbidity zone (defined by a SPM >30 mg l 1) (HTZ) which have been reported by Chiffoleau et al. (1994) to be responsible for the release of Zn from resuspended sediments in estuaries and coastal waters. 3.2. Trace metals in suspended solids and surface sediments along St. Louis River The linearity for the calibration lines for Cr, Zn and Pb showed correlation coefficient ranging from 0.95 to 0.99. The reported results of Cr, Zn and Pb in suspended particulate matter and surface sediments along St. Louis River for summer and winter are the mean values of duplicate determinations as shown in Tables 5 and 6 and Figs. 3 and 4. Both Cr and Zn in sediments along St. Louis River exceeded the target values of 100 and 180 mg kg 1 (Van Veen and Stortelder, 1998), respectively, at some stations. However, Pb was within the 50 mg kg 1 target value (Van Veen and Stortelder, 1998) and much less than values exceeding 700 mg kg 1 as indicated by Mungur and Choong Kwet Yive (1998) for soil samples pollution collected near the M1 motorway adjacent to the GRNW
Fig. 4. Particulate Cr, Zn and Pb along St. Louis River in summer and winter 2000.
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Table 7 Pairwise comparisons of trace metal concentration in sediments upstream, downstream and estuary (S3, S6 and S9) for both summer and winter 2000 collected along St. Louis using t-test at 5% significance level (ns—not significant) Comparisons at 5% significance level using t-test (two-tail) Upstream summer and winter
Downstream summer and winter
Estuary summer and winter
Upstream and downstream
Upstream and estuary
Downstream and estuary
Chromium
ns
ns
ns
Zinc Lead
ns ns
Significant t = 2.325 t(crit) = 2.306 n = 10 ns ns
ns Significant t = 3.295 t(crit) = 2.306 n = 10
ns ns
Significant t = 3.767 t(crit.) = 2.101 n = 10 ns Significant t = 2.865 t(crit.) = 2.101 n = 10
Significant t = 3.223 t(crit.) = 2.101 n = 10 ns Significant t = 2.305 t(crit) = 2.101 n = 10
catchment area which has a daily traffic density of about 20 000 vehicles. For comparison purposes, the values of Pb and Cr in the sediments in St. Louis River were also below those reported by Miller et al. (2000) who reported high Pb (33 – 996 mg kg 1) and Cr (120 –300 mg kg 1) concentrations in Holy Loch sediments in Scotland. Particulate Cr, Zn and Pb along St. Louis River were also below the values reported by Vaithiyanathan et al. (1993) and Williams and Millward (1998) for rivers in Cauvery Basin, India and the Humber estuary and metal transport by suspended particulate matter along St. Louis River was estimated < 40% as SPM was negligible ( < 5 mg l 1). The mean concentration of Cr, Zn and Pb in surface sediments upstream (station 3), downstream (station 6) and in the estuary (station 9) along St. Louis River in summer and winter are shown in Fig. 5 and pairwise comparisons of trace metal concentration in sediments from upstream, downstream and estuary for both summer and winter 2000 collected along St. Louis using t-test at 5% significance level is shown in Table 7. A correlation matrix and correlation at 5% significance level for trace metals in sediments and master variables are shown in Tables 8 and 9. Zn and Pb in the sediments along St. Louis River were significantly positively correlated at 5% significance level (r2 = 0.59, n = 10) as shown in Fig. 6. Further, both Zn and Pb were significantly negatively correlated to D.O in summer at 5%
Table 8 Correlation matrix for trace metals in both sediments and SPM and master variables along St. Louis River pH pH D.O SPM Mean Cr Mean Zn Mean Pb
1 0.316 0.209 0.205 0.374 0.541
D.O
1 0.328 0.286 0.563 0.299
SPM
1 0.970 0.333 0.108
Mean Cr
1 0.310 0.183
Mean Zn
1 0.766
Mean Pb
1
significance level (Zn: 0.73, n = 10; Pb: r2 = 0.54, n = 10) as shown in Fig. 7. 3.3. Cr, Zn and Pb in sediments along St. Louis River The mean concentration of Cr (105 F 30 mg kg 1), Zn (167 F 30 mg kg 1) and Pb (14 F 7 mg kg 1) in the sediments along St. Louis River were well below the limits of 600, 2500 and 700 mg kg 1 quoted for contaminated sediments adopted from the draft standards (24% clay and 10% organic matter by weight) from Netherlands (Van Veen and Stortelder, 1998). Both Cr and Zn exceeded the target values of 100 and 180 mg kg 1 (Van Veen and Stortelder, 1998), respectively, at some stations. Implications can be that large quantities of Cr and Zn may be stored in the St. Louis River system and could be released into the river as similarly reported by Rees et al. (1998) for Pb and Zn in rivers discharging into the Humber estuary. However, Pb was within the 50 mg kg 1 target value (Van Veen and Stortelder, 1998) and much less than values exceeding 700 mg kg 1 as indicated by Mungur and Choong Kwet Yive
Table 9 Correlation at 5% significance for trace metals and master variables along St. Louis River (Par—particulate; ns—not significant; Sig—significant) pH
D.O
pH D.O SPM Mean Cr
1 ns ns ns
1 ns ns
Mean Zn Mean Pb
ns ns
ns ns
SPM
1 Sig F = 127.1 F crit. = 5.32 n = 10 r2 = 0.94 ns ns
Mean Cr
Mean Zn
Mean Pb
1
ns ns
1 Sig F = 11.4 F crit. = 5.32 n = 10 r2 = 0.59
1
322
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Fig. 6. Correlation of Zn and Pb in sediments along St. Louis river January – October 2000.
(1998) for soil samples pollution collected near the M1 motorway adjacent to the GRNW catchment area which has a daily traffic density of about 20 000 vehicles. For comparison purposes, the values of Pb and Cr in the sediments in St. Louis River were also below those reported by Miller et al. (2000) who reported high Pb (33 –996 mg kg 1) and Cr (120 –300 mg kg 1) concentrations in Holy Loch sediments in Scotland as compared to 86– 187 mg kg 1 for Pb and 46 – 84 mg kg 1 for Cr, respectively in the Tyne, Humber, Thames and Mersey estuaries. Pb was significantly lower in the estuary in summer compared to winter which may indicate that there could be a significant change in organic profile by resuspension/deposition of cleaner background sediments and removal of organic rich sediments in the estuary during periods of heavy rains in summer as similarly indicated by Wei and Morisson (1993) for a small urban river in Goteborg during storm events. Cr was also significantly lower in sediments downstream in summer compared to winter at the 5% significance level suggesting removal of sediments or deposition of cleaner background sediments downstream during periods of heavy rain in summer as similarly reported by Wei and Morisson (1993) for a river in Goteborg during
storm events. Zn was however not significantly different upstream, downstream and in the estuary for both summer and winter. The significant levels of Zn in sediments from upstream to the estuary suggest that the potential sources could be from the adjacent motorway and road runoff causing significant quantities to be trapped within the St. Louis River. Pb was two folds significantly higher in the sediments in the estuary of St. Louis River compared to upstream and downstream at 5% significance level indicating accumulation of Pb in estuarine sediments which could continuously be released into the lagoon. The potential sources of Pb in sediments from upstream to the estuary could be from the adjacent motorway and road runoff causing significant quantities to be trapped within the St. Louis River. This has similarly been reported by Fernandez et al. (1994) who assessed heavy metal pollution in Jacarepagua basin, Rio de Janeiro and found that Pb in sediments outside areas of industrial releases and close to motorways were high and originated from the combustion of leaded petrol Pb was not significantly different in sediments collected upstream and downstream St. Louis River at the 5% significance level. Cr was significantly lower in estuarine sediments compared to
Fig. 7. Correlation between D.O and trace metals in sediments in summer along St. Louis River.
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upstream and downstream St. Louis River at 5% significance level which may indicate that Cr accumulated both upstream and downstream St. Louis River and could arise from industries situated in the EPZ area adjacent midway along St. Louis River. Cr was, however, not significantly different in sediments collected upstream and downstream St. Louis River at the 5% significance level. Industrial contamination appeared to undergo rapid dilution in the estuary as Cr had high levels near point sources from industries, but decreased rapidly in amount in the estuary possibly because of dilution by other sediments. Cr, Zn and Pb which were stored in sediments along St. Louis River could have a greater influence on biological uptake than those in waters, particularly in the case of benthic faunas as reported by Breward et al. (1998) and the accumulation of toxic and persistent substances in the coastal environment may continuously increase owing to anthropogenic activities as the estuary and the adjacent coast are the focus for many economic activities. According to Miller et al. (2000), sediment quality criteria should be regarded as flexible tools for evaluating the toxicological significance of sediment chemistry data and care must be taken to choose the most appropriate set of criteria according to the application. Pb and Zn were significantly positively correlated in the sediments along St. Louis River indicating that the cycling of Pb and Zn were linked as similarly reported by Vasconcelos et al. (1995) who found strong positive correlations between Pb and Zn for sediments in a tidal estuary in Britanny, France. Significant negative correlations were found for both Pb and Zn with D.O in summer along St. Louis River which indicated that the presence of anoxic waters influenced the trapping of Zn and Pb in the sediment phase which seemed to be controlled by scavenging with iron sulphide according to Skei (1983). Particulate Cr, Zn and Pb along St. Louis River were below the values reported by Vaithiyanathan et al. (1993) and Williams and Millward (1998) for rivers in Cauvery Basin, India and the Humber estuary and metal transport by suspended particulate matter along St. Louis River was < 40% as SPM was negligible ( < 5 mg l 1) as contrary to the findings of Vaithiyanathan et al. (1993) where more than 80% Cr, Zn and Pb were carried in suspension in particulate form in the Cauvery river Basin, India. At present, wastewater management in Mauritius increasingly needs to understand the assimilative capacity of sediments which are major sink and the threat of reworking of contaminated sediments through natural and anthropogenic processes poses a long-term hazard which requires further evaluation. The relatively low values obtained for the trace metals suggest that industrialization and urbanization have not yet had a major impact on the St. Louis River system. This study has also highlighted that a phasedown of Pb in petrol is necessary and with the introduction of unleaded petrol and vehicles equipped with catalytic converters, studies on levels of Pd and Pt to provide baseline data need to be done in the near future. There is also a need for long-
323
term perspectives in water resource management to involve integrated strategies in which water – sediment interactions need to be considered and international standards for sediment quality developed. Such studies undertaken in the future would then further the understanding of the biogeochemical processes of the St. Louis River system which could be used in environmental development schemes and effective integrated coastal zone management of small island states.
Acknowledgements Thanks to Mr. V. Ramsahye and Mr. S. Radha for assistance during sampling work and analysis and the University of Mauritius for providing Funds under the HTEP World Bank Program.
References Anon. Water related environmental issues and problems of Mauritius and their potential regional and transboundary importance. Country report. Ministry of Environment, Mauritius 1998;200 pp. Breward N, Rees JG, Rawlins B, Weeks JM, Williams TM, Harris JRW, Stevenson N. Manual for contaminant flux assessment in tropical coastal environments. Volume 1: Guidelines. British Geological Survey, Overseas Geology Series, Technical report WC/98/41 1998;56 pp. Calmano W, Hong J, Forstner U. Binding and mobilization of heavy metals in contaminated sediments affected by pH and redox potential. Water Sci Technol 1993;28(8 – 9):223 – 35. Chiffoleau JF, Cossa D, Auger D, Truquet I. Trace metal distribution, partition and fluxes in the Seine estuary (France) in low discharge regime. Mar Chem 1994;47:145 – 58. Fernandez HM, Conti LFC, Patchineelam SR. An assessment of the pollution of heavy metals in Jacarepagua basin, Rio de Janeiro, Brazil: a statistical approach. Environ Technol 1994;15:87 – 94. Miller BS, Pirie DJ, Redshaw CJ. An assessment of the contamination and toxicity of marine sediments in the Holy Loch, Scotland. Mar Pollut Bull 2000;40(1):22 – 35. Millward GE. Processes affecting trace element speciation in estuaries. Analyst 1995;120:609 – 14. Mungur P, Choong Kwet Yive NS. Soil lead pollution alongside roads in Mauritius: preliminary results. Univ Mauritius Sci Technol J 1998; 1:123 – 9. Pereira ME, Duarte AC, Millward GE, Abreu SN, Vale C. An estimation of industrial mercury stored in sediments of a confined area of the lagoon of Aveiro (Portugal). Water Sci Technol 1998;37(6 – 7):125 – 30. Ramessur RT, Parry SJ, Jarvis KE. Characterization of some trace metals from the Export Processing Zone and a coastal tourist area in Mauritius. Environ Int, 326 1998;24(7):773 – 81. Ramessur RT, Parry SJ, Ramjeawon T. The relationship of dissolved Pb to some dissolved trace metals (Al, Cr, Mn and Zn) and to dissolved nitrate and phosphate in a freshwater aquatic system in Mauritius. Environ Int 2001;26(4):223 – 30. Rees JC, Ridgway J, Knox RWOB, Wiggans G, Breward N. Sedimentborne contaminants in rivers discharging into the Humber estuary, UK. Mar Pollut Bull 1998;37(3 – 7):316 – 29. Skei JM. Permanently anoxic marina basins-exchange of substances across boundaries. Ecol Bull 1983;35:419 – 29. Vaithiyanathan P, Ramanathan AI, Subramanian V. Transport and distribution of heavy metals in Cauvery river. Water, Air, Soil Pollut 1993; 71:13 – 28.
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Van Veen RJ, Stortelder PBM. Research on contaminated sediments in the Netherlands. In: Wolf K, Van de Brink WJ, Colon FJ, editors. Contaminated soil. London: Academic Publisher, 1998. p. 1263 – 75. Vasconcelos FP, Piron-Frenet M, Perthuisot JP, Ben Haj S, Alliot A. Tracemetal dynamics in tidal estuaries. J Coast Res 1995;11(3):763 – 75. Wei C, Morisson GM. Effect of stormwater runoff on metal distribution in the sediment and interstitial waters of an urban river. Environ Technol 1993;14:1057 – 64.
Williams MR, Millward GE. Dynamics of particulate trace metals in the tidal reaches of the Ouse and Trent, UK. Mar Pollut Bull 1998;37(3 – 7): 306 – 15. Windom HL. Contamination of the marine environment from land-based sources. Mar Pollut Bull 1992;25(1 – 4):32 – 6.
Determination of lead, chromium and zinc in sediments from an urbanized river in Mauritius Roshan T. Ramessur *, Toolseeram Ramjeawon Faculty of Science, University of Mauritius, Reduit, Mauritius Received 1 June 2002; accepted 7 June 2002
Abstract The mean concentration of Cr (105 F 30 mg kg 1), Zn (167 F 30 mg kg 1) and Pb (14 F 7 mg kg 1) in the sediments along St. Louis River situated in an urbanized and industrialized area in Mauritius were well below the limits of 600, 2500 and 700 mg kg 1 quoted for contaminated sediments adopted from the draft standards (24% clay and 10% organic matter by weight) from the Netherlands [Van Veen RJ, Stortelder PBM. Research on contaminated sediments in the Netherlands. In: Wolf K, Van de Brink WJ, Colon FJ, editors. Contaminated soil. Academic Publisher, 1998. p. 1263 – 1275.]. Industrial contamination appeared to undergo rapid dilution in the estuary as Cr had high levels near point sources from industries, but decreased rapidly in amount in the estuary possibly because of dilution by other sediments. The significant levels of Zn in sediments from upstream to the estuary suggest that the potential sources could be from the adjacent motorway and road runoff causing significant quantities to be trapped within the St. Louis River. Pb was two folds higher in the sediments in the estuary of St. Louis River compared to upstream and downstream indicating accumulation of Pb in estuarine sediments, which could be released continuously into the lagoon. The potential sources of Pb in sediments from upstream to the estuary were from the adjacent motorway and road runoff causing significant quantities to be trapped within the St. Louis River. Pb and Zn were significantly positively correlated in the sediments along St. Louis River indicating a common source for Pb and Zn. Significant negative correlations were also found for both Pb and Zn with dissolved oxygen in summer along St. Louis River which indicated that the presence of anoxic waters influenced the trapping of Zn and Pb in the sediment phase. This study has also highlighted that a phasedown of Pb in petrol is necessary and with the introduction of unleaded petrol and vehicles equipped with catalytic converters, studies on levels of Pd and Pt to provide baseline data need to be done in the near future and integrated in environmental development schemes and effective coastal zone management of small island states. D 2002 Elsevier Science Ltd. All rights reserved. Keywords: Trace metals; River; Estuarine sediments; Correlation
1. Introduction The sediment environment provides a long term record of the effects of anthropogenic discharges, such as industrial and domestic effluents and is an area where impacts often occur before effects are manifest in the water column. In contrast to the strong temporal and spatial variability in the aqueous concentrations of contaminants, sediments inteAbbreviations: AAS, Atomic Absorption Spectrometry; km2, square kilometres; mbar, millibars; mm, millimetres; ms 1, metres per second; mg l 1, milligrams per litre; nm, nonametres; ppm, parts per million (mg kg 1); ng ml 1, nanogrammes per millilitre (or ppb); jC, degrees Centigrade. * Corresponding author. Tel.: +230-454-1041; fax: +230-465-6928. E-mail address: [email protected] (R.T. Ramessur).
grate contaminant concentrations over time and that by focusing on sediments the number of samples needed in contamination assessment can be reduced (Breward et al., 1998). The accumulation of toxic and persistent substances in the coastal environment continuously increases owing to anthropogenic activities as the estuaries and their adjacent coasts are the focus for many economic activities. (Chiffoleau et al., 1994; Windom, 1992; Pereira et al., 1998). The observed presence of anoxic water significantly influences the trapping of Zn and Pb in the sediment phase and seem to be controlled by scavenging with iron sulphide according to various authors (Skei, 1983). However, Calmano et al. (1993) reported that pH and redox conditions controlled the mobilization of Zn and Pb from contaminated sediments in a study from Hamburg harbour. Rees et al. (1998)
0160-4120/02/$ - see front matter D 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 1 6 0 - 4 1 2 0 ( 0 2 ) 0 0 0 4 0 - 5
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reported that large quantities of Pb and Zn, arising from mining related waste trapped in sediments within fluvial systems in the Trent and Ouse draining the Pennine orefields, remained high between source areas and the Humber estuary, whereas Cr which is associated with manufacturing industries had high levels near source cities, but decreased rapidly down the river systems because of dilution by other sediments. Fernandez et al. (1994) assessed heavy metal pollution in Jacarepagua basin, Rio de Janeiro and found that Pb in sediments outside areas of industrial releases and close to motorways were high and originated from the combustion of leaded petrol. Further, Wei and Morisson (1993) found that storm events led to significant changes in organic profile by resuspension/deposition of cleaner background sediments and removal of organic rich sediments which were being washed downstream in a small urban river in Goteborg and reported that interstitial water during periods of dry and light rain combined with storm resuspension played a key role in metal diffusion and distribution in the river. Owing to increasing urbanisation and industrialisation in the Eastern African area, river catchment areas are being subject to significant conflicts in uses with development. The threat of contamination of surface waters and surface sediments in Mauritius and deterioration in water quality by industrial wastes, in particular, metal pollution is only recent as compared to industrialized countries where problems of heavy metals from industrial wastes have been ascendant since the 1960s (Ramessur et al., 1998, 2001). The major areas for environmental and public concern in Mauritius are pollution of the surface water and underground waters by industrial and domestic effluents, untreated sewage, agricultural and storm-runoff. Sources of input of trace metals (in particular, Pb, Zn and Cr) in Mauritius in recent years include industrial effluents, urban and road runoff, sewage sludge, landfill leachates including scrap metal and solid waste leachates. The use of leaded fuel (200 million litres annually with an average Pb content of 0.4 g l 1) is potentially dangerous both as an atmospheric pollutant and through its introduction into foodchains via rain induced run-off from roads and measures are being taken for the introduction of unleaded petrol on the island (Anon., 1998). Mungur and Choong Kwet Yive (1998) indicated that vehicular emission was a major source of Pb in soil samples pollution ( > 700 mg kg 1) collected near the M1 motorway adjacent to the GRNW catchment area which has a daily traffic density of about 20 000 vehicles. Metal contaminants, which are stored in sediments may have a greater influence on biological uptake than those in waters, particularly in the case of benthic faunas as reported by Breward et al. (1998). In this paper, three trace metals (Cr, Zn and Pb) in both suspended solids and surface sediments and master variables along an axial transect along St. Louis River in the GRNW lower urbanised catchment area were assessed and any common linking in biogeochemical cycles investigated during the
wet and dry months during the period January– October 2000 using atomic absorption spectrometry with graphite furnace attachment. Correlation of some trace metals (Cr, Zn and Pb) in the surface sediments along the St. Louis River and some master variables were investigated and specific questions were formulated as below:
Are there any significant trace metal deposition (Cr, Zn and Pb) along St. Louis River? Are there any significant correlations between the trace metals in surface sediments and master variables along St. Louis River? Is there a common influential linking among Cr, Zn and Pb in surface sediments along St. Louis River?
2. Materials and methods 2.1. Study sites The St. Louis River has a catchment area of 14.5 km2 and is about 5 km long. The river runs adjacent to the Trunk Road M2 with both St. Louis Generating Power Station and Camp Chapelon, an urbanised area situated upstream whereas the river crosses the Port Louis St. Jean Road A1 downstream, Fig. 1. The St. Louis estuary indirectly receives wastewater discharges from road and urban runoff (Zn and Pb) and both domestic and industrial origins such as dye houses (Cr), battery manufacturers (Ni, Zn, Cd, Pb), soap and detergent industries, ethanol distilleries, chemical industries, paint manufacturers (Zn), food and beverages processors, galvanising and electroplating plants (Fe, Ni, Cu, Zn, Cr), downstream from industries situated midway adjacent to the St. Louis River in the Export Processing Zone in Plaine Lauzun. The average rainfall at a nearby meteorological station (Fort Williams, Port Louis) were 100 mm during summer 2000 and 25 mm during winter 2000. 2.2. Sediments Surface sediment and suspended particulate matter were collected from 10 stations (S1– S10) along an axial transect from upstream to the estuary of St. Louis River at fortnightly intervals from January to October 2000. A number of master variables (salinity, temperature, pH, D.O and SPM) were also determined along the river together with three trace metals (Cr, Zn and Pb) in both surface sediments and SPM using atomic absorption spectrometry with graphite furnace attachment. The establishment of stations was directed in a typical aquatic system in Mauritius which could be used as an example in a regional perspective. Sampling was done at a sufficiently high sampling density to ensure data integrity and also to maintain a sufficiently broad spatial coverage to include relatively uncontaminated
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317
Fig. 1. Map showing sampling stations along St. Louis River, Mauritius.
areas for reference purposes as recommended by Breward et al. (1998). The potential for a significant degree of localised distributional heterogeneity at each sampling incidence was considered and replicate sampling was done to obtain results within specified confidence limits.
2.3. Trace metals in suspended solids and surface sediments Replicate water samples were collected in 1000 ml polythene bottles along St. Louis River in the GRNW catchment area during the period January– October 2000,
Table 1 Draft standards for contaminated sediments, SPM and trace metal concentration in sediments in rivers and estuaries Targeta mg kg
1
Standarda mg kg
1
Limita mg kg
Chromium
100
125
600
Zinc
180
750
2500
Lead
50
125
700
Particulate Cr, mg kg 1 Particulate Zn, mg kg 1 Particulate Pb, mg kg 1 Suspended Particulate Matter mg l a b c d e
1
Van Veen and Stortelder (1998). Rees et al. (1998), rivers discharging in the Humber estuary. Fernandez et al. (1994), lagoon sediments. Vaithiyanathan et al. (1993), Cauvery Basin, India. Williams and Millward (1998), Humber estuary.
1
River
Estuary b
22 – 295 20 – 250d 75 – 1264b 15 – 230d 22 – 484b 5 – 60d 200 – 5600d 500 – 2000e 80 – 450d 5 – 200e
5.6 – 15c 62 – 470c 12 – 64c
500 – 1000e 1000 – 15 000e
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Table 2 Detection limits for the trace metals and recovery of SRM 1646a estuarine sediment using AAS Trace metal
Detection limit, mg kg 1
SRM 1646a certified, mg kg 1
SRM 1646a analysed, mg kg 1
% Recovery
Cr Zn Pb
2.3 2.2 1.2
40.9 F 1.9 48.9 F 1.6 11.7 F 1.2
25.6 F 1.5 40.5 F 2.2 8.7 F 1.6
62.8 82.8 74.4
transported in ice boxes and filtered using 0.45 Am Whatman cellulose nitrate membrane filter for trace metal determination in SPM. Surface sediments (0– 1 cm) were handcollected at 10 sampling stations along St. Louis River with the help of a polythene scoop. Sediment samples were homogenised, stored in plastic bags and frozen till analysis. 2.4. Analytical procedures: field measurements 2.4.1. Master variables Salinity was recorded in the different compartments along the St. Louis River using a Bellingham and Stanley field refractometer with a precision level of 0.2%. Replicate measurements of pH and temperature in situ the river were done using a Whatman pH portable meter A270. Replicate measurements of dissolved oxygen (D.O) were made in situ using a portable D.O meter Jenway Model 9071 consisting of a ‘Clark’ type polarographic oxygen electrode after calibration using a solution of 2% sodium sulphite. 2.4.2. Surface sediments Sediment samples were air dried for 3 days and oven dried at 70 jC for 1 day. After drying, the sediments were disaggregated and dry sieved using a 63-Am sieve. The dried sediment leaching were made in triplicate. A total of 0.5 g of the fine fraction was weighed into a teflon beaker cleaned with nitric acid (65%) and deionised water. Analar nitric acid (4 ml, 65%, BDH, UK) was added to remove most organic matter in the sample followed by 2 ml hydrofluoric acid and the mixture swirled until the vigorous
Table 3 Mean values of master variables along St. Louis River in summer 2000 (n = 8) Stations
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10
1
7.8 F 0.1 7.7 F 0.1 7.4 F 0.3 7.5 F 0.2 7.0 F 0.2 6.4 F 0.1 5.9 F 0.2 6.0 F 0.1 6.7 F 0.2 7.1 F 0.1
% Saturation
pH
Suspended particulate matter, mg l 1
Salinity, x
94.6 93.4 89.8 91.0 84.9 77.6 82.3 76.2 89.0 100.0
7.3 F 0.1 7.7 F 0.1 7.5 F 0.1 7.7 F 0.2 7.6 F 0.1 7.8 F 0.1 7.3 F 0.1 7.3 F 0 8.0 F 0 8.1 F 0.1
0.5 F 0.4 0.8 F 0.9 1.7 F 0.7 1.9 F 0.5 1.8 F 1.3 1.7 F 1.9 1.7 F 2 1.1 F 1.1 0.7 F 0.6 0.8 F 0.8
0 0 0 0 0 0 3 F 3.8 8.3 F 1.5 14.5 F 2.6 27.5 F 2.4
action stopped. All samples were digested in duplicate using a Milestone microwave digestion system. The samples were then left to cool and the contents filtered into 25 ml plastic flasks and the volume made up to the mark with deionised water. The extracted solution was then analysed in terms of total Cr, Zn and Pb, respectively. A blank consisting of only nitric acid (4 ml, 65%) and hydrofluoric acid (2 ml) and digested in the same way as the samples was also prepared. 2.4.3. Suspended solids Suspended solids were also determined for stations along the St. Louis River –estuary transect. Well mixed 200 ml replicate samples were filtered through weighed standard glass-fiber filters (0.45 Am) and the suspended particulate matter determined as the oven dry weight of particles retained on the replicate filters. All samples were digested in duplicate using a Milestone microwave digestion system after filters were oven dried at 60 jC overnight and dissolved in Analar nitric acid (4 ml, 65%) and hydrofluoric acid (2 ml). Twenty-five millilitres extracted solution was
Table 4 Mean values of master variables along St. Louis River in winter 2000 (n = 8) Stations
Fig. 2. Mean values for salinity, pH, D.O and SPM along St. Louis River, January – October 2000.
D.O, mg l
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10
D.O, mg l
1
7.9 F 0.1 7.8 F 0.1 8.8 F 0.8 8.8 F 0.8 7.9 F 0.5 7.7 F 0.3 6 F 0.1 6 F 0.1 7.6 F 0.4 7.2 F 0.6
% Saturation
pH
Suspended particulate matter, mg l 1
Salinity, x
92.3 91.2 100 100 92.3 90.0 70.9 73.4 94.4 98.2
7.4 F 0.1 7.8 F 0.1 7.8 F 0.3 7.8 F 0.3 7.6 F 0.2 7.8 F 0.1 7.4 F 0.1 7.4 F 0.1 8 F 0.1 8.1 F 0.1
0.5 F 0.1 0.6 F 0.1 1.2 F 0.2 1.5 F 0.3 1.4 F 0.5 0.6 F 0.1 0.7 F 0.1 0.4 F 0.1 0.4 F 0.1 0.3 F 0
0 0 0 0 0 0 2.5 F 1.4 7.7 F 4.8 14.5 F 9.1 30.7 F 2.1
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Table 5 Mean trace metal concentration in sediments and SPM along St. Louis River in summer 2000 (n = 8) 1
Stations
Cr, mg kg
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10
78.6 F 9.4 85.8 F 9.4 112.3 F 5 130.1 F 42.5 132.1 F 84.6 52.3 F 49.6 98.9 F 8.0 82.7 F 6.2 75.8 F 54.7 73.5 F 10.7
Zn, mg kg
1
134.6 F 8.6 143.3 F 18.3 145.4 F 21.6 154 F 22.4 163.2 F 22.9 173.9 F 39.2 177.1 F 41.2 170.2 F 21.6 179.2 F 60.2 170 F 30.4
Pb, mg kg
1
3.8 F 0.6 4.5 F 0.7 7.0 F 4.3 10 F 2.5 14.5 F 3.6 15.3 F 3.8 13.1 F 3.2 10.5 F 2 12.2 F 1.9 7.9 F 1.8
then analysed in terms of total Cr, Zn and Pb, respectively, using a Unicam 929 AAS with graphite furnace attachment. Blank filters digested in the same way as the samples were also prepared.
Particulate Cr, mg kg 1
Particulate Zn, mg kg 1
Particulate Pb, mg kg 1
3735 F 305 3800 F 140 4845 F 666 4055 F 455 4010 F 810 4825 F 910 3595 F 190 4070 F 230 3940 F 955 3930 F 700
1842 F 0.188 1796 F 126 2178 F 134 1988 F 60 2114 F 104 2328 F 144 1984 F 156 1802 F 186 1840 F 94 1908 F 70
239 F 29 270 F 30 299 F 28 328 F 57 270 F 90 480 F 60 510 F 58 540 F 57 270 F 29 240 F 28
aligned and the spectrometer optical system set up. For chromium, zinc and lead, 1000 ng ml-1 working solutions were prepared by 10 fold dilution of 10 mg l-1 stock solutions. The working solutions were further diluted 20– 100 folds in a 100-ml volumetric flask by adding distilled and deionised water with standards prepared in the range of 0 –20 ng ml 1.
2.4.4. Trace metal analysis in suspended particulate matter and surface sediments Cr, Zn and Pb were determined in the extracted solution from surface sediments and suspended particulate matter collected along the 10 stations during the period January – October 2000 using a UNICAM 929 Atomic Absorption Spectrometer (AAS) (Analytical Technology, 1993) with graphite furnace attachment. Standards used for calibration for trace metal determination in sediments were prepared from standard 1000 mg l 1 stock solution. The working solutions (10 mg l 1) of each Zn, Pb and Cr were prepared by diluting the stock solution in a 100 ml volumetric flask with deionised water. Standards were then prepared in the range of 1.0 –4.0 mg l 1 for Zn and Pb and 1.0 – 5.0 mg l 1 for Cr. Flame AAS used for trace metal analysis in surface sediments involved the use of a mixture of acetylene and nitrous oxide for Cr at a wavelength of 357.9 nm and an air-acetylene flame for Pb and Zn at a wavelength of 217.0 and 213.9 nm, respectively. The graphite furnace was used for determining particulate metals in the suspended solids. Cr, Zn and Pb hollow cathode lamps were installed and
2.5. Quality control Values for contaminated sediments adopted from the draft standards (24% clay and 10% organic matter by weight) from the Netherlands (Van Veen and Stortelder, 1998) and values for SPM in the Cauvery Basin, India and the Humber estuary (Vaithiyanathan et al., 1993; Williams and Millward, 1998) are shown in Table 1 as no international standards for sediment quality yet exist. The accuracy and precision of the method was evaluated using three replicate determination of National Institute of Science and Technology, (NIST) Standard Reference Material, SRM 1646a for estuarine sediments which yielded 25.6 F 1.8 mg kg 1 (Cr); 40.5 F 1.2 mg kg 1 (Zn); 8.7 F 1.3 mg kg 1 (Pb) compared to certified values of 40.9 F 1.9 mg kg 1 (Cr); 48.9 F 1.6 mg kg 1 (Zn) and 11.7 F 1.2 mg kg 1 (Pb), respectively (mean and standard deviation) as determined using ICP-MS by NIST with %
Table 6 Mean trace metal concentration in sediments and SPM along St. Louis River in winter 2000 (n = 12) 1
Stations
Cr, mg kg
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10
79.4 F 7.6 91.8 F 5 135.3 F 20.8 150.1 F 46.9 139.9 F 54.9 139.6 F 30.7 132.4 F 28.1 76.5 F 13.6 75.8 F 27.6 73.5 F 39.3
Zn, mg kg
1
123.3 F 29.8 128.4 F 34.2 171 F 90.5 103.6 F 48.9 182.6 F 73.8 183.8 F 106.2 162.3 F 89 205.8 F 72.8 238.4 F 89.5 204.1 F 71.9
Pb, mg kg 3.8 F 0.8 6.2 F 1.2 13.5 F 5.6 21.5 F 8.1 12.2 F 3.3 12.7 F 7.5 11.3 F 4.8 24.1 F 7.4 34.9 F 13.4 28.3 F 5.1
1
Particulate Cr, mg kg 1
Particulate Zn, mg kg 1
Particulate Pb, mg kg 1
3495 F 510 4155 F 160 4855 F 325 3745 F 450 3640 F 215 4305 F 235 3880 F 110 3660 F 265 3435 F 295 3245 F 250
1614 F 78 1702 F 200 2110 F 134 2254 F 156 2162 F 44 2110 F 128 1830 F 152 1708 F 124 1590 F 120 1472 F 92
209 F 29 241 F 31 328 F 28 359 F 58 300 F 30 510 F 30 540 F 60 570 F 30 330 F 120 240 F 120
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Fig. 3. Cr, Zn and Pb in sediments along St. Louis River in summer (s) and winter (w) 2000.
recovery as 62.8%, 82.8% and 74.4% for Cr, Zn and Pb, respectively. The main causes of losses could be attributed during the digestion procedure and could also include random and systematic sources of uncertainty during analysis using the atomic absorption spectrometer. Necessary corrections were made accordingly. The detection limits were 2.3, 2.2 and 1.2 mg kg 1 for Cr, Zn and Pb, respectively, as shown in Table 2. (The limits of detection were taken as 3r of three replicates of the procedural blank digested filter paper (3 standard deviation about the mean). The limits of detection were 1.5, 0.5 and 0.1 ng ml 1 for Cr, Zn and Pb using AAS with graphite furnace attachment.
3. Results and discussion 3.1. Master variables Mean values for master variables (D.O, pH, SPM and salinity) along St. Louis River are shown in Fig. 2 and Tables 3 and 4. Dissolved oxygen fell from 7 mg l 1 upstream St. Louis River to about 6 mg l 1 downstream and in the estuary. Salinity increased to 10 –15 g l 1 (S8 and S9) in the estuary and was about 25 – 30 g l 1 at S10. The mean suspended particulate matter in samples collected from an upstream-estuarine along St. Louis River was < 5.0 mg l 1 and was considered low compared to European
Fig. 5. Cr, Zn and Pb in sediments along stations S3 (Upstream), S6 (downstream) and S9 (estuarine) in St. Louis.
estuaries where values up to 400 mg l 1 in the turbidity maximum zone (TMZ) have been reported by Millward (1995). A TMZ for the uptake of dissolved constituents onto particles was however not observed along St. Louis River. SPM along St. Louis River was not high compared to suspended particulate matter in the high turbidity zone (defined by a SPM >30 mg l 1) (HTZ) which have been reported by Chiffoleau et al. (1994) to be responsible for the release of Zn from resuspended sediments in estuaries and coastal waters. 3.2. Trace metals in suspended solids and surface sediments along St. Louis River The linearity for the calibration lines for Cr, Zn and Pb showed correlation coefficient ranging from 0.95 to 0.99. The reported results of Cr, Zn and Pb in suspended particulate matter and surface sediments along St. Louis River for summer and winter are the mean values of duplicate determinations as shown in Tables 5 and 6 and Figs. 3 and 4. Both Cr and Zn in sediments along St. Louis River exceeded the target values of 100 and 180 mg kg 1 (Van Veen and Stortelder, 1998), respectively, at some stations. However, Pb was within the 50 mg kg 1 target value (Van Veen and Stortelder, 1998) and much less than values exceeding 700 mg kg 1 as indicated by Mungur and Choong Kwet Yive (1998) for soil samples pollution collected near the M1 motorway adjacent to the GRNW
Fig. 4. Particulate Cr, Zn and Pb along St. Louis River in summer and winter 2000.
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321
Table 7 Pairwise comparisons of trace metal concentration in sediments upstream, downstream and estuary (S3, S6 and S9) for both summer and winter 2000 collected along St. Louis using t-test at 5% significance level (ns—not significant) Comparisons at 5% significance level using t-test (two-tail) Upstream summer and winter
Downstream summer and winter
Estuary summer and winter
Upstream and downstream
Upstream and estuary
Downstream and estuary
Chromium
ns
ns
ns
Zinc Lead
ns ns
Significant t = 2.325 t(crit) = 2.306 n = 10 ns ns
ns Significant t = 3.295 t(crit) = 2.306 n = 10
ns ns
Significant t = 3.767 t(crit.) = 2.101 n = 10 ns Significant t = 2.865 t(crit.) = 2.101 n = 10
Significant t = 3.223 t(crit.) = 2.101 n = 10 ns Significant t = 2.305 t(crit) = 2.101 n = 10
catchment area which has a daily traffic density of about 20 000 vehicles. For comparison purposes, the values of Pb and Cr in the sediments in St. Louis River were also below those reported by Miller et al. (2000) who reported high Pb (33 – 996 mg kg 1) and Cr (120 –300 mg kg 1) concentrations in Holy Loch sediments in Scotland. Particulate Cr, Zn and Pb along St. Louis River were also below the values reported by Vaithiyanathan et al. (1993) and Williams and Millward (1998) for rivers in Cauvery Basin, India and the Humber estuary and metal transport by suspended particulate matter along St. Louis River was estimated < 40% as SPM was negligible ( < 5 mg l 1). The mean concentration of Cr, Zn and Pb in surface sediments upstream (station 3), downstream (station 6) and in the estuary (station 9) along St. Louis River in summer and winter are shown in Fig. 5 and pairwise comparisons of trace metal concentration in sediments from upstream, downstream and estuary for both summer and winter 2000 collected along St. Louis using t-test at 5% significance level is shown in Table 7. A correlation matrix and correlation at 5% significance level for trace metals in sediments and master variables are shown in Tables 8 and 9. Zn and Pb in the sediments along St. Louis River were significantly positively correlated at 5% significance level (r2 = 0.59, n = 10) as shown in Fig. 6. Further, both Zn and Pb were significantly negatively correlated to D.O in summer at 5%
Table 8 Correlation matrix for trace metals in both sediments and SPM and master variables along St. Louis River pH pH D.O SPM Mean Cr Mean Zn Mean Pb
1 0.316 0.209 0.205 0.374 0.541
D.O
1 0.328 0.286 0.563 0.299
SPM
1 0.970 0.333 0.108
Mean Cr
1 0.310 0.183
Mean Zn
1 0.766
Mean Pb
1
significance level (Zn: 0.73, n = 10; Pb: r2 = 0.54, n = 10) as shown in Fig. 7. 3.3. Cr, Zn and Pb in sediments along St. Louis River The mean concentration of Cr (105 F 30 mg kg 1), Zn (167 F 30 mg kg 1) and Pb (14 F 7 mg kg 1) in the sediments along St. Louis River were well below the limits of 600, 2500 and 700 mg kg 1 quoted for contaminated sediments adopted from the draft standards (24% clay and 10% organic matter by weight) from Netherlands (Van Veen and Stortelder, 1998). Both Cr and Zn exceeded the target values of 100 and 180 mg kg 1 (Van Veen and Stortelder, 1998), respectively, at some stations. Implications can be that large quantities of Cr and Zn may be stored in the St. Louis River system and could be released into the river as similarly reported by Rees et al. (1998) for Pb and Zn in rivers discharging into the Humber estuary. However, Pb was within the 50 mg kg 1 target value (Van Veen and Stortelder, 1998) and much less than values exceeding 700 mg kg 1 as indicated by Mungur and Choong Kwet Yive
Table 9 Correlation at 5% significance for trace metals and master variables along St. Louis River (Par—particulate; ns—not significant; Sig—significant) pH
D.O
pH D.O SPM Mean Cr
1 ns ns ns
1 ns ns
Mean Zn Mean Pb
ns ns
ns ns
SPM
1 Sig F = 127.1 F crit. = 5.32 n = 10 r2 = 0.94 ns ns
Mean Cr
Mean Zn
Mean Pb
1
ns ns
1 Sig F = 11.4 F crit. = 5.32 n = 10 r2 = 0.59
1
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Fig. 6. Correlation of Zn and Pb in sediments along St. Louis river January – October 2000.
(1998) for soil samples pollution collected near the M1 motorway adjacent to the GRNW catchment area which has a daily traffic density of about 20 000 vehicles. For comparison purposes, the values of Pb and Cr in the sediments in St. Louis River were also below those reported by Miller et al. (2000) who reported high Pb (33 –996 mg kg 1) and Cr (120 –300 mg kg 1) concentrations in Holy Loch sediments in Scotland as compared to 86– 187 mg kg 1 for Pb and 46 – 84 mg kg 1 for Cr, respectively in the Tyne, Humber, Thames and Mersey estuaries. Pb was significantly lower in the estuary in summer compared to winter which may indicate that there could be a significant change in organic profile by resuspension/deposition of cleaner background sediments and removal of organic rich sediments in the estuary during periods of heavy rains in summer as similarly indicated by Wei and Morisson (1993) for a small urban river in Goteborg during storm events. Cr was also significantly lower in sediments downstream in summer compared to winter at the 5% significance level suggesting removal of sediments or deposition of cleaner background sediments downstream during periods of heavy rain in summer as similarly reported by Wei and Morisson (1993) for a river in Goteborg during
storm events. Zn was however not significantly different upstream, downstream and in the estuary for both summer and winter. The significant levels of Zn in sediments from upstream to the estuary suggest that the potential sources could be from the adjacent motorway and road runoff causing significant quantities to be trapped within the St. Louis River. Pb was two folds significantly higher in the sediments in the estuary of St. Louis River compared to upstream and downstream at 5% significance level indicating accumulation of Pb in estuarine sediments which could continuously be released into the lagoon. The potential sources of Pb in sediments from upstream to the estuary could be from the adjacent motorway and road runoff causing significant quantities to be trapped within the St. Louis River. This has similarly been reported by Fernandez et al. (1994) who assessed heavy metal pollution in Jacarepagua basin, Rio de Janeiro and found that Pb in sediments outside areas of industrial releases and close to motorways were high and originated from the combustion of leaded petrol Pb was not significantly different in sediments collected upstream and downstream St. Louis River at the 5% significance level. Cr was significantly lower in estuarine sediments compared to
Fig. 7. Correlation between D.O and trace metals in sediments in summer along St. Louis River.
R.T. Ramessur, T. Ramjeawon / Environment International 28 (2002) 315–324
upstream and downstream St. Louis River at 5% significance level which may indicate that Cr accumulated both upstream and downstream St. Louis River and could arise from industries situated in the EPZ area adjacent midway along St. Louis River. Cr was, however, not significantly different in sediments collected upstream and downstream St. Louis River at the 5% significance level. Industrial contamination appeared to undergo rapid dilution in the estuary as Cr had high levels near point sources from industries, but decreased rapidly in amount in the estuary possibly because of dilution by other sediments. Cr, Zn and Pb which were stored in sediments along St. Louis River could have a greater influence on biological uptake than those in waters, particularly in the case of benthic faunas as reported by Breward et al. (1998) and the accumulation of toxic and persistent substances in the coastal environment may continuously increase owing to anthropogenic activities as the estuary and the adjacent coast are the focus for many economic activities. According to Miller et al. (2000), sediment quality criteria should be regarded as flexible tools for evaluating the toxicological significance of sediment chemistry data and care must be taken to choose the most appropriate set of criteria according to the application. Pb and Zn were significantly positively correlated in the sediments along St. Louis River indicating that the cycling of Pb and Zn were linked as similarly reported by Vasconcelos et al. (1995) who found strong positive correlations between Pb and Zn for sediments in a tidal estuary in Britanny, France. Significant negative correlations were found for both Pb and Zn with D.O in summer along St. Louis River which indicated that the presence of anoxic waters influenced the trapping of Zn and Pb in the sediment phase which seemed to be controlled by scavenging with iron sulphide according to Skei (1983). Particulate Cr, Zn and Pb along St. Louis River were below the values reported by Vaithiyanathan et al. (1993) and Williams and Millward (1998) for rivers in Cauvery Basin, India and the Humber estuary and metal transport by suspended particulate matter along St. Louis River was < 40% as SPM was negligible ( < 5 mg l 1) as contrary to the findings of Vaithiyanathan et al. (1993) where more than 80% Cr, Zn and Pb were carried in suspension in particulate form in the Cauvery river Basin, India. At present, wastewater management in Mauritius increasingly needs to understand the assimilative capacity of sediments which are major sink and the threat of reworking of contaminated sediments through natural and anthropogenic processes poses a long-term hazard which requires further evaluation. The relatively low values obtained for the trace metals suggest that industrialization and urbanization have not yet had a major impact on the St. Louis River system. This study has also highlighted that a phasedown of Pb in petrol is necessary and with the introduction of unleaded petrol and vehicles equipped with catalytic converters, studies on levels of Pd and Pt to provide baseline data need to be done in the near future. There is also a need for long-
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term perspectives in water resource management to involve integrated strategies in which water – sediment interactions need to be considered and international standards for sediment quality developed. Such studies undertaken in the future would then further the understanding of the biogeochemical processes of the St. Louis River system which could be used in environmental development schemes and effective integrated coastal zone management of small island states.
Acknowledgements Thanks to Mr. V. Ramsahye and Mr. S. Radha for assistance during sampling work and analysis and the University of Mauritius for providing Funds under the HTEP World Bank Program.
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