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Dynamics of Particulate Trace Metals in the Tidal Reaches of the Ouse and Trent, UK
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Marine Pollution Bulletin Vol. 37, Nos. 3±7, pp. 306±315, 1998 Ó 1999 Elsevier Science Ltd. All rights reserved Printed in Great Britain S0025-326X(99)00054-5 0025-326X/99 $ ± see front matter
Dynamics of Particulate Trace Metals in the Tidal Reaches of the Ouse and Trent, UK M. R. WILLIAMS* and G. E. MILLWARD Department of Environmental Science, University of Plymouth, Drake Circus, Plymouth, Devon, PL4 8AA UK
Samples of suspended particulate matter (SPM) have been collected in the tidal reaches of the Ouse and Trent over seven surveys covering the annual cycle of riverine conditions. Settling experiments were carried out on water samples to dierentiate permanently suspended particulate material (PSPM) from temporarily suspended particulate material (TSPM). The results indicated that PSPM, additional to the riverine component, was created within the turbidity maximum zone (TMZ) during tidal resuspension. The PSPM varied between 10% and 50% of the total SPM and could be transported quasi-conservatively down-estuary. The concentrations of particulate Cd, Fe, and Zn (available to 1M HCl) were determined in PSPM and TSPM, as well as samples of the bulk SPM. The trace metal concentrations in PSPM were seasonally variable and generally higher than in TSPM, which had relatively constant concentrations. The trace metal concentrations in PSPM and TSPM were used to develop a two-component, particle mixing model, which was applied to the interpretation of the bulk SPM concentrations of Cd and Zn. It is postulated that as the PSPM encounters the salinity gradient labile Cd and Zn desorb from the particles. On the basis of these results an additional mechanism for generating estuarine maxima in dissolved trace metals has been proposed. Ó 1999 Elsevier Science Ltd. All rights reserved Keywords: trace metals; particles; turbidity maximum; rivers; estuary; Ouse; Trent.
Introduction Processes controlling estuarine trace metal geochemistry at low salinities are complex, involving water and sediment hydrodynamics, particle-water interactions and biological mediation (Morris et al., 1978; Millward and Turner, 1995). Hydrodynamic processes in macrotidal *Corresponding author. Present address. Dunstanage Marine Laboratory, P.O. Box 3, Oban, Argyll, Scotland PA34 4AD, UK.
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estuaries give rise to turbidity maxima, where physical processes combine to generate a wide range of particle concentrations and types, upon which changes in trace metal concentrations are superimposed, as a result of sorption reactions and re-partitioning (Yeats and Loring, 1991). In situ laser particle sizing measurements in the Tamar Estuary (Bale et al., 1989) have shown that mean particle diameters decrease in the vicinity of the TMZ, which arise as a result of particle selection processes during resuspension, combined with the disaggregation of loosely bound ¯ocs by tidal shear. As a consequence, particles within the TMZ are potentially composed of two types (a) TSPM (temporary suspended particulate matter), with higher settling velocities, which could undergo limited axial transport and (b) PSPM (permanently suspended particulate matter), with relatively low settling velocities, which could have signi®cant down-estuary transport. Subsequently, the PSPM ¯occulates, settles to the sediments and becomes involved in further internal estuarine cycles (Morris et al., 1986). However, as the PSPM passes the estuarine interphase, labile particulate trace metals undergo salinityinduced desorption (Hegeman et al., 1992; Paalman et al., 1994), thereby contributing to estuarine maxima in dissolved trace metals, for example in the Tamar Estuary (Ackroyd et al., 1986), the St Lawrence (Yeats and Loring, 1991) and the Gironde (Kraepiel et al., 1997). Since the dissolved and particulate phases are transported in entirely dierent ways, an understanding of physico-chemical processes within the TMZ is vital to the accurate quanti®cation of trace metal ¯uxes from the land to the oceans. In the Humber Estuary, SPM concentrations in the freshwaters of the Ouse and Trent are in the range 5±200 mg lÿ1 , whereas SPM concentrations within the salinity range 0.5±10, are of the order 1±15 g lÿ1 (Ferreira et al., 1998) and 75±85% of the SPM have particle sizes <16 lm (Uncles et al., 1998). There only a few axial transects of reliable concentrations of dissolved trace metals for the Humber and some show mid-estuarine peaks for Zn, with apparent sources contributing from 10±20 lg lÿ1 (Coey, 1994; Coey and Jickells, 1995; Comber et al.,
Volume 37/Numbers 3±7/March±July 1998 TABLE 1 Timings of surveys of the Ouse and Trent. Date October 1994 July 1995 February 1996 April 1996 July 1996 February 1997 July 1997
Season Autumn Summer Winter Spring Summer Winter Summer
1995). For Cd, the additional contribution is between 0.2 and 0.4 lg lÿ1 (Comber et al., 1995; Ng et al., 1996). Thus, this work was designed to investigate what role particle selection processes in the TMZ may play in controlling trace metal transport in the tidal reaches of a highly turbid system. The study formed part of the Natural Environment Research Council, Land-Ocean Interaction Study (NERC-LOIS).
Methods Axial surveys were performed in the low salinity region of the Humber Estuary from October 1994 through July 1997 (Table 1) and covered the seasonal cycle. Riverine samples were taken near the tidal limits of the Ouse and Trent and at sites of higher salinity as determined by the conductivity (Fig. 1). Transmissometer measurements during surveys 1±3 indicated that the maximum turbidities were located at salinities <5. The samples were collected in acid-washed carboys by immersing them just below the surface of the water and rinsing several times prior to sample collection.
The suspended particles were separated into two components, PSPM and TSPM, based on their settling characteristics. Samples were homogenised by gentle agitation of the carboy and immediately a sub-sample of 1 l was transferred into an acid-cleaned measuring cylinder, in which the particles were allowed to settle under gravity for 3 h. This time was selected because it is representative of the water residence time within this region of the estuary and assuming Stokesian settling the particle diameters that would comprise TSPM are estimated to be >15 lm. The majority of the suspension in the measuring cylinder was carefully decanted, giving a fraction containing only PSPM. The settled material contained the TSPM fraction, together with PSPM. Knowing the concentration of PSPM in the decanted sample, it was possible to correct the TSPM concentration for the presence of PSPM. The two fractions were ®ltered through acidcleaned 47 mm cellulose acetate ®lters (Sartorius). The ®lters were stored at ÿ18°C until required. The ®lters were thawed at room temperature and dried inside a laminar ¯ow cabinet. The ®lters were weighed and digested for 24 h using 25 ml 1M HCl (BDH, Aristar). The digested solution was ®ltered using a cellulose acetate ®lter (Sartorius, 0.45 lm poresize, 47 mm diameter) and the solution stored in high density polyethylene bottles prior to analysis. The trace metals were determined using a microcup attached to a Varian SpectrAA in ¯ame mode for Fe, Mn, Zn using linear calibration, and using a graphite furnace atomic absorption spectrometer (Perkin Elmer SIMAA 6000) optimised for Cd and calibrated using method of standard additions. The acceptance criteria for the determination of trace metal concentrations were (a) a relative standard deviation for replicate peaks of <10%
Fig. 1 Schematic diagram showing in the sampling region. The diagram indicates the three main sampling sites within the rivers Ouse and Trent (d).
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and (b) a correlation coecient for the calibration of >0.995. The detection limits were taken as being three times the standard deviation of the blank values. Quality Assurance was achieved by co-digesting a sediment of known metal concentration (RSD (n 9): Fe, 8.5%; Mn, 9.1%; Zn, 9.3%; Cd, 10.9%).
Results and Discussion Results of settling experiments The estuarine conditions for the February 1997 collections of SPM in the Ouse and the Trent are shown in the top diagram of Figs. 2 and 3, respectively. The river¯ow in the Trent was 150 m3 sÿ1 and the tidal range was 6 m. The conditions in both rivers were generally governed by turbidities in the range 10±2000 mg lÿ1 and salinities were <1. The SPM concentrations in the Ouse during February 1997 varied little in the freshwaters encompassing Naburn and Kel®eld (Fig. 2). The settling experiments show that, although TSPM had the higher concentration, the concentrations of PSPM and TSPM were approximately constant and, therefore, it is concluded that both fractions were dominated by riverine particles. However, as the turbidity rose towards Boothferry the concentrations of both fractions increased but the increase was greater for TSPM. In the Ouse (Fig. 2), a riverine PSPM concentration of about 20±25 mg lÿ1 was augmented by a further 50 mg lÿ1 due to resuspension processes at Boothferry. Similar observations were made for the Trent (Fig. 3), although there is only one riverine sample at Cromwell. Nevertheless, the increased turbidities recorded at Gainsborough and Keadby gave enhanced concentrations of both PSPM and TSPM, compared to those at Cromwell. The conclusion from these observations is that resuspension processes in the TMZ generate PSPM, additional to that originating purely from the rivers. Inspection of the data from other surveys shows that the enhancement of the PSPM concentrations occurred at other times of the year. The variation in the percentage of PSPM generated in the TMZ was dependent on the SPM concentration and there was generally more TSPM than PSPM except at turbidities of less than 20 mg lÿ1 . Generally, the settling experiments show a two- to fourfold increase in concentrations of PSPM at higher turbidities, compared to the riverine concentrations. Measurements of in situ laser particle size within the TMZ during July 1995 showed that mean particle diameters were 131 lm in freshwater at a turbidity of 13 g lÿ1 and they increased to values >230 lm at turbidities of 0.5 g lÿ1 and salinity 5 (D. Law, personal communication). The in situ particle diameters were relatively large compared to mean values of 8.6 1.5 lm obtained on SPM samples following disaggregation by ultrasoni®cation in the laboratory. Thus, the lower mean particle diameters at the highest turbidities, and the low mean particle diameters obtained after ultrasoni®cation, support the contention that disaggregation 308
of resuspending particles, by tidal shear, could be mainly responsible for producing the additional PSPM within the TMZ. Particles which comprise PSPM have slow settling velocities and are more likely to be transported down-estuary, quasi-conservatively, with the residual ¯ow of the water. Furthermore, the additional PSPM may also be implicated in trace metal sorption processes, thereby providing an additional transport mechanism from the river into the low salinity zone. Particulate trace metal concentrations in PSPM and TSPM The range of and trends in particulate Fe, Cd and Zn concentrations for three sites the Ouse and Trent are exempli®ed in Figs. 2 and 3, respectively. Comparison of the relative concentrations of trace metals in PSPM (lefthand diagram) and TSPM (right-hand diagram) can be made in Figs. 2 and 3. Iron The concentrations of Fe in PSPM and TSPM (Figs. 2 and 3; Table 2) were approximately constant throughout the tidal reaches of both the Ouse and Trent. These examples are supported by data in Fig. 4a, which has the Fe concentrations for all the Ouse settling experiments. There are no signi®cant dierences in Fe concentrations between PSPM and TSPM, each having a relatively low coecient of variation (means and standard deviations given in Table 2). The Fe concentrations in bulk SPM (Fig. 4b) are invariant above 50 mg lÿ1 and below this value there are only few data points with lower Fe concentrations. The results of the settling experiments for the Trent samples (Fig. 4c), indicate that the TSPM has a lower Fe concentration compared to the PSPM and the concentrations of Fe for both particle fractions are more variable (means and standard deviations in Table 2). This suggests that the TSPM in the Trent originates from a dierent source compared to the PSPM, a hypothesis also supported by the fact that TSPM from the Trent has a lower Fe concentration than that from the Ouse. The bulk Fe concentrations (Fig. 4d) show greater variability, probably associated with intermittency in the sources of Fe. The uniformity of particulate Fe concentrations could be a consequence of inputs from the mineralised catchments, although Neal et al. (1997) reported no signi®cant source of Fe from historic mining activities in the hinterland. More likely Fe sources are the acid-iron waste discharges near the mouth of the Humber, the magnitudes of which dwarf any of the natural inputs of Fe (Millward and Glegg, 1997). Since Fe is a particle-reactive element, the acid-iron wastes could ultimately precipitate as Fe(III) coatings on SPM and/or as Fe(III) ¯ocs. A large fraction of the SPM in the estuary is mainly clay particles (Ferreira et al., 1998), which could have originated from the coastal erosion of boulder clays, 70% of which have grain sizes <63 lm (McCave, 1987). Thus, a major source of clay particles to the Humber
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Fig. 2 Salinity (. . .. . .) and SPM concentrations (AA) in the tidal reaches of the river Ouse during February 1997. Variation of PSPM (left-hand column) and TSPM (right-hand column) concentrations and associated Fe, Cd and Zn concentrations.
Estuary and the anthropogenic Fe inputs are in close geographical proximity. It is, therefore, reasonable to suggest that the surfaces of the uncontaminated boulder clays are modi®ed by coatings of anthropogenic Fe. Tidal dynamics pump the Fe-coated particles up-estuary to the limit of salt intrusion, as sediment provenance studies have con®rmed (Grant and Middleton, 1990). However, does the tidal pumping mechanism disperse particles from down-estuary equally between the Ouse and the Trent? Since the Ouse has (a) a higher concentration of Fe in TSPM and (b) relatively constant concentration of PSPM and TSPM it is possible that there is
a preferred hydrodynamic pathway at Trent Falls. At the con¯uence of the rivers the water transport is constrained by estuarine geomorphology because the Humber and Trent are at 90° to one another, whereas the Humber and Ouse form a straight channel (Fig. 1). Another contributory factor is the generally higher freshwater ¯ows originating from the Trent, which are likely to push the salt intrusion down-estuary. Thus, Fe in TSPM from the Trent may have originated preferentially from its catchment rather than from industrial sources down-estuary, whereas for the Ouse the higher concentrations of Fe in TSPM implicate the industrial 309
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Fig. 3 Salinity (. . .. . .) and SPM concentrations (AA) in the tidal reaches of the river Trent during February 1997. Variation of PSPM (left-hand column) and TSPM (right-hand column) and associated Fe, Cd and Zn concentrations.
sources down-estuary. PSPM is assumed to behave quasi-conservatively and, therefore, its transport is less in¯uenced by the geomorphological and hydrodynamical factors constraining the movement of TSPM. Consequently, PSPM had similar Fe concentrations in both rivers and the Fe was mainly of anthropogenic origin. Cadmium and Zinc The changes in particulate Cd and Zn concentrations in the Ouse are shown in Fig. 2. The Cd and Zn concentrations, in both PSPM and TSPM, were similar at Naburn and Kel®eld, because the SPM at both sites was predominantly of riverine character. At Boothferry, 310
where salinity 1, the Cd and Zn concentrations, in both fractions, decrease signi®cantly as a result of mixing and dilution by metal-depleted particles resuspended from the bed and from desorption processes as particles encountered the salinity gradient. Similarly, for the Trent, the highest concentrations were in the riverine material at Cromwell followed by decreases in the Cd and Zn concentrations at higher turbidities and salinities (Fig. 3). The concentrations of Cd in the two fractions for the Ouse and Trent are shown in Figs. 5a and c, respectively. Relatively high concentrations of Cd in PSPM were obtained at low turbidity, whereas the concentration of Cd in TSPM is almost constant. For the Ouse the
Volume 37/Numbers 3±7/March±July 1998 TABLE 2 Mean standard deviations of Fe concentrations from the settled fractions shown in Fig. 4. Seasonal extremes of Cd and Zn concentrations in settled fractions used to produce model curves in Fig. 5b,c and Fig. 6b,c, respectively. Metal concentration, lg gÿ1
Fe Ouse Trent Cd Ouse Trent
Zn Ouse Trent
Overall Overall Summer Winter Summer Winter
Summer Winter Summer Winter
TSPM
PSPM
23.6 5.0 14.9 7.6
24.8 6.9 28.6 11.5
0.6 0.7 0.9 0.7
220 230 210 200
5.2 0.9 11 1.1
280 420 480 600
mean concentrations of Cd in PSPM were 4.3 5.1 lg gÿ1 and TSPM 3.3 5.4 lg gÿ1 were similar. However, in the Trent the mean concentration of Cd in PSPM (9.7 17 lg gÿ1 ) was generally higher than Cd in TSPM (2.6 5.6 lg gÿ1 ). The comparison of the mean con-
SPM concentration, mg lÿ1 Bulk
PSPM
180 380 140 1700
34 120 18 130
180 380 140 1700
34 120 18 130
Max. Desorbable Metal on PSPM, lg lÿ1
0.2 0.1 0.2 0.1
9 50 9 78
centrations is not straightforward because of the large spread of values. Similarly, the PSPM-associated Zn (390 240 lg gÿ1 ) in the Ouse (Fig. 6a) had similar concentrations to the TSPM (410 350 lg gÿ1 ). However, in the Trent, the concentration of Zn was higher in
Fig. 4 Particulate Fe concentrations derived from settling experiments (s P-SPM; d T-SPM) for (a) river Ouse and (c) river Trent as a function of SPM concentration. Indicated in (b) and (d) are the Fe concentrations in bulk suspended particles as a function of SPM concentration. Survey symbols ± crossed box, October 1994; star, July 1995; diamond, February 1996; square, April 1996; inverted triangle, July 1996; circle, February 1997; triangle, July 1997.
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Fig. 5 Cd concentrations derived from settling experiments (s PSPM; d T-SPM) for (a) river Ouse and (c) river Trent. Indicated in (b) and (d) are the Cd concentrations in bulk suspended particles as a function of SPM concentration. The lines are the model predictions from the particle mixing model The full line is for winter conditions and the dashed line for summer. Survey symbols- crossed box, October 1994; star, July 1995; diamond, February 1996; square, April 1996; inverted triangle, July 1996; circle, February 1997; triangle, July 1997.
PSPM (590 410 lg gÿ1 ) (Fig. 6c) compared to Zn in TSPM (210 83 lg gÿ1 ). The reason for this is not clear, although particulate Zn may be adsorbed onto Fe oxides (Millward and Moore, 1982), associated with the elevated Fe concentrations in PSPM (Fig. 4c). Particle mixing model The particulate Cd and Zn concentrations suggests a variable trace metal concentration of PSPM mixing with an invariant trace metal concentration of TSPM typical of the estuary. Assuming that the concentrations of particulate trace metals in the bulk SPM are only a consequence of the mixing of PSPM with TSPM, the data has been ®tted using a two-component particle mixing model (Eqs. (1) and (2)) (Turner et al, 1994) PS
M
PS
SPM-SPM M TSSPM M PS ; SPM
1
where SPM SPMPS SPMTS :
2
M is the particulate metal concentration; SPM is the suspended particulate matter concentration; SPMPS is the permanently suspended particulate matter concen312
tration; SPMTS is the temporary suspended particulate matter concentration. The end-member concentrations for the trace metal concentrations (i.e. MPS and MTS ), during summer and winter, were taken from the results of the settling experiments (Table 2). However, the Cd concentrations of the bulk samples for the Ouse (Fig 5b) and Trent (Fig. 5d) show that the application of the model ®ts the bulk trace metal data reasonably well. The bold line has been estimated using end-member data from winter surveys, while the dashed line utilises summer end-member concentrations. Thus, the model prediction covers a signi®cant part of the seasonal change in particulate concentrations of Cd. Similarly, for Zn (Figs. 6b and d) show that the end-member concentrations in Table 2 ®t the data for Zn over a seasonal cycle. Although the model assumes no particle±water interactions, sorption processes will aect the observed particulate trace metal concentrations and may account for some of the deviations shown in Figs. 5 and 6. Trace metal desorption from seaward ¯uxing PSPM Desorption of Cd is caused by a combination of its complexation with chloride anions in seawater and competition from Ca2 and Mg2 for active surface sites
Volume 37/Numbers 3±7/March±July 1998
Fig. 6 Zn concentrations derived from settling experiments (s PSPM; d T-SPM) for (a) river Ouse and (c) river Trent. Indicated in (b) and (d) are the Zn concentrations in bulk suspended particles as a function of SPM concentration. The lines are the model predictions from the particle mixing model The full line is for winter conditions and the dashed line for summer. Survey symbols ± crossed box, October 1994; star, July 1995; diamond, February 1996; square, April 1996; inverted triangle, July 1996; circle, February 1997; triangle, July 1997.
(Comans and van Dijk, 1988; Paalman et al., 1994; Kraepiel et al., 1997). Similar particle±water exchange processes may also apply to Zn but to a lesser extent (Grieve and Fletcher, 1997; Hegeman et al., 1992; Kraepiel et al., 1997). The 1M HCl digest used in this study probably overestimates the concentrations of easily exchangeable trace metals on the particle surfaces. Therefore, this digest gives an upper limit to the amount of desorbable metal that could be contributed to the dissolved phase. The trace metals added to the standing concentrations of dissolved Cd and Zn are in the range 0.1±0.2 and 9±78 lg lÿ1 (Table 2), respectively and are of a magnitude which could explain the mid-estuarine maxima in the Humber observed for Cd (Comber et al., 1995; Ng et al., 1996) and Zn (Coey, 1994; Coey and Jickells, 1995; Comber et al., 1995). An alternative approach to assessing the amounts of trace metal desorbed involves the use of a mass balance of labile metals on suspended particles encountering the salinity gradient (Li et al., 1984), which is given by Eq. (3). The partition coecient, KD , is known to be salinity dependent and is given by Eq. (4) (Turner and Tyler, 1997) C SPM KD0 1 ; CR SPM KDS 1
3
where KDS KD0 S 1ÿb :
4
C is the dissolved trace metal concentration in estuarine water of salinity S, CR is the dissolved trace metal concentration in river water; SPM is the concentration of suspended particulate matter; KD0 is the partition coecient in river water; KDS is the partition coecient in estuarine water of salinity S and b is the slope factor. Radiochemical experiments, involving the adsorption of radioactive 65 Zn onto Humber Estuary SPM, as a function of salinity, gave values of KD0 5; 300 l kgÿ1 and b 0.95. Using Eqs. (3) and (4) in combination the extent of desorption from varying amounts of seaward ¯uxing PSPM can be estimated. Fig. 7 shows the concentration of dissolved Zn, relative to its river water concentration, as a function of salinity. Thus, if 10 mg lÿ1 of PSPM crosses the salinity gradient dissolved Zn concentrations increase by about 5% over those in river water. For 50 mg lÿ1 of PSPM the dissolved metal increases and reaches a constant value after a salinity of about 10, adding about another 25% of dissolved Zn to the riverine concentration. If 100 mg lÿ1 of PSPM passes the estuarine interphase the increase is proportionally more, although a plateau in dissolved Zn concentration 313
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Fig. 7 The ratio of the concentration of dissolved Zn in estuarine water to the dissolved Zn concentration in river water (C/CR) as a function of salinity.
is not reached by a salinity of 20. Thus, desorption of Zn from PSPM could make a signi®cant contribution to development of mid-estuarine maxima in dissolved Zn.
Conclusions The turbidity maximum zone of an estuary is a dynamic and complex region in which particle selection processes and disaggregation by tidal shear modify the population of particles in suspension (Millward et al., 1990). Sampling campaigns in the rivers Ouse and Trent, UK, in which the particle populations have been separated by settling and have been used to evaluate estuarine chemical processes within the TMZ. Iron concentrations in PSPM and TSPM are dominated by Fe waste inputs in the lower estuary, except for TSPM in the Trent, which had its origins in the catchment. The distribution of particulate Fe in the Ouse and Trent strongly suggests that the Ouse is the preferred pathway for the up-estuary transport of SPM, which has implications for the fate of particle-associated contaminants from down estuary. The behaviour of particulate trace metals in the low salinity region of the Humber Estuary is a function of the mixing of spatially and temporally invariant trace metal concentrations of TSPM with seasonally variable trace metal concentrations of PSPM. The results suggest that processes within the TMZ dominate the particulate trace metal concentrations found down estuary, rather than seasonal variability in the trace metal concentrations of riverine SPM. Processes aecting the concentrations of particulate trace metals within was interpreted in terms of a two-component particle mixing algorithm which predicted Cd 314
and Zn concentrations over a wide range of annual estuarine variability. If the concentration of PSPM and its associated Cd and Zn concentrations are taken into account then values of the potential for desorption of trace metals into the water column can be estimated. Desorption was shown to account for some of the increases observed for dissolved trace metals in mid estuary. However, other sources cannot be discounted and better quanti®cation of benthic inputs from porewaters and anthropogenic inputs is required. Evidence of mid-estuarine maxima in dissolved Zn and Cd concentrations have been observed in other turbid estuarine systems, such as the Severn (Apte et al., 1990), the Loire (Boutier et al., 1993), the Gironde (Kraepiel et al., 1997) and the Gironde, Rhone and the Chinese Yellow River (Elbaz-Poulichet et al., 1987). Thus, PSPM, and its sorption reactions at low salinity, may have a generic role within turbid estuarine systems and more should be done to understand the mechanisms involved in its generation and promulgation. This study was funded by the Natural Environment Research Council Land±Ocean Interaction Study (Special Topic Number GST/02/750). The authors wish to thank the members of the LOIS community, particularly Dr. E. Tipping and his colleagues from the Institute of Freshwater Ecology, and the crew of Sea Vigil for their assistance in this work. This is LOIS Publication Number 626. Ackroyd, D. R., Bale, A. J., Howland, R. J. M., Knox, S., Millward, G. E. and Morris, A. W. (1986) Distributions and behaviour of dissolved Cu, Zn and Mn in the Tamar Estuary. Estuarine and Coastal Shelf Science 23, 621±640. Apte, S. C., Gardner, M. J., Gunn, A. M., Ravenscroft, J. E. and Vale, J. (1990) Trace metals in the Severn Estuary: a reappraisal. Marine Pollution Bulletin 21, 393±396.
Volume 37/Numbers 3±7/March±July 1998 Bale, A. J., Barret, C. D., West, J. R. and Oduyemi, K. O. K. (1989) Use of in situ laser diraction particle sizing for particle transport studies in estuaries. In: Developments in Estuarine and Coastal Study Techniques eds J. McManus and M. Elliot, Olsen and Olsen, Fredensborg. pp. 133±138. Boutier, B., Chioleau, J. F., Auger, D. and Truquet, I. (1993) In¯uence of the Loire River on dissolved lead and cadmium concentrations in coastal waters of Brittany. Estuarine and Coastal Shelf Science, 36, 133±145. Coey, M. J. (1994) The Behaviour of Trace Metals in the Humber Estuary, U.K. Ph.D Thesis, University of East Anglia, p. 154. Coey, M. J. and Jickells, T. D. (1995) Ion chromatographyinductively coupled plasma-atomic emission spectrometry (ICICP-AES) as a method for determining trace metals in estuarine water. Estuarine and Coastal Shelf Science 40, 379±386. Comans, R. N. and van Dijk, C. P. J. (1988) Role of complexation processes in cadmium mobilisation during estuarine mixing. Nature 336, 151±154. Comber, S. D. W., Gunn, A. M. and Whalley, C. (1995) Comparison of the partitioning of trace metals in the Humber and Mersey estuaries. Marine Pollution Bulletin 30, 851±860. Elbaz-Poulichet, F., Martin, J. M., Huang, W. W. and Zhu, J. X. (1987) Dissolved Cd behaviour in some selected French and Chinese estuaries. Consequences on Cd supply to the ocean. Marine Chemistry 22, 125±136. Ferreira, J. R, Lawlor, A. J., Bates, J. M., Clarke, K. J. and Tipping, E. (1998) Chemistry of riverine and estuarine suspended particles from the Ouse-Trent system. Colloid Surface, 120, 183±198. Grant, A. and Middleton, R. (1990) An assessment of metal contamination of sediments in the Humber Estuary, UK. Estuarine Coastal Shelf Science 31, 71±85. Grieve, D. A. and Fletcher, W. K. (1977) Interactions between zinc and suspended sediment in the Frazer River Estuary, British Columbia. Estuarine and Coastal Marine Science 5, 415±419. Hegeman, W. J. M., van der Weijden, C. H. and Zwolsman, J. J. G. (1992) Sorption of zinc on suspended particles along a salinity gradient: A laboratory study using illite and suspended matter from the river Rhine. Netherlands Journal of Sea Research 28, 285±292. Kraepiel, A. M. L., Chioleau, J. -F., Martin, J. -M. and Morel, F. M. M. (1997) Geochemistry of trace metals in the Gironde estuary. Geochim. Cosmochim. Acta 61, 1421±1436. Li, Y. -H., Burkhardt, L. and Teraoka, H. (1984) Desorption and coagulation of trace elements during estuarine mixing. Geochim. Cosmochim. Acta 48, 1879±1884. McCave, I. N. (1987) Fine sediment sources and sinks around the East Anglian coast. Journal of the Geological Society (London) 144, 149± 152.
Millward, G. E. and Moore, R. M. (1982) The adsorption of Cu, Mn and Zn by iron oxyhydroxides in model estuarine systems. Water Research 16, 981±985. Millward, G. E., Turner, A., Glasson, D. R. and Glegg, G. A. (1990) Intra-and inter-estuarine variability of particle microstructure. Science of Total Environment 97/98, 289±300. Millward, G. E. and Turner, A. (1995) Trace Metals in Estuaries. In: Trace Elements in Natural Waters, eds B. Salbu and E. Steinnes, pp. 223±245. CRC press, London. Millward, G. E. and Glegg, G. A. (1997) Fluxes and retention of trace metals in the Humber Estuary, Estuarine and Coastal Shelf Science 44, 97±105. Morris, A. W., Mantoura, R. F. C., Bale, A. J. and Howland, R. J. M. (1978) Very low salinity regions of estuaries: important sites for chemical and biological reactions. Nature 274, 678±680. Morris, A. W., Bale, A. J., Howland, R. J. M., Millward, G. E., Ackroyd, D. R., Loring, D. H. and Rantala, R. T. T. (1986) Sediment mobility and its contribution to trace metal cycling and retention in a macrotidal estuary. Water Science and Technology 18, 111±119. Neal, C., Robson, A. J., Jerey, H. A., Harrow, M. L., Neal, M., Smith, C. J. and Jarvie, H. P. (1997) Trace element interrelationships for the Humber rivers: inferences for hydrological and chemical controls. Science of Total Environment 194/195, 321± 343. Ng, B., Turner, A., Tyler, A. O., Falconer, R. A. and Millward, G. E. (1996) Modelling contaminant geochemistry in estuaries. Water Research 30, 63±74. Paalman, M. A. A., van der Weijden, C. H. and Loch, J. P. G. (1994) Sorption of cadmium on suspended matter under estuarine competition and complexation with major sea-water ions. Water, Air and Soil Pollution 73, 49±60. Turner, A., Millward, G. E. and Tyler, A. O. (1994) The distribution and chemical composition of particles in a macrotidal estuary. Estuarine and Coastal Shelf Science 38, 1±17. Turner, A. and Tyler, A. O. (1997) Modelling adsorption and desorption processes in estuaries. In: Biogeochemistry of Intertidal Sediments eds T. D. Jickells and J. E. Rae, pp. 42±58. Cambridge University Press, Cambridge. Uncles, R. J., Stephens, J. A. and Plummer, D. H. (1998) Sediment characteristics and transport in the Humber-Ouse Estuary, UK. In: Physics of Estuaries and Coastal Seas, ed J. Dronkers, RIKZ, The Netherlands, In press. Yeats, P. A. and Loring, D. H. (1991) Dissolved and particulate metal distributions in the St. Lawrence Estuary. Canadian Journal of Earth Science 28, 729±742.
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Marine Pollution Bulletin Vol. 37, Nos. 3±7, pp. 306±315, 1998 Ó 1999 Elsevier Science Ltd. All rights reserved Printed in Great Britain S0025-326X(99)00054-5 0025-326X/99 $ ± see front matter
Dynamics of Particulate Trace Metals in the Tidal Reaches of the Ouse and Trent, UK M. R. WILLIAMS* and G. E. MILLWARD Department of Environmental Science, University of Plymouth, Drake Circus, Plymouth, Devon, PL4 8AA UK
Samples of suspended particulate matter (SPM) have been collected in the tidal reaches of the Ouse and Trent over seven surveys covering the annual cycle of riverine conditions. Settling experiments were carried out on water samples to dierentiate permanently suspended particulate material (PSPM) from temporarily suspended particulate material (TSPM). The results indicated that PSPM, additional to the riverine component, was created within the turbidity maximum zone (TMZ) during tidal resuspension. The PSPM varied between 10% and 50% of the total SPM and could be transported quasi-conservatively down-estuary. The concentrations of particulate Cd, Fe, and Zn (available to 1M HCl) were determined in PSPM and TSPM, as well as samples of the bulk SPM. The trace metal concentrations in PSPM were seasonally variable and generally higher than in TSPM, which had relatively constant concentrations. The trace metal concentrations in PSPM and TSPM were used to develop a two-component, particle mixing model, which was applied to the interpretation of the bulk SPM concentrations of Cd and Zn. It is postulated that as the PSPM encounters the salinity gradient labile Cd and Zn desorb from the particles. On the basis of these results an additional mechanism for generating estuarine maxima in dissolved trace metals has been proposed. Ó 1999 Elsevier Science Ltd. All rights reserved Keywords: trace metals; particles; turbidity maximum; rivers; estuary; Ouse; Trent.
Introduction Processes controlling estuarine trace metal geochemistry at low salinities are complex, involving water and sediment hydrodynamics, particle-water interactions and biological mediation (Morris et al., 1978; Millward and Turner, 1995). Hydrodynamic processes in macrotidal *Corresponding author. Present address. Dunstanage Marine Laboratory, P.O. Box 3, Oban, Argyll, Scotland PA34 4AD, UK.
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estuaries give rise to turbidity maxima, where physical processes combine to generate a wide range of particle concentrations and types, upon which changes in trace metal concentrations are superimposed, as a result of sorption reactions and re-partitioning (Yeats and Loring, 1991). In situ laser particle sizing measurements in the Tamar Estuary (Bale et al., 1989) have shown that mean particle diameters decrease in the vicinity of the TMZ, which arise as a result of particle selection processes during resuspension, combined with the disaggregation of loosely bound ¯ocs by tidal shear. As a consequence, particles within the TMZ are potentially composed of two types (a) TSPM (temporary suspended particulate matter), with higher settling velocities, which could undergo limited axial transport and (b) PSPM (permanently suspended particulate matter), with relatively low settling velocities, which could have signi®cant down-estuary transport. Subsequently, the PSPM ¯occulates, settles to the sediments and becomes involved in further internal estuarine cycles (Morris et al., 1986). However, as the PSPM passes the estuarine interphase, labile particulate trace metals undergo salinityinduced desorption (Hegeman et al., 1992; Paalman et al., 1994), thereby contributing to estuarine maxima in dissolved trace metals, for example in the Tamar Estuary (Ackroyd et al., 1986), the St Lawrence (Yeats and Loring, 1991) and the Gironde (Kraepiel et al., 1997). Since the dissolved and particulate phases are transported in entirely dierent ways, an understanding of physico-chemical processes within the TMZ is vital to the accurate quanti®cation of trace metal ¯uxes from the land to the oceans. In the Humber Estuary, SPM concentrations in the freshwaters of the Ouse and Trent are in the range 5±200 mg lÿ1 , whereas SPM concentrations within the salinity range 0.5±10, are of the order 1±15 g lÿ1 (Ferreira et al., 1998) and 75±85% of the SPM have particle sizes <16 lm (Uncles et al., 1998). There only a few axial transects of reliable concentrations of dissolved trace metals for the Humber and some show mid-estuarine peaks for Zn, with apparent sources contributing from 10±20 lg lÿ1 (Coey, 1994; Coey and Jickells, 1995; Comber et al.,
Volume 37/Numbers 3±7/March±July 1998 TABLE 1 Timings of surveys of the Ouse and Trent. Date October 1994 July 1995 February 1996 April 1996 July 1996 February 1997 July 1997
Season Autumn Summer Winter Spring Summer Winter Summer
1995). For Cd, the additional contribution is between 0.2 and 0.4 lg lÿ1 (Comber et al., 1995; Ng et al., 1996). Thus, this work was designed to investigate what role particle selection processes in the TMZ may play in controlling trace metal transport in the tidal reaches of a highly turbid system. The study formed part of the Natural Environment Research Council, Land-Ocean Interaction Study (NERC-LOIS).
Methods Axial surveys were performed in the low salinity region of the Humber Estuary from October 1994 through July 1997 (Table 1) and covered the seasonal cycle. Riverine samples were taken near the tidal limits of the Ouse and Trent and at sites of higher salinity as determined by the conductivity (Fig. 1). Transmissometer measurements during surveys 1±3 indicated that the maximum turbidities were located at salinities <5. The samples were collected in acid-washed carboys by immersing them just below the surface of the water and rinsing several times prior to sample collection.
The suspended particles were separated into two components, PSPM and TSPM, based on their settling characteristics. Samples were homogenised by gentle agitation of the carboy and immediately a sub-sample of 1 l was transferred into an acid-cleaned measuring cylinder, in which the particles were allowed to settle under gravity for 3 h. This time was selected because it is representative of the water residence time within this region of the estuary and assuming Stokesian settling the particle diameters that would comprise TSPM are estimated to be >15 lm. The majority of the suspension in the measuring cylinder was carefully decanted, giving a fraction containing only PSPM. The settled material contained the TSPM fraction, together with PSPM. Knowing the concentration of PSPM in the decanted sample, it was possible to correct the TSPM concentration for the presence of PSPM. The two fractions were ®ltered through acidcleaned 47 mm cellulose acetate ®lters (Sartorius). The ®lters were stored at ÿ18°C until required. The ®lters were thawed at room temperature and dried inside a laminar ¯ow cabinet. The ®lters were weighed and digested for 24 h using 25 ml 1M HCl (BDH, Aristar). The digested solution was ®ltered using a cellulose acetate ®lter (Sartorius, 0.45 lm poresize, 47 mm diameter) and the solution stored in high density polyethylene bottles prior to analysis. The trace metals were determined using a microcup attached to a Varian SpectrAA in ¯ame mode for Fe, Mn, Zn using linear calibration, and using a graphite furnace atomic absorption spectrometer (Perkin Elmer SIMAA 6000) optimised for Cd and calibrated using method of standard additions. The acceptance criteria for the determination of trace metal concentrations were (a) a relative standard deviation for replicate peaks of <10%
Fig. 1 Schematic diagram showing in the sampling region. The diagram indicates the three main sampling sites within the rivers Ouse and Trent (d).
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and (b) a correlation coecient for the calibration of >0.995. The detection limits were taken as being three times the standard deviation of the blank values. Quality Assurance was achieved by co-digesting a sediment of known metal concentration (RSD (n 9): Fe, 8.5%; Mn, 9.1%; Zn, 9.3%; Cd, 10.9%).
Results and Discussion Results of settling experiments The estuarine conditions for the February 1997 collections of SPM in the Ouse and the Trent are shown in the top diagram of Figs. 2 and 3, respectively. The river¯ow in the Trent was 150 m3 sÿ1 and the tidal range was 6 m. The conditions in both rivers were generally governed by turbidities in the range 10±2000 mg lÿ1 and salinities were <1. The SPM concentrations in the Ouse during February 1997 varied little in the freshwaters encompassing Naburn and Kel®eld (Fig. 2). The settling experiments show that, although TSPM had the higher concentration, the concentrations of PSPM and TSPM were approximately constant and, therefore, it is concluded that both fractions were dominated by riverine particles. However, as the turbidity rose towards Boothferry the concentrations of both fractions increased but the increase was greater for TSPM. In the Ouse (Fig. 2), a riverine PSPM concentration of about 20±25 mg lÿ1 was augmented by a further 50 mg lÿ1 due to resuspension processes at Boothferry. Similar observations were made for the Trent (Fig. 3), although there is only one riverine sample at Cromwell. Nevertheless, the increased turbidities recorded at Gainsborough and Keadby gave enhanced concentrations of both PSPM and TSPM, compared to those at Cromwell. The conclusion from these observations is that resuspension processes in the TMZ generate PSPM, additional to that originating purely from the rivers. Inspection of the data from other surveys shows that the enhancement of the PSPM concentrations occurred at other times of the year. The variation in the percentage of PSPM generated in the TMZ was dependent on the SPM concentration and there was generally more TSPM than PSPM except at turbidities of less than 20 mg lÿ1 . Generally, the settling experiments show a two- to fourfold increase in concentrations of PSPM at higher turbidities, compared to the riverine concentrations. Measurements of in situ laser particle size within the TMZ during July 1995 showed that mean particle diameters were 131 lm in freshwater at a turbidity of 13 g lÿ1 and they increased to values >230 lm at turbidities of 0.5 g lÿ1 and salinity 5 (D. Law, personal communication). The in situ particle diameters were relatively large compared to mean values of 8.6 1.5 lm obtained on SPM samples following disaggregation by ultrasoni®cation in the laboratory. Thus, the lower mean particle diameters at the highest turbidities, and the low mean particle diameters obtained after ultrasoni®cation, support the contention that disaggregation 308
of resuspending particles, by tidal shear, could be mainly responsible for producing the additional PSPM within the TMZ. Particles which comprise PSPM have slow settling velocities and are more likely to be transported down-estuary, quasi-conservatively, with the residual ¯ow of the water. Furthermore, the additional PSPM may also be implicated in trace metal sorption processes, thereby providing an additional transport mechanism from the river into the low salinity zone. Particulate trace metal concentrations in PSPM and TSPM The range of and trends in particulate Fe, Cd and Zn concentrations for three sites the Ouse and Trent are exempli®ed in Figs. 2 and 3, respectively. Comparison of the relative concentrations of trace metals in PSPM (lefthand diagram) and TSPM (right-hand diagram) can be made in Figs. 2 and 3. Iron The concentrations of Fe in PSPM and TSPM (Figs. 2 and 3; Table 2) were approximately constant throughout the tidal reaches of both the Ouse and Trent. These examples are supported by data in Fig. 4a, which has the Fe concentrations for all the Ouse settling experiments. There are no signi®cant dierences in Fe concentrations between PSPM and TSPM, each having a relatively low coecient of variation (means and standard deviations given in Table 2). The Fe concentrations in bulk SPM (Fig. 4b) are invariant above 50 mg lÿ1 and below this value there are only few data points with lower Fe concentrations. The results of the settling experiments for the Trent samples (Fig. 4c), indicate that the TSPM has a lower Fe concentration compared to the PSPM and the concentrations of Fe for both particle fractions are more variable (means and standard deviations in Table 2). This suggests that the TSPM in the Trent originates from a dierent source compared to the PSPM, a hypothesis also supported by the fact that TSPM from the Trent has a lower Fe concentration than that from the Ouse. The bulk Fe concentrations (Fig. 4d) show greater variability, probably associated with intermittency in the sources of Fe. The uniformity of particulate Fe concentrations could be a consequence of inputs from the mineralised catchments, although Neal et al. (1997) reported no signi®cant source of Fe from historic mining activities in the hinterland. More likely Fe sources are the acid-iron waste discharges near the mouth of the Humber, the magnitudes of which dwarf any of the natural inputs of Fe (Millward and Glegg, 1997). Since Fe is a particle-reactive element, the acid-iron wastes could ultimately precipitate as Fe(III) coatings on SPM and/or as Fe(III) ¯ocs. A large fraction of the SPM in the estuary is mainly clay particles (Ferreira et al., 1998), which could have originated from the coastal erosion of boulder clays, 70% of which have grain sizes <63 lm (McCave, 1987). Thus, a major source of clay particles to the Humber
Volume 37/Numbers 3±7/March±July 1998
Fig. 2 Salinity (. . .. . .) and SPM concentrations (AA) in the tidal reaches of the river Ouse during February 1997. Variation of PSPM (left-hand column) and TSPM (right-hand column) concentrations and associated Fe, Cd and Zn concentrations.
Estuary and the anthropogenic Fe inputs are in close geographical proximity. It is, therefore, reasonable to suggest that the surfaces of the uncontaminated boulder clays are modi®ed by coatings of anthropogenic Fe. Tidal dynamics pump the Fe-coated particles up-estuary to the limit of salt intrusion, as sediment provenance studies have con®rmed (Grant and Middleton, 1990). However, does the tidal pumping mechanism disperse particles from down-estuary equally between the Ouse and the Trent? Since the Ouse has (a) a higher concentration of Fe in TSPM and (b) relatively constant concentration of PSPM and TSPM it is possible that there is
a preferred hydrodynamic pathway at Trent Falls. At the con¯uence of the rivers the water transport is constrained by estuarine geomorphology because the Humber and Trent are at 90° to one another, whereas the Humber and Ouse form a straight channel (Fig. 1). Another contributory factor is the generally higher freshwater ¯ows originating from the Trent, which are likely to push the salt intrusion down-estuary. Thus, Fe in TSPM from the Trent may have originated preferentially from its catchment rather than from industrial sources down-estuary, whereas for the Ouse the higher concentrations of Fe in TSPM implicate the industrial 309
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Fig. 3 Salinity (. . .. . .) and SPM concentrations (AA) in the tidal reaches of the river Trent during February 1997. Variation of PSPM (left-hand column) and TSPM (right-hand column) and associated Fe, Cd and Zn concentrations.
sources down-estuary. PSPM is assumed to behave quasi-conservatively and, therefore, its transport is less in¯uenced by the geomorphological and hydrodynamical factors constraining the movement of TSPM. Consequently, PSPM had similar Fe concentrations in both rivers and the Fe was mainly of anthropogenic origin. Cadmium and Zinc The changes in particulate Cd and Zn concentrations in the Ouse are shown in Fig. 2. The Cd and Zn concentrations, in both PSPM and TSPM, were similar at Naburn and Kel®eld, because the SPM at both sites was predominantly of riverine character. At Boothferry, 310
where salinity 1, the Cd and Zn concentrations, in both fractions, decrease signi®cantly as a result of mixing and dilution by metal-depleted particles resuspended from the bed and from desorption processes as particles encountered the salinity gradient. Similarly, for the Trent, the highest concentrations were in the riverine material at Cromwell followed by decreases in the Cd and Zn concentrations at higher turbidities and salinities (Fig. 3). The concentrations of Cd in the two fractions for the Ouse and Trent are shown in Figs. 5a and c, respectively. Relatively high concentrations of Cd in PSPM were obtained at low turbidity, whereas the concentration of Cd in TSPM is almost constant. For the Ouse the
Volume 37/Numbers 3±7/March±July 1998 TABLE 2 Mean standard deviations of Fe concentrations from the settled fractions shown in Fig. 4. Seasonal extremes of Cd and Zn concentrations in settled fractions used to produce model curves in Fig. 5b,c and Fig. 6b,c, respectively. Metal concentration, lg gÿ1
Fe Ouse Trent Cd Ouse Trent
Zn Ouse Trent
Overall Overall Summer Winter Summer Winter
Summer Winter Summer Winter
TSPM
PSPM
23.6 5.0 14.9 7.6
24.8 6.9 28.6 11.5
0.6 0.7 0.9 0.7
220 230 210 200
5.2 0.9 11 1.1
280 420 480 600
mean concentrations of Cd in PSPM were 4.3 5.1 lg gÿ1 and TSPM 3.3 5.4 lg gÿ1 were similar. However, in the Trent the mean concentration of Cd in PSPM (9.7 17 lg gÿ1 ) was generally higher than Cd in TSPM (2.6 5.6 lg gÿ1 ). The comparison of the mean con-
SPM concentration, mg lÿ1 Bulk
PSPM
180 380 140 1700
34 120 18 130
180 380 140 1700
34 120 18 130
Max. Desorbable Metal on PSPM, lg lÿ1
0.2 0.1 0.2 0.1
9 50 9 78
centrations is not straightforward because of the large spread of values. Similarly, the PSPM-associated Zn (390 240 lg gÿ1 ) in the Ouse (Fig. 6a) had similar concentrations to the TSPM (410 350 lg gÿ1 ). However, in the Trent, the concentration of Zn was higher in
Fig. 4 Particulate Fe concentrations derived from settling experiments (s P-SPM; d T-SPM) for (a) river Ouse and (c) river Trent as a function of SPM concentration. Indicated in (b) and (d) are the Fe concentrations in bulk suspended particles as a function of SPM concentration. Survey symbols ± crossed box, October 1994; star, July 1995; diamond, February 1996; square, April 1996; inverted triangle, July 1996; circle, February 1997; triangle, July 1997.
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Fig. 5 Cd concentrations derived from settling experiments (s PSPM; d T-SPM) for (a) river Ouse and (c) river Trent. Indicated in (b) and (d) are the Cd concentrations in bulk suspended particles as a function of SPM concentration. The lines are the model predictions from the particle mixing model The full line is for winter conditions and the dashed line for summer. Survey symbols- crossed box, October 1994; star, July 1995; diamond, February 1996; square, April 1996; inverted triangle, July 1996; circle, February 1997; triangle, July 1997.
PSPM (590 410 lg gÿ1 ) (Fig. 6c) compared to Zn in TSPM (210 83 lg gÿ1 ). The reason for this is not clear, although particulate Zn may be adsorbed onto Fe oxides (Millward and Moore, 1982), associated with the elevated Fe concentrations in PSPM (Fig. 4c). Particle mixing model The particulate Cd and Zn concentrations suggests a variable trace metal concentration of PSPM mixing with an invariant trace metal concentration of TSPM typical of the estuary. Assuming that the concentrations of particulate trace metals in the bulk SPM are only a consequence of the mixing of PSPM with TSPM, the data has been ®tted using a two-component particle mixing model (Eqs. (1) and (2)) (Turner et al, 1994) PS
M
PS
SPM-SPM M TSSPM M PS ; SPM
1
where SPM SPMPS SPMTS :
2
M is the particulate metal concentration; SPM is the suspended particulate matter concentration; SPMPS is the permanently suspended particulate matter concen312
tration; SPMTS is the temporary suspended particulate matter concentration. The end-member concentrations for the trace metal concentrations (i.e. MPS and MTS ), during summer and winter, were taken from the results of the settling experiments (Table 2). However, the Cd concentrations of the bulk samples for the Ouse (Fig 5b) and Trent (Fig. 5d) show that the application of the model ®ts the bulk trace metal data reasonably well. The bold line has been estimated using end-member data from winter surveys, while the dashed line utilises summer end-member concentrations. Thus, the model prediction covers a signi®cant part of the seasonal change in particulate concentrations of Cd. Similarly, for Zn (Figs. 6b and d) show that the end-member concentrations in Table 2 ®t the data for Zn over a seasonal cycle. Although the model assumes no particle±water interactions, sorption processes will aect the observed particulate trace metal concentrations and may account for some of the deviations shown in Figs. 5 and 6. Trace metal desorption from seaward ¯uxing PSPM Desorption of Cd is caused by a combination of its complexation with chloride anions in seawater and competition from Ca2 and Mg2 for active surface sites
Volume 37/Numbers 3±7/March±July 1998
Fig. 6 Zn concentrations derived from settling experiments (s PSPM; d T-SPM) for (a) river Ouse and (c) river Trent. Indicated in (b) and (d) are the Zn concentrations in bulk suspended particles as a function of SPM concentration. The lines are the model predictions from the particle mixing model The full line is for winter conditions and the dashed line for summer. Survey symbols ± crossed box, October 1994; star, July 1995; diamond, February 1996; square, April 1996; inverted triangle, July 1996; circle, February 1997; triangle, July 1997.
(Comans and van Dijk, 1988; Paalman et al., 1994; Kraepiel et al., 1997). Similar particle±water exchange processes may also apply to Zn but to a lesser extent (Grieve and Fletcher, 1997; Hegeman et al., 1992; Kraepiel et al., 1997). The 1M HCl digest used in this study probably overestimates the concentrations of easily exchangeable trace metals on the particle surfaces. Therefore, this digest gives an upper limit to the amount of desorbable metal that could be contributed to the dissolved phase. The trace metals added to the standing concentrations of dissolved Cd and Zn are in the range 0.1±0.2 and 9±78 lg lÿ1 (Table 2), respectively and are of a magnitude which could explain the mid-estuarine maxima in the Humber observed for Cd (Comber et al., 1995; Ng et al., 1996) and Zn (Coey, 1994; Coey and Jickells, 1995; Comber et al., 1995). An alternative approach to assessing the amounts of trace metal desorbed involves the use of a mass balance of labile metals on suspended particles encountering the salinity gradient (Li et al., 1984), which is given by Eq. (3). The partition coecient, KD , is known to be salinity dependent and is given by Eq. (4) (Turner and Tyler, 1997) C SPM KD0 1 ; CR SPM KDS 1
3
where KDS KD0 S 1ÿb :
4
C is the dissolved trace metal concentration in estuarine water of salinity S, CR is the dissolved trace metal concentration in river water; SPM is the concentration of suspended particulate matter; KD0 is the partition coecient in river water; KDS is the partition coecient in estuarine water of salinity S and b is the slope factor. Radiochemical experiments, involving the adsorption of radioactive 65 Zn onto Humber Estuary SPM, as a function of salinity, gave values of KD0 5; 300 l kgÿ1 and b 0.95. Using Eqs. (3) and (4) in combination the extent of desorption from varying amounts of seaward ¯uxing PSPM can be estimated. Fig. 7 shows the concentration of dissolved Zn, relative to its river water concentration, as a function of salinity. Thus, if 10 mg lÿ1 of PSPM crosses the salinity gradient dissolved Zn concentrations increase by about 5% over those in river water. For 50 mg lÿ1 of PSPM the dissolved metal increases and reaches a constant value after a salinity of about 10, adding about another 25% of dissolved Zn to the riverine concentration. If 100 mg lÿ1 of PSPM passes the estuarine interphase the increase is proportionally more, although a plateau in dissolved Zn concentration 313
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Fig. 7 The ratio of the concentration of dissolved Zn in estuarine water to the dissolved Zn concentration in river water (C/CR) as a function of salinity.
is not reached by a salinity of 20. Thus, desorption of Zn from PSPM could make a signi®cant contribution to development of mid-estuarine maxima in dissolved Zn.
Conclusions The turbidity maximum zone of an estuary is a dynamic and complex region in which particle selection processes and disaggregation by tidal shear modify the population of particles in suspension (Millward et al., 1990). Sampling campaigns in the rivers Ouse and Trent, UK, in which the particle populations have been separated by settling and have been used to evaluate estuarine chemical processes within the TMZ. Iron concentrations in PSPM and TSPM are dominated by Fe waste inputs in the lower estuary, except for TSPM in the Trent, which had its origins in the catchment. The distribution of particulate Fe in the Ouse and Trent strongly suggests that the Ouse is the preferred pathway for the up-estuary transport of SPM, which has implications for the fate of particle-associated contaminants from down estuary. The behaviour of particulate trace metals in the low salinity region of the Humber Estuary is a function of the mixing of spatially and temporally invariant trace metal concentrations of TSPM with seasonally variable trace metal concentrations of PSPM. The results suggest that processes within the TMZ dominate the particulate trace metal concentrations found down estuary, rather than seasonal variability in the trace metal concentrations of riverine SPM. Processes aecting the concentrations of particulate trace metals within was interpreted in terms of a two-component particle mixing algorithm which predicted Cd 314
and Zn concentrations over a wide range of annual estuarine variability. If the concentration of PSPM and its associated Cd and Zn concentrations are taken into account then values of the potential for desorption of trace metals into the water column can be estimated. Desorption was shown to account for some of the increases observed for dissolved trace metals in mid estuary. However, other sources cannot be discounted and better quanti®cation of benthic inputs from porewaters and anthropogenic inputs is required. Evidence of mid-estuarine maxima in dissolved Zn and Cd concentrations have been observed in other turbid estuarine systems, such as the Severn (Apte et al., 1990), the Loire (Boutier et al., 1993), the Gironde (Kraepiel et al., 1997) and the Gironde, Rhone and the Chinese Yellow River (Elbaz-Poulichet et al., 1987). Thus, PSPM, and its sorption reactions at low salinity, may have a generic role within turbid estuarine systems and more should be done to understand the mechanisms involved in its generation and promulgation. This study was funded by the Natural Environment Research Council Land±Ocean Interaction Study (Special Topic Number GST/02/750). The authors wish to thank the members of the LOIS community, particularly Dr. E. Tipping and his colleagues from the Institute of Freshwater Ecology, and the crew of Sea Vigil for their assistance in this work. This is LOIS Publication Number 626. Ackroyd, D. R., Bale, A. J., Howland, R. J. M., Knox, S., Millward, G. E. and Morris, A. W. (1986) Distributions and behaviour of dissolved Cu, Zn and Mn in the Tamar Estuary. Estuarine and Coastal Shelf Science 23, 621±640. Apte, S. C., Gardner, M. J., Gunn, A. M., Ravenscroft, J. E. and Vale, J. (1990) Trace metals in the Severn Estuary: a reappraisal. Marine Pollution Bulletin 21, 393±396.
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