Behaviour of manganese in the Rhine and Scheldt estuaries

Estuarine and Coastal 2~larlne Science 0979) 9, 727--738

Behaviour of Manganese in the Rhine and Scheldt Estuaries H. Geochemical Cycling

J. C. Dulnker, R. Wollast and G. BiUen Nederlands Instltuut voor Onderzoek der Zee, P O B 59, den Burg, Texel, The Netherlands Laboratoire d'OcLanographle, Unlversltd Libre de Bruxelles, Av. F. D. Roosevelt, 5o; Io5o Brussels, Belgium Received 25 2~Iay x978 and in revised form ~8 August x978

Keywords: manganese compounds; geochemical cycle; estuaries; pH; salinity; North Sea coast Measurements of dissolved and particulate suspended ]kin concentrations throughout the Rhine and Scheldt estuaries a r e interpreted in terms of cycling processes of Mn. Dissolved Mn is removed in the lower estuary into particulate form. "/'his gives rise to elevated ]kin concentrations in coastal suspended matter; particulate ]k//n is also partially returned to the upper estuary by estuarine circulation processes. Dissolved IX,in is produced in the upper estuary by dissolution of particulate ]kin either in the water column owing to the low pH and Eh prevailing at low salinities or in the anoxic sediments, from where it subsequently diffuses into the overlying water. Budget calculations and other arguments show that a significant part of the Mn carried down by the river is recycled between the lower and upper estuaries. An important part accumulates within the sediments; the fraction that escapes to the marine environment is mainly in particulate form. Very similar conclusions can be drawn for the two estuaries with quite different residence times of both water and particulates.

Introduction Manganese, as well as iron, is a key element in geochemical processes since the various oxide and hydroxide species of these elements act as scavengers for a series of trace elements (Goldberg, I954; Krauskopf, x956 ). The strong gradients of chemical parameters occurring in estuaries may induce transitions between dissolved and particulate forms of these two metals. Thus, dissolved iron is removed from solution in the early stages of estuarine mixing (Coonley et al., x97I ; Boyle et al., x974; Aston & Chester, x973; Duinker & Nolting, x976; Holliday & Liss, x976; Sholkovitz, 1976 ). Less agreement between different estuaries is found for manganese. Windom et aL (x97t) indicated removal to some extent in the Satilla estuary; conservative behaviour was reported for the Beaulicu estuary with possible desorption at low salinities (Holliday & Liss, x976 ). Removal from solution was indicated for the lower Rhine estuary (Duinker & Nolting, x976 ). Desorption may occur at low salinities in the tidal rivers that enter into Narragansett Bay but manganese in the Bay is predominantly particulate, probably due to the rapid manganese oxidation at Bay water pH (Graham et al., i97.6). 727

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3 t. C. Duh,ker, R. Wollast ~ G. Billen

In a previous paper ~Vollast, Billen & Duinker, x979) primarily based on thermodynamical considerations, we have shown that the physico-chemical behaviour of manganese in estuaries is essentially controlled by the variations of pH, Eh (or ox3'gen concentration), ionic strength and total dissolved carbonate content. In the present paper we shall combine this approach with the available information on the hydrodynamics and sediment circulation in the Rhine/ Meuse ~ and Scheldt estuaries in order to describe the geochemical cycling of manganese in these two environments. This work is a combination of the results of independent studies on manganese, performed for several years, in the Rhine and Scheldt estuaries by the two presently reporting laboratories. Biotopes a n d m e t h o d s Estuaries

The Rhine and Scheldt rivers transport 60 and 3"5 km3 water and 4X x 9 e and x.2• to e ton suspended material respectively into the estuary per year. The mean residence time of water in the Rhine estuary from Rotterdam to Hook of Holland is in the order of one to two days; for the Scheldt estuary it is roughly two months for the area where the mLxing of freshwater and seawater occurs (80 km long). An area with zero dissolved oxygen concentration is nearly always present in the Scheldt estuary, except under high fresh water supply conditions. Dissolved oxygen levels are restored toward the mouth of the estuary: a minimum of 4 mg 1-x 02 is found at salinity values between 4 and X6~ooS, the higher salinity values occurring ia the summer period. Dissolved oxygen levels in the Rhine estuary are usually not below 40% of saturation. The Seheldt estuary is well mixed while strong salinity stratification is found in the Rhine estuary. Sampling stations are given in Figure x . i

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Geochemical cycling of estuarine nzanganese

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Analytical methods Samples were obtained either by polypropylene bucket sampling (surface samples) or with glass bottles according to Postma (I954) , and were filtered immediately through 0.45/am Milllpore membranes. The filtrate was kept deep frozen. The filters were leached with o.I N-HC1 during z8 h (Duinker et at., x974), l~Ianganese concentration was determined by flameless atomic absorption spectrometry in the original sea water matrix. Standard addition techniques were applied to each individual sample. 2000

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Figure z. Concentration of suspended ~in per weight of suspended matter in lag g-I plotted against salinity in samples obtained at stations z-ur (0) and :tz-38 (It---m) in October x973, at stations 39-56 (m . I) in April z972 and at stations 39-54 (I . . . . . I) in April z974. Results Dissolved and particulate ~,in concentrations were measured during several cruises covering a wide range of salinity values within the estuaries (Figures 2, 3, 4, 5, and 6). Additionally, measurements were made at a fixed station in the Rhine estuary (Station 5 in Figure z), characterized by permanent stratification, salinity in the surface layer ranging from the fresh water value (o.5) to 2O~ooS, and salinity in the near bottom layer (x m above the bed) ranging from z5 to 3o~oo S (Figures 7 and 8). A positive deviation from the ideal dilution line is observed for manganese at low salinities (Figures 3, 4, 5, 6 and 8); a pronounced maximum is present in the data of several cruises (Figures 3, 4 and 8). A negative deviation may be present at higher salinities. The positive deviations occur in the salinity range where minima in pH are observed (Figures 3 and 4) (see also Wollast et aL, x979). In both estuaries, particulate M n concentrations (pg g-l) increase, or at least do not decrease with salinity up to roughly 3o~ooS, where a' sharp decrease toward higher salinities is observed (Figure z). Similar observations are made for the amounts of particulate ~ n (lag 1-1) versus salinity relationships (Figures 3 and 4)" The effect is more pronounced for the

730

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Scheldt than for the Rhine estuary. T h e data of the fixed station show that large amounts of particulate matter (up to 4oo mg 1-x) are transported up and down the estuary during the tidal cycle, being part of the turbidity cloud. Furthermore, the concentrations (in lttg g - l ) of particulate ~ I n in the near-bottom layer are considerably higher than in the surface layer (Figure 7). T h e dissolved 3In-salinity relation for the measurements at the fixed station (Figure 8) show the presence of a maximum at low salinities and a z o ~ lower concentration in the lower layer than in the surface layer in the salinity range x5-2o~oo S. Other trace dements have been measured with iX/In during the cruises and the tidal station. Unique features of manganese were, the occurrence of: (i) a positive deviation from the ideal dilution line for the dissolved species at low salinities, (ii) a negative deviation at higher

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salinities, (iii) the high particulate concentrations in the lower estuary, and (iv) the higher particulate concentrations inthe near-bottom layer than in the surface layer during the tidal cycle measurements. Discussion Removal of dissolved manganeseand.highparticulate manganeseconcentrations in the lower estuary Particulate Mn concentrations (lag g-a) in the salinity range up to 3o,5t~S are laigher than would be expected on the basis of pure mixing of river-borne and marine derived suspended

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particles (Figures 2, 3, 4 and 5)- This is most likely to be associated with the removal of dissolved Mn in the lower estuary. The correlation between the loss from solution and gain in particulate ~,in is demonstrated for tile Scheldt in Figure 5- It is less evident for the Rhine estuary. The difference in residence time and the degree of exchange of bottom sediment and suspended particles may be mainly responsible for this observation. A similar increase in l-Mn concentration versus salinity up to 3o~ooS was observed in the Newport estuary by Evans et aL (z977). Net removal of dissolved bin appears to occur in the lower part of both estuaries (Figures 3, 4, 5, 6 and 8). It was demonstrated in Part x that the dissolved ]VIn concentrations in the Scheldt estuary could be reproduced reasonablywell by equilibrium model calculations taking into account the data on pH, dissolved O 3 and chlorinity (Wollast et aL, x979). Following this model, the removal can be explained by transitions from Mn(II) to oxidized and particulate species. The salinity range where removal of bin is apparent depends on the season, at least in the Scheldt (Figure 6). This may be related to the lower dissolved oxygen levels in summer, shifting the removal to tiigher salinity values. Similar data are not available for the Rhine estuary. Addition of dissolved manganese in the upper estuary A positive deviation from the ideal dilution line for dissolved manganese is observed for all cruises between o--IO~ooS within the Rhine and Scheldt estuaries (Figures 3, 4, 5, 6 and 8). We are not aware of any large scale input of'manganese into the estuary from tributary streams or industrial activities that might account for this observation. In the presently reported cruises, the freshwater end member concentrations for the Rhine estuary are in the

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range 40-80, 6o--xoo, 30--4~ and 8o-zoo lag 1-1, for the Scheldt estuary z7o-3oo , 370--400, z7o--34o and z7o--3zolag 1-1. The marine end members show much smaller variations: o-z-x.o lag 1-1. The fluctuations in the end member concentrations cannot account for the observed maxima therefore. These can only be interpreted in terms of in situ production ef dissolved Mn. Three possible sources must be considered: (i) release from river-borne suspended particles, (ii) release from marine derived particles once in contact with estuarine water and (iii) diffusion from the interstitial water of the sediments into the overlying water.

(i) Release from river-borne suspended matter In the Scheldt estuary, considerable amounts of particulate manganese are transported by the river into the upper estuary. In the reducing conditions prevailing there as well as in the river, MnCO a must be the dominant form of particulate manganese, in equilibrium with high dissolved manganese concentration. The thermodynamical calculations presented in the first paper of this series show indeed that the solubility of l'VinCOa entirely explains the observed tV[n++ concentration in the Scheldt (WoUast et aI., x979). As long as the reducing conditions are maintained, d{lution of river water in sea water induces dissolution of part of the suspended ~In until saturation is reached again. This sole process could explain the observed positive deviation from the theoretical dissolved manganese-salinity dilution line. In the Rhine on the contrary, the amount of suspended M n transported with the river is relatively much smaller. If all particulate Win transported by the river were to be released upon contact with sea water, the contribution could not be more than 3o-4 o lag 1-1, being the river supply. The relation between suspended M n in the surface layer (lag 1-1) and salinity is practically linear apart froin a small decrease in the range o.5-2~oo S (Figures 3 and 4). If this decrease were caused by mobilization of part of the particulate river-borne particulates, the maximum contribution to the dissolved Mn concentration in that salinity range could be some zo lag 1-1 or essentially zero. This is considerably less than the actual increase. It is more probable that the decrease in the amount of particulate 2VInper litre can be attributed to the transfer of total particulate matter from the surface layer into the more saline layer. In order to check on the possible release of mafiganese from suspended matter upon contact with estuarine water, a series of river and estuarine samples was filtered on board ship in duplicate immediately after sampling. One of the filter contents was shaken with x I filtered

734

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coastal sea water (26~oo S) for i h and filtered again. T h i s filter and the untreated one were analyzed in the laborator?. T h e concentration of M n in the particulate matter after seawater treatment was practically identical to that in the duplicate original suspended matter samples (difference being within -4-Io%). T h i s confirms that short-term iriteraction of river-borne

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particulate ~in with estuarlne water is not likely to explain a significant part of the observed increase in dissolved ~In. In the Columbia estuary, Evans and Cutshall (x973) found that a considerable fraction (30-60%) of river suspended S4Mrt is released upon contact with sea water. The difference with our findings may be interpreted in terms of different chemical forms for more or less natural particulate ~ln and freshly introduced ~aMn, derived from human activities. (ii) Releasefrom marine particles brought into the estuarine region A significant transport of marine panicles occurs in b.oth the Scheldt and Rhine estuaries. As demonstrated above, the manganese content of these particles is considerably higher than those of the upper estuary. In the Rhine, the high amounts of particulate ~In per litre in the near-bottom layer (pH 8"z-8"4) might well be the source of dissolved Mn once in contact

736

.7. C. Duinker, R. Wollast & G. 23illen

with estuarine water with lower pH. The process may be enhanced in water with the minimum pH values such as those detected at low salinities (Figures 3 and 4). The amounts of particulate ?,In in the lower layer of the tidal station varying periodically within the range 3o-45o lag 1- t would be sufficient to account for the obsen'ed increase of dissolved)In in the surface layer with 7o-8o lag I -t. Although the Scheldt estuary is far less stratified, similar processess could occur there, the low redox potential in the upper estuary playing the most important role in the dissolution of the particulate manganese. (iii) Diffusion from the sediments Interstitial water in the anaerobic bottom mud of both estuaries is another possible source. In the Scheldt and Rhine estuaries, interstitially dissolved ?,In concentrations in the reduced layer (usually less than x cm below the sediment-water interface) can be as high as 5-xo mg 1-1. Considerable amounts of bottom derived ?,in may return into the overlying water, especially during periods of high current velocity or during storms (Hartmann, x964; Duinker et aL, I974). However, any contribution from interstitial water to overlying water should pass through the near-bottom layer. The data of the tidal station in the Rhine estuary show that dissolved ?,in concentrations in the near-bottom layer, although quite substantial, are lower than in the surface layer. These observations do not support a dominant role of interstitially dissolved 3/In in contributing to the increase in the surface layer water. It is not improbable however that this process takes place more upstream in the estuary and our sampling scheme may have left this mechanism undetected. In the case of the Scheldt estuary, a comparison of the ?,In/Al ratio in the suspended matter and in the recent sediments indicates that approximately 2o% of the manganese is rcmobilized after deposition. The mean flux of dissolved ?,in from the sediments corresponding to this remobilization should then be about x lag cm -z d -t, in close agreement with direct measurements made by Graham et al. (t976) in Narragansett Bay (24- x lag cm -2 d-t). More experimental data are required throughout the estuaries covering a wider range of sampling stations in surface, subsurface and interstitial water before the importance of the interstitial water phase can be assessed more fully. Cycling of manganese within the estuary The data above strongly suggest that the overall behaviour of M n in the Scheldt and Rhine estuaries is determined by cycling processes; removal from solution at salinities above 1o-I5~oo 8 accounts for the high suspended ~,in concentrations in the lower estuaries and along the Belgian and Dutch coasts. Part of the newly formed particulate l'VIn is transported back into the upper estuary. Once in contact with low pH or low Eh estuarine water or after deposition in sediment and subsequent reduction, ~,lln may return into the dissolved compartment within the water column. This cyclic behaviour of manganese was already suggested for the Rhine estuary by earlier data of Duinker & Nolting 0976). Very similar cyclic behaviour of l~in was reported for the Newport estuary (Evans et at., x977) and for Jcrvis Inlet (Grill, I978 ) . In order to estimate the relative importance of the processes considered above, an attempt was made to establish a mass balance for manganese. Only for the Scheldt sufficient data were available. In a previous work (Peters & Wollast, I976), we have estimated annual mass balances of suspended matter in the Scheldt from intensive measurements of input, transport and accumulation by sedimentation during a" three-year period. Taking into account the physical characteristics of the Scheldt, the estuary was divided into two zones: an upper one from km Ioo to km 55 and a lower one from km 55 to the mouth. From a total load of x52o •

Geochemical cycling of estuarlne manganese

737

xO3 tons/year of suspended matter discharged in the first zone, x z o o • 3 tons/year are deposited in the upper zone and 3 2 0 • Io 3 tons/year are transported into the second one; only x z o • xo a tons/year reach the North Sea. 800)< xo 8 tons/year of sand are transported upstream b y strong bottom currents. T h e mass balances for manganese (Figure 9) were constructed for each zone by considering, both for dissolved and suspended ?,In, the net flow due to river discharge, the longitudinal turbulent dispersion (estimated from the salinity profile), the sedimentation process and the lateral input due to tributaries and sewers. T h e results given in Figure 9 are based on measurements of the longitudinal profiles of dissolved and suspended manganese, and of the manganese content in the sediments during x974 and i975. T h e value used for the lateral input of dissolved 1Vln into the upper zone (3oo tons/year) satisfies the mass balance; it is close to the value based on actual measurements (around z5o tons/year). Similarly, a value for the transfer of dissolved M n from the pore water to the surface water in the lower zone has been evaluated by difference, thus satisfying mass conservation. T h e transfer between dissolved and particulate M n in each zone is obtained by difference between the fluxes of the two species in both zones. I n the upper zone, where anaerobic conditions prevail, reduction of particulate M n contributes to the increase of dissolved Mn. T h e increase of dissolved g i n is mainly due to remobilization of this compound in strongly reducing sediments and to a lower extent (zo%) to the dissolution o f suspended g i n . T h e accumulation of Nln in the sediments constitutes approximately 3 o % of the total input in the upper zone. I n the lower zone, where aerobic conditions are restored, oxidation and precipitation of dissolved g i n is the dominant process. T h e strong gradient of particulate g i n and the high bottom currents near the limit of the two zones transport a fairly large amount of the precipitated g i n upwards and as a result the net flux of particulate ~ i n is directed from the lower to the upper zone. T h e remobilization of M n in the sediments is more restricted in this zone probably due to themore oxidizing conditions as a result of the lower organic matter content there. A mechanism of mobilization and precipitation of this type been suggested recently (Sundby, I977) in order to explain the origin of manganese-rich particulate matter in a coastal marine environment.

References Aston, S. R. & Chester, R. x973 The influence of suspended particles on the precipitation of iron in natural waters. Estuarine and Coastal Marine Science x, ezS-z3I. Boyle, E., Collier, R., Dengler, A. T., Edmond, J. M., Ng, A. C. & Stellard, R. F. I974 On the chemical mass balance in estuaries. Geoehimlea et Cosmochimica Acta 38, I719-x7z8. Coonley, L. S., Baker, E. B. & Holland, H. D. x97I Iron in the Mullica river and Great Bay N.Y. Chemical Geology 7, 51-63. Duinker, J. C., Eek, G. T. M. van & Nolting, R. F. x974 On the behaviour of Copper, Zinc, Iron and Manganese in the Dutch ~Vadden Sea; evidence for mobilization processes. Netherlands Journal of Sea Research 8, 2 t4-239. Duinker, J. C. & Nolting, R. F. x976 Distribution model for particulate trace metals in the Rhine estuary, Southern Bight and Dutch Wadden Sea. Netherlands Journal of Sea Research xo, 7x-Io2. Evans, D. W. & Cutshall, N. H. x973 Effects of ocean water on the soluble suspended distribution of Columbia river radionuclides. In Radioactive Contamination of the marine environment pp. I25x4o. International Atomic Energy Agency, Vienna. Evans, D. W., Cutshall, N. H., Cross, F. A. & Wolfe, D. A. x977 Manganese cycling in the Newport estuary, North Carolina. Estuarine and Coastal Marine Science 5, 7x-8o. Goldberg, E. D. I954 l~,~arine geochemistry x. Chemical scavengers of the sea. Journal of Geology 62, 249-z65. Graham, x,V.F., Bender, M. L. & Klinkhammer, G. P. x976 Manganese in Narragansett Bay. Limnology and Oceanography 2x, 665-673.

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Grill, E. V. 1978 T h e effect of sediment-water exchange on manganese deposition and nodule growth in Jervis Inlet, British Columbia. Geochlmica et Cosmechlmica Acta 42, 485-494. Hartmann, M. 1964 Zur Geochemie yon Mangan und Eisen in der Ostsee. BIes'viana I4, 3-2o. Holliday, L. M. & Liss, P. S. x976 The behaviour of dissolved iron, manganese and zinc in the Beaulieu estuary, S. England. Estuarine and Coastal 2tlarine Science 4, 349-353. Krauskopf, K. B. t956 Factors controlling the concentrations of thirteen rare metals in sea water. Geochlmica et Cosmochimica Acta 9, 1-32B. Peters, J. J. & Wollast, R. x976 Role of the sedimentation in the self purification of the Schcldt estuary. In Proceedings of the third Federal Inter-Agency Sedimentation Conference (Sedimentation Committee of the Water Resource Council, eds). Dent'er x976. Postma, II. i954 ltydrography of the Dutch Wadden Sea. Archit'es n&rlandalses deZoologle xo, x-xo6. Sholkovitz, E. R. x976 Flocculation of dissolved and inorganic matter during the mixing of river water and sea water. Geochlmlca et Cosmochimica Acta 40, 83x-845. Sundhy, B. x977 Manganese-rich particulate matter in a coastal marine environment. Nature 27o ~ 4x7-419. Windom, H. L., Beck, K. C. & Smith, R. x97 Z Transport of trace elements to the Atlantic ocean by three Southeastern rivers. Southeastern Geology x xo9-x x8 x. Wollast, R., Billen, G. & Duinker, J. C. I979 Behaviour of manganese in the Rhine and Scheldt estuaries. Part x. Physico-chemieal behaviour. Estuarine and Coastal 3larlne Science 8, x6I-X69.