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Mercury in estuarine sediments: Spatial distribution and redistribution
Environment International, Vol. 4, pp. 265-267, 1980
0160-4120/80/090265-03502.00/0 Copyright©1981 Pergamon Press Ltd.
Printed in the USA. All rights reserved.
MERCURY IN ESTUARINE SEDIMENTS: SPATIAL DISTRIBUTION AND REDISTRIBUTION
C. L. So Department of Geography and Geology Universityof Hong Kong Pokfulam Road, Hong Kong (Received 17 January 1980; Accepted 13 September 1980)
Mercury distribution in estuarine sediments is strongly influenced by the effect of desorption and adsorption on metal retention and transfer, the reactivity of mercury with particulate matter, and the close association of mercury with the fine suspended particulate fraction of the sedimentary load. The significance of this lies in that resuspension, initiated in diverse circumstances, can be an effective process in mercury remobilization. As one of the processes of mercury transfer in the estuarine environment, resuspension merits close attention, regular monitoring, and proper documentation as a prelude to judicious planning for environmental management. 1973), mercury is considered to be bound to an inorganic iron-phosphate complex probably adsorbed onto a hydrated iron oxide coating on clay particles. Estuarine mercury concentrations in sediments vary spatially. These variations depend on a number of controlling parameters which may include, among others, the type of sedimentary load, grain size, mineral constituents, chemical composition, organic content, salinity of overlying water, and estuarine hydrology. Analysis for mercury from unconsolidated surface sediments in the outer Thames estuary, using a flameless atomic absorption technique, reveals mercury contents of the sediments ranging from 0.012 to 0.550 ppm, with the higher concentrations usually occurring in sediments containing a high proportion of fine particles (Smith, 1973). Concurrently, mercury values in the Mersey estuary measured for 136 samples of the top 5 cm of sediment reveal a mean of 2.23 p p m and a m a x i m u m of 14.30 ppm, the high values occurring in fine silt sediments with a large amount of organic material (Craig, 1975).
Introduction
The postulation that estuarine sediments act as a sink and provide pathways for rapid mercury-to-biota transfer is a matter of increasing concern. When mercury enters an estuary with any type of mixing dynamics, in either dissolved or particulate associated form, it tends to be incorporated into the dynamic equilibrium of the water-sediment system in the estuarine environment. Its subsequent fate in this system must be viewed against the conditions and processes affecting the behaviour of pollutants in general and heavy metals in particular. This study reviews the available data base on mercury in estuarine sediments as a framework for monitoring pollution. Forms and Concentrations
Mercury occurs in sediments in association with several functional groups. Mercury shows a strong affinity for NH4, SH, C O O H , and phenol groups (Cotton, 1962; Gavis, 1972; Jenne, 1970; Keckes, 1970). Because these groups occur naturally (Jackson, 1972; Rashid, 1970; 1971; Schnitzer, 1965), the predominant manner in which mercury is bound in sediment is through association with organic material (Keckes, 1970). In another form, mercury occurs as an insoluble sulphide (Kennedy, 1971) or is adsorbed onto the surfaces of sulphide minerals such as FeS 2 (Thomas, 1972), as indicated by statistical correlations between mercury and total sediment sulphur in lake sediments. Thirdly, from observed relationships between total Hg, Fe, and P (Vernet, 1972; Thomas,
Associated Sedimentary Loads Mercury association with fine grades of materials in the bed load is also found, as in Southampton water, where a significant source of mercury for some organisms may be the bottom deposit (Raymont, 1972). Ranging from 0.188 to 5.680 p p m (dry mud) in value, the total mercury concentration in mud from Southampton water tends to be higher in anoxic mud (depth 10 cm) and farther up the estuary. 265
266 Study of the bed sediments of the Ottawa River reveals the presence of mercury in the sediments and the influence of some environmental factors on its desorption (Kudo, 1975). Thus, the desorption rate of mercury from the bed sediments ranges from 0.100 to 1.00 ng/cmE/day, the rate decreasing with an increase of exposure period to the water, but increasing with an increase in the depth of bed sediments. The amount of mercury desorbed from the bed sediments to the overlying water is highly dependent on the volume of the bed sediments. Calculations based on experiments show the half-lives of total mercury associated with bed sediments from 2.1 to 180 years, depending on the depth of the bed sediments. Mercury concentrations found in cores taken from an estuary associated with fine sediments have been reported to display maximum values in an area where the saltwater influence on the river becomes significant; this is interpreted as a result of mercury adsorption on sedimenting particulate matter (Cranston, 1976). The suspended load of the estuaries, which is composed of mineral and organic material and may include a substantial amount of fine sedimentary particles, is found to adsorb trace elements (Duursma, 1967; de Groot, 1966; Jenne, 1968; Turekian, 1971). Many heavy metals present in the aqueous environment, including mercury in the Rhine (So, 1978), are dominantly associated with suspended particles due to the tendency of coarse-grained particles and aggregates to settle more quickly, while suspended matter comprising very fine grains and colloidal material, may remain in suspension for long periods of time and be transported over great distances. For most heavy metals there is a higher concentration in the suspended matter compared with that in the deposited sediment (de Groot, 1973; Duinker, 1974; Forstner, 1974). The amount of adsorption achieved may play a part in determining the relative amounts of mercury present in dissolved loads and in suspended loads. The low mercury concentration in the Everglades, for example, is considered to arise from the fact that the dissolved organomercury complexes are not efficiently adsorbed onto the suspended matter, thus leaving more mercury in solution (Lindberg, 1975). It is, however, difficult to evaluate how much of the adsorbed mercury is exchangeable from the particulate matter in the estuaries. Because of the reactivity of mercury with particulate matter, the suspended load exerts a strong influence on its aqueous behavior. In the estuarine environment, this is enhanced by the widespread occurrence of sediment particles and other particulate matter in suspension, especially where tides, currents, and waves are active. Moreover, in estuaries, the amount of sediment in suspension is far greater than that in the river upstream and that in the sea (Guilcher, 1958). The downstream increase in the particulate-mercury content in La Have estuary, Nova
C.L. So Scotia, with a suspended load of 3.50 mg/1, but not in the Mississippi, with a suspended load of 250 mg/l, is attributed to decrease in particle size following sedimentation of large particles with lower mercury content as the estuary is approached (Cranston, 1972). Further comparative studies of this nature confirm the significance of the suspended particulate fraction of the sedimentary load. The Mobile Bay estuarine system shows a smaller amount of mercury associated with particulate matter than the Mississippi and differs from it in showing an increase in mercury content as salinity and dis,ance from the point of discharge of the river increase. This, together with concomitant drastic reduction of the suspended load indicating sedimentation of larger particles as water of higher salinity is approached, points to similar increase in mercury concentration with increase in finer suspended particles of higher mercury content. On the basis of these findings, it is not unreasonable to postulate that estuaries where much sedimentation occurs act as reservoirs for considerable mercury-laden particulate matter. Since the dissolved-mercury concentration in estuaries is fairly constant, the total amount of mercury in a water sample is largely governed by the amount of suspended matter, that is, the higher the suspended load, the higher the mercury content of the water sample.
Resuspension and Remobilization The association of mercury content with the fine suspended particulate fraction of the sedimentary load is significant in that resuspension can be an effective process in remobilization and transfer of mercury. Large-scale resuspension of sediment that permits pore water and particulate matter with relatively high mercury concentration to mix with surface waters of low mercury content might lead to release of mercury. This resuspension process may be effected in different ways. Natural mechanisms operate in response to seasonal rainfall regime marked by fluctuations in terrestrial runoff. Rapid increase in the discharge of fresh water into river systems and estuaries has been found to cause much suspension of unconsolidated sediments. An anthropogenic form of resuspension results from dredging, and to some extent dumping, through which engineering projects in rivers, estuaries, or near-shore marine sites may involve physical changes in the quantity, suspension, or deposition of sediment. This resuspension of bottom material by dredging and dumping is enhanced by reagitation of materials during storms and floods. When effluents contain suspended organic-rich sediments, because of suspension of fine particulate matter, the total amount of released organic matter in the water column may increase by many times over the dissolved concentrations.
Mercury in estuarine sediments T h e a s s o c i a t i o n of m e r c u r y with o r g a n i c m a t t e r further turns this into a n i m p o r t a n t m e c h a n i s m of m e r c u r y m o b i l i z a t i o n . A field e x p e r i m e n t d e s i g n e d to investigate the kinetics of m e r c u r y d i s s o l u t i o n in perio d s of intense w a t e r - s e d i m e n t m i x i n g has b e e n c a r r i e d o u t ( L i n d b e r g , 1973) b y s i m u l a t i n g r e s u s p e n s i o n of n e a r - s h o r e m a r s h surface s e d i m e n t s a n d o p e n - b a y , dredge-spoil sediments collected from Mobile Bay and f r o m the E v e r g l a d e s in the G u l f of Mexico. Results of the studies, agreeing with field data, suggest that largescale r e s u s p e n s i o n of e s t u a r i n e s e d i m e n t s b y such processes as d r e d g i n g causes a sizable s h o r t - t e r m release of m e r c u r y i n t o the s u r r o u n d i n g w a t e r f o l l o w e d b y a d e c r e a s e to levels close to those p r e d i c t e d b y ideal d i l u t i o n calculations. In the case of organic-rich, previously u n d r e d g e d , m a r s h sediments, the m a g n i t u d e of the m e r c u r y p e a k a n d the s t e a d y - s t a t e level w o u l d exceed the a m b i e n t dissolved m e r c u r y c o n c e n t r a t i o n of the s u r r o u n d i n g w a t e r b y a larger factor t h a n for the d r e d g e - s p o i l sediments. Thus, s i m u l a t e d d r e d g i n g resuits in a slight increase in t o t a l dissolved m e r c u r y in the overlying w a t e r d u e to m i x i n g with m e r c u r y - e n r i c h e d s e d i m e n t p o r e w a t e r a n d s h o r t - t e r m release of a small f r a c t i o n of m e r c u r y a s s o c i a t e d with p a r t i c u l a t e material.
Discussion and Conclusion O n c e m e r c u r y h a s b e e n d e p o s i t e d in the s e d i m e n t a r y e n v i r o n m e n t of the estuary, its s u b s e q u e n t b e h a v i o u r a n d a n y i n t e r a c t i o n s it m a y u n d e r g o w o u l d d e t e r m i n e its u l t i m a t e fate a n d a n y l o n g - t e r m e n v i r o n m e n t a l imp a c t it m i g h t i m p o s e o n the biota. W h e t h e r the sedim e n t s act as a sink or as a source for m e r c u r y in the e s t u a r i n e e n v i r o n m e n t m a y b e s t be viewed in terms of the stabilities of v a r i o u s s e d i m e n t a n d p o r e - w a t e r m e r c u r y c o m p l e x e s ( L i n d b e r g , 1975). A l t h o u g h retention b y the s e d i m e n t s c a n be a v a l u a b l e buffer b e t w e e n the l a n d a n d the sea, the p o l l u t a n t s in the s e d i m e n t s d o n o t a l w a y s r e m a i n p e r m a n e n t l y l o c k e d - u p . If c o n d i tions change, they m a y be m o b i l i z e d again. Processes like i o n e x c h a n g e are reversible a n d , a l t h o u g h sedim e n t s m a y h a v e a greater affinity for ions of h e a v y m e t a l s t h a n for alkali metals, a n y a b r u p t increase in salinity c o u l d release the h e a v y m e t a l s b a c k into solution. It is p o s t u l a t e d t h a t p o s t d e p o s i t i o n a l m i g r a t i o n occurs in sediments, with m e r c u r y going into s o l u t i o n in the s u r r o u n d i n g w a t e r as a n o r g a n o m e t a l l i c c o m p l e x . T h e c o b w e b of p a t h w a y s t h r o u g h w h i c h m e r c u r y is thus r e d i s t r i b u t e d s h o u l d b e the focus of a t t e n t i o n a n d of m o n i t o r i n g efforts.
References Cotton, F. A. and Wilkinson, G. (1962) Advanced Inorganic Chemistry, Interscience, New York.
267 Craig, P. J. and Morton, S. F. (1975) Mercury in Mersey estuary sediments, and the analytical procedure for mercury, Nature 261, 125-126. Cranston, R. E. (1976) Accumulation and distribution of total mercury in estuarine sediments, Estuarine Coastal Mar. Sci. 4, 695-700. Cranston, R. E. and Buckle),, E. D. (1972) Mercury pathways in a river and estuary, Environ. Sci. Technol. 6, 274-278. de Groot, A. J. (1966) Mobility of trace elements in deltas, Comm. II and IV, Internat. Soc. Soil Sci. Trans., Aberdeen. de Groot, A. J. (1973) Occurrence and behaviour of heavy metals in river deltas, with special reference to the Rhine and Ems rivers, in E. D. Goldberg, ed. North Sea Science, pp. 308-325, MIT Press, Cambridge, MA. Duinker, J. C., van Eck, G.T.N., and Nolting, R. F. (1974) On the behaviour of copper, zinc, iron and manganese, and evidence for mobilization processes in the Dutch Wadden Sea, Neth. J. Sea Res. 8, 214-239. Duursma, E. R. and Hoede, C. (1967) Theoretical, experimental and field studies concerning diffusion of radioisotopes in sediments and suspended solid particles of the sea, Part A, Neth. J. Sea Res. 3, 423-457. Forstner, U. and Muller, G. (1974) Schwermetalle in Fliissen und Seen, Springer-Vedag, Berhn. Gavis, J. and Ferguson, J. F. (1972) The cycling of mercury through the environment, Water Res. 6, 989-1008. Guilcher, A. (1958) Coastal and Submarine Morphology, Methuen, London. Jackson, M. P., Swift, R., Posner, A., and Knox, J. R. (1972) Phenobiodegradation of humic acid, Soil Sci. 114, 75-78. Jenne, E. A. (1970) Atmospheric and fluvial transport of mercury, U.S.G.S. Prof. Paper 713, Washington, DC. Jenne, E. A. and Wahlberg, J. S. (1968) Role of certain stream sediment components in radio-ion sorption, U.S.G.S. Prof. Paper 433-F, Washington, DC. Keckes, S. and Miettinen, J. (1970) Review of mercury as a marine pollutant, Laboratory of Marine Radioactivity, International Atomic Agency, Rome. Kennedy, E. J., Ruch, R., and Shimp, N.F. (1971) Distribution of mercury in unconsolidated sediments from southern Lake Michigan,
Illinois State Geol. Survey Environ. Geol. Notes, No. 44. Kudo, A., Mortimer, D.C., and Hart, J.S. (1975) Factors influencing desorption of mercury from bed sediments, Can. J. Earth Sci. 12, 1036-1040. Lindberg, S. E. (1973) Mercury in interstitial solutions and associated sediments from estuarine areas in the Gulf of Mexico, M.Sc. Thesis, Florida State University. Lindberg, S. E., Andren, A. W., and Harriss, R. C. (1975) Geochemistry of mercury in the estuarine environment, in L. E. Cronin, ed, Estuarine Research Vol. 1, pp. 64-107, Academic, New York. Rashid, M. A. and King, L. H. (1970) Major oxygen-containing functional groups present in humic and fulvic acid fractious isolated from contrasting marine environments, Geochim. Cosmochim. Acta 34 193-201. Rashid, M. A. and King, L. H. (1971) Chemical characteristics of fractionated humic acids associated with marine sediments, Chem Geol. 7, 37-43. Raymont, J. E. C. (1972) Some aspects of pollution in Southampton Water, Proc. R. Soc. Lond B. 180, 451-468. Schnitzer, M. and Skinner, S. I. M. (1965) Organo-metallic interactions in soils, 4, Soil Sci. 99, 278-284. Smith, J. D., Nicholson, R. A., and Moore, P. J. (1973) Mercury in sediments from the Thames estuary, Environ. Poll. 4, 153-158. So, C. L. (1978) Environmental pollution of estuaries--a problem of hazards, Environ. Conservation 5, 205-211. Thomas, R. L. (1972) The distribution of mercury in the sediments of Lake Ontario, Can. J. Earth Sci. 9, 636-651. Thomas, R. L. (1973) The distribution of mercury in the surficial sediments of the Lake Huron, Can. J. Earth Sci. 10, 194-204. Turekian, K. K. (1971) Rivers, tributaries, and estuaries, in D. W. Hood, ed, Impingement of Man on the Oceans, pp. 9-74, Wiley, New York. Vernet, J. P. and Thomas, R. (1972) The occurrence and distribution of mercury in the sediments of the Petit-Lac, Eclogae Geol. Heir. 65, 307-316.
0160-4120/80/090265-03502.00/0 Copyright©1981 Pergamon Press Ltd.
Printed in the USA. All rights reserved.
MERCURY IN ESTUARINE SEDIMENTS: SPATIAL DISTRIBUTION AND REDISTRIBUTION
C. L. So Department of Geography and Geology Universityof Hong Kong Pokfulam Road, Hong Kong (Received 17 January 1980; Accepted 13 September 1980)
Mercury distribution in estuarine sediments is strongly influenced by the effect of desorption and adsorption on metal retention and transfer, the reactivity of mercury with particulate matter, and the close association of mercury with the fine suspended particulate fraction of the sedimentary load. The significance of this lies in that resuspension, initiated in diverse circumstances, can be an effective process in mercury remobilization. As one of the processes of mercury transfer in the estuarine environment, resuspension merits close attention, regular monitoring, and proper documentation as a prelude to judicious planning for environmental management. 1973), mercury is considered to be bound to an inorganic iron-phosphate complex probably adsorbed onto a hydrated iron oxide coating on clay particles. Estuarine mercury concentrations in sediments vary spatially. These variations depend on a number of controlling parameters which may include, among others, the type of sedimentary load, grain size, mineral constituents, chemical composition, organic content, salinity of overlying water, and estuarine hydrology. Analysis for mercury from unconsolidated surface sediments in the outer Thames estuary, using a flameless atomic absorption technique, reveals mercury contents of the sediments ranging from 0.012 to 0.550 ppm, with the higher concentrations usually occurring in sediments containing a high proportion of fine particles (Smith, 1973). Concurrently, mercury values in the Mersey estuary measured for 136 samples of the top 5 cm of sediment reveal a mean of 2.23 p p m and a m a x i m u m of 14.30 ppm, the high values occurring in fine silt sediments with a large amount of organic material (Craig, 1975).
Introduction
The postulation that estuarine sediments act as a sink and provide pathways for rapid mercury-to-biota transfer is a matter of increasing concern. When mercury enters an estuary with any type of mixing dynamics, in either dissolved or particulate associated form, it tends to be incorporated into the dynamic equilibrium of the water-sediment system in the estuarine environment. Its subsequent fate in this system must be viewed against the conditions and processes affecting the behaviour of pollutants in general and heavy metals in particular. This study reviews the available data base on mercury in estuarine sediments as a framework for monitoring pollution. Forms and Concentrations
Mercury occurs in sediments in association with several functional groups. Mercury shows a strong affinity for NH4, SH, C O O H , and phenol groups (Cotton, 1962; Gavis, 1972; Jenne, 1970; Keckes, 1970). Because these groups occur naturally (Jackson, 1972; Rashid, 1970; 1971; Schnitzer, 1965), the predominant manner in which mercury is bound in sediment is through association with organic material (Keckes, 1970). In another form, mercury occurs as an insoluble sulphide (Kennedy, 1971) or is adsorbed onto the surfaces of sulphide minerals such as FeS 2 (Thomas, 1972), as indicated by statistical correlations between mercury and total sediment sulphur in lake sediments. Thirdly, from observed relationships between total Hg, Fe, and P (Vernet, 1972; Thomas,
Associated Sedimentary Loads Mercury association with fine grades of materials in the bed load is also found, as in Southampton water, where a significant source of mercury for some organisms may be the bottom deposit (Raymont, 1972). Ranging from 0.188 to 5.680 p p m (dry mud) in value, the total mercury concentration in mud from Southampton water tends to be higher in anoxic mud (depth 10 cm) and farther up the estuary. 265
266 Study of the bed sediments of the Ottawa River reveals the presence of mercury in the sediments and the influence of some environmental factors on its desorption (Kudo, 1975). Thus, the desorption rate of mercury from the bed sediments ranges from 0.100 to 1.00 ng/cmE/day, the rate decreasing with an increase of exposure period to the water, but increasing with an increase in the depth of bed sediments. The amount of mercury desorbed from the bed sediments to the overlying water is highly dependent on the volume of the bed sediments. Calculations based on experiments show the half-lives of total mercury associated with bed sediments from 2.1 to 180 years, depending on the depth of the bed sediments. Mercury concentrations found in cores taken from an estuary associated with fine sediments have been reported to display maximum values in an area where the saltwater influence on the river becomes significant; this is interpreted as a result of mercury adsorption on sedimenting particulate matter (Cranston, 1976). The suspended load of the estuaries, which is composed of mineral and organic material and may include a substantial amount of fine sedimentary particles, is found to adsorb trace elements (Duursma, 1967; de Groot, 1966; Jenne, 1968; Turekian, 1971). Many heavy metals present in the aqueous environment, including mercury in the Rhine (So, 1978), are dominantly associated with suspended particles due to the tendency of coarse-grained particles and aggregates to settle more quickly, while suspended matter comprising very fine grains and colloidal material, may remain in suspension for long periods of time and be transported over great distances. For most heavy metals there is a higher concentration in the suspended matter compared with that in the deposited sediment (de Groot, 1973; Duinker, 1974; Forstner, 1974). The amount of adsorption achieved may play a part in determining the relative amounts of mercury present in dissolved loads and in suspended loads. The low mercury concentration in the Everglades, for example, is considered to arise from the fact that the dissolved organomercury complexes are not efficiently adsorbed onto the suspended matter, thus leaving more mercury in solution (Lindberg, 1975). It is, however, difficult to evaluate how much of the adsorbed mercury is exchangeable from the particulate matter in the estuaries. Because of the reactivity of mercury with particulate matter, the suspended load exerts a strong influence on its aqueous behavior. In the estuarine environment, this is enhanced by the widespread occurrence of sediment particles and other particulate matter in suspension, especially where tides, currents, and waves are active. Moreover, in estuaries, the amount of sediment in suspension is far greater than that in the river upstream and that in the sea (Guilcher, 1958). The downstream increase in the particulate-mercury content in La Have estuary, Nova
C.L. So Scotia, with a suspended load of 3.50 mg/1, but not in the Mississippi, with a suspended load of 250 mg/l, is attributed to decrease in particle size following sedimentation of large particles with lower mercury content as the estuary is approached (Cranston, 1972). Further comparative studies of this nature confirm the significance of the suspended particulate fraction of the sedimentary load. The Mobile Bay estuarine system shows a smaller amount of mercury associated with particulate matter than the Mississippi and differs from it in showing an increase in mercury content as salinity and dis,ance from the point of discharge of the river increase. This, together with concomitant drastic reduction of the suspended load indicating sedimentation of larger particles as water of higher salinity is approached, points to similar increase in mercury concentration with increase in finer suspended particles of higher mercury content. On the basis of these findings, it is not unreasonable to postulate that estuaries where much sedimentation occurs act as reservoirs for considerable mercury-laden particulate matter. Since the dissolved-mercury concentration in estuaries is fairly constant, the total amount of mercury in a water sample is largely governed by the amount of suspended matter, that is, the higher the suspended load, the higher the mercury content of the water sample.
Resuspension and Remobilization The association of mercury content with the fine suspended particulate fraction of the sedimentary load is significant in that resuspension can be an effective process in remobilization and transfer of mercury. Large-scale resuspension of sediment that permits pore water and particulate matter with relatively high mercury concentration to mix with surface waters of low mercury content might lead to release of mercury. This resuspension process may be effected in different ways. Natural mechanisms operate in response to seasonal rainfall regime marked by fluctuations in terrestrial runoff. Rapid increase in the discharge of fresh water into river systems and estuaries has been found to cause much suspension of unconsolidated sediments. An anthropogenic form of resuspension results from dredging, and to some extent dumping, through which engineering projects in rivers, estuaries, or near-shore marine sites may involve physical changes in the quantity, suspension, or deposition of sediment. This resuspension of bottom material by dredging and dumping is enhanced by reagitation of materials during storms and floods. When effluents contain suspended organic-rich sediments, because of suspension of fine particulate matter, the total amount of released organic matter in the water column may increase by many times over the dissolved concentrations.
Mercury in estuarine sediments T h e a s s o c i a t i o n of m e r c u r y with o r g a n i c m a t t e r further turns this into a n i m p o r t a n t m e c h a n i s m of m e r c u r y m o b i l i z a t i o n . A field e x p e r i m e n t d e s i g n e d to investigate the kinetics of m e r c u r y d i s s o l u t i o n in perio d s of intense w a t e r - s e d i m e n t m i x i n g has b e e n c a r r i e d o u t ( L i n d b e r g , 1973) b y s i m u l a t i n g r e s u s p e n s i o n of n e a r - s h o r e m a r s h surface s e d i m e n t s a n d o p e n - b a y , dredge-spoil sediments collected from Mobile Bay and f r o m the E v e r g l a d e s in the G u l f of Mexico. Results of the studies, agreeing with field data, suggest that largescale r e s u s p e n s i o n of e s t u a r i n e s e d i m e n t s b y such processes as d r e d g i n g causes a sizable s h o r t - t e r m release of m e r c u r y i n t o the s u r r o u n d i n g w a t e r f o l l o w e d b y a d e c r e a s e to levels close to those p r e d i c t e d b y ideal d i l u t i o n calculations. In the case of organic-rich, previously u n d r e d g e d , m a r s h sediments, the m a g n i t u d e of the m e r c u r y p e a k a n d the s t e a d y - s t a t e level w o u l d exceed the a m b i e n t dissolved m e r c u r y c o n c e n t r a t i o n of the s u r r o u n d i n g w a t e r b y a larger factor t h a n for the d r e d g e - s p o i l sediments. Thus, s i m u l a t e d d r e d g i n g resuits in a slight increase in t o t a l dissolved m e r c u r y in the overlying w a t e r d u e to m i x i n g with m e r c u r y - e n r i c h e d s e d i m e n t p o r e w a t e r a n d s h o r t - t e r m release of a small f r a c t i o n of m e r c u r y a s s o c i a t e d with p a r t i c u l a t e material.
Discussion and Conclusion O n c e m e r c u r y h a s b e e n d e p o s i t e d in the s e d i m e n t a r y e n v i r o n m e n t of the estuary, its s u b s e q u e n t b e h a v i o u r a n d a n y i n t e r a c t i o n s it m a y u n d e r g o w o u l d d e t e r m i n e its u l t i m a t e fate a n d a n y l o n g - t e r m e n v i r o n m e n t a l imp a c t it m i g h t i m p o s e o n the biota. W h e t h e r the sedim e n t s act as a sink or as a source for m e r c u r y in the e s t u a r i n e e n v i r o n m e n t m a y b e s t be viewed in terms of the stabilities of v a r i o u s s e d i m e n t a n d p o r e - w a t e r m e r c u r y c o m p l e x e s ( L i n d b e r g , 1975). A l t h o u g h retention b y the s e d i m e n t s c a n be a v a l u a b l e buffer b e t w e e n the l a n d a n d the sea, the p o l l u t a n t s in the s e d i m e n t s d o n o t a l w a y s r e m a i n p e r m a n e n t l y l o c k e d - u p . If c o n d i tions change, they m a y be m o b i l i z e d again. Processes like i o n e x c h a n g e are reversible a n d , a l t h o u g h sedim e n t s m a y h a v e a greater affinity for ions of h e a v y m e t a l s t h a n for alkali metals, a n y a b r u p t increase in salinity c o u l d release the h e a v y m e t a l s b a c k into solution. It is p o s t u l a t e d t h a t p o s t d e p o s i t i o n a l m i g r a t i o n occurs in sediments, with m e r c u r y going into s o l u t i o n in the s u r r o u n d i n g w a t e r as a n o r g a n o m e t a l l i c c o m p l e x . T h e c o b w e b of p a t h w a y s t h r o u g h w h i c h m e r c u r y is thus r e d i s t r i b u t e d s h o u l d b e the focus of a t t e n t i o n a n d of m o n i t o r i n g efforts.
References Cotton, F. A. and Wilkinson, G. (1962) Advanced Inorganic Chemistry, Interscience, New York.
267 Craig, P. J. and Morton, S. F. (1975) Mercury in Mersey estuary sediments, and the analytical procedure for mercury, Nature 261, 125-126. Cranston, R. E. (1976) Accumulation and distribution of total mercury in estuarine sediments, Estuarine Coastal Mar. Sci. 4, 695-700. Cranston, R. E. and Buckle),, E. D. (1972) Mercury pathways in a river and estuary, Environ. Sci. Technol. 6, 274-278. de Groot, A. J. (1966) Mobility of trace elements in deltas, Comm. II and IV, Internat. Soc. Soil Sci. Trans., Aberdeen. de Groot, A. J. (1973) Occurrence and behaviour of heavy metals in river deltas, with special reference to the Rhine and Ems rivers, in E. D. Goldberg, ed. North Sea Science, pp. 308-325, MIT Press, Cambridge, MA. Duinker, J. C., van Eck, G.T.N., and Nolting, R. F. (1974) On the behaviour of copper, zinc, iron and manganese, and evidence for mobilization processes in the Dutch Wadden Sea, Neth. J. Sea Res. 8, 214-239. Duursma, E. R. and Hoede, C. (1967) Theoretical, experimental and field studies concerning diffusion of radioisotopes in sediments and suspended solid particles of the sea, Part A, Neth. J. Sea Res. 3, 423-457. Forstner, U. and Muller, G. (1974) Schwermetalle in Fliissen und Seen, Springer-Vedag, Berhn. Gavis, J. and Ferguson, J. F. (1972) The cycling of mercury through the environment, Water Res. 6, 989-1008. Guilcher, A. (1958) Coastal and Submarine Morphology, Methuen, London. Jackson, M. P., Swift, R., Posner, A., and Knox, J. R. (1972) Phenobiodegradation of humic acid, Soil Sci. 114, 75-78. Jenne, E. A. (1970) Atmospheric and fluvial transport of mercury, U.S.G.S. Prof. Paper 713, Washington, DC. Jenne, E. A. and Wahlberg, J. S. (1968) Role of certain stream sediment components in radio-ion sorption, U.S.G.S. Prof. Paper 433-F, Washington, DC. Keckes, S. and Miettinen, J. (1970) Review of mercury as a marine pollutant, Laboratory of Marine Radioactivity, International Atomic Agency, Rome. Kennedy, E. J., Ruch, R., and Shimp, N.F. (1971) Distribution of mercury in unconsolidated sediments from southern Lake Michigan,
Illinois State Geol. Survey Environ. Geol. Notes, No. 44. Kudo, A., Mortimer, D.C., and Hart, J.S. (1975) Factors influencing desorption of mercury from bed sediments, Can. J. Earth Sci. 12, 1036-1040. Lindberg, S. E. (1973) Mercury in interstitial solutions and associated sediments from estuarine areas in the Gulf of Mexico, M.Sc. Thesis, Florida State University. Lindberg, S. E., Andren, A. W., and Harriss, R. C. (1975) Geochemistry of mercury in the estuarine environment, in L. E. Cronin, ed, Estuarine Research Vol. 1, pp. 64-107, Academic, New York. Rashid, M. A. and King, L. H. (1970) Major oxygen-containing functional groups present in humic and fulvic acid fractious isolated from contrasting marine environments, Geochim. Cosmochim. Acta 34 193-201. Rashid, M. A. and King, L. H. (1971) Chemical characteristics of fractionated humic acids associated with marine sediments, Chem Geol. 7, 37-43. Raymont, J. E. C. (1972) Some aspects of pollution in Southampton Water, Proc. R. Soc. Lond B. 180, 451-468. Schnitzer, M. and Skinner, S. I. M. (1965) Organo-metallic interactions in soils, 4, Soil Sci. 99, 278-284. Smith, J. D., Nicholson, R. A., and Moore, P. J. (1973) Mercury in sediments from the Thames estuary, Environ. Poll. 4, 153-158. So, C. L. (1978) Environmental pollution of estuaries--a problem of hazards, Environ. Conservation 5, 205-211. Thomas, R. L. (1972) The distribution of mercury in the sediments of Lake Ontario, Can. J. Earth Sci. 9, 636-651. Thomas, R. L. (1973) The distribution of mercury in the surficial sediments of the Lake Huron, Can. J. Earth Sci. 10, 194-204. Turekian, K. K. (1971) Rivers, tributaries, and estuaries, in D. W. Hood, ed, Impingement of Man on the Oceans, pp. 9-74, Wiley, New York. Vernet, J. P. and Thomas, R. (1972) The occurrence and distribution of mercury in the sediments of the Petit-Lac, Eclogae Geol. Heir. 65, 307-316.