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Removal of dissolved boron and silicon during estuarine mixing of sea and river waters
Geochimice t:tCnsmochimica Acta,1973, Vol.37,pp.1493 to 1498.Pergamon Press.Printed inNorthern Ireland
Removal of dissolved boron and silicon during estuarine mixing of sea and river waters P.
S. LISS and M. J. POINTON
School of Environmental
Sciences, University NOR 88C, England
(Received 5 July 1972;
of East
accepted in revised form
Anglia, Norwich,
11 December
1972)
Abstract--The behaviour of dissolved boron and silicon during mixing of sea and river waters Removal has been studied in two surveys of the estuary of the River Alde in Suffolk, England. This appears to be the fist report of approximately 25-30 per cent was found for both elements. of estuarjne removal of dissolved boron. The extent of removal of silicon in the Alde is somewhat higher
than that found in other estuaries.
Ix A WELL stirred estuary rapid mixing takes place between natural waters which Furthermore, stirring by tidal generally have very different chemical compositions. currents often produces a high concentration of suspended sediment in the estuarine water. These factors suggest that, as well as straightforward mixing of fresh and saline waters, reactions between dissolved species and solid phases in suspension may also be important. One ‘way of investigating whether such reactions do take place is to collect a suite of water samples covering the whole salinity range found in the estuary, and to analyse them for the dissolved constituents of interest. The measured concentrations of each element are then plotted graphically against a conservative index of mixing, such as salinity. To aid the interpretation of such graphs a theoretical dilution line (TDL) is often plotted. This is the straight line joining the end members of the estuarine mixing series (i.e. corresponding to 100 per cent river water and 100 per cent sea water). If the only process occurring during mixing is straightforward dilution, the measured points should lie on this line. The extent of any deviation is a measure of the amount of the dissolved constituent removed from (points plot below TDL) or added to (points plot above TDL) the water during mixing in the estuary. Up t’o the present this approach has been used to investigate the behaviour of only a small number of elements during estuarine mixing. Of these, the most widely studied is dissolved silicon. In summary, abiological removal of 10-20 per cent of the silicon in solution occurs in some estuaries (BIEN et al., 1958; BURTON, 1970; Lrss and SPENCER, 1970; HOSOKAWA et al., 1970), but where biological production is high even larger removal can occur (WOLLAST and DE BROEU, 1971). Laboratory simulation of the natural inorganic removal process indicates that both suspended matter and sea water electrolytes are required for maximum removal to take place (&EN e2 al., 1968; LISS and SPENCER, 1970). At least part of the silicon removed appears to be adsorbed on to the suspended sediment (LISS and SPENCER, 1970). In contrast, in other estuaries little or no abiological removal is found (STEFANSSON ~~1x1 RICHARDS, 1963; BURTON, LISS and VENUGOPALAN, 1970). The reason for this difference in behaviour between estuaries is not clear; but it seems likely that the nature and degree of equilibration of the solid phase with the water are probably more important than the total quantity of suspended material. 1493
1494
P.
S. Lrss and M. J.
POINTON
A number of laboratory studies have shown that dissolved boron may be removed from natural waters by reaction with clay minerals (HARDER, 1961; FLEET, 1965; LTCRXAN,1966; COUCH and GRIM, 1968). The purpose of these studies has been to investigate the use of boron content as a palaeosalinity indicator in sedimentary rocks. Most theories of how boron is incorporated into sediments have adsorption of dissolved boron on to the clay particles as a first stage, LEVINSONand LUDWICK (1966) go further and suggest that during estuarine mixing boron is differentially adsorbed and deposited according to particle size, i.e. the smaller particles remove more boron than the Iarger ones, and in turn are transported further seawards before settling out. However, very little effort seems to have been devoted to seeing whether such adsorption does actually occur in estuaries. One relevant field study is that of BOON and MACINTYRE (1968) who measured the total amount of boron in a range of bottom sediment samples collected from the estua,ry of the Rappahannock river. They found that the boron content of the sediment showed a positive correlation with salinity over the range S-18%,. It is not the purpose of the present paper to discuss the validity, or otherwise, of boron as an indicator of palaeosalinity; detailed discussions of this subject have recently been provided by a number of workers (HAZDER, 1970; PERRY, 1972). Rather, our purpose has been to examine simultaneously the behaviour of boron and silicon during estuarine mixing in order to identify any similarities or differences. To this end, measurements of salinity, dissolved boron and silicon, as well as quantity and type of suspended sediment have been made on samples collected on surveys of an estuary in East Angha. EXPERIMENTAL Two surveys were carried out in the estuary of the River Alde in Suffolk, England. The estuary is virtually unpolluted. Both surveys took place in winter (Survey &-27/l/71, Survey II--8/3/71) when biological activity in the water should be at a minimum. A plastic bucket More than was used for coheetion of sampIes which were then transferred to polythene bottles. thirty samples were collected on each survey. Due to the strong tidal currents the estuary water is well mixed vertically. On return to the laboratory samples were filtered through ‘Oxoid’ cellulose ester membranes (mean porosity 0.5 p). Where analysis could not be started straight after filtration, samples were kept in a deep freeze. Frozen samples were allowed to thaw and then stored overnight prior to analysis, to ensure depolymerisation of any polymeric silicon formed during the freezing process (BURTON,LEATHERLAND and LISS, 1970). Salinity was measured both in the field and laboratory using an Electionic Switchgear salinity-temperature bridge. Reactive silicate was determined using a modification (Lrss and SPENCER, 1969) of tllotechnique described by STRICKLAND and PAILSONS (1968). The ooefficient of variation for this method as applied to the estuarine samples was approximately 1 per cent. DissoIved boron was measured using the method of BULTIIE, UPPSTR~M and ~STLUNG (1970). Although originally developed for use with a discrete sample automatic analyser, in the present study it was applied manually. The method involves reacting the boron with curcumin to form an orange complex (rosoc~ran~) which is then determined spectrophotometrioally. As efficient formation of the complex only takes place in the absence of water, propionic anhydride with oxalyl chloride as catalyst is added to dehydrate the reaction mixture. Reagent blanks were determined using distilled water in place of the sample, and the method was calibrated by adding an increment of 2.0 mg B/l to replicates of each sample analysed. The coefficient of variation determined on samples from the estuary was approximately 2 per cent. In the analysis for both silicon rend boron, reagent blanks, samples and samples with calibration increments were a11 processed in triplicate.
Removal of dissolved boron and silicon during estuarine mixing of sea and river waters
1495
The amount of suspended sediment w&s measured for nine samples collected on Survey II. This involved using a, gmvimetric procedure in which the sediment wa.s collected by f&ration onto an ‘Oxoid’ membrane. The ‘total’ &mo~t of suspended material was determ~ed by drying the sediment loaded membrane at 100°C, and the ‘inorganic’ fraction by ashing s,t 750%. The difference between these two weights gave the amount of ‘organio’ (strictly organic plus bound water) material in the suspended sediment. The precision of the method is about 1 mg/l. The suspended sediment from eight of the samples collected on Survey II was examined by X-ray diffraction to obtain a qualitative indication of the type of solid material in suspension in the sstuarine water.
RESULTS Figure 1 (a)-(d) shows graphs of the variation with salinity of dissolved boron and silicon found in each survey.
S.
%a
Fig. 1. Variation of dissolved constituent concentrations with salinity for samples collected from the Alde estuary. The theoretical dilution line (TDL) is shown dotted. (a) Survey I-silicon. (b) Survey I-boron. (c) Survey II-silicon. (d) Survey II-boron.
1496
P. S. LISS and M. J. POINTON
In all cases an identical procedure has been adopted for plotting the theoretical dilution line (TDL). For the low salinity end member this involves calculating the mean salinity and dissolved boron or silicon concentration for all samples of salinity less than 1x0. A similar calculation for all samples of salinity greater than 30%, gives the upper salinity end member of the dilution line. It is apparent from Fig. 1 (a)-(d) that most of the points corresponding to samples of intermediate salinity lie below the TDL, This implies removal of some boron and silicon from the water during mixing in the estuary, The extent of this removal has been calculated from the deviation of the observed concentrations from the TDL. For all four graphs the amount of removal calculated is in the range 25-30 per cent. showing little variation either between surveys or between the two elements studied. Because the number of samples defining the end members of the TDL is on occasions rather small, numerical values for the extent of removal must be treated with some caution. The total amount of suspended sediment in the in~owing river water was approximately 10 mg/l, and samples up to salinity 18%, contained a similar quantity. Above IS%, the amount of suspended sediment generally increased, probably due to the more vigorous tidal stirring, reaching about 80 mg/l at the estuary mouth. Between 10-20 per cent of the total sediment load was ‘organic’ and this percentage showed no consistent variation with water salinity. Analysis of the suspended sediment by X-ray di~ra~tion revealed the presence of the following minerals: quartz, calcite, ill&, chlorite and kaolinite. There did not appear to be any significant change in the relative proportions of these minerals with changing salinity. DISCUSSIONS In other estuaries where silicon removal has been observed, the extent of such removal is usually in the range lo-20 per cent. The magnitude of the removal found in the estuary of the Aide (25-30 per cent) is rather higher than this. As far as we are aware there are no previously published reports of removal of dissolved boron during estuarine mixing of sea and river waters. In some other estuaries there is evidence that removal of boron does not occur. We have made nleasuremeilts on samples collected from the estuary of the Beaufieu river in Hampshire, England, and these show a good linear relationship between dissolved boron and salinity. A similar result was obtained by HOSOKAWA et al. (1970) in a survey of the Chikugogawa estuary in Japan. It is still an unresolved problem as to why removal of dissolved silicon or boron should occur in some estuaries but not in others. Factors which would seem to warrant further study include : type of clay mineral present, degree of equilibrium established between the solid phase and the estuarine water, and possible formation of new hydrous metal (M) oxide phases during the mixing process. With regard to this last factor, it has been shown in laboratory experiments (HARDER, 1965) that such phases (where M = Al, Fe, Mn, Mg) can remove dissolved silicon from natural waters, and there is some evidence for their formation (where M = Fe) in estuarine waters (CooNLEY et al., 1971). The simultaneous removal of silicon and boron observed in the Alde estuary, coupled with the similar magnitude of removal for both elements, might be taken
Removal of dissolved boron and silicon during estuarine mixing of sea and river waters 1497 to indicate a common removal mechanism. Certainly the laboratory studies mentioned previously, indicate that boron and silicon behave similarly, with adsorption on to solid phases as the first step in the removal process. However, there is almost nothing known about the fate of elements after adsorption; and furthermore, the applicability of laboratory simulations to estuarine processes is open to question. In spite of these difficulties, the present evidence seems to indicate an adsorption mechanism for the estuarine removal of dissolved silicon and boron. Any silicon removed in estuarine mixing is usually assumed to come from the silicon rich river water flowing into the estuary. For boron the opposite seems more likely, with the boron removed coming from the sea water which is rich in this eIement relative to most river waters. Whatever its source, once in the estuary, the dissolved component is in an environment where the ionic strength changes rapidly, and turbulent mixing produces a higher concentration of suspended sediment than is found in the sea or in many rivers. Such conditions should be conducive to rapid reaction between dissolved species and solids in suspension. The results presented in this paper indicate that estuarine removal can occur for silicon and boron. It seems likely that estuarine mixing must also play an important part in the geochemical cycle of many other dissolved constituents, and by implication in the geochemistry of estuarine sediments. Furthermore, sedimentwater reactions would seem likely to have a very considerable effect on the fate of many substances introduced into estuaries by human activities. would like to thank Drs. J. R. CANNand I. N. MCCAVEfor their helpful comments on the manuscript, and Miss B. THORPEand Mr. C. K. WINTER for analysis of the suspended sediment. M. J. POINTONwas supported in this researchby a S.R.C. studentship.
Acknowledgemeltt.s-We
REVERENCES G. S., CONTOISD. E. and THOMASW. H. (1958) The removal of soluble silica from fresh water entering the sea. Geochim. Cosmochim.Acta 14, 35-54. BOONJ. D. and MYACINTYRE W. G. (1968) The boron-salinity relationshipin estuarinesediments of the Rappahennock river, Virginia. Chesapeake Sci. 9, 21-26. BUXTON J. D. (1970) The behaviour of dissolved silicon during estuarinemixing II. Preliminary investigation in the Vellar estuary, Southern India. J. Cons. Perma. Int. Explor. MET 93, 141-148. BURTONif. D., LEATAZRU~ T. M. and LISS I?. S. (1970) The reactivity of dissolved silicon in some natural waters. .Limraol. ~~~~~g~. l&473-476. BURTOX ,J. D., LISS P. S. and VENUGOPALAN V. K. (1970) The behaviour of dissolved silicon during estuarine mixing I. Investigations in Southampton Water. J. Cons. Perma. Int.
BIEN
Explor.
Mer
33, 134-140.
COONLEYL. S., BAEERE. B. and HOLLANDH. D. (1971) Iron in tho Mullica River and in Great 13ay, New Jersey. Chem. Geol. 7, 51-63. Coma E. L. and GRIMR. E. (1968) Boron fixation by illites. CZuysClay &f&?-& 16, 249-256. FLEETM. E. L. (1965) Preliminary investigations into the sorption of boron by clay minor&. Clay M~~er~~
6,
3-16.
HARDERH. (1961) Einbau von borin detritische tonminerale. Experimente zur eklgrung des borgehaltes toniger sedimente. Geochim.Cosmochim. Acta 21, 284-294. HARDERH. (1965) Experimente zur “ausfallung” der Kiesilsiiure. Geochim. Cosmochim. Acta 29,429-442.
HARDERH. (1970) Boron content of sediments as a tool in facies analysis. Sediment. ffeol. 4, 153-175. HOSOKAWAI., OHSH~MA F. and KONDON. (1970) On the concentrationsof the dissolved chemical elements in the estuary water of the Chikugogawariver. J. Ocealaogr. Sot. Jup. 29, l-5.
I.498
P. S. LISS and BE. J. POINTON
HIXTHE P., UPPSTR~ML. and &TLTJN~ G. (1970) An automatic procedure for the determination of boron in sea water. Anal. China. Acta 51,31-37. LERMAN A. (1966) Boron in clays and estimation of paleosalinities. Sedirnentology 6, 267-286. LEVINSON A. A. and LUDWICK J. C. (1966) Speculation on the incorporation of boron into argillaceous sediments. Geochim. Cosmochim. Acta 30, 8X-861. LISS P. S. and SPENCERC. P. (1969) An investigation of some methods used for the determination of silicate in sea water. J. Nap. Biol. Ass. U.K. 49, 589-601. LISS P. S. and SPENCER C. P. (1970) Abiological processes in the removal of silicate from sea water. ffeochim. Cosmochim. Acta 34, 1073-1088. PERRY E. A. (1972) L&genesis and the validity of the boron paleosalinity technique. Amer. J. Sci. 2’92, X0-160. STIZFANSSONU. and RIGHAR~S F. A. (1963) Processes contributing to the nutrient distributions off the Columbia river and Strait of Juan de Fuca. Limnol. Oceanogr. 2, 394-410. S~ICKLAND J. D. H. and PARSONS T. R. (1968) A practical handbook of seawater analysis. &UK Fish. Res. Board Can. 167, 311 pp. WOLLAST R. and DE BROEU F. (1971) Study of the behaviour of dissolved silica in the estuary of the Scheldt. Gwchim. ~o~o~h~rn. Acta 35,613-620.
Removal of dissolved boron and silicon during estuarine mixing of sea and river waters P.
S. LISS and M. J. POINTON
School of Environmental
Sciences, University NOR 88C, England
(Received 5 July 1972;
of East
accepted in revised form
Anglia, Norwich,
11 December
1972)
Abstract--The behaviour of dissolved boron and silicon during mixing of sea and river waters Removal has been studied in two surveys of the estuary of the River Alde in Suffolk, England. This appears to be the fist report of approximately 25-30 per cent was found for both elements. of estuarjne removal of dissolved boron. The extent of removal of silicon in the Alde is somewhat higher
than that found in other estuaries.
Ix A WELL stirred estuary rapid mixing takes place between natural waters which Furthermore, stirring by tidal generally have very different chemical compositions. currents often produces a high concentration of suspended sediment in the estuarine water. These factors suggest that, as well as straightforward mixing of fresh and saline waters, reactions between dissolved species and solid phases in suspension may also be important. One ‘way of investigating whether such reactions do take place is to collect a suite of water samples covering the whole salinity range found in the estuary, and to analyse them for the dissolved constituents of interest. The measured concentrations of each element are then plotted graphically against a conservative index of mixing, such as salinity. To aid the interpretation of such graphs a theoretical dilution line (TDL) is often plotted. This is the straight line joining the end members of the estuarine mixing series (i.e. corresponding to 100 per cent river water and 100 per cent sea water). If the only process occurring during mixing is straightforward dilution, the measured points should lie on this line. The extent of any deviation is a measure of the amount of the dissolved constituent removed from (points plot below TDL) or added to (points plot above TDL) the water during mixing in the estuary. Up t’o the present this approach has been used to investigate the behaviour of only a small number of elements during estuarine mixing. Of these, the most widely studied is dissolved silicon. In summary, abiological removal of 10-20 per cent of the silicon in solution occurs in some estuaries (BIEN et al., 1958; BURTON, 1970; Lrss and SPENCER, 1970; HOSOKAWA et al., 1970), but where biological production is high even larger removal can occur (WOLLAST and DE BROEU, 1971). Laboratory simulation of the natural inorganic removal process indicates that both suspended matter and sea water electrolytes are required for maximum removal to take place (&EN e2 al., 1968; LISS and SPENCER, 1970). At least part of the silicon removed appears to be adsorbed on to the suspended sediment (LISS and SPENCER, 1970). In contrast, in other estuaries little or no abiological removal is found (STEFANSSON ~~1x1 RICHARDS, 1963; BURTON, LISS and VENUGOPALAN, 1970). The reason for this difference in behaviour between estuaries is not clear; but it seems likely that the nature and degree of equilibration of the solid phase with the water are probably more important than the total quantity of suspended material. 1493
1494
P.
S. Lrss and M. J.
POINTON
A number of laboratory studies have shown that dissolved boron may be removed from natural waters by reaction with clay minerals (HARDER, 1961; FLEET, 1965; LTCRXAN,1966; COUCH and GRIM, 1968). The purpose of these studies has been to investigate the use of boron content as a palaeosalinity indicator in sedimentary rocks. Most theories of how boron is incorporated into sediments have adsorption of dissolved boron on to the clay particles as a first stage, LEVINSONand LUDWICK (1966) go further and suggest that during estuarine mixing boron is differentially adsorbed and deposited according to particle size, i.e. the smaller particles remove more boron than the Iarger ones, and in turn are transported further seawards before settling out. However, very little effort seems to have been devoted to seeing whether such adsorption does actually occur in estuaries. One relevant field study is that of BOON and MACINTYRE (1968) who measured the total amount of boron in a range of bottom sediment samples collected from the estua,ry of the Rappahannock river. They found that the boron content of the sediment showed a positive correlation with salinity over the range S-18%,. It is not the purpose of the present paper to discuss the validity, or otherwise, of boron as an indicator of palaeosalinity; detailed discussions of this subject have recently been provided by a number of workers (HAZDER, 1970; PERRY, 1972). Rather, our purpose has been to examine simultaneously the behaviour of boron and silicon during estuarine mixing in order to identify any similarities or differences. To this end, measurements of salinity, dissolved boron and silicon, as well as quantity and type of suspended sediment have been made on samples collected on surveys of an estuary in East Angha. EXPERIMENTAL Two surveys were carried out in the estuary of the River Alde in Suffolk, England. The estuary is virtually unpolluted. Both surveys took place in winter (Survey &-27/l/71, Survey II--8/3/71) when biological activity in the water should be at a minimum. A plastic bucket More than was used for coheetion of sampIes which were then transferred to polythene bottles. thirty samples were collected on each survey. Due to the strong tidal currents the estuary water is well mixed vertically. On return to the laboratory samples were filtered through ‘Oxoid’ cellulose ester membranes (mean porosity 0.5 p). Where analysis could not be started straight after filtration, samples were kept in a deep freeze. Frozen samples were allowed to thaw and then stored overnight prior to analysis, to ensure depolymerisation of any polymeric silicon formed during the freezing process (BURTON,LEATHERLAND and LISS, 1970). Salinity was measured both in the field and laboratory using an Electionic Switchgear salinity-temperature bridge. Reactive silicate was determined using a modification (Lrss and SPENCER, 1969) of tllotechnique described by STRICKLAND and PAILSONS (1968). The ooefficient of variation for this method as applied to the estuarine samples was approximately 1 per cent. DissoIved boron was measured using the method of BULTIIE, UPPSTR~M and ~STLUNG (1970). Although originally developed for use with a discrete sample automatic analyser, in the present study it was applied manually. The method involves reacting the boron with curcumin to form an orange complex (rosoc~ran~) which is then determined spectrophotometrioally. As efficient formation of the complex only takes place in the absence of water, propionic anhydride with oxalyl chloride as catalyst is added to dehydrate the reaction mixture. Reagent blanks were determined using distilled water in place of the sample, and the method was calibrated by adding an increment of 2.0 mg B/l to replicates of each sample analysed. The coefficient of variation determined on samples from the estuary was approximately 2 per cent. In the analysis for both silicon rend boron, reagent blanks, samples and samples with calibration increments were a11 processed in triplicate.
Removal of dissolved boron and silicon during estuarine mixing of sea and river waters
1495
The amount of suspended sediment w&s measured for nine samples collected on Survey II. This involved using a, gmvimetric procedure in which the sediment wa.s collected by f&ration onto an ‘Oxoid’ membrane. The ‘total’ &mo~t of suspended material was determ~ed by drying the sediment loaded membrane at 100°C, and the ‘inorganic’ fraction by ashing s,t 750%. The difference between these two weights gave the amount of ‘organio’ (strictly organic plus bound water) material in the suspended sediment. The precision of the method is about 1 mg/l. The suspended sediment from eight of the samples collected on Survey II was examined by X-ray diffraction to obtain a qualitative indication of the type of solid material in suspension in the sstuarine water.
RESULTS Figure 1 (a)-(d) shows graphs of the variation with salinity of dissolved boron and silicon found in each survey.
S.
%a
Fig. 1. Variation of dissolved constituent concentrations with salinity for samples collected from the Alde estuary. The theoretical dilution line (TDL) is shown dotted. (a) Survey I-silicon. (b) Survey I-boron. (c) Survey II-silicon. (d) Survey II-boron.
1496
P. S. LISS and M. J. POINTON
In all cases an identical procedure has been adopted for plotting the theoretical dilution line (TDL). For the low salinity end member this involves calculating the mean salinity and dissolved boron or silicon concentration for all samples of salinity less than 1x0. A similar calculation for all samples of salinity greater than 30%, gives the upper salinity end member of the dilution line. It is apparent from Fig. 1 (a)-(d) that most of the points corresponding to samples of intermediate salinity lie below the TDL, This implies removal of some boron and silicon from the water during mixing in the estuary, The extent of this removal has been calculated from the deviation of the observed concentrations from the TDL. For all four graphs the amount of removal calculated is in the range 25-30 per cent. showing little variation either between surveys or between the two elements studied. Because the number of samples defining the end members of the TDL is on occasions rather small, numerical values for the extent of removal must be treated with some caution. The total amount of suspended sediment in the in~owing river water was approximately 10 mg/l, and samples up to salinity 18%, contained a similar quantity. Above IS%, the amount of suspended sediment generally increased, probably due to the more vigorous tidal stirring, reaching about 80 mg/l at the estuary mouth. Between 10-20 per cent of the total sediment load was ‘organic’ and this percentage showed no consistent variation with water salinity. Analysis of the suspended sediment by X-ray di~ra~tion revealed the presence of the following minerals: quartz, calcite, ill&, chlorite and kaolinite. There did not appear to be any significant change in the relative proportions of these minerals with changing salinity. DISCUSSIONS In other estuaries where silicon removal has been observed, the extent of such removal is usually in the range lo-20 per cent. The magnitude of the removal found in the estuary of the Aide (25-30 per cent) is rather higher than this. As far as we are aware there are no previously published reports of removal of dissolved boron during estuarine mixing of sea and river waters. In some other estuaries there is evidence that removal of boron does not occur. We have made nleasuremeilts on samples collected from the estuary of the Beaufieu river in Hampshire, England, and these show a good linear relationship between dissolved boron and salinity. A similar result was obtained by HOSOKAWA et al. (1970) in a survey of the Chikugogawa estuary in Japan. It is still an unresolved problem as to why removal of dissolved silicon or boron should occur in some estuaries but not in others. Factors which would seem to warrant further study include : type of clay mineral present, degree of equilibrium established between the solid phase and the estuarine water, and possible formation of new hydrous metal (M) oxide phases during the mixing process. With regard to this last factor, it has been shown in laboratory experiments (HARDER, 1965) that such phases (where M = Al, Fe, Mn, Mg) can remove dissolved silicon from natural waters, and there is some evidence for their formation (where M = Fe) in estuarine waters (CooNLEY et al., 1971). The simultaneous removal of silicon and boron observed in the Alde estuary, coupled with the similar magnitude of removal for both elements, might be taken
Removal of dissolved boron and silicon during estuarine mixing of sea and river waters 1497 to indicate a common removal mechanism. Certainly the laboratory studies mentioned previously, indicate that boron and silicon behave similarly, with adsorption on to solid phases as the first step in the removal process. However, there is almost nothing known about the fate of elements after adsorption; and furthermore, the applicability of laboratory simulations to estuarine processes is open to question. In spite of these difficulties, the present evidence seems to indicate an adsorption mechanism for the estuarine removal of dissolved silicon and boron. Any silicon removed in estuarine mixing is usually assumed to come from the silicon rich river water flowing into the estuary. For boron the opposite seems more likely, with the boron removed coming from the sea water which is rich in this eIement relative to most river waters. Whatever its source, once in the estuary, the dissolved component is in an environment where the ionic strength changes rapidly, and turbulent mixing produces a higher concentration of suspended sediment than is found in the sea or in many rivers. Such conditions should be conducive to rapid reaction between dissolved species and solids in suspension. The results presented in this paper indicate that estuarine removal can occur for silicon and boron. It seems likely that estuarine mixing must also play an important part in the geochemical cycle of many other dissolved constituents, and by implication in the geochemistry of estuarine sediments. Furthermore, sedimentwater reactions would seem likely to have a very considerable effect on the fate of many substances introduced into estuaries by human activities. would like to thank Drs. J. R. CANNand I. N. MCCAVEfor their helpful comments on the manuscript, and Miss B. THORPEand Mr. C. K. WINTER for analysis of the suspended sediment. M. J. POINTONwas supported in this researchby a S.R.C. studentship.
Acknowledgemeltt.s-We
REVERENCES G. S., CONTOISD. E. and THOMASW. H. (1958) The removal of soluble silica from fresh water entering the sea. Geochim. Cosmochim.Acta 14, 35-54. BOONJ. D. and MYACINTYRE W. G. (1968) The boron-salinity relationshipin estuarinesediments of the Rappahennock river, Virginia. Chesapeake Sci. 9, 21-26. BUXTON J. D. (1970) The behaviour of dissolved silicon during estuarinemixing II. Preliminary investigation in the Vellar estuary, Southern India. J. Cons. Perma. Int. Explor. MET 93, 141-148. BURTONif. D., LEATAZRU~ T. M. and LISS I?. S. (1970) The reactivity of dissolved silicon in some natural waters. .Limraol. ~~~~~g~. l&473-476. BURTOX ,J. D., LISS P. S. and VENUGOPALAN V. K. (1970) The behaviour of dissolved silicon during estuarine mixing I. Investigations in Southampton Water. J. Cons. Perma. Int.
BIEN
Explor.
Mer
33, 134-140.
COONLEYL. S., BAEERE. B. and HOLLANDH. D. (1971) Iron in tho Mullica River and in Great 13ay, New Jersey. Chem. Geol. 7, 51-63. Coma E. L. and GRIMR. E. (1968) Boron fixation by illites. CZuysClay &f&?-& 16, 249-256. FLEETM. E. L. (1965) Preliminary investigations into the sorption of boron by clay minor&. Clay M~~er~~
6,
3-16.
HARDERH. (1961) Einbau von borin detritische tonminerale. Experimente zur eklgrung des borgehaltes toniger sedimente. Geochim.Cosmochim. Acta 21, 284-294. HARDERH. (1965) Experimente zur “ausfallung” der Kiesilsiiure. Geochim. Cosmochim. Acta 29,429-442.
HARDERH. (1970) Boron content of sediments as a tool in facies analysis. Sediment. ffeol. 4, 153-175. HOSOKAWAI., OHSH~MA F. and KONDON. (1970) On the concentrationsof the dissolved chemical elements in the estuary water of the Chikugogawariver. J. Ocealaogr. Sot. Jup. 29, l-5.
I.498
P. S. LISS and BE. J. POINTON
HIXTHE P., UPPSTR~ML. and &TLTJN~ G. (1970) An automatic procedure for the determination of boron in sea water. Anal. China. Acta 51,31-37. LERMAN A. (1966) Boron in clays and estimation of paleosalinities. Sedirnentology 6, 267-286. LEVINSON A. A. and LUDWICK J. C. (1966) Speculation on the incorporation of boron into argillaceous sediments. Geochim. Cosmochim. Acta 30, 8X-861. LISS P. S. and SPENCERC. P. (1969) An investigation of some methods used for the determination of silicate in sea water. J. Nap. Biol. Ass. U.K. 49, 589-601. LISS P. S. and SPENCER C. P. (1970) Abiological processes in the removal of silicate from sea water. ffeochim. Cosmochim. Acta 34, 1073-1088. PERRY E. A. (1972) L&genesis and the validity of the boron paleosalinity technique. Amer. J. Sci. 2’92, X0-160. STIZFANSSONU. and RIGHAR~S F. A. (1963) Processes contributing to the nutrient distributions off the Columbia river and Strait of Juan de Fuca. Limnol. Oceanogr. 2, 394-410. S~ICKLAND J. D. H. and PARSONS T. R. (1968) A practical handbook of seawater analysis. &UK Fish. Res. Board Can. 167, 311 pp. WOLLAST R. and DE BROEU F. (1971) Study of the behaviour of dissolved silica in the estuary of the Scheldt. Gwchim. ~o~o~h~rn. Acta 35,613-620.