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The distribution of zinc and reactive silicate in the Otago Harbour, New Zealand
Marine Chemistry, 20 (1987) 377 387
377
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
THE D I S T R I B U T I O N OF ZINC A N D R E A C T I V E SILICATE IN THE OTAGO H A R B O U R , N E W Z E A L A N D
KEITH A. HUNTER and SIMON R. TYLER
Chemistry Department, University of Otago, Box 56, Dunedin (New Zealand) (Received April 22, 1986; revision accepted October 20, 1986)
ABSTRACT Hunter, K.A. and Tyler, S.R., 1987. The distribution of zinc and reactive silicate in the Otago Harbour, New Zealand. Mar. Chem., 20:377 387. The horizontal distributions of reactive silicate and zinc in the Otago Harbour, New Zealand, a shallow, vertically welLmixed estuary, have been compared. Within the harbour silicate displayed a linear salinity profile on two sampling occasions in October and early November. Consideration of the silicate budget suggested some removal of freshwater silicate in the headwaters of the estuary. Zinc concentrations, measured using state-of-the-art clean laboratory techniques were orders of magnitude lower t h a n previously published data for coastal waters. Typical values of 5 20 n g l 1 Zn were recorded in the seaward endmember. Zinc salinity profiles were approximately linear. Relationships between zinc and silicate concentrations for the estuarine and freshwater samples suggest a coupling between the rates of supply of these elements to the ocean and their utilisation by plankton. INTRODUCTION
There have been very few reliable studies of the geochemistry of zinc in marine and fresh waters, largely because contamination problems involved in sample collection, handling and analysis (Patterson and Settle, 1976) are especially severe for this element. Recent application of clean room methodology has successfully minimised these problems to the point where the oceanic geochemistry of this element can be discerned for the first time. Results for the North Pacific (Bruland, 1980), confirmed by a later study in the North Atlantic (Bruland and Franks, 1983), indicate that Zn closely mimics silicate in behaviour. Both elements are depleted in surface waters as a result of incorporation into the opaline tests of surface plankton, and undergo a deep water regeneration cycle as this skeletal material dissolves. There are few published studies of Zn behaviour in shelf and estuarine waters where the same careful control of sample contamination has been maintained during collection and analysis. The expectation that trace metal concentrations will be significantly higher in these waters than in offshore waters is based largely on erroneous data, and is inconsistent with the likelihood that many trace metals, including zinc, may be regulated by internal
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~ 1987 Elsevier Science Publishers B.V.
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biological and scavenging processes that may swamp the effects of external sources, except on a very local scale. Indeed some processes known to regulate trace metal behaviour in the ocean, for example adsorptive scavenging by suspended particles or biological assimilation during photosynthetic growth, take place more rapidly in productive shelf waters, and could therefore immobilise a greater fraction of a reactive element. In this paper we report on a study of zinc in Otago Harbour, a shallow marine embayment that has been the subject of earlier trace metal studies (Dickson and Hunter, 1981; Hunter et al., 1984; Hunter and Lee, 1986), Clean laboratory working methods were used throughout the study to ensure freedom from contamination problems imposed by more classical techniques. Because of the clear relationship between Zn and Si in the ocean, reactive silicate was also investigated. STUDY AREA AND METHODS Otago Harbour is a sectionally homogeneous shallow estuary with its only important freshwater input source, the Water of Leith, situated near its head (Fig. 1). Salinity and micronutrient distributions indicate that the head waters in the Upper Harbour Basin (UHB) comprise a well-mixed water mass of volume 20 × 106 m ~ having a flushing time of 20-30 days. The freshwater discharge has an annual mean flow rate of 1.0 m 3s ' and is very low in relation to the rate of tidal water exchange. Consequently salinities in the UHB rarely fall below 31%o except within the immediate mixing zone of the river. Downharbour, salinities progressively increase to around 34.5 in a series of vertically homogeneous water types. Because of the low and often variable flow rate of the Water of Leith, it is not possible nor very realistic to attempt collection of samples throughout the full salinity range down to that of the river endmember. The true low salinity
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379 endmember for water mixing within the Harbour system is the UHB water mass at its landward extremity (Dickson and Hunter, 1981; Hunter and Lee, 1986). Seawater samples for analysis of soluble zinc, reactive silicate and salinity were collected from the Otago Harbour on two occasions: on 4 October 1983 following a prolonged period of fine weather and a low river flow rate of 0.13m3s ', and on 2 November 1983, following a brief period of rain during which the flow rate peaked at 1.0m3s -' (on 30 October) declining to around 0.13m3s 1 on the day of sampling. These two sampling events therefore represent extremes, one in which no major recent input of freshwater above dry weather flow had taken place and the second in which a major freshwater pulse had recently entered the system. Samples were collected over a 2-3 h period centred on the tidal maximum. Samples for zinc analysis were collected in bottles made of linear polyethylene. These were cleaned by soaking for at least 10 days in 10M nitric acid, rinsed with deionised water and left filled with a 1% solution of high-purity HC1 for at least 2 weeks before use. After filling with the HC1, the bottles were sealed in 2 polyethylene bags to protect them from transfer of airborne and surface zinc contaminants. The inner bag was acid-cleaned in the same way as the samples. Samples were collected using a small, powered runabout moving slowly forward into clean water. The sample bottle was unwrapped from its protective bags in such a way that only a clean polyethylene-gloved hand came into contact with the successive inside surfaces of the bag or with the outside of the bottle. A second worker wearing shoulder-length gloves removed the bottle from the inside bag, making sure that neither his hands nor the bottle came into contact with any other objects or surfaces. This worker then opened the bottle, poured out the final acid-rinse solution and recapped the bottle. It was then passed under the water surface, re-opened and allowed to fill with seawater. Finally the bottle was capped, raised through the water and returned to the inner bag. During this transfer care was taken to hold the bottle uppermost in the air to prevent seawater that had been in contact with the workers' gloved arms from dripping onto the sample bottle. The sample bottle was then resealed in the 2 polythene bags by a reversal of the opening procedure. All subsequent sample handling and analysis was conducted in a positive pressure class 100 clean laboratory. Workers were clothed and hooded in polyethylene clean room suits and wore face masks and polyethylene gloves at all times. In the clean lab, subsamples for silicate analysis were transferred to 180-mi polyethylene bottles, and analysed immediately using the molybdenum blue colorimetric technique of Koroleff (1976). Absorbances were corrected for the ionic strength effect of this method using measured salinity values for each sample (Koroleff, 1976). The rest of the sample, destined for Zn analysis, was acidified by addition of 2ml high-purity HC1 per litre of sample and resealed in clean polythene bags for storage at 1°C until analysis. To avoid use of 'dirty' glass equipment in the clean lab, separate samples for salinity determination were collected in 30O-ml screw-capped bottles and later
380
measured using an Autolab salinometer calibrated with IAPSO standard seawater. Soluble Zn was preconcentrated for analysis by quantitative solvent extraction in chloroform of the zinc chelates formed with the ligands ammonium pyrolidene dithiocarbamate and sodium diethyldithiocarbamate (Danielsson et al., 1982) after buffering to pH 4.5 using ammonium acetate. Extractions were performed on 100-g atiquots of sample in 250-ml acid-cleaned Teflon separatory funnels. The Zn chelates were concentrated into 2 successive 10-ml aliquots of chloroform in a polyethylene vial. Subsequently the Zn was back-extracted into 0.4 M nitric acid to yield a final concentration factor of between 20 and 50 depending on the Zn concentration. The Zn concentration in the final extract was then determined by flameless atomic absorption spectrometry (AAS) using an Instrumentation Laboratories IL251 spectrophotometer with model 655 flameless atomiser. Background correction was applied to all samples. Samples were not filtered before Zn analysis to avoid contamination problems associated with filters and filtration apparatus. Thus the results reported below relate to zinc not only in solution, but also particulate and colloidal forms of zinc rendered soluble by the prior acidification and/or the chelating agents used for preconcentration. These fractions will be collectively termed 'soluble Zn' in this paper. All reagents used required extensive purification to reduce the blank to a level suitable for determination of the low levels of Zn found in the samples studied. Mineral acids were purified by sub-boiling distillation in quartz (Kuehnen et al., 1972). Chloroform was purified by repeated extraction with 6 M high-purity HC1 in Teflon. The ammonium acetate buffer solution was prepared by mixing high-purity acetic acid from the quartz sub-boiling still with ammonia solution prepared by isothermal distillation in Teflon vessels (Veillon and Reamer, 1981). The chelatingagents were purified by extracting a freshly prepared 0.5% aqueous solution of each ligand 8-10 times with purified TABLE I Temperature, salinity, reactive silicate and soluble zinc for Otago Harbour samples, 4 October 1983 Station number
Temperature (°C)
Salinity (%o)
Silicate (umolkg 1)
Zinc (ngkg :~
1 2 3 4 5 7 8 10 12 15 17
13.70 13.72 1.2.31 11.70 11.69 12.06 11.80 12.30 12.30 12.00 12.45
33.90 33.82 33.92 33.76 33.72 33.36 33.24 32.44 31.68 a0.85 31.65
2.46 1.93 1.36 1.80 2.02 3.12 2.86 3.82 5.50 7.08 5.85
8.0 7.0 9.0 14.0 17.0 13.0 t8.0 36.0 28.0 25.0 63.0
381
chloroform. The purity of each reagent used was monitored regularly before use by flameless AAS and contaminated reagents were rejected. A reagent blank of less t h a n 4 ng kg 1was typically achieved by these means. Analyses of waters low in dissolved Zn indicated that the overall procedural blank including contamination from sample bottles, reagents and handling amounted to about 5 ng kg 1 of Zn. Samples of water were also collected at selected times from the Water of Leith for analysis of Zn and silicate. These were collected by hand using similar procedures as those used for the small boat sampling in the harbour. Samples were collected at a small bridge 1 km from the mouth of the river. A Foxboro continuous flow gauge sited next to the bridge was used to measure river flow rates. It was only possible to sample the river by hand for Zn analysis at low to moderate flow rates. RESULTS AND DISCUSSION
The experimental results for the two study periods are shown in Tables I (4 October 1983) and II (2 November 1983). The salinity data reveal a consistent decrease in salinity upharbour from station 1 onwards as a result of the progressive dilution of the coastal water endmember with runoff from the Water of Leith. Reactive silicate distribution
The reactive silicate data reflect the influence of freshwater runoff, with levels increasing systematically upharbour and reaching a maximum in the freshwater plume. The silicate-salinity profiles for both study periods are shown in Figs. 2a and b, respectively. The profile for the October samples shows T A B L E II T e m p e r a t u r e , s a l i n i t y , r e a c t i v e silicate a n d s o l u b l e zinc for O t a g o H a r b o u r samples, 2 N o v e m b e r 1983 Station number
Temperature (°C)
Salinity (%o)
Silicate (ttmol kg ~)
Zinc (ng kg i)
1 4 6 7 8 9 11 12 15 16 17 18
11.70 11.70 12.40 11.90 12.50 12.30 i3.40 13.40 13.20 13.15 13.55 13.40
34.18 34.25 33.71 33.61 33.72 33.50 32.74 32.73 32.70 32.56 32.51 32.68
1.61 2.58 2.94 1.98 2.48 2.71 6.35 6.21 6.81 6.72 9.84 5.98
12.0 18.0 10.0 62.0 40.0 68.0 88.0 84.0 98.0 92.0 60.0 84.0
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Fig. 2. Reactivesilicate-salinity profilesfor the Otago Harbour: (a) 4 October 1983,(b) 2 November 1983. Least-squares regression lines are shown. a reasonably good linear correlation between silicate and salinity with a least-squares correlation parameter, r 2 = 0.97. Although a reasonable correlation parameter (r 2 = 0.83) was calculated for the November data, the results shown in Fig. 2b do suggest a pronounced negative curvature as salinity increases. This may not be significant because of the scatter of the data and the limited number of samples. Boyle et al. (1974) discussed the use of concentration vs. salinity profiles to determine the mixing behaviour of dissolved constituents in estuaries. When the concentration-salinity profile is linear it is reasonably certain t h a t mixing is conservative on the time scale of the estuarine residence time. Curvature of the concentration profile can be caused by a number of processes: input or removal processes operating within the estuary, fluctuations in the riverine input or the presence of more than one freshwater endmember. In the context of the present study, it is important to assess the possibility of fluctuations in the riverine supply rate of silicate. Moreover, this analysis must extend over a period of several months prior to the present study period since the residence time of water in the Upper Harbour is 20-30 days. Weekly measurements of freshwater silicate levels in the Water of Leith were conducted throughout 1983 as part of a larger study focussing on silicate cycling (Grundy, 1985). Figure 3 shows silicate concentrations for the period May-November 1983. The results show a steady increase in riverine silicate from June to November which corresponds closely to the seasonal temperature increase (winter to spring) in the Leith catchment. Such a dependence of silicate levels on catchment temperature has been already noted (Meybeck, 1981). The flow rates fluctuated less regularly, with maxima in May and J u l y and minima in August and October. The data in Fig. 2a for silicate in the Otago Harbour at the beginning of
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October extrapolate to a hypothetical freshwater concentration of 60gmolkg 1, which is considerably lower than the average levels recorded during the previous three-month period (July-September inclusive). This suggests that a significant proportion of the silicate may have been removed from solution within the upper harbour basin through biological utilisation. A similar trend was observed with the November data, which extrapolate to a zero salinity intercept of 130 gmol kg 1, again much lower than most recorded values for the freshwater input during the previous three-month period (August-October inclusive). Soluble zinc distribution
The soluble Zn data reported in Tables I and II contrast markedly with most previously published data for this element in coastal or estuarine waters. Samples collected in the Lower Harbour (stations 1-5 inclusive) have soluble Zn levels lower than 2 0 n g k g -1, i.e. at least one and generally two or more orders of magnitude lower than data reported for similar waters elsewhere (Bewers et al., 1975; Yeats et ah, 1978). By contrast, the present data are close to those reported recently by Bruland and co-workers for shelf and offshore waters; e.g. 4 6 ng kg 1for surface waters on a transect from Hawaii to Monterey, California (Bruland, 1980), 2.6-7.0ngkg 1 for surface waters of the Sargasso Sea, and 10ngkg 1 on the continental slope adjacent to Long Island Sound, increasing to 160 ng kg-1 in shelf waters (Bruland and Franks, 1983). The low concentrations of Zn found in the lower Otago Harbour, close to surface oceanic levels, are consistent with the low nutrient status of these waters and the generally low concentrations
384
of other trace metals such as Cu, Ni and Cd, whose oceanic distributions are also related to micronutrient cycling (Dickson and Hunter, 1981; Hunter et al., 1984). The comparisons just made suggest that many published data for Zn in coastal and estuarine waters are probably compromised by contamination artifacts introduced during the sampling and sample handling operations. The soluble zinc-salinity profiles for both study periods are shown in Figs. 4a and b, respectively. The October data, for which there are only 4 points at the lower salinity range, are less linearly related to salinity than the corresponding silicate data and there is no strong evidence for a linear profile. There is, however, an increasing trend of soluble Zn concentration with freshwater fraction. The November data do show a linear relationship with salinity with a regression coefficient (r 2 = 0.74) not greatly different from that found for the corresponding silicate data. The regression line intercept for this set of results corresponds to a hypothetical freshwater Zn level of 1500 ng kg -1. This may be compared with actual measurements made on freshwater samples collected from the Water of Leith on several days preceding the November study which are shown in Table III and indicate an average concentration of 860 ng kg Bruland and Franks (1983) made a similar extrapolation of Zn-salinity data for surface samples collected along a transect from the Sargasso Sea towards Long Island Sound, obtaining a zero salinity intercept of 1000ngkg-~ Zn, similar to the values found in this work. Because of contamination problems, they adjudged that there were no reliable data for Zn in freshwaters entering the North Atlantic with which to compare their extrapolated result.
Relationship between Zn and silicate We do not have sufficient information about temporal changes in freshwater Zn concentrations to assess the degree of Zn removal in the upper harbour ,
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385 TABLE III Reactive silicate and soluble zinc c o n c e n t r a t i o n s in the W a t e r of Leith Date of collection (day/month/year)
Silicate (gmol kg 1)
Zinc (ng k g - 1)
Flow rate (m 3s ~)
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216 239 251 241
750 930 820 950
0.080 0.995 0.273 0.130
basin, as was done earlier for silicate. However, it can be noted that the freshwater Zn levels recorded in the last week of November (Table III) are about 60% of the zero-salinity intercept for the November harbour study data. In contrast, for silicate the average measured freshwater level (236 pmol kg-1 Table III) is almost twice the corresponding zero-salinity intercept of 126 pmol kg- 1. A possible explanation for this difference is t h a t soluble Zn has an additional, perhaps diagenetic, source within the upper harbour basin t h a t involves a higher Zn:Si ratio than opaline diagenesis. On the basis of existing knowledge of this system, the most likely source would appear to be diagenetic release of particle-bound Zn of riverine origin within the fine, organic-rich sediments of the upper harbour basin (Dickson and Hunter, 1981; Hunter and Lee, 1986). Most of the dissolved Mn(II) in Otago Harbour waters is of diagenetic origin rather t h a n from direct input of riverine Mn(II) (Hunter and Lee, 1986). The distribution of dissolved Cu in this system appears to be controlled by the same process (Dickson and Hunter, 1981; Hunter et al., 1984). Much the same situation may prevail for Zn, and could involve Zn associated with colloidal organics. The difference in Zn and Si behaviour would also be expected if soluble Zn levels had undergone a recent decline in the Water of Leith during October at the same time as silicate levels were increasing. Unfortunately there are few reliable data on zinc in freshwaters to test this hypothesis, in this or any other freshwater system. However, the data that do exist suggest a positive relationship between Zn and Si in freshwater that would mitigate against large changes in the Zn:Si ratio. Figure 5 compares Zn:Si data for this catchment with other data obtained by Ahlers and Hunter (1986) for the Manuherikia River, Central Otago, New Zealand, and a number of its tributaries within a high-altitude subalpine catchment. The mean and standard deviation of reactive silicate and dissolved Zn (Nuclepore 0.4pro filtered) results for 7 samples in May 1983 and 10 samples in December 1983 are shown. These results suggest a positive relationship between Zn and Si in freshwater. The Otago Harbour results (Tables I and II) are all too small to plot clearly on this diagram, and have a mean Zn:Si ratio of 14ngpmo1-1. This is significantly higher than ratios observed for the Water of Leith (3.6 ng #mol 1) as a
386
consequence of the changes in Zn and Si concentrations in the estuary already discussed. Figure 5 also shows the Zn-Si correlations observed by Bruland (1980) and Bruland and Franks (1983) for the North Pacific and the North Atlantic. respectively. There are no other reliable data on Zn concentrations in uncontaminated river waters of which we are aware. Although the database is still limited, it is nonetheless interesting that the Zn:Si ratio established in oceanic waters through the agency of phytoplankton growth and remineralisation should fall within the range of ratios observed in freshwaters. This would suggest a coupling between the rates of supply of Zn and Si to the ocean from terrestrial weathering and their rates of utilisation by diatoms and radiolaria in the ocean. Clearly, more data on Zn and Si in freshwaters are required to substantiate this trend. Silicon is used by these phytoplankton to secrete their opaline exoskeleta. It is difficult to conceive of an active role for Zn in this process. It is more likely that Zn becomes passively incorporated as a trace constituent of the silica mineral. Thus, the apparent similarity of Zn:Si ratios in plankton and freshwaters may mean that the degrees of incorporation of Zn into diatomaceous opal and terrestrial source sediments are similar. Clearly, future work on the mechanism and rates of incorporation of Zn into phytoplanktom and the corresponding rates of incorporation into freshwater through terrestrial weathering, will be required to elucidate details of the marine geochemical cycle of this trace element. Finally, if such a causal relationship between Zn:Si ratios in plankton and N At]ant;c
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387 f r e s h w a t e r i n p u t s t o t h e o c e a n e x i s t s , i t is p o s s i b l e t o s u g g e s t r e a l i s t i c l i m i t s f o r the c o n c e n t r a t i o n of Zn in u n c o n t a m i n a t e d r i v e r s in o r d e r to assess e x i s t i n g data. Reactive silicate concentrations in freshwater depend largely on rock t y p e a n d c a t c h m e n t t e m p e r a t u r e ( M e y b e c k , 1981) a n d r a r e l y e x c e e d t h e e q u i l i b r i u m s o l u b i l i t y o f a l u m i n o s i l i c a t e m i n e r a l s o f a b o u t 400 # m o l k g 1 (Liss, 1976). W i t h t h i s m a x i m u m [Si], t h e c o r r e l a t i o n s i n F i g . 5 w o u l d s u g g e s t t h a t f r e s h w a t e r d i s s o l v e d Zn l e v e l s g r e a t e r t h a n a b o u t 3 0 0 0 n g Zn k g 1 w o u l d n o t be expected. The Zn:Si ratio of a river water may thus provide a useful criterion for assessing the l i k e l i h o o d of possible sample c o n t a m i n a t i o n d u r i n g c o l l e c t i o n or analysis. REFERENCES Ahlers, W.W. and Hunter, K.A., 1986. Water quality and trace metals in the Manuherikia River, Central Otago. Aust. J. Mar. Freshwater Res., in preparation. Bewers, J.M., Sundby, B. and Yeats, P.A., 1975. The distribution of trace metals in the western North Atlantic off Nova Scotia. Geochim. Cosmochim. Acta, 40: 687~96. Boyle, E., Collier, R., Dengler, A.T., Edmond, J.M., Ng, A.C. and Stallard, R.F., 1974. On the chemical mass balance in estuaries. Geochim. Cosmochim. Acta, 41: 1719-1728. Bruland, K.W., 1980. Oceanographic distributions of cadmium, zinc, nickel and copper in the North Pacific. Earth Planet. Sci. Lett., 47: 176~198. Bruland, K.W. and Franks, R.P., 1983. Mn, Ni, Cu, Zn and Cd in the western North Atlantic. NATO Conf. Ser. 4, 9 pp.; Trace Met. Seawater: 395414. Danielsson, L., Magnusson, B., Westerlund, S. and Zhang, K., 1982. Trace metal determinations in estuarine waters by electrothermal atomic absorption spectroscopy after extraction of dithiocarbamate complexes into Freon, Anal. Chim. Acta, 144: 183~188. Dickson, R.J. and Hunter, K.A., 1981. Copper and nickel in surface waters of Otago Harbour. N.Z. J. Mar. Freshwater Res., 15:475 480. Grundy, R.G., 1985. The behaviour of dissolved silica within Blueskin Bay and its tributary waters. M.Sc. Thesis, University of Otago, 142 pp. Hunter, K.A. and Lee, S.L., 1986. The dynamic balance of manganese transport and diagenesis in the Otago Harbour, New Zealand. Mar. Chem., 19: 17~192. Hunter, K.A., Ho, F.W.T., Lee, S.L. and Tyler, S.R., 1984. Trace metal geochemistry in coastal and shelf waters. In: K.M. Gawne (Editor), Proc. Syrup. on Trace Metals in the Environment. Deakin University. Koroleff, I., 1976. Analysis of micronutrients. In: K. Grasshof (Editor), Methods of Seawater Analysis. Verlag-Chimie, Berlin, pp. 134 145. Kuehnen, E.C., Alvarez, R., Paulson, P.J. and Murphy, T.J., 1972. Production and analysis of special high purity acids purified by subboiling distillation. Anal. Chem., 44:2050 2056. Liss, P.S., 1976. Conservative and non-conservative behaviour of dissolved constituents during estuarine mixing. In: J.D. Burton and P.S. Liss (Editors), Estuarine Chemistry. Academic Press, London, pp. 98-130. Meybeck, M., 1981. Pathways of major elements from land to ocean through rivers. In: River Inputs to Ocean Systems. United Nations, New York, pp. 1~30. Patterson, C. and Settle, D.M., 1976. The reduction of orders of magnitude errors in lead analysis of biological materials and natural waters by evaluating and controlling external sources of industrial lead contamination introduced during sample collection, handling and analysis. In: D.M. la Fleur (Editor), N.B.S. Special Publication No. 422, National Bureau of Standards, Washington, DC, pp. 321-323. Veillon, C. and Reamer, D.C., 1981. Preparation of high-purity volatile acids and bases by isothermal distillation. Anal. Chem., 53: 549-550. Yeats, P.A., Bewers, J.M. and Walton, A., 1978. Sensitivity of coastal waters to anthropogenic trace metal emissions. Mar. Pollut. Bull., 9:264 268.
377
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
THE D I S T R I B U T I O N OF ZINC A N D R E A C T I V E SILICATE IN THE OTAGO H A R B O U R , N E W Z E A L A N D
KEITH A. HUNTER and SIMON R. TYLER
Chemistry Department, University of Otago, Box 56, Dunedin (New Zealand) (Received April 22, 1986; revision accepted October 20, 1986)
ABSTRACT Hunter, K.A. and Tyler, S.R., 1987. The distribution of zinc and reactive silicate in the Otago Harbour, New Zealand. Mar. Chem., 20:377 387. The horizontal distributions of reactive silicate and zinc in the Otago Harbour, New Zealand, a shallow, vertically welLmixed estuary, have been compared. Within the harbour silicate displayed a linear salinity profile on two sampling occasions in October and early November. Consideration of the silicate budget suggested some removal of freshwater silicate in the headwaters of the estuary. Zinc concentrations, measured using state-of-the-art clean laboratory techniques were orders of magnitude lower t h a n previously published data for coastal waters. Typical values of 5 20 n g l 1 Zn were recorded in the seaward endmember. Zinc salinity profiles were approximately linear. Relationships between zinc and silicate concentrations for the estuarine and freshwater samples suggest a coupling between the rates of supply of these elements to the ocean and their utilisation by plankton. INTRODUCTION
There have been very few reliable studies of the geochemistry of zinc in marine and fresh waters, largely because contamination problems involved in sample collection, handling and analysis (Patterson and Settle, 1976) are especially severe for this element. Recent application of clean room methodology has successfully minimised these problems to the point where the oceanic geochemistry of this element can be discerned for the first time. Results for the North Pacific (Bruland, 1980), confirmed by a later study in the North Atlantic (Bruland and Franks, 1983), indicate that Zn closely mimics silicate in behaviour. Both elements are depleted in surface waters as a result of incorporation into the opaline tests of surface plankton, and undergo a deep water regeneration cycle as this skeletal material dissolves. There are few published studies of Zn behaviour in shelf and estuarine waters where the same careful control of sample contamination has been maintained during collection and analysis. The expectation that trace metal concentrations will be significantly higher in these waters than in offshore waters is based largely on erroneous data, and is inconsistent with the likelihood that many trace metals, including zinc, may be regulated by internal
0304-4203/87/$03.50
~ 1987 Elsevier Science Publishers B.V.
378
biological and scavenging processes that may swamp the effects of external sources, except on a very local scale. Indeed some processes known to regulate trace metal behaviour in the ocean, for example adsorptive scavenging by suspended particles or biological assimilation during photosynthetic growth, take place more rapidly in productive shelf waters, and could therefore immobilise a greater fraction of a reactive element. In this paper we report on a study of zinc in Otago Harbour, a shallow marine embayment that has been the subject of earlier trace metal studies (Dickson and Hunter, 1981; Hunter et al., 1984; Hunter and Lee, 1986), Clean laboratory working methods were used throughout the study to ensure freedom from contamination problems imposed by more classical techniques. Because of the clear relationship between Zn and Si in the ocean, reactive silicate was also investigated. STUDY AREA AND METHODS Otago Harbour is a sectionally homogeneous shallow estuary with its only important freshwater input source, the Water of Leith, situated near its head (Fig. 1). Salinity and micronutrient distributions indicate that the head waters in the Upper Harbour Basin (UHB) comprise a well-mixed water mass of volume 20 × 106 m ~ having a flushing time of 20-30 days. The freshwater discharge has an annual mean flow rate of 1.0 m 3s ' and is very low in relation to the rate of tidal water exchange. Consequently salinities in the UHB rarely fall below 31%o except within the immediate mixing zone of the river. Downharbour, salinities progressively increase to around 34.5 in a series of vertically homogeneous water types. Because of the low and often variable flow rate of the Water of Leith, it is not possible nor very realistic to attempt collection of samples throughout the full salinity range down to that of the river endmember. The true low salinity
Ofago Harbour
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Fig. 1. Map of Otago Harbour showingsampling stations used in the study.
379 endmember for water mixing within the Harbour system is the UHB water mass at its landward extremity (Dickson and Hunter, 1981; Hunter and Lee, 1986). Seawater samples for analysis of soluble zinc, reactive silicate and salinity were collected from the Otago Harbour on two occasions: on 4 October 1983 following a prolonged period of fine weather and a low river flow rate of 0.13m3s ', and on 2 November 1983, following a brief period of rain during which the flow rate peaked at 1.0m3s -' (on 30 October) declining to around 0.13m3s 1 on the day of sampling. These two sampling events therefore represent extremes, one in which no major recent input of freshwater above dry weather flow had taken place and the second in which a major freshwater pulse had recently entered the system. Samples were collected over a 2-3 h period centred on the tidal maximum. Samples for zinc analysis were collected in bottles made of linear polyethylene. These were cleaned by soaking for at least 10 days in 10M nitric acid, rinsed with deionised water and left filled with a 1% solution of high-purity HC1 for at least 2 weeks before use. After filling with the HC1, the bottles were sealed in 2 polyethylene bags to protect them from transfer of airborne and surface zinc contaminants. The inner bag was acid-cleaned in the same way as the samples. Samples were collected using a small, powered runabout moving slowly forward into clean water. The sample bottle was unwrapped from its protective bags in such a way that only a clean polyethylene-gloved hand came into contact with the successive inside surfaces of the bag or with the outside of the bottle. A second worker wearing shoulder-length gloves removed the bottle from the inside bag, making sure that neither his hands nor the bottle came into contact with any other objects or surfaces. This worker then opened the bottle, poured out the final acid-rinse solution and recapped the bottle. It was then passed under the water surface, re-opened and allowed to fill with seawater. Finally the bottle was capped, raised through the water and returned to the inner bag. During this transfer care was taken to hold the bottle uppermost in the air to prevent seawater that had been in contact with the workers' gloved arms from dripping onto the sample bottle. The sample bottle was then resealed in the 2 polythene bags by a reversal of the opening procedure. All subsequent sample handling and analysis was conducted in a positive pressure class 100 clean laboratory. Workers were clothed and hooded in polyethylene clean room suits and wore face masks and polyethylene gloves at all times. In the clean lab, subsamples for silicate analysis were transferred to 180-mi polyethylene bottles, and analysed immediately using the molybdenum blue colorimetric technique of Koroleff (1976). Absorbances were corrected for the ionic strength effect of this method using measured salinity values for each sample (Koroleff, 1976). The rest of the sample, destined for Zn analysis, was acidified by addition of 2ml high-purity HC1 per litre of sample and resealed in clean polythene bags for storage at 1°C until analysis. To avoid use of 'dirty' glass equipment in the clean lab, separate samples for salinity determination were collected in 30O-ml screw-capped bottles and later
380
measured using an Autolab salinometer calibrated with IAPSO standard seawater. Soluble Zn was preconcentrated for analysis by quantitative solvent extraction in chloroform of the zinc chelates formed with the ligands ammonium pyrolidene dithiocarbamate and sodium diethyldithiocarbamate (Danielsson et al., 1982) after buffering to pH 4.5 using ammonium acetate. Extractions were performed on 100-g atiquots of sample in 250-ml acid-cleaned Teflon separatory funnels. The Zn chelates were concentrated into 2 successive 10-ml aliquots of chloroform in a polyethylene vial. Subsequently the Zn was back-extracted into 0.4 M nitric acid to yield a final concentration factor of between 20 and 50 depending on the Zn concentration. The Zn concentration in the final extract was then determined by flameless atomic absorption spectrometry (AAS) using an Instrumentation Laboratories IL251 spectrophotometer with model 655 flameless atomiser. Background correction was applied to all samples. Samples were not filtered before Zn analysis to avoid contamination problems associated with filters and filtration apparatus. Thus the results reported below relate to zinc not only in solution, but also particulate and colloidal forms of zinc rendered soluble by the prior acidification and/or the chelating agents used for preconcentration. These fractions will be collectively termed 'soluble Zn' in this paper. All reagents used required extensive purification to reduce the blank to a level suitable for determination of the low levels of Zn found in the samples studied. Mineral acids were purified by sub-boiling distillation in quartz (Kuehnen et al., 1972). Chloroform was purified by repeated extraction with 6 M high-purity HC1 in Teflon. The ammonium acetate buffer solution was prepared by mixing high-purity acetic acid from the quartz sub-boiling still with ammonia solution prepared by isothermal distillation in Teflon vessels (Veillon and Reamer, 1981). The chelatingagents were purified by extracting a freshly prepared 0.5% aqueous solution of each ligand 8-10 times with purified TABLE I Temperature, salinity, reactive silicate and soluble zinc for Otago Harbour samples, 4 October 1983 Station number
Temperature (°C)
Salinity (%o)
Silicate (umolkg 1)
Zinc (ngkg :~
1 2 3 4 5 7 8 10 12 15 17
13.70 13.72 1.2.31 11.70 11.69 12.06 11.80 12.30 12.30 12.00 12.45
33.90 33.82 33.92 33.76 33.72 33.36 33.24 32.44 31.68 a0.85 31.65
2.46 1.93 1.36 1.80 2.02 3.12 2.86 3.82 5.50 7.08 5.85
8.0 7.0 9.0 14.0 17.0 13.0 t8.0 36.0 28.0 25.0 63.0
381
chloroform. The purity of each reagent used was monitored regularly before use by flameless AAS and contaminated reagents were rejected. A reagent blank of less t h a n 4 ng kg 1was typically achieved by these means. Analyses of waters low in dissolved Zn indicated that the overall procedural blank including contamination from sample bottles, reagents and handling amounted to about 5 ng kg 1 of Zn. Samples of water were also collected at selected times from the Water of Leith for analysis of Zn and silicate. These were collected by hand using similar procedures as those used for the small boat sampling in the harbour. Samples were collected at a small bridge 1 km from the mouth of the river. A Foxboro continuous flow gauge sited next to the bridge was used to measure river flow rates. It was only possible to sample the river by hand for Zn analysis at low to moderate flow rates. RESULTS AND DISCUSSION
The experimental results for the two study periods are shown in Tables I (4 October 1983) and II (2 November 1983). The salinity data reveal a consistent decrease in salinity upharbour from station 1 onwards as a result of the progressive dilution of the coastal water endmember with runoff from the Water of Leith. Reactive silicate distribution
The reactive silicate data reflect the influence of freshwater runoff, with levels increasing systematically upharbour and reaching a maximum in the freshwater plume. The silicate-salinity profiles for both study periods are shown in Figs. 2a and b, respectively. The profile for the October samples shows T A B L E II T e m p e r a t u r e , s a l i n i t y , r e a c t i v e silicate a n d s o l u b l e zinc for O t a g o H a r b o u r samples, 2 N o v e m b e r 1983 Station number
Temperature (°C)
Salinity (%o)
Silicate (ttmol kg ~)
Zinc (ng kg i)
1 4 6 7 8 9 11 12 15 16 17 18
11.70 11.70 12.40 11.90 12.50 12.30 i3.40 13.40 13.20 13.15 13.55 13.40
34.18 34.25 33.71 33.61 33.72 33.50 32.74 32.73 32.70 32.56 32.51 32.68
1.61 2.58 2.94 1.98 2.48 2.71 6.35 6.21 6.81 6.72 9.84 5.98
12.0 18.0 10.0 62.0 40.0 68.0 88.0 84.0 98.0 92.0 60.0 84.0
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Fig. 2. Reactivesilicate-salinity profilesfor the Otago Harbour: (a) 4 October 1983,(b) 2 November 1983. Least-squares regression lines are shown. a reasonably good linear correlation between silicate and salinity with a least-squares correlation parameter, r 2 = 0.97. Although a reasonable correlation parameter (r 2 = 0.83) was calculated for the November data, the results shown in Fig. 2b do suggest a pronounced negative curvature as salinity increases. This may not be significant because of the scatter of the data and the limited number of samples. Boyle et al. (1974) discussed the use of concentration vs. salinity profiles to determine the mixing behaviour of dissolved constituents in estuaries. When the concentration-salinity profile is linear it is reasonably certain t h a t mixing is conservative on the time scale of the estuarine residence time. Curvature of the concentration profile can be caused by a number of processes: input or removal processes operating within the estuary, fluctuations in the riverine input or the presence of more than one freshwater endmember. In the context of the present study, it is important to assess the possibility of fluctuations in the riverine supply rate of silicate. Moreover, this analysis must extend over a period of several months prior to the present study period since the residence time of water in the Upper Harbour is 20-30 days. Weekly measurements of freshwater silicate levels in the Water of Leith were conducted throughout 1983 as part of a larger study focussing on silicate cycling (Grundy, 1985). Figure 3 shows silicate concentrations for the period May-November 1983. The results show a steady increase in riverine silicate from June to November which corresponds closely to the seasonal temperature increase (winter to spring) in the Leith catchment. Such a dependence of silicate levels on catchment temperature has been already noted (Meybeck, 1981). The flow rates fluctuated less regularly, with maxima in May and J u l y and minima in August and October. The data in Fig. 2a for silicate in the Otago Harbour at the beginning of
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October extrapolate to a hypothetical freshwater concentration of 60gmolkg 1, which is considerably lower than the average levels recorded during the previous three-month period (July-September inclusive). This suggests that a significant proportion of the silicate may have been removed from solution within the upper harbour basin through biological utilisation. A similar trend was observed with the November data, which extrapolate to a zero salinity intercept of 130 gmol kg 1, again much lower than most recorded values for the freshwater input during the previous three-month period (August-October inclusive). Soluble zinc distribution
The soluble Zn data reported in Tables I and II contrast markedly with most previously published data for this element in coastal or estuarine waters. Samples collected in the Lower Harbour (stations 1-5 inclusive) have soluble Zn levels lower than 2 0 n g k g -1, i.e. at least one and generally two or more orders of magnitude lower than data reported for similar waters elsewhere (Bewers et al., 1975; Yeats et ah, 1978). By contrast, the present data are close to those reported recently by Bruland and co-workers for shelf and offshore waters; e.g. 4 6 ng kg 1for surface waters on a transect from Hawaii to Monterey, California (Bruland, 1980), 2.6-7.0ngkg 1 for surface waters of the Sargasso Sea, and 10ngkg 1 on the continental slope adjacent to Long Island Sound, increasing to 160 ng kg-1 in shelf waters (Bruland and Franks, 1983). The low concentrations of Zn found in the lower Otago Harbour, close to surface oceanic levels, are consistent with the low nutrient status of these waters and the generally low concentrations
384
of other trace metals such as Cu, Ni and Cd, whose oceanic distributions are also related to micronutrient cycling (Dickson and Hunter, 1981; Hunter et al., 1984). The comparisons just made suggest that many published data for Zn in coastal and estuarine waters are probably compromised by contamination artifacts introduced during the sampling and sample handling operations. The soluble zinc-salinity profiles for both study periods are shown in Figs. 4a and b, respectively. The October data, for which there are only 4 points at the lower salinity range, are less linearly related to salinity than the corresponding silicate data and there is no strong evidence for a linear profile. There is, however, an increasing trend of soluble Zn concentration with freshwater fraction. The November data do show a linear relationship with salinity with a regression coefficient (r 2 = 0.74) not greatly different from that found for the corresponding silicate data. The regression line intercept for this set of results corresponds to a hypothetical freshwater Zn level of 1500 ng kg -1. This may be compared with actual measurements made on freshwater samples collected from the Water of Leith on several days preceding the November study which are shown in Table III and indicate an average concentration of 860 ng kg Bruland and Franks (1983) made a similar extrapolation of Zn-salinity data for surface samples collected along a transect from the Sargasso Sea towards Long Island Sound, obtaining a zero salinity intercept of 1000ngkg-~ Zn, similar to the values found in this work. Because of contamination problems, they adjudged that there were no reliable data for Zn in freshwaters entering the North Atlantic with which to compare their extrapolated result.
Relationship between Zn and silicate We do not have sufficient information about temporal changes in freshwater Zn concentrations to assess the degree of Zn removal in the upper harbour ,
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385 TABLE III Reactive silicate and soluble zinc c o n c e n t r a t i o n s in the W a t e r of Leith Date of collection (day/month/year)
Silicate (gmol kg 1)
Zinc (ng k g - 1)
Flow rate (m 3s ~)
29/10/83 30/10/83 31/10/83 1/11/83
216 239 251 241
750 930 820 950
0.080 0.995 0.273 0.130
basin, as was done earlier for silicate. However, it can be noted that the freshwater Zn levels recorded in the last week of November (Table III) are about 60% of the zero-salinity intercept for the November harbour study data. In contrast, for silicate the average measured freshwater level (236 pmol kg-1 Table III) is almost twice the corresponding zero-salinity intercept of 126 pmol kg- 1. A possible explanation for this difference is t h a t soluble Zn has an additional, perhaps diagenetic, source within the upper harbour basin t h a t involves a higher Zn:Si ratio than opaline diagenesis. On the basis of existing knowledge of this system, the most likely source would appear to be diagenetic release of particle-bound Zn of riverine origin within the fine, organic-rich sediments of the upper harbour basin (Dickson and Hunter, 1981; Hunter and Lee, 1986). Most of the dissolved Mn(II) in Otago Harbour waters is of diagenetic origin rather t h a n from direct input of riverine Mn(II) (Hunter and Lee, 1986). The distribution of dissolved Cu in this system appears to be controlled by the same process (Dickson and Hunter, 1981; Hunter et al., 1984). Much the same situation may prevail for Zn, and could involve Zn associated with colloidal organics. The difference in Zn and Si behaviour would also be expected if soluble Zn levels had undergone a recent decline in the Water of Leith during October at the same time as silicate levels were increasing. Unfortunately there are few reliable data on zinc in freshwaters to test this hypothesis, in this or any other freshwater system. However, the data that do exist suggest a positive relationship between Zn and Si in freshwater that would mitigate against large changes in the Zn:Si ratio. Figure 5 compares Zn:Si data for this catchment with other data obtained by Ahlers and Hunter (1986) for the Manuherikia River, Central Otago, New Zealand, and a number of its tributaries within a high-altitude subalpine catchment. The mean and standard deviation of reactive silicate and dissolved Zn (Nuclepore 0.4pro filtered) results for 7 samples in May 1983 and 10 samples in December 1983 are shown. These results suggest a positive relationship between Zn and Si in freshwater. The Otago Harbour results (Tables I and II) are all too small to plot clearly on this diagram, and have a mean Zn:Si ratio of 14ngpmo1-1. This is significantly higher than ratios observed for the Water of Leith (3.6 ng #mol 1) as a
386
consequence of the changes in Zn and Si concentrations in the estuary already discussed. Figure 5 also shows the Zn-Si correlations observed by Bruland (1980) and Bruland and Franks (1983) for the North Pacific and the North Atlantic. respectively. There are no other reliable data on Zn concentrations in uncontaminated river waters of which we are aware. Although the database is still limited, it is nonetheless interesting that the Zn:Si ratio established in oceanic waters through the agency of phytoplankton growth and remineralisation should fall within the range of ratios observed in freshwaters. This would suggest a coupling between the rates of supply of Zn and Si to the ocean from terrestrial weathering and their rates of utilisation by diatoms and radiolaria in the ocean. Clearly, more data on Zn and Si in freshwaters are required to substantiate this trend. Silicon is used by these phytoplankton to secrete their opaline exoskeleta. It is difficult to conceive of an active role for Zn in this process. It is more likely that Zn becomes passively incorporated as a trace constituent of the silica mineral. Thus, the apparent similarity of Zn:Si ratios in plankton and freshwaters may mean that the degrees of incorporation of Zn into diatomaceous opal and terrestrial source sediments are similar. Clearly, future work on the mechanism and rates of incorporation of Zn into phytoplanktom and the corresponding rates of incorporation into freshwater through terrestrial weathering, will be required to elucidate details of the marine geochemical cycle of this trace element. Finally, if such a causal relationship between Zn:Si ratios in plankton and N At]ant;c
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387 f r e s h w a t e r i n p u t s t o t h e o c e a n e x i s t s , i t is p o s s i b l e t o s u g g e s t r e a l i s t i c l i m i t s f o r the c o n c e n t r a t i o n of Zn in u n c o n t a m i n a t e d r i v e r s in o r d e r to assess e x i s t i n g data. Reactive silicate concentrations in freshwater depend largely on rock t y p e a n d c a t c h m e n t t e m p e r a t u r e ( M e y b e c k , 1981) a n d r a r e l y e x c e e d t h e e q u i l i b r i u m s o l u b i l i t y o f a l u m i n o s i l i c a t e m i n e r a l s o f a b o u t 400 # m o l k g 1 (Liss, 1976). W i t h t h i s m a x i m u m [Si], t h e c o r r e l a t i o n s i n F i g . 5 w o u l d s u g g e s t t h a t f r e s h w a t e r d i s s o l v e d Zn l e v e l s g r e a t e r t h a n a b o u t 3 0 0 0 n g Zn k g 1 w o u l d n o t be expected. The Zn:Si ratio of a river water may thus provide a useful criterion for assessing the l i k e l i h o o d of possible sample c o n t a m i n a t i o n d u r i n g c o l l e c t i o n or analysis. REFERENCES Ahlers, W.W. and Hunter, K.A., 1986. Water quality and trace metals in the Manuherikia River, Central Otago. Aust. J. Mar. Freshwater Res., in preparation. Bewers, J.M., Sundby, B. and Yeats, P.A., 1975. The distribution of trace metals in the western North Atlantic off Nova Scotia. Geochim. Cosmochim. Acta, 40: 687~96. Boyle, E., Collier, R., Dengler, A.T., Edmond, J.M., Ng, A.C. and Stallard, R.F., 1974. On the chemical mass balance in estuaries. Geochim. Cosmochim. Acta, 41: 1719-1728. Bruland, K.W., 1980. Oceanographic distributions of cadmium, zinc, nickel and copper in the North Pacific. Earth Planet. Sci. Lett., 47: 176~198. Bruland, K.W. and Franks, R.P., 1983. Mn, Ni, Cu, Zn and Cd in the western North Atlantic. NATO Conf. Ser. 4, 9 pp.; Trace Met. Seawater: 395414. Danielsson, L., Magnusson, B., Westerlund, S. and Zhang, K., 1982. Trace metal determinations in estuarine waters by electrothermal atomic absorption spectroscopy after extraction of dithiocarbamate complexes into Freon, Anal. Chim. Acta, 144: 183~188. Dickson, R.J. and Hunter, K.A., 1981. Copper and nickel in surface waters of Otago Harbour. N.Z. J. Mar. Freshwater Res., 15:475 480. Grundy, R.G., 1985. The behaviour of dissolved silica within Blueskin Bay and its tributary waters. M.Sc. Thesis, University of Otago, 142 pp. Hunter, K.A. and Lee, S.L., 1986. The dynamic balance of manganese transport and diagenesis in the Otago Harbour, New Zealand. Mar. Chem., 19: 17~192. Hunter, K.A., Ho, F.W.T., Lee, S.L. and Tyler, S.R., 1984. Trace metal geochemistry in coastal and shelf waters. In: K.M. Gawne (Editor), Proc. Syrup. on Trace Metals in the Environment. Deakin University. Koroleff, I., 1976. Analysis of micronutrients. In: K. Grasshof (Editor), Methods of Seawater Analysis. Verlag-Chimie, Berlin, pp. 134 145. Kuehnen, E.C., Alvarez, R., Paulson, P.J. and Murphy, T.J., 1972. Production and analysis of special high purity acids purified by subboiling distillation. Anal. Chem., 44:2050 2056. Liss, P.S., 1976. Conservative and non-conservative behaviour of dissolved constituents during estuarine mixing. In: J.D. Burton and P.S. Liss (Editors), Estuarine Chemistry. Academic Press, London, pp. 98-130. Meybeck, M., 1981. Pathways of major elements from land to ocean through rivers. In: River Inputs to Ocean Systems. United Nations, New York, pp. 1~30. Patterson, C. and Settle, D.M., 1976. The reduction of orders of magnitude errors in lead analysis of biological materials and natural waters by evaluating and controlling external sources of industrial lead contamination introduced during sample collection, handling and analysis. In: D.M. la Fleur (Editor), N.B.S. Special Publication No. 422, National Bureau of Standards, Washington, DC, pp. 321-323. Veillon, C. and Reamer, D.C., 1981. Preparation of high-purity volatile acids and bases by isothermal distillation. Anal. Chem., 53: 549-550. Yeats, P.A., Bewers, J.M. and Walton, A., 1978. Sensitivity of coastal waters to anthropogenic trace metal emissions. Mar. Pollut. Bull., 9:264 268.