Estuarine and Coastal Marine .f%%‘nce (1977) 5, 4X5-419

Notes and Discussions

Interactions Between Zinc and Suspended Sediments in the Fraser River Estuary, British Columbia

D. Grieve and K. Fletcher Department of Geological Sciences, University of British Columbia, 2075, Wesbrook Place, Vancouver, B.C., Canada V6T IW~

Received 16 March 1976

Behaviour of Zn has been studied in the Fraser River estuary. Increases in dissolved and suspended Zn in the mixing zone between fresh and brackish waters demonstrate the importance of both adsorption and desorption phenomena in estuarine waters. Together with estuarine circulation these processes provide a mechanism for retention of heavy metals in coastal zone sediments and waters.

Introduction

Despite the importance of estuaries as transition zones through which materials pass on their way to the oceans, few data are available on trace element geochemistry in estuarine environments. Desorption of trace metals from suspended sediments coming into contact with saline waters has been reported in experimental (Kharkar et al., 1968; Murray & Murray, 1973) and field (Evans & Cutshall, 1973; Groot, 1973) studies but appears to be

VANCOUVER

Figure I. Sampling stations in the Fraser River estuary with conductivity (umho/cm at I m depth) in parentheses. Insert diagram shows sampling times relative to tidal height at Vancouver; extremes are progressively delayed upriver by up to approxi- mately 5 h at Station 9.

4x5

416 I). Griew & K. Fletcllel

inconsistent with overall retention of trace metals in coastal regions (Turekian, 1971).

Preliminary data for Zn in the estuary of the Fraser River are relevant to this problem. The Fraser River drains approximately 233 ooo km2 of the interior of British Columbia

(Hoos & Packman, 1974). Discharge varies from more than 8500 m3/s during the summer freshet to less than 850 m3/s in January and February. In the estuary a wedge of salt water penetrates upriver beneath brackish surface waters, the extent of penetration depending on riverflow and tide. This study was undertaken in August 1975 when river discharge was approximately 2800 m3/s and brackish surface waters could be detected as far inland as Station 5 (Figure I).

Materials and methods

Duplicate 2-l water samples were collected from a depth of I m. Water temperature and conductivity were measured in situ. Waters were filtered under N, pressure through 0.45 pm cellulose nitrate (Sartorius) filters and the filtrate analysed for Zn (Zn,) and Fe (Fe,) by atomic absorption after extraction into APDC-chloroform at pH 2.5. After drying and weighing, suspended sediments were digested with either HF-HNO,-HClO, or by leaching with I M-hydroxylamine hydrochloride in 25% acetic acid to remove hydrous oxide coatings of Fe and Mn as recommended by Chester & Hughes (1967). Zn, Fe and Mn were deter- mined on the resulting solutions by atomic absorption.

Results and discussion

Results (Table I, Figure 2) show a decrease in suspended sediments from 25 mg/l at Station 9 to 4 mg/l at Station 2. Only a small change in the suspended sediment load was found between Stations 5 and 4, and Milliman (personal communication) has observed considerable resuspension of bottom sediments in this area during ebb tides. Visual inspection of the filters indicates that suspended sediments are principally silts.

TABLE I. Conductivity at I m depth together with total (HF-HNO,-HC104) and hydroxylamine hydrochloride-acetic acid extractable Zn, Mn and Fe for suspended sediments from the Fraser River estuary

Zn’Gw/k) Mn b-x/W Fe (73 Conductivity -

Station bho/cm) Tot. Ext. Tot. Ext. Tot. Ext. ~-

i 106 120 ‘65 191 43 40 838 919 232 257 4’4 4.8 0.26 0.30

2 122 12s 150 178 4’ 57 921 839 290 188 4’2 4’7 0.16 0.35 5 570 167 31 819 275 4’3 0.29 4 4oo 349 190 794 3’3 4’1 0.35 3 go00 418 219 798 3x0 4’5 0.43 2 27 500 542 464 858 289 4’3 0.30

Changes in concentration of suspended Fe (Fe,), together with Mn, closely follow variations in suspended sediment load. Suspended Zn (Zn,) follows the same trend in the freshwaters between Stations 9 and 5, However, in the brackish waters between Stations 5 and 4 there is an increase from 2.8 pg/l to 5.8 yg/l. Th is is equivalent to a change in Zn content of 167 mg/kg to 349 mg/kg dryweight. From Stations 4 to 2 there is a further increase to 542 mg/kg.

Zinc and suspended sediments in Fraser River Estuary 417

Assuming an average concentration of 170 mg/kg Zn for river-borne sediments, approxi- mately 50% and 70% of the Zn associated with suspended sediments at Stations 4 and 2

respectively, must have been adsorbed within the brackish waters of the estuary. However, concentrations of Zn, also increase in passing from fresh ((‘-1.4 c(g/l) to brackish (2+6- 3.3 pg/l) waters, although the underlying salt water intrusion contains only 1.4 pg/l Zn.

The increase in both Zn, and Zn, on entering brackish waters cannot be explained by mixing with seawater or by simple sorption-desorption reactions and implies an increased residence time for Zn in the estuary relative to that of freshwater. Recycling of suspended sediments and resuspension of bottom sediments are well-known phenomena in estuaries (Schubel, 1968; Postma, 1967; Dyer, 1972). Large-scale removal of dissolved iron also occurs in estuaries (Boyle et al., 1974) and experimental studies (Aston & Chester, 1973)

suggest that turbidity favours its precipitation as hydrous oxide coatings on suspended sediments. The scavenging ability of such coatings for trace elements is well known (Jenne, 1968; Gibbs, 1973). Detailed movement of suspended sediments in the Fraser River estuary is not yet established. However, resuspension of bottom sediments and a decrease in con- centrations of Fe, (from 3 yg/l to <I pg/l) all occur between Stations 5 and 3. Behaviour of Zn might, therefore, be explained by its adsorption onto hydrous iron-oxide coatings developed as suspended or resuspended sediments encounter new parcels of freshwater.

H----- 'Suspended sediments

P IO

5 t/

500 f I'

/’

statm Figure z. Variations in suspended sediment load; total suspended Fe (Fe.) and Zn (Zn.) ; hydroxylamine hydrochloride-acetic acid extractable Zn (Zn,) ; and dissolved Zn (Zn,).

To test this model, Zn associated with hydrous oxide coatings was determined by leaching the duplicate set of suspended sediments with I M-hydroxylamine hydrochloride in zs’+G acetic acid (Chester & Hughes, 1967). There is a more than fivefold increase of leachable Zn (Zn,) in passing from freshwaters (average of 42 mg/kg Zn,) to brackish waters (more than 200 mg/kg), with the greatest increase occurring between Stations 5 and 4 (Table I,

Figure 2). Corresponding changes in the Zn, : Zn, ratio are from zo”/o Zn, in freshwaters, to 50% at Stations 4 and 3, and to 86% at Station 2-in good agreement with calculated values based on an average of 170 mg/kg for river-borne sediments. Five per cent of the Fe and 25% of the Mn are also extracted from suspended sediments in freshwaters, with an

418 D. Grieoe & K. Fletcher

increase in the amount extracted, though to a much lesser extent than for Zn, between Stations 5 and 4.

These findings, together with decreased concentrations of Fe,, in brackish waters, are consistent with scavenging of Zn by hydrous oxides of Fe and Mn. Levels of leachable Fe, Mn and Zn on suspended sediments in freshwater-s presumably reflect oxide coatings developed during weathering or transport of material downriver. As such they can be compared with results of Gibbs (1973) who found that nearly half of the Fe and Mn associated with suspended sediments in the Amazon and Yukon rivers is present as precipi- tate and coprecipitate coatings. On entering brackish waters the growth of new hydrous oxide coatings promotes further scavenging of Zn. Burrell (1973) has suggested a similar process as sediments settle through the halocline of an Alaskan fjord.

Although the increase in adsorbed Zn values between Stations 4 and 2 suggests that Zn is preferentially associated with the finer fractions of river-borne material remaining in suspension, it is not clear if scavenging takes place in brackish waters throughout the estuary or only in a narrow zone at the transition from fresh to brackish waters. However, since the Zn, : Zn, ratio increases seawards and scavenging is probably closely related to supply of dissolved Fe by the river, the latter seems more likely. Increasing Zn, values would then reflect desorption of Zn as suspended sediments move seawards and encounter more saline waters.

Previous studies (Kharkar et al., 1968; Murray & Murray, 1973 ; Evans & Cutshall, 1973 ;

Groot, 1973) have emphasized desorption of metals from suspended sediments mixed with seawater to account for increased concentrations of dissolved metals in estuarine and coastal waters. This study, though providing further evidence for desorption, suggests that such a view may be an oversimplification. Adsorption and desorption of Zn, together with move- ment of suspended sediments in the estuary, provide a mechanism for increasing concentra- tions of both Zn, and Zn, in estuarine waters compared to those in inflowing freshwaters. Depending on the extent of desorption, eventual deposition of suspended sediments will result in the coastal zone acting as a sink for Zn. This is corroborated by related studies which have shown that distribution of Zn in sediments on the Fraser River delta-front closely reflects the distribution of Fe and Mn (Grieve & Fletcher, 1975).

These conclusions suggest that non-conservative behaviour of dissolved iron and sus- pended sediments in estuaries is significant with respect to the natural flux of trace elements to the oceans and also to the fate of heavy metal pollutants discharged into rivers or estuaries. Results further emphasize the need to simultaneously consider dissolved and particulate phases and both physical and chemical processes in relation to behaviour of trace metals in estuaries.

Acknowledgements

We thank J. Milliman of the Geological Survey of Canada for information on suspended sediments and for many helpful comments. Research was supported by the Geological Survey of Canada, the Department of Lands, Forests and Water Resources of British Columbia, and by the Greater Vancouver Regional District.

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