an assessment of metal contamination of sediments in the humber estuary, u.k

Estuarine, Coastal and Shelf Science (1990) 31,71-85

An Assessment of Metal Contamination of Sediments in the Humber Estuary, U.K.

Alastair GranP and Richard Middletox# “Department of Applied Biology and Institute of Estuarine and Coastal Studies,

“School of Earth Resources, Hull University, Hull, HU6 7RX, U.K.

Received 17 November 1989 and in revisedform 21 June 1990

Keywords: chemical pollution; contaminant measurements; estuaries; heavy metals; phosphorus; sediments; titanium dioxide industry

It is difficult to make an overall assessment of the degree of metal contamination in estuarine and marine sediments. This is a consequence of variations in analytical procedures between studies and the presence of an unknown natural background in the sediment. Measurement of total (rather than extractable) metal and normalization of concentrations as ratios to an element associated with clays provides a solution to the first difficulty. Expressing these values as enrichment factors relative to pre-industrial sediments from the same environment solves the second.

Levels of anthropogenic enrichment of intertidal sediments in the Humber Estuary have been assessed relative to a baseline provided by sediments deposited in the Humber approximately 5000 years BP. A sample of consolidated Holocene mud estimated to be at least 100 years old confirmed the appro- priateness of this baseline. Normalization relative to Rb, which is not anthropogenically enriched, was the most suitable way to adjust for grain size.

Levels of Ti, Fe, P, V, Cr, Mn, Ni, Cu, Zn, As, Y, Nb and Pb are elevated above this baseline. The most marked enrichments (between 3% and 6-fold) are of P, As, Pb, Cu and Zn. Normalized concentrations were spatially rather uniform with two exceptions. A single sample from the north bank showed elevated levels of Pb, Cu, Zn and Cr. An area receiving effluents from an industrialized zone on the south bank, including two titanium dioxide processing factories, showed high levels of a number of elements, particularly Nb. It is suggested that Nb may be a valuable tracer for effluents from the sulphate process of TiO, extraction.

Introduction

A recent report on the quality status of the North Sea (Department of the Environment, 1987) identified several difficulties in assessing the degree of metal contamination of marine and estuarine sediments. These include the variety of extraction methods and adjustments for grain size effects, the presence in sediments of an unknown natural background and difficulties in assessing trends resulting from inconsistencies of analytical procedure. Here we present procedures which circumvent these difficulties, and illustrate

&Present address: School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, U.K.

O272-7714/90/070071+ 15 $03.00/O @ 1990 Academic Press Limited

72 A. Grant ~3 R. Middleton

Figure 1. (a) Location of sampling sites on Humber Estuary. The confluence of the Trent and the Ouse, known as Trent Falls, is indicated. Numbers indicate distances below Trent Falls in kilometres. + , area near Easington from which samples used as baseline were obtained; l location of consolidated mud sampled from erosion surface at Hessle, just west of Hull. (b) Location of (a) in Great Britain.

the methodology with analyses of sediments from the Humber Estuary. Concentrations of a range of metals in samples of intertidal sediments are compared with a baseline provided by samples which were laid down in creeks of the Humber approximately 5000 years B.P.

The Humber, one of the largest British estuaries, has been variously described as ’ one of the cleanest of the deepwater British estuaries ’ (Shell U.K., 1987) and as the third most polluted estuary in the U.K. (Greenpeace, 1987). That such contradictory views can be expressed is perhaps a reflection of the fact that the estuary has received relatively little scientific study. The relative sparsity of data on, and lack of a baseline for, heavy metals in the estuary has been previously identified (Greenpeace, 1986). Levels of trace metals have been surveyed on an area of the south bank of the outer estuary (Jaffe & Walters, 1977) and wider scale surveys of metals in sediment and biota have been carried out by Water Authorities (Barnett & Ashcroft, 1985; Barnett et al., 1989; Edwards et al., 1987).

Materials and methods

Sample collection and preparation Samples of surface mud were taken from 30 sites on the Humber (Figure I). The majority were collected in the period 14 June to 18 July 1988, although five were collected on 6 October 1988. Samples were located on both north and south banks of the estuary, from near its mouth to just above Trent Falls, the confluence of its two major tributaries, the Trent and the Ouse. Most samples were taken on the upper half of the intertidal area. On the graphs presented here, locations of sites are indicated by their distance from Trent Falls.

A further sample was taken from an erosion surface of consolidated mud estimated from its position to be at least a century old (Figure 1). Further information on the stratigraphy of this sample will be presented later as part of a study of the history of metal contamination in the Humber. The estuarine muds used to erect the baseline were sampled near Easington on the North Sea coast (Figure I), from intertidal exposures of Scrobiculariu clays laid down in former channels of the Humber (Bisat, 1952; Gaunt & Tooley, 1974;

Metal contamination of sediments 73

TABLE 1. Elemental composition of a pre-industrial mud containing 100 pg g ’ Rb; calculated from data on baseline samples from Easington (Middleton & Grant, 1990)

Major elements PO) Trace elements (W8 ‘1

A&O, TiO Fe& K$ PZO,

so,

13.4 0.7 5.0 2.5 0.1 0.8

V Cr

Mn Ni CU Zn As Rb Sr Y

Nb La Ce Pb

109 99

425 38 17 a4

(1::) 120

27 14 34 74 22

Middleton & Grant, 1990). Elemental concentrations for this baseline are shown in Table 1.

Immediately after collection, sediments were dried at 95 -C, disaggregated with a pestle and mortar and screened at 500 pm to remove coarse particles. Organic carbon levels were determined by titration with dichromate (Gaudette et al., 1974). Samples for X-ray fluorescence (XRF) analysis were ashed for 1 hat 500 ‘C, then micronized in an agate mill. Analysis of ashed and untreated subsamples of the same sediment showed no significant loss of even the more volatile elements such as As during this process.

XRF analysis All analyses were carried out using a Phillips PW1410 X-ray fluorescence spectrometer. For 24 samples, a comprehensive XRF analysis was carried out, starting with a major element analysis on a fusion disc (Norrish & Hutton, 1969). Levels of major elements were then used to calculate X-ray mass absorbances for the sample and these were used in carrying out trace element determinations on pressed pellets made with 5 g of sediment held with an organic binder. Calibrations were based on a series of international silicate rock standards. Too few suitable reference materials were available for arsenic, so this calibration was performed with two spiked muds. The following spectral overlaps have been compensated for: Rb on Y; Ti on V; Sr on Zr; Pb on As; Ce on Ba.

The fusion method yields accurate and precise determinations of major elements, but involves fairly lengthy and painstaking sample preparation. A marked saving in sample preparation time would be achieved if major elements could be determined on the pressed powder samples used for trace elements. Preliminary investigations showed that satisfactory calibrations could be obtained for the heavier major elements (e.g. Fe and Ti) but not for the lighter elements (including Al and I’).

The remaining samples were therefore analysed using an alternative approach. The intensity of a Compton scattered X-ray line from the spectrum of the spectrometer tube target, in this instance the Rh Ku line, can provide a good measure of mass absorbances for elements with analytical wavelengths shorter than that of the iron absorption edge. This makes it possible to analyse pressed pellets for a range of heavy metals without having a

74 A. Grant & R. Middleton

major element analysis available. Satisfactory calibrations were obtained for Zn, Cu, Ni, Nb, Zr, Sr, Rb, Y, As and Pb using least squares calibration, again based on international silicate rock standards for all elements except As. Agreement between the results of the two approaches was close. Replicate analyses and replicate samples from the same site generally agreed to between 2 and 5q.;,, except for Cr where agreement was better than 200,.

In this paper, we have chosen to normalize metal concentrations using rubidium as a grain-size proxy. The reasoning leading to this choice is given in the results section. Enrichment factors have been calculated as:

with the Holocene estuarine clays from Easington being used as the baseline (Middleton & Grant, 1990). The ratio [fl/[Rb] has been calculated for each baseline sample and the mean of these used. Equivalences between these enrichment factors and metal levels in a typical Humber mud (right hand axis on Figures 3 and 4) have been calculated on the basis of a Rb content of 100 pg g-‘, which would correspond to an A&O, content of about 13.59;. This is close to the mean Rb content for the muds sampled [see Figure 2(c)].

Results

Effect of grain size on metal concentrations In estuarine and marine sediments, there is usually a dependence of metal levels on grain size, resulting from the association of metals with the finer particles. Some workers circumvent this by separating out the finer particles for analysis, but there is no general agreement on procedures for this (Ackermann et al., 1983; Forstner & Witmann, 1981) and to do so increases sample preparation time.

In this study we have compensated for this effect by normalizing metal concentrations as ratios to another constituent of the sediment. The constituent chosen for this purpose should also be associated with finer particles and its concentration should not be anthropo- genitally altered (Ackermann, 1980). Selection of such a ‘ grain size proxy ’ is best done by inspection of scatter diagrams showing the relationship between levels of pairs of elements. The processes which have led to our choice is illustrated using Cu, but patterns for other metals are very similar. Al represents an obvious choice, as it is a major constituent of clay minerals and has been used successfully elsewhere (Kemp et aZ., 1976; Ryan & Windom, 1988). Cu and Al show a reasonably close linear relationship [Figure 2(a)] indicating that Al would be a useful grain size proxy. However a satisfactory calibration for Al was not obtainable on pressed pellets, so this element can only be used for the samples on which full major element analyses were carried out. There is also some evidence for an anthropogenic input of Al to the estuary (see below).

After Al, the next most obvious candidates are Fe (used by Sinex & Wright, 1988) and Ti (which is also associated with clay minerals). A potential difficulty with using Fe is that in certain circumstances this element can be mobile during diagenesis (Finney & Huh, 1989). A very real difficulty with using either Fe or Ti in the Humber is that the estuary is noticeably and heterogeneously contaminated with both elements (Middleton & Grant, 1990 and see below). A plot of Cu levels against Fe [Figure 2(b)] shows a good linear relationship, except for seven samples, which have anomalously high levels of Fe. These


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samples also show high levels of Ti. They are taken from an area of shore which receives large discharges of both these elements in effluent from two titanium dioxide processing plants (House of Lords, 1984). For these reasons, neither of these elements are suitable for making comparisons with the baseline. It is worth noting that these difficulties would not be revealed simply by examining correlation coefficients. Away from the area influenced by this discharge there is a closer relationship between Fe and Cu than between Rb and Cu. Cu concentrations are locally elevated in the area of the discharge, so the correlation between Fe and Cu is not markedly reduced. A more detailed statistical analysis of sediment geochemistry is given elsewhere (Grant, 1990).

The most suitable element proves to be Rb. This shows a good correlation with levels of most metals [see Table 2 and, for example, Figure 2(c)]. It can be measured accurately on pressed pellets and shows no evidence of anthropogenic enrichment (Middleton & Grant, 1990; see also Allen & Rae, 1986; Allen, 1987). The only other common element which fulfils these criteria is K. It is perhaps significant that these are both Group I elements


TABLE 2. Non-parametric correlations of element concentrations with concentranon of Rb, excluding two sands and samples from area influenced by discharge from TiO, processing industry

Al$, SiO, TiO, Fe,O, MgO CaO Na20 K,O PzO, SO, Zn Cu Ni

0.98 -0.87 0.95 0.97 0.87 0.66 0.42 0.98 0.87 0.88 0.85 0.79 o-90

Mn 0.78

Cr V Nb Zr Y Sr Pb As Ce Ba La Carbon 0.84 0.93 0.98 -0.83 0.61 0.88 0.90 0.80 0.60 0.22 O-76 0.71

which are associated with clays. Ackermann (1980) found that Cs proved most suitable for grain size normalization, while Loring (1990) has favoured Li, both of which are also in Group I. Cs is however usually present at rather low concentrations, so its measurement requires the use of a rather sensitive analytical method such as neutron activation analysis and Li is difficult to measure using XRF. Relationships between metal concentrations and organic carbon (used by, for example, Luoma & Phillips, 1988) are much poorer [e.g. Figure 2(d)] so this is not the best choice in the Humber. There are also some indications that wet oxidation gives erroneously high values for organic carbon in the region of the shore receiving titanium dioxide effluent, probably due to the presence of large amounts of iron II in the effluents. The need for grain size normalization can be clearly seen from Figure 2. Even amongst sediments which would be classified as muds (Rb content greater than 75 ppm), a roughly two-fold variation in metal levels may be a consequence entirely of grain size effects. If sands are included, this rises to more than four-fold.

We can examine how closely levels of an element are associated with grain size by examining the correlation of its concentration with Rb (Table 2). Zr and Si are negatively correlated with Rb, and the correlation with Ba is not significant. High Si values presumably indicate the presence of relatively coarse grains of quartz. Zr and Ba are likely to be present as the heavy minerals zircon and barytes respectively which, because of their high density, will behave hydrodynamically as coarser grains. Other elements are positively correlated with Rb, and the correlation is very strong for all except Ce, Y, Mn, Ca and Na. The strength of these relationships suggests that when studying metal contamination, a good first approximation is to consider Humber sediments to be a two component mixture. Metals are associated with the finer component (particularly clays). This is

diluted with varying amounts of quartz. Reporting metal concentrations as ratios to Rb will therefore substantially reduce the level of variability in comparison with raw data.

Evidence in support of the baseline The presence of an unknown natural background of metal in sediments makes it very difficult to assess the degree of anthropogenic enrichment of metal concentrations. The composition of ‘ average shale ’ (Turekian & Wedepohl, 1961) is often used as a baseline, but a pre-industrial baseline for the area under study is clearly the preferred choice. A full discussion of the 5000 B.P. baseline for the Humber has been presented elsewhere (Middleton & Grant, 1990), showing that for some elements there can be up to a two-fold difference between this and average shale, with one of the largest differences being for copper, which is one of the most toxic common contaminants.

Here we present further evidence in favour of the use of this baseline. The sample taken from a consolidated mud platform at Hessle is compared with the 5000 B.P. baseline in


TABLE 3. Enrichment factors for consolidated mud sample from Hessle, relative to baseline at 5000 B.P.

Element Pb As Zn Cu Ni Cr Nb Sr Y

Enrichment factor 1.5 1.0 1.1 1.5 1.0 0.9 1.1 1.6 1.1

TABLE 4. Concentrations of metals remaining in single mud sample after leaching with acids

Metal

Initial concentration

(M?g ‘1

o,i of initial after treatment with

Concentrated 1MHCl HNO, (“o) (“,a 1

Rb 86 107 114 Nb 24 116 108 Zn 336 16 34 cu 67 13 28 Ni 65 28 57 Cr 164 46 74 As 79 32 49 Pb 132 15 17

Table 3. As, Zn, Ni, Cr, Nb and Y are at levels very close to the baseline. Pb, Cu and Sr are present at slightly higher values than in the baseline samples. The elevation of Pb and Cu may indicate some spatial variation in primitive metal levels, or may indicate that the sample dates from the early industrial period and is slightly contaminated by man’s activities.

Extractability of metals

X-ray fluorescence measures the total quantity of metal present and gives results comparable with those of neutron activation analysis or total dissolution in HF. This avoids difficulties resulting from the dependence of the results of extraction techniques upon sample pre-treatment (Rapin et al., 1986; Kersten & Forstner, 1987). The proportion of metal which is nitric acid labile is likely to be less severely affected by sample pretreatment than gentler extractions which are sensitive to the redox state of the samples. Given that HCl available metal has been shown to correlate more closely with biological availability than other extraction methods for several metals (Luoma, 1983) a case could be argued in favour of concentrating effort on totals and HCl extractable metals.

To estimate the proportion of metals extractable by acids, and to enable comparison with other work, batches of a single sediment were leached with 1 M HCl and HNO, with a third batch being washed with distilled water. The residue was separated by centrifugation, washed with distilled water, then metal concentrations were measured. The results are presented in Table 4. Cu, Zn and Pb are rather loosely bound; As, Ni and Cr more strongly so. The extraction of other elements from the sediment (presumably Fe in particular) leads to an increase in Rb and Nb content. The fact that acids have little

78 A. Grant Q R. Middleton

impact on Rb concentrations suggests that the majority of Rb present substitutes for Na and K in clay minerals and further supports its use as a grain size proxy.

Spatialpattern of contamination Enrichment factors are plotted against distance from Trent Falls in Figures 3 and 4. Fe, Ti, V, Nb, Y, and SO, show a similar pattern with elevated levels on the south bank of the outer estuary, adjacent to the industrial zone [Figures 3(a)-(e), 4(a),(b)]. Cr appears to be enriched in this area, although there are also locally elevated levels elsewhere [Figure 4(b)]. There may also be some enrichment with Al and Y [Figures 3(e) and 4(b)]. This part of the Humber receives effluent from two titanium dioxide processing plants which make use of the sulphate process and produce effluents containing substantial levels of most of these metals (Jaffe & Walters, 1977; House of Lords, 1984). The influence of these discharges can be seen from red iron staining on the littoral zone and sediment chemical composition in this area is distinctive (Grant, 1990). Away from this area, levels of enrichment are rather uniform, ranging from 1.3 for Y to 2.2 for Nb. Maximum and median enrichments are shown in Table 5. These indicate that the effluents have had a major impact on the whole estuary, even raising levels of the rather abundant elements such as Fe and Ti by more than 500,, . The degree of enrichment for a particular element will depend upon the relative magnitude of the amount discharged and the level of natural background. Although large amounts of Fe and Ti are discharged, background levels of these elements are also high. Concentrations of S in this area are substantially elevated above those in the remainder of the estuary [Figure 3(d)], reflecting the discharge of sulphuric acid in effluent from the sulphate process. Enrichment factors are however < 1 in most of the estuary, suggesting that the Easington samples do not provide a realistic baseline for this element.

The most marked enrichment is of Nb, as background levels of this element are low in the Humber, and generally low in sedimentary rocks, with an average of 11 ppm in shales (Turekian & Wedepohl, 1961). Nb has been reported at very high levels in ilmenite ore from which titanium dioxide is extracted (Wedepohl, 1978). Nb compounds have extremely low solubilities (Wedepohl, 1978; see also information on extractability above) and this, together with grain size relationships, suggests that Nb occurs in the Humber as insoluble fine particulates. This element may be useful elsewhere as an indicator of the influence of waste from titanium dioxide processing. The samples from the Trent and the Ouse are very similar in Nb content to those from lower down the estuary, indicating substantial mixing and upstream transport of sediments to beyond Trent Falls.

The only other site showing substantially elevated levels of several elements comes from the north bank just east of Hull. This shows very high Pb levels [enrichment factor (E.F.) = 23.71 and lower levels of Cu, Zn, As and Cr (E.F. 12.9, 11.8, 7.9 and 3.3, respectively). This enrichment may be associated with the presence nearby of a sewage outfall, dock or industrial discharges.

In the estuary as a whole, Pb is substantially enriched above baseline, with a median enrichment of 5.8. In addition to the sample noted above, the single sample from the river Hull showed elevated levels [Figure 4(c)]. Median enrichment of As is 5.0, with the highest value being at the site discussed previously. Levels show a slight peak in the inner estuary [Figure 4(d)], near a known discharge on the north bank. Apart from this, levels are largely uniform, although concentrations appear to be more heterogeneous than those of other metals. Concentrations of As are rather higher than in many other British estuaries, but more than an order of magnitude lower than those in some of the estuaries in


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TABLE 5. Median and maximum enrichments for elements elevated in sediments adjacent to industrial zone of the south bank of the Humber

Median enrichment Maximum enrichment

Ti 1.7 4.4 Fe 1.6 2.5 V 1.8 3.1 Nb 2.7 8.1 Y 1.6 2.2 so3 0.7 1.3 A&O, 1.1 1.3 Cr 2.2 29

,‘Maximum level in samples from this area, highest values of Cr are elsewhere in the estuary.

the mineralized areas of south-west Britain (Langston, 1985, 1986). Because of the input of large amounts of iron to the outer estuary, normalizing As levels using Fe (as done by Langston, 1980) gives misleading results, with the As:Fe ratio being depressed at the seaward end. Coupled with the slight elevation due to an As discharge to the inner estuary, the impression given is of a gradual decline of As:Fe moving seawards.

The patterns for Cu, Zn and Ni are similar [Figure 4(e)-(g)], with no other locally elevated concentrations and median enrichments of 3.9,3.9 and 1.5, respectively. There is little enrichment of Cr (median E.F. = 2.1). Away from the industrial zone of the south bank, concentrations are fairly uniform [Figure 4(h)]. A single high value of 3.7 is not associated with any known discharges. When adjusted for grain size effects, Mn levels are highest at the upstream end of the estuary and reduce towards the mouth [Figure 3(i)]. There is some elevation of levels at two sites on the outer estuary, but this is not as marked as might be expected given the high discharges of Mn here (House of Lords, 1984). High Mn levels have been found at the inland end of Chesapeake Bay (Helz et al., 1985) and the Tamar Estuary (Ackroyd et al., 1987-these authors give an explanation of the pattern in terms of the known behaviour of Mn). Several other studies have, however, found no marked longitudinal gradients of this element (Bryan & Hummerstone, 1973; De Groot t’~ ul., 1976; Langston, 1986). It is not clear whether the observed gradient is due to remobilization of Mn from sediments (Morris et al., 1982) or to large inputs of this element in the tidal rivers. In view of the dependence of Mn levels on position in the estuary, enrichment factors are not strictly meaningful as levels in the baseline samples are unlikely to be typical of the estuary as a whole.

P is substantially enriched (median E.F. = 4,7), presumably as a result of inputs from domestic sewage, and levels are largely uniform throughout the estuary [Figure 3(h)]. When expressed as elemental P, the concentrations range from 0.18 to 0.32”” for the muds and are within the range quoted for Netherlands estuaries by Salomons and Gerritse (1981), and substantially lower than their maximum value of 0,89O,,.

Levels of Ce, La and Sr are largely uniform [Figures 3(f),(g), 4(i)], although two outer estuary sites on the south bank show some elevation of Sr levels. These are both sands, and the high Sr levels probably reflect the presence of molluscan shell debris. The enrichment of Sr in current sediments (median 1.6) may reflect some loss of Sr in early diagenesis from the muds used as a baseline or perhaps an accumulation of shell debris in the estuary. Table 3 shows the same level of Sr in the consolidated mud from Hessle, so this is unlikely to represent anthropogenic enrichment.

a2 A. Grant & R. Middleton

TABLE 6. Comparison of metal levels in this study (excluding two sands) with literature data for the Humber and maxima recorded in south-west Britain by Bryan et al. (1980)

Metal

Humber South-west

Jaffi and Edwards Bryan Walters et al. et al.

Present study (1977)” (1987) (1980)

Median Maximum Mean Mean Maximum

Zn 319 914 cu 70 206 Ni 55 81 Cr 212 422 Pb 127 469 As 103 173 TiOz 1.3”,, 30,, V 209 328 Mn 677’ 2386 Fe,Ol 8,8”,, 12.2”,,

314 118 59

163 158

1.38”,,h 1292 951

11.55”,,b

260 3515 49 2540 39 39 79 799

104 2175 54 3520

1160 9.0”,,1

Levels in ug g I, except where indicated. dMean of analyses of four samples classified as ‘ anoxic muds ’ by Jai% and Walters (1977). Converted to a percentage of metal oxide for comparability. {Based on mean of muds from same area as Jaffe and Walters in view of effect of position in estuary on sediment Mn concentrations.

Discussion

This study shows the importance of adjusting for particle size when studying metal contamination. Normalization using ratios to an element associated with fine particles is a considerably easier process than using methods which involve an initial separation of fine particles. The use of XRF allows the straightforward evaluation of a range of possible methods of grain size normalization. The method also allows measurement of elements such as Nb which would otherwise require total dissolution in HF.

Metal concentrations in the sample of consolidated mud from Hessle support the baseline used. Levels of Pb and Cu in this sample are slightly higher than the Easington baseline but are still very much lower than current levels in the Humber. Sr concentration is higher than in the baseline samples, but comparable to current Humber levels. This is despite the considerable spatial separation between Hessle and Easington.

Metal levels measured in this study are broadly comparable to those reported previously for the Humber (Table 6). The only marked exception is V, for which our measurements are substantially lower than those of Jaffe and Walters (1977). A possible explanation for this is that these workers do not appear to have accounted for spectral overlap between V and Ti. The somewhat lower concentrations reported by Edwards et al. (1987) can be explained by their use of nitric acid digestion. An additional factor is that the majority of our samples have been ashed, leading to a weight loss of up to 12”,, .

When the influence of grain size is removed, our results show that levels of metals are remarkably uniform, with the exception of those elements released by TiO, processing. This uniformity is not apparent in other studies of the Humber estuary which have not compensated for grain size effects (Jaffk & Walters, 1977; Edwards et al., 1987). For


elements such as Cu and Zn this may simply reflect input from multiple sources, including sewage, trade effluents and the tidal rivers (Edwards et al., 1987). The major source of As is however a single trade effluent, and this element is also distributed rather uniformly throughout the estuary. This implies that in a macrotidal estuary such as the Humber tidal mixing renders sediment metal levels a poor indicator of point sources of contamination. This uniformity is however an advantage in assessing the degree of enrichment relative to a baseline for the river as a whole. The Severn Estuary is similarly rather uniform in metal concentrations once grain size effects have been removed (Allen, 1987).

Levels of most elements are elevated above baseline, with the greatest enrichments being of As, I’, Pb, Cu and Zn. Metal concentrations are however not high in comparison with maximum concentrations reported for south-west Britain by Bryan et al. (1980), with the exception of Fe, Mn and Ni (Table 6). Levels of Fe are notably high in the Humber (c.f. Bryan, 1984). Mn and Ni levels in south-west Britain are rather low (Langston, 1986) and while Mn concentrations are difficult to compare with the baseline, Ni is only moderately enriched in the Humber. The Humber is clearly not the third most contaminated estuary in the U.K. as judged by heavy metal concentrations in sediments. It is substantially enriched with metals when compared to its original state. This enrichment is, however, only about a third of the enrichment which has taken place in the Rhine (Salomons & De Groot, 1978). The observed enrichments are much greater than would result from the roughly loo,, enhancement of metals in surface muds which can result from diagenetic processes (Elderfield & Hepworth, 1975), and so presumably derive from industrial sources.

Acknowledgements

This work was carried out while A.G. was supported by a grant from Capper Pass, Ltd to N. V. Jones. We are grateful to Chris Park for assistance with sample preparation. The maps in Figure I have been produced using data relating to digitized boundary information and these data remain the property and copyright of the Crown.

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84 A. Grant&R. Middleton

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