Continental Shell Research, Vol. 7, Nos 11/12. pp. 1489-1493, 1987. 0278~.343/87 $3.(X) + 0.1)0 Printed in Great Britain. © 1987 Pergamon Journals Ltd.

Influence of long-term sediment transport on contaminant dispersal in a turbid estuary

R. J. UNCLES,* T. Y. WOODROW* and J. A. STEPHENS*

(Received 5 September 1986; accepted 28 March 1987)

Abstract--Theoretical calculations are made of the long-term transport of fine sediment in a turbid estuary, and its possible consequences for the tidally averaged distribution of a contami- nant whose partitioning between dissolved and particulate phases is dependent on salinity. It is found that the partitioning has a crucial effect on the levels of dissolved contaminant, in agreement with the observations of MORRIS (1986, The Science of the Total Environment, 49, 297-304), Calculations also imply that the vertical fluxes of particulate contaminant between water column and bed have a profound influence on these levels.

I N T R O D U C T I O N

A PRACTICAL requirement exists for predicting the long-term (of order years) behaviour of contaminants released into estuaries. This is a particularly complex problem when partitioning can occur between dissolved and particulate forms of contaminant. The objective of this paper is to present results of a simulation of sediment and contaminant transport in a model estuary. Topography and physical conditions approximate those of the Tamar Estuary, U.K. The sensitivity of the contaminant distributions to various aspects of the sediment transport processes is investigated. The cases considered are: (a) all sediment transport processes included; (b) zero horizontal sediment transport; (c) zero vertical flux of particulate contaminant between the water column and bed; (d) zero partitioning.

Observations in the Tamar Estuary (MORRIS, 1986) have shown that the removal of a substantial proportion of the freshwater inputs of dissolved trace metals is a consistent feature of the very low salinity, high turbidity region of the upper estuary. Simple theoretical predictions imply that this removal occurs through rapid uptake onto suspended sediment in the turbidity maximum (MORRIS, 1986). Observations have also shown that sediment in the central reaches of the Tamar can act as a source of dissolved trace metals (MORRIS et al., 1986). Combined sediment and contaminant transport models have an important role to play in increasing our understanding of these processes. Some results from a long-term, one-dimensional, tidally averaged model of contaminant dispersal in the Tamar Estuary have been published (HARRIS et al., 1984). However, in that model the axial fluxes of suspended sediment are based on empirical estimates of the monthly averaged transport during 1982 (BALE et al., 1985); these data

* Natural Environment Research Council, Institute for Marine Environment Research, Prospect Place, The Hoe, Plymouth PLI 3DH, U.K.

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1490 R.J . UNCLES el al.

provide no information on vertical fluxes of sediment, or on daily or spring-neap variations, and cannot be extrapolated to other years.

S E D I M E N T AND C O N T A M I N A N T MODELS

Currents are computed using a one-dimensional, hydrodynamical model (UNCLES and JORDAN, 1980). These are driven by predicted tidal oscillations in water level at the mouth of the estuary, and measured freshwater inputs at the head. The space-step in the model is 1 km; the time-step varies, but is of the order of several minutes. The one- dimensional equation of sediment conservation includes the effects of advection by currents, local resuspension and deposition (UNCLES et al., 1985); it is based on formulations given by ODD and OWEN (1972). Although more sophisticated and higher dimensional sediment transport models exist (e.g. RODGER and ODD, 1985; ONISHI and THOMPSON, 1984), such models could not be used to simulate the long time-scales considered here. Moreover, data are not available which would allow us to take into account properties such as particle (floc) size spectra or armouring and compaction of the bed.

The one-dimensional model described by HARRIS et al. (1984) has been extended to incorporate data from our simulation of sediment transport. This model has a 1 km space-step, and a time-step of the order of 0.5 h. Axial residual fluxes (averaged over each tidal cycle) and vertical fluxes of resuspended and deposited sediment are computed and stored for the period simulated by the contaminants model. Contaminant chemistry is treated in a very simplified way for the calculations presented here, in order to bring out the essential features. Chemical transformations within the sediment are excluded. Total dissolved and total particulate forms only are considered. Speciation is not treated explicity. It is assumed that exchanges of contaminant are sufficiently rapid such that equilibrium exists everywhere between dissolved and suspended particulate states according to a partition coefficient K:

K = Cp/Cd = K1 exp(KzS) (1)

in which Cp is the particulate concentration of contaminant (g g-~), Ca is the dissolved concentration (g 1-l), and S is salinity (g 1-1). KI is the partition coefficient for fresh water. Regression of In(K) on S for the data given by SALOMONS (1980) for cadmium, at pH 8 with a particle concentration of 100 ppm of Rhine mud, gives an estimate of 4.9 1 g-l for Kl and -0.12 1 g-~ for the empirical curve fitting constant K2 (HARRIS, in press). These values are used here. Cadmium is used only as an example of a contaminant for which partitioning information is available.

RESULTS AND D ISCU SSIO N

The combined hydrodynamical and sediment transport model was run for a 4 year period to provide realistic initial conditions for the axial mass distribution of bed sediment. The model was then run using tidal and run-off data for 1982. Fast tidal currents and high bed shear stresses during spring tides lead to high rates of sediment erosion. This is illustrated in Fig. la, which shows the modelled axial distributions of suspended sediment concentration at high water during the largest spring tide and smallest neap tide of March 1982. The spring tide concentrations are approximately

Contaminant dispersal in a turbid estuary 1491

d

(s)

3 0 0 -

250"

2 0 0 •

1 5 0 .

1 0 0 -

5 0 :

0 " , . . . . . . J . . . . . . . . . . . . . . . . 0 5 1 0 1 5 2 0 2 5 3 0

Distance from head ( k m )

(b )

2-

- 4 -

- 6 -

o A W

v - 2 -

. . . . i . . . . i . . . . i . . . . i . . . . i . . . . i

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Distance lrom head ( k i n )

Fig. 1. Axial distribution of (a) suspended sediment (ppm), and (b) residual transport of suspended sediment (kg s -~, positive up-estuary) for the highest spring tide (full line) and smallest neap tide (dashed line) of March 1982. The tidal range and run-off for the two cases are

5.2 m and 17 m 3 s t, and 1.5 m and 41 m -~ s ].

10 ppm at the head (freshwater value) and 5 ppm at the mouth (coastal value). Concentrations rise steeply and reach a turbidity maximum of about 300 ppm at 7 km from the head. Concentrations of suspended sediment during the neap tide are much lower, and show a gradual decrease from a freshwater input value of about 25 ppm at the head to about 10 ppm at the mouth. Thus, net sediment resuspended during spring tides is largely deposited during subsequent smaller tides, with very little net erosion occurring during neap tides. These distributions are very similar to observed data in the Tamar (HARRIS et al., 1984).

The axial residual rates of transport of suspended sediment for these two tides are shown as functions of distance along the estuary in Fig. lb. The spring tide residual transport is directed down-estuary in the upper 8 km, and up-estuary between 8 and 17 km. Thus, tidal pumping of suspended sediment (UNCLES et al., 1985) in this latter region sustains a supply of sediment and enhances the turbidity maximum, despite the opposing influence of freshwater flow. Down-estuary of 18 km the residual transport is generally directed out of the estuary and reflects localized redistribution and deposition of sediment within the lower estuary. The residual transport of suspended sediment at neap tides is generally directed down-estuary and decreases in magnitude away from the head. Therefore, sediment derived from freshwater inputs is being deposited within the estuary. Data on axial and vertical fluxes of suspended sediment are derived for each simulated tidal cycle of 1982, and used for input to the contaminants model.

We consider the hypothetical case of contaminant released into the upper 1 km of estuary at a continuous and constant rate of 1 kg d -I. The contaminant is initially released during the last week of 1981. Concentrations are highest at the source point, and rapidly decrease down-estuary. Particulate contaminant deposited over each 1 km section of bed is assumed to be completely mixed throughout the pool of mobile sediment which currently occupies that section. The temporal behaviour of the dissolved concen- tration of contaminant at a mid-estuarine position during 1982 is shown as the continuous line in Fig. 2a. Strong spring-neap oscillations are present, with maximum dissolved concentrations during neap tides when suspended sediment concentrations are lowest.

1492 R . J . UNCLES et al.

1 4 0 -

120-

dm 1 0 0 - i 0 80-

"~ 6 0 -

0

(a)

/,.; !:'!:'

40-

20- " "'; " /

0 i I 0 3 6 9 1 '2

250-

200-

1502

o 1 0 0

a

(b)

5 0 ~ " " ' ' '" : ~:'

o i i 0 3 6

i 9 1'2

Time (months) Time (months)

Fig. 2. Concentrat ions of dissolved contaminant at a mid-estuarine position during 1982; (a) solutions with all processes included (full line) and with zero partit ioning (dashed line); (b) solutions with all processes included (full line) and with zero flux of particulate contaminant

between the bed and water column (dashed line).

Levels are higher during summer neap tides because of the longer flushing time of the estuary under low run-off conditions, and because of the lower partitioning associated with increased salinity during summer (equation 1). These effects are much less evident during summer spring tides because of the higher concentrations of suspended sediment in the turbidity maximum during low run-of conditions. During the last 3 months of the year, enhanced concentrations of dissolved contaminant occur in mid estuary due to erosion and down-estuary transport (driven by high run-off) of contaminant-rich sedi- ment derived from the upper reaches. Concentrations in the bed sediment show a gradual build-up during the year superimposed upon strong seasonal variations.

We now consider the case where the contaminant does not partition between dissolved and particulate phases, but remains in the dissolved form (K = 0, equation 1). The temporal behaviour is shown by the broken line in Fig. 2a. During summer months the dissolved concentrations are almost an order of magnitude greater than when partition- ing is taken into account. This demonstrates the importance of sediment (suspended and deposited) as a trapping medium for contaminant within the estuary.

To illustrate the effects of vertical fluxes of particulate contaminant from the bed, Fig. 2b shows results from a calculation of the dissolved concentrations (broken line) in which these fluxes are taken to be zero. This is equivalent to assuming that suspended sediment releases its contaminant load to the dissolved phase just before deposition, so that bed sediment has zero concentration of contaminant. Therefore, the mass of contaminant held in particulate form during spring tides is released to the water column during neap tides, rather than stored in the bed sediment, which leads to the very high neap tide concentrations shown in Fig. 2b.

Finally, concentrations have been computed for the case in which horizontal transport of particulate contaminant is assumed to be zero. A comparison between this and the full solution (not plotted) shows almost no difference during neap tides (owing to the small sediment transport), and at most a factor of two in dissolved concentrations during spring tides. Therefore, in addition to hydrodynamical flushing, the crucial processes which govern the tidally averaged levels of dissolved contaminant are the partitioning between

Contaminant dispersal in a turbid estuary 1493

dissolved and particulate phases and the fluxes of particulate contaminant between the water column and the bed. Horizontal, axial residual transport of suspended sediment plays a secondary role. Thus, axial transport of dissolved contaminant dominates that due to the particulate form.

R E F E R E N C E S

BALE A. J., A. W. MORRIS and R. J. M. HOWLAND (1985) Seasonal sediment movement in the Tamar Estuary. Oceanologica Acta, 8, 1-6.

HARRIS J. R. W. (in press) Sink or drain; a simulation study of factors affecting the role of an estuary subject to toxic inputs. Water Research.

HARRIS J. R. W., A. J. BALE, B. L. BAYNE, R. F. C. MANTOURA, A. W. MORRIS, L. A. NELSON, P. J. RADFORD, R. J. UNCLES, S. A. WESTON and J. WIDDOWS (1984) A preliminary model of the dispersal and biological effect of toxins in the Tamar Estuary, England. Ecological Modelling, 22,253-284.

MORRIS A. W. (1986) Removal of trace metals in the very low salinity region of the Tamar Estuary, England. The Science of the Total Environment, 49,297-304.

MORRIS A. W., A. J. BALE, R. J. M. HOWLAND, G. E. MILLWARD, D. R. ACKROYD, D. H. LORING and R. T. T. RANTALA (1986) Sediment mobility and its contribution to trace metal cycling and retention in a macrotidal estuary. Water Science Technology, 18, 111-119.

ODD N. V. M. and M. W. OWEN (1972) A two-layer model of mud transport in the Thames Estuary. Proceedings of the Institute of Civil Engineers, paper 7517S, 175-205.

ONISHI Y. and F. L. THOMPSON (1984) Mathematical simulation of sediment and radionuclide transport in coastal waters. Vol. 1, Pacific Northwest Laboratory, Richland, WA 99352, U.S.A.

RODGER J. G. and N. V. M. ODD (1985) Sludge disposal in coastal waters--mathematical modelling of the transport of heavy metals in Liverpool Bay. Report No. SR70, Hydraulics Research, Wallingford, U K .

SALOMONS W. (1980) Adsorption processes and hydrodynamic conditions in estuaries. Environmental Technology Letters, 1,356-365.

UNCLES R. J. and M. B. JORDAN (1980) A one-dimensional representation of residual currents in the Severn Estuary and associated observations. Estuarine and Coastal Marine Science, 10, 39-60.

UNCLES R. J., R. C. A. ELLIO'I-F and S. A. WESTON (1985) Observed fluxes of water, salt and suspended sediment in a partly mixed estuary. Estuarine and Coastal Marine Science, 20, 147-167.