Simulating the impact of technological change on dissolved cadmium distribution in the Severn Estuary

Water Resee.rch Vo[. 15, p ~ 1045 to 105~ - 1981 Printed in Great Britain

0043-1354 8I 09104.5-08502.00:0 Pergamon Press Ltd

SIMULATING THE IMPACT OF TECHNOLOGICAL CHANGE ON DISSOLVED CADMIUM DISTRIBUTION IN THE SEVERN ESTUARY P. J. RADFORD, R. J. UNCLES and A. W. MORRIS Natural Environment Research Council, Institute for Marine Environmental Research, The Hoe, Plymouth PL1 3DH, England (Received June 1980)

Abstract--An estuarine model is described which computes the dispersive and advective properties of the Severn Estuary. It was calibrated and validated using 50 measured salinity distributions and then used to predict the magnitude and sitings of the major inputs of dissolved cadmium levels throughout the estuary. The results provided an impetus for implementing tighter controls on effluents and for improving estimates of cadmium discharges from industrial sources. The model has also been used to investigate the sensitivity of the estuarine system to changes in dispersion; by considering large reductions in the dispersion coefficients it is hoped that the results might be indicative of the environmental consequences following the construction of a tidal power generating scheme.

INTRODUCTION The Severn Estuary is situated in the south west of the British Isles, dividing south Wales from the English counties of Avon and Somerset (Fig. 1), and extending from the tidal limit at Maisemore Weir (Gloucester) to a hypothetical line joining Nash Point to Hurtstone Point some 137.5km seawards. Ten major rivers enter the estuary, including the Wye, Avon, Usk, Taft and the Severn itself, all of which are affected by the activities of man which include coalmining, smelting, manufacturing and agriculture. The run-off from these rivers mixes with the incoming saline water at rates which change markedly according to river flow rates, resulting in flushing times of between 50 and 250 days (IMER Annual Report, 1975). The high tidal range of up to 14 m for spring tides at Avonmouth, together with an associated tidal excursion of about 25 kin, ensure thorough vertical mixing of the water column in all but exceptional circumstances. Also, lateral heterogeneity is small compared with the axial gradients of all constituents of the water column, except in the immediate vicinity of industrial outfalls. Although the system is not considered to be highly polluted, possible expansion of population in the region (Central Unit for Environmental Planning, 1971), and the implementation of plans for the construction of a major tidal power generation scheme (Select Committee on Science and Technology, 1977), could radically affect the levels of contaminants in the future. SIMULATION MODEL OF SALINITY DISTRIBUTIONS

A one-dimensional model of the estuary was devised, in which axial dispersion between successive

5 km slices (Fig. 1) was assumed to vary according to river run-off and tidal range, and residual advection was equated with cumulative river flows. Surface salinity distributions, measured at high water on 29 separate occasions over a 6 year period (1971-1976), were transformed into tidally averaged data by interpolating the position of each data point through a complete tidal excursion and averaging the computed salinity at each point. Dispersion coefficients between each slice were subsequently estimated. Full details of the computational methods and assumptions have been given by Uncles & Radford (1980). A computer simulation model, HYDROBASE, driven by historic data of daily river flows and tidal range predictions was constructed. This utilised the derived dispersion relationships and reproduced the implied daily average salinity distributions. The key equations in this computer program are given in Table I. The first day of June 1971 was taken as the starting date for the simulations, with initial conditions for salinity typical o f measured summer distributions in the estuary. Thereafter, the model was driven by measured daily values of flow for each of the ten rivers and predicted daily values of tidal range at Avonmouth. The boundary conditions for salinity were taken as zero at the tidal limit (Maisemore Weir), and as 31.5%, at the seaward end (Nash Point-Hurtstone Point), the latter being the long term average salinity for that location. SIMULATION RESULTS FOR SALINITY The results of the model have been presented in two ways to emphasise different aspects of the agreement between observed and simulated results. In Fig. 2 the measured geographical distributions of surface salinity along the estuary are presented for 36 separate sampling occasions, together with the equivalent

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simulated distributions. In general, the agreement between model and measurement is very good, not only for the calibration (pre-1977), but also for the validation (1977) data set. The most serious discrepancies (11.6.73, 14.5.74, 20.5.76 and 16.8.76) occur at low salinities and are believed to be caused by the

problems of extrapolating high water measurements into tidally averaged values. One other major discrepancy, at the seaward end of the distribution of January 1977 (26.1.77), can be attributed to abnormal weather conditions which caused marked variations in salinity through the water column. The river run-

Table 1. Key equations from the simulation model HYDROBASE (i) For calculating the Ith dispersion coefficient S(I) from a multiple linear regression with coefficients I0, MI and M 2 S(/) = V ( / ) . (tO(t) + MI(/), (FO(1) - 25.92) + M 2 ( I ) . (RO(I) - 9.4)) where 25.92 is the long term mean daily freshwater flow ( m 3 . 1 0 6) 9.4 is the long term mean daily tidal range (m) V(I) is the tidally averaged volume of the lth slice for that day FO(1) is the moving average of daily flow RO(I) is the tidal range for that day (ii) For updating the quantity of freshwater (QI) in the lth slice for each time interval Q(I) = Q ( I ) + (F(1 - 1), e ( / - 1) - F ( / ) , P(I) + R(1) + S ( l - 1). (P(1 - 1) - P(1)) - S ( l ) . (P(I) - e ( l + 1)), D T where D T is the length of the time step in days F(I) is the advective transport of the Ith slice P(1) is the proportion of freshwater in the lth slice

P(/) = Q(/)/v(/) R(1) is the lateral inflow of rivers into the lth slice

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SALINITY DISTRIBUTIONS IN THE SEVERN ESTUARY

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1075 I 1976 ! 1977 ] 1978 ! TR TR-Tidol River IF-Inner F'stuory OE-Outer Estuary IC-Inner Channel Fig. 3. Daily simulated salinities averaged over the four large regions given in Fig. l are plotted against time as continuous lines for the period 1975-1978. Measured salinities averaged in the same way are shown as points. The first 2 yr of measurements were used for calibration of HYDROBASE. and the second 2 yr for validation. An explanation of the circled data point is given in the text.

off which had prevailed over the previous month was equivalent to twice the long term average daily rate, and the preceding day's flow had been 4 times as great. Fortunately, a research vessel was stationed in the region during the latter part of this e~cent which recorded salinity at a number of depths. Around the neap tide (tidal range <9m), coincident with high river run-off, differences in salinity of up to 2,',,, between surface and bottom were recorded. The region of lower salinity being quite shallow (< 5 m), this resulted in a depth averaged salinity almost 2'% higher than the surface measurement; a figure which is in agreement with the discrepancy demonstrated in Fig. 2 (26.1.77). A second aspect of the comparison between observed and simulated salinities is presented in Fig. 3 where time series data for four geographical regions of the estuary are given. This illustrates the considerable fluctuation of salinity with time caused by seasonal changes in river run-off, and the marked effect of regular spring-neap tidal cycles on the salinities in the Tidal River region. The model adequately reproduces both the slow build-up of salinity over the summer and autumn periods and its rapid decrease during periods of heavy rainfall. Once again the marked variation through the water column of the Inner Channel observed in January 1977 produced a large divergence of that datum point ((~)) from the simulation line. In general the agreement between observed and simulated data is good both for the calibration (pre-1977) and the validation (1977-1978) data set. We interpret this, with other evidence (Uncles, 1979) as justification for applying the model to any dissolved constituent of the water column using the derived dispersion coefficients.

S I M U L A T I O N RESULTS FOR DISSOLVED CADMIUM

Dissolved cadmium distributions in the estuary, measured at high water on 30 occasions over a 4 year period (1975-1978), were transformed into tidally averaged data in the same way as for the salinity observations. Typical examples of earlier distributions, which are representative of low (9.7.75) and high (24.3.76) river run-off, are given in Fig, 4(a) and 4(b), respectively. The distributions always showed elevated concentrations in the Outer Estuary region centred around Avonmouth, which were indicative of major loca[ised inputs. Elsewhere, subsidiary inputs were insignificant and a comparison of dissolved cadmium concentrations with simultaneously recorded salinities indicated that the behaviour of dissolved cadmium was essentially conservative. During the course of this investigation, average dissolved cadmium concentrations in the estuary have declined considerably (Fig. 5) although the general features of the distributions have remained unaltered. Originally, simulations of these dissolved cadmium distributions by the validated HYDROBASE model utilised the measured dissolved cadmium distribution on 3.6.75 as initial conditions and values for inputs of dissolved cadmium which were derived from a pilot survey of discharges to the estuary carried out during the summer and autumn of 1976 (Technical Working Party of the Severn Estuary Joint Committee, 1977). However, these measured inputs of dissolved cadmium (23 kg d a y - 1 to the whole estuary) were grossly insufficient to generate mid-estuarine concentrations similar to the observed conditions. To obtain the reasonable fits between the observed and simulated distributions given in Fig. 4(a) and (b), it was necess-

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Fig. Observed and simulated distributions of dissolved cadmium assuming measured input concentrations for the Taft, the Wye and the Severn but an extra input at Avonmouth of 70 kg in 1975-1976 and 35 kg day- t in 1977. The mismatch of the peak concentrations on 28 February 1977 indicates the existence of significant input in the region of the Taft. ary to raise, hypothetically, the input to the Avonmouth region to 70 kg day- t. This amount was not far short of Preston's (1973) estimate of about 100 kg day-t input to the region, a value which was corroborated by the distributions recorded in 1971 by Abdullah & Royle (1974), although substantial reductions in identified major sources had followed these reports. These early simulations indicated the magnitude and regional siting of major dissolved cadmium sources in the estuary and, thereby, emphasised the need both for a more intensive survey of contaminant

inputs and for a more stringent control of effluent discharges. This latter strategy brought about an appreciable drop in the mid-estuarine levels of dissolved cadmium in the estuary, as demonstrated by the observed data for 1977 shown in Fig. 4(c)-(e). Cadmium distributions during 1977 were found to be compatible with a halving (to ~ 35 kg day- t) of the previously inferred dissolved cadmium input to the Avonmouth region. Moreover, the principal observed peak in dissolved cadmium was not now consistently located in the immediate vicinity of Avonmouth, but occasionally occurred some 30km seaward; this is clearly demonstrated in the data for 28.2.77 (Fig. 4c). A comprehensive survey of the principal inputs to so*s i ~ , o t t ) E ~ l the Severn Estuary was carried out between August 3 ,ITs La,*.otc) • ...... • 1977 and March 1979 (Technical Working Party of lot7 (Jm. ~ 1 >~; the Severn Estuary Joint Committee, 1980). This ~l?t l~ll -~I,} O-------e study confirmed the existence of a major, but erratic, source of cadmium close to the mouth of the river Taft which had been deduced from the distributions 2 observed during 1977. However, this reassessment of inputs to the estuary again failed to produce estimates for dissolved cadmium inputs which could account for the levels of dissolved cadmium observed in the I. l + estuary. Summary data from this input survey is given in Table 2, which lists the mean daily input of both dissolved and particulate cadmium to each of the three major geographic areas: Inner Estuary, Outer Estuary and Inner Channel (cf. Fig. I). In order to . + 2. 0 . . . . m.o . . do ' ' ' 6 obtain the close simulations of observed data for kms from Miiisemoe'e Weir average dissolved cadmium concentrations within Fig. 5. The average dissolved cadmium concentrations in + the estuary derived from all measurements made in each of these regions shown in Fig. 6, we found it necessary to the years 1975-1978. A general decline in dissolved concen- multiply the measured inputs of dissolved cadmium tration is observable over this period. by 3 for pre-1977 data and by 2 for subsequent data.

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Table 2. Regional mass inputs of cadmium to the Severn Estuary. (Data obtained from the Second Report of the Technical Working Party of the Severn Estuary Joint Committee. 1980)

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It was deduced that the unidentified sources of dissolved cadmium were closely associated geographically with the known dissolved cadmium inputs, and because it was most improbable that significant effluents to the estuary had been disregarded in the comprehensive inputs survey, we were ted to the conclusion that most of the particulate cadmium in the recognised inputs was being rapidly solubilised within the estuary. Support for this conclusion can be found in the work of Rohatgi & C h e n (1975), who demonstrated that highly diluted primary and secondary effluents and digested sludges released, within three weeks, more than 90% of their particulate cadmium to the soluble ph'ase in oxygenated sea water at 15°C. These results suggest that particulate cadmium inputs to the Severn Estuary can be extensively solubilised within time scales which are short compared to their rate of dispersion out of the primary input region. Accordingly, simulations of dissolved cadmium distributions in the Severn Estuary were recalculated under this assumption. The resultant comparison between measured and simulated distributions for 1977 and

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Fig. 7. A comparison of observed and simulated dissolved cadmium distributions along the axis of the Severn Estuary. The model is driven by the measured inputs of cadmium, both dissolved and particulate, from all known sources including natural, industrial, domestic and atmospheric (Technical Working Party of the Severn Estuary Joint Committee. 1980).

Simulating dissolved cadmium distributions distribution is also closely followed on most occasions. Deviations can be attributed to the erratic input which emanates close to the mouth of the river Taft; only by utilising detailed time-series data in place of averaged data for this highly variable input could a better fit be obtained. We conclude that the general goodness of fit obtained with this final model confirms its applicability and, thereby, validates the assumptions made concerning the behaviour of cadmium in the estuary. It appears that particulate cadmium from anthropogenic sources is rapidly dissolved under estuarine conditions and that dissolved cadmium behaves conservatively elsewhere within the system.

EFFECTS

OF

A TIDAL

BARRIER

It has been suggested that the tidal energy of the Severn Estuary could be harnessed to generate electrical power by the construction of a semi-permeable barrier fitted with submerged generators (Shaw, 1980). The effect of such a barrage upon the flushing time (or retention time) of the estuary could lead to significantly increased levels of pollutants. HYDROBASE can be used to give a direct prediction of post-barrage conditions if it is assumed that the presence of a barrier would not alter fundamentally the dispersive characteristics of the estuary. If the system t~emains homogeneous both laterally and with depth, then sensitivity analysis of the model with respect to changes in dispersion coefficients can give a valuable indication of the magnitude of the estuarine response. Such an assumption is reasonable for the Severn Estuary because the modified tidal range would remain large compared with that of known permanently stratified estuaries. HYDROBASE has been used to study the sensitivity of the system to the range of dispersion modification that would be likely to occur if a tidal

barrage were built: (a) near the boundary between the Outer Estuary and the Inner Channel (Fig. I), i.e. 97.5km from Gloucester. (b) across the Inner Channel near to a line joining Nash Point and Hurtstone Point (Fig. 1), i.e. 132.5 km from Gloucester. Two adjustments were made in the model to simulate the possible impact of such structures upon the estuary: (a) The average volume of water above the barrage was increased to the proposed average volume, which conicides with the volume presently attained at high water level on an average neap tide. The change in volume from spring to neap tide allows for the very different projected average volumes which would be propagated by the barrier. (b) The dispersion coefficients were altered to test the sensitivity of the model to modifications in the transport properties of the system. Although the physical causes of dispersion in the Severn are not yet well understood, it is reasonable to consider the worst possible effects that a barrage is likely to have on the dispersion in order to predict the worst environmental consequences of such a structure. Figure 8(a) shows, as a continuous line, the dissolved cadmium distribution expected along the estuary, in the absence of a barrage, for a neap tide (range 6.7m) coincident with a total river run-off of 10 x 10 -6 m ~ day -z with the given cadmium inputs for the Taft, the Usk and the Severn, but with an additional 35 kg day-S entering in the Avonmouth region. These are typical of low flow summer conditions, and the cadmium input is similar to that experienced in 1977-1978. The two discontinuous lines represent the predicted levels of dissolved cadmium if the dispersion coefficients are reduced to one quarter

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of natural ',alues. landward of the two possible barrage sites. In the worst case, the peak cadmium concentration is almost double the pre-barrage value, an effect which is of the same order of magnitude as increasing the Avonmouth input to 70 kg day'-~, i.e. returning inputs to the levels experienced in 1975-1976 (Fig. 8b). The sensitivity of the model to changes in dispersion is demonstrated in Fig. 8(b) which shows, as a continuous line. the expected cadmium distribution for conditions similar to those of Fig. 8(a), except for a doubled input at Avonmouth. The two discontinuous lines demonstrate the effect of reducing dispersion to 50 and 25",, of natural values above a barrage sited at 132.5 km from Gloucester. The worst effect is again to double the peak cadmium concentration, raising general levels to more than 3,~,gl -~ for some 25-30km of the estuary around Avonmouth. It is beyond the scope of this paper to discuss likely harmful effects of these cadmium levels on organisms but there is evidence that macroalgae and both primary and secondary consumers concentrate cadmium in their tissues (Butterworth et ~d., 1972, Morris & Bale, 1975). CONCLUSIONS Good estimates of the dispersive properties of unstratified estuaries may be obtained from tidally averaged distributions of salinity. Simulation models driven by measured river run-off and predicted tidal range may be used to reproduce the dynamics of salinity changes with good precision. The same dispersion coefficients may be used to simulate the fate of any conservative pollutant and thus highlight inconsistencies between measured estuarine levels and estimates of polluting loads. The existence of a model such as H Y D R O B A S E therefore provides an incentive for a more critical evaluation of known sources and a search for alternative causes for any observed discrepancies. A further advantage is that the effects of proposed modifications to the system may be investigated, sometimes with absolute condidence (e.g. a study of the modification of river flows within historical ranges) or alternatively with more reservations (e.g. the study of the effects of a tidal barrage). Acknowledgements--HYDROBASE is a sub-system model of a General Ecosystem Model of the Bristol Channel and

g r d!

Severn Estuary {GEMBASE) which is a product ~t' the Estuarine and Near Shore Systems Ecolog), Group of the Institute for Marine En',ironmental Research, a component of the UK. Natural En;ironmen: Re~earch Council. The authors of this paper wish to thank Mrs Young fur the considerable computing effort which she contributed towards the success of this project. The work recei',ed partial financial support from the U.K. Department of the Environment under contract number DGR a80,48. The majority of the data presented in this paper ~ere collected on helicopter surveys initiated by the Technical Working Party--Survey and Systems Panel of the Severn Estuary Joint Committee. This committee is composed of representatives of the four Water Authorities of England and Wales which ha~e statutory responsibilities for the Severn Estuary. We are most grateful to them for allowing us free use of these data. REFERENCES Abdultah M. I. & Royle L. G. {1974) A study of the dissolved and particulate trace elements in the Bristol Channel. J. Mar. Biol. Ass., U.K, 54, 58t-597. Butterworth J., Lester P. & Nickless G. (1972) Distribution of heavy metals in the Severn Estuary. Mar, Poilu,. Bull. 3, 72-74. Central Unit for Environmental Planning (1971) Severnside: a feasibility study. H.M. Stationery Office, London. Institute for Marine Environmental Research (1975). Estuarine and near shore ecosystems. Annual Report 1974-1975, 85 pp. Morris A. W. & Bale A. J. (1975) The accumulation of cadmium, copper, manganese and zinc by Fucus t'esiculosus in the Bristol Channel. Estuar. Cot,st. Mar. Sci. 3, 153-163. Preston A. (1973) Cadmium in the marine environment of the United Kingdom. Mar. Pollut. Bull. 4, 105-107. Rohatgi N. &Chen K. Y. (1975) Transport of trace metals by suspended particulates on mixing with seawater, d. War. Poilu,. Control Fed. 47, 2298-2316. Select Committee on Science and Technology (1977) The development of alternative sources of energy in the United Kingdom. Third Report 1976-1977 (2 Vols). H.M. Stationery Office, London. Shaw, T. L. (Ed.). (1980). At, Environmental Appraisal of Tidal Power Stations with Particuhtr Reference to the Serern Barraoe. Pitman. London.

Technical Working Party of the Severn Estuary Joint Committee (1977) First Report. Severn Estuary Survey and Systems Panel. Technical Working Party of the Severn Estuary Joint Committee (1980) Second Report. Severn Estuary Survey and Systems Panel. Uncles, R. J. (1979). A comparison of the axial distributions of salt and t3:Cs in the Severn Estuary during August 1974. Estuar Coast. Mar. Sci. 9, 585-594. Uncles, R. J. & Radford, P. J. (1980). Seasonal and spring neap dependence of axial dispersion coefficients in the Severn Estuary. J. FI,id Mech. 98, 703-726.