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The environmental impact of paper mill waste discharges to the Upper Medway Estuary, Kent, England
Environmental Pollution (Series A) 40 (1986) 345-357
The Environmental Impact of Paper Mill Waste Discharges to the Upper Medway Estuary, Kent, England J. R . W h a r f e , R . A. D i n e s & L. A. Bird Southern Water Authority, Kent Division, Capstone Road, Chatham, Kent ME5 7QA, Great Britain
ABSTRACT The Upper Medway Estuary is considered as the narrow,funnel-shaped channel from the tidal limit at Allington downstream to Lower Upnor. Mathematical models are used to predict the depletion of dissolved oxygen resulting from organic waste inputs to the upper estuary. Surveys of the soft sediment fauna together with sediment redox potentials, organic carbon and cellulose content showed that conditions were generally poor and that faunal diversity was low, although afew surviving oligochaetes proliferated in the absence of predation and competition, with numbers in excess of 1.0 x 106 individuals per m 2 of sediment being recorded. The bottom sediments were relatively mobile, although a combination of sediment organic content and redox potential profiles clearly demonstrated the occurrence of reduced sediment conditions at depths below 6 cm at some sites in the upper estuary, with the greatest effect at sites downstream of the paper mill discharges. A significant reduction in the amount of organic waste discharged to the upper estuary should increase faunal diversity, although high species richness is considered unlikely given the naturally harsh conditions.
INTRODUCTION F o r regional water authorities whose resources are stretched by the pressures of recent legislation and international directives there is a need for techniques which allow the rapid and effective assessment of the impact of waste discharges to tidal waters. 345 Environ. Pollut. Ser. A. 0143-1471/86/$03-50 © Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Great Britain
346
J. R. Wharfe, R. A. Dines, L. A. Bird
The measurement of selected sediment parameters as a rapid means of assessing the effects of organic waste inputs has been evaluated (Pearson & Stanley, 1979; Wharfe et al., 1985) and successfully employed in environmental impact assessments of paper mill waste discharges to the Swale (Dines & Wharfe, 1985) and of partially treated domestic sewage released from a long sea outfall (Institute of Offshore Engineering, 1984). In estuaries where the bottom deposits are less stable, however, interpretation of the data requires greater care. This paper describes studies undertaken to assess the impact of paper mill discharges to the Upper Medway Estuary, Kent. The physiography and hydrography of the Medway Estuary have been described elsewhere (Gascoine & Wildish, 1971; Wharfe, 1977). For the purpose of this study the Upper Medway Estuary is considered to be the narrow, funnel-shaped channel from the tidal limit at Allington Sluice (TQ 745581) downstream to Lower Upnor (TQ 764713) (Fig. 1). The banks in the upper reaches are typically steep-sided and the horizontal area of intertidal sediment exposed at low water is small compared with that below Wouldham, where the channel widens. Here the banks become N
Lower Frindsbury Rochester ~ " ~ ~_ f
% H
Medway/--F/
I Wouldham 4aUing ~)S nodl,and
Paper Mill Discharge Pulp & Paper Mill @ Discharge • Sampling Points 1lkml
Burham ew
ythe ,,.Avlesford
Fig. 1. The Upper Medway Estuary showing the location of sampling points and paper mill discharges.
Paper mill waste discharge to Medway
347
more stable, though the gradient remains relatively steep and it is not until the outer estuary is reached below Lower Upnor that extensive intertidal mudflats appear. Fine deposits which characterise the upper reaches are deposited and resuspended by tidal action, creating a high suspended particulate load. At Allington the tide ebbs for three-quarters of the tidal cycle and salinities range from freshwater at the head of the estuary to 30%0 at Lower Upnor. The waters are generally well mixed, though point source organic inputs from two paper mills at Aylesford and Snodland cause severe, periodic, deoxygenation. The biochemical oxygen demand (BOD) of the paper mill discharges (average load of 5.35 tonnes day- 1 at Aylesford and 5.25 tonnes day- 1 at Snodland) added to that from the river (average load 6.5 tonnes day- 1) accounts for more than 90 % of the organic input to the upper estuary. The mill discharges contain large amounts of fibrous cellulose material and together they contribute more than 55 % of the total suspended particulate load entering the upper estuary. At some states of the tide these fibres settle out and combine with the sediments where their rate of decomposition is slow. Mathematical models of water quality in the Upper Medway Estuary, developed by the Water Research Centre (1980), are employed by the Southern Water Authority to predict distributions of chloride and dissolved oxygen. It is recognised, however, that conventional water quality data alone are inadequate for the sensible formulation of environmental quality objectives as anoxic sediment conditions are often not reflected by the overlying water. Therefore as part of the Southern Water Authority's pollution control programme a more detailed study of the effects of the mill discharges was initiated in 1981. The objectives of the programme were: (i)
to assess the environmental impact of paper mill discharges to the Upper Medway Estuary; (ii) to provide baseline data for monitoring changes resulting from anticipated improvements in effluent quality.
METHODS In order to determine the maximum impact of the mill discharges, samples of the intertidal sediments were taken close to the low water
348
J. R. Wharfe, R. A. Dines, L. A. Bird
mark. Eleven sites were selected of which nine were located in the upper estuary between Allington and Lower Upnor. Two sites in the outer estuary, at Gillingham and Horrid Hill, were included for comparative purposes (Fig. 1). The sites were sampled in March and September 1981, using sections of plastic pipe to obtain sediment cores 51 mm in diameter and 100ram deep. From each site, one core was taken for the measurement of sediment particle size distribution, redox potential profiles, organic carbon and cellulose content and five cores for identification and enumeration of the macrofauna. Techniques for sample preservation and subsequent analysis have been previously described (Dines & Wharfe, 1985). In addition, four sites in the upper estuary and one site in the outer estuary were sampled monthly during 1982, for sediment organic carbon content and redox potential measurements, to assess seasonal variability. Water quality data were obtained from routine surveys undertaken monthly by Southern Water Authority staff.
RESULTS
Water quality The principal feature of the paper mill effluent is a high particulate content and the suspended solids concentration of surface water samples from the vicinity of the paper mill discharges ranged from 25 to 190 mg litre -x during 1981. By contrast, the suspended solids concentration below Lower Upnor ranged from 10 to 40 mg litre-1 during the same period. The 5-day allylthiourea-inhibited BOD showed a similar contrast, ranging from 6 to 17 mg litre- 1 in the vicinity of the mill discharges and from 1 to 3 mg litre-x in the outer estuary. At Allington the freshwater flow ensures dissolved oxygen concentrations in excess of 80~o saturation at all states of the tide. Downstream in the vicinity of the mill discharges the dissolved oxygen concentration drops dramatically and values lower than 10 ~o saturation have been recorded under extreme conditions. Downstream of Rochester Bridge dissolved oxygen concentrations were generally in excess of 50 saturation at all states of the tide and above 75 ~o saturation in the outer estuary.
Paper mill waste discharge to Medway
349
The freshwater flow into the upper estuary maintains salinities below 1%o 5 km below Allington, and at Wouldham they rarely exceed 5%0. As the channel widens, the salinity regime changes rapidly in response to the tidal influence and at Medway Bridge salinities ranged from 1 to 20%0and at Lower Upnor from 15 to 30%0. Particle size distribution Particle size distributions are summarised in Table 1. Fine deposits are predominant in the upper estuary, though a high suspended particulate load, which is continuously deposited and resuspended, produces variable bottom types. This is particularly true between Allington and Burham where river scour is most noticeable. Here MD~b values varied TABLE 1 A Summary of the Particle Size Distribution of Sediments from the
Upper Medway Estuary Site~Season
Allington Aylesford New Hythe Burham Snodland Hailing Medway Bridge Frindsbury Lower Upnor Gillingham Horrid Hill
S A S A S A S A S A S A S A S A S A S A S A
MDO
Q 1(a-Q3 q~
3.00 8-25 1.10 1.30 4.00 7.00 2.95 2.10 8.10 7.80 8-30 7.90 6-05 7.60 7-75 7-35 8.75 4.60 11.00 8.50 7-65 7-25
1.85-6.30 6.60-9.60 -2.00-6.70 -0.10-2.60 -0.02-8.40 3.50-9.80 1.95-5.25 1.20-3.90 5.70-11.60 5.25-10.00 6.20-10.80 5.70-10.60 2.95-10.00 5.15-10.90 5.45-11.05 4.55-11.30 4-95-12.00 2-60-9.00 4-00-11.00 6-60-9.95 5-00-11.70 5.00-12-00
S, spring 1981; A, autumn 1981.
% < O"125 mm 48.26 93.55 33.51 21.36 55.23 77.12 47.81 30.77 92.41 86.12 94.36 93.88 72.78 91.77 98.80 96.72 98-04 65.10 80.26 90.85 96.05 95.15
350
J. R. Wharfe, R. A. Dines, L. A. Bird
from + 1 to + 8 with as little as 20 %, or as great as 90 %, of the particles being smaller than 0.125 mm. Downstream of Snodland the bottom types are more stable and MDq~ values ranged from + 6 to +8 (with the exception of a single result of + 11 at Gillingham in spring 1981). At these sites the proportion of particles of less than 0-125mm was generally greater than 90 %. Redox profiles Redox potential profiles are presented in Fig. 2. September 1981 sampling coincided with storm conditions and the data for some sites are atypical of
Eh(m~ -200
• :SPRING • :AUTUMN
o. ALLINGT'ON O"
0
AYLESFORD
+200
*400
_~__~~ --
~ S N O ~ #
HAL~~_
~
FR,N S URY
HORRIDHILL
~
-
r t Fig. 2. Sedimentredox potentials for the Upper Medway Estuary, spring and autumn 1981.
351
Paper mill waste discharge to Medway A
B
H
F
HH. .Jan / Mar
10
Jun
c
Sept
'~
-/.,00
0
/-i-I / Y .400
Oct
/ Dec
Eh (rn~)
Fig. 3. Quarterly average redox profiles for the Upper Medway Estuary, 1982. (A, Allington; B, Burham; H, Halling; F, Frindsbury; HH, Horrid Hill.) those recorded in succeeding years when samples collected during autumn corresponded more closely with those from the spring. The 1982 seasonal data presented in Fig. 3 show a typical pattern with sediment conditions generally more reduced in the late summer months due to high summer temperatures. Proceeding downstream from Allington there was a gradual decline in potentials and in the vicinity o f the mill discharges the sediments showed reducing conditions within a few millimetres of the surface. Negative potentials of lower than - 100 mV were regularly recorded at depths of TABLE 2
Percentage Organic Carbon Content (Quarterly Averages and Range) of Sediments from the Upper Medway Estuary 1982/Site
January March April-June July-September October-December
Aylesford
Burham
4.42 (2.9-5.8) 5.77 (4-2 8. l) 4.75 (1.9 7.4) 5.12 (2.9-7.0)
3.93 (2-5-6.3) 4.03 (2.7.-4.9) 3.96 (2.6-4.7) 5.19 (3.0-6.6)
Hailing
6.62 (6.1-7.0) 6.29 (5.9-6.6) 6. l l (5.8 6-5) 6.10 (5.5-6.6)
Frindsbury Horrid Hill
3.59 (3.2 3.8) 4.49 (4.2-4.8) 3.91 (3.5-4.6) 4.23 (3-7-4.9)
1.97 (1.3 2.3) 2.4 l (2.4-2.5) 2.42 (2.4-2-5) 2.50 (2.4-2.5)
352
J. R. Wharfe, R. A. Dines, L. A. Bird
6 cm and below from Burham to Hailing. There was a marked recovery below Medway Bridge, where positive values to a depth of 6 cm indicated well-oxidised sediments. Organic carbon and cellulose content Sediment cellulose and organic carbon content for auttlmn 1981 and spring 1982 are shown in Fig. 4 and the 1982 seasonal organic carbon data in Table 2. The associated distributions of both determinands showed elevated concentrations in the upper estuary. Values at Aylesford (organic carbon 1.9-7.4 %, cellulose 4.7-18.5/~g m g - 5 , where they were "~118-46
-lZ,
SPRING
~
6-
.12 "10
4~ I:1
~
2-
"8 "6 "4
d
o~
"2
cO
-0 --"
o,_
AUTUMN
10 E Cl.
U
O
6 2-
~
./, -2
o
g
~"
1'0
l's
~0
2's
o 30
Distance Downstream from Tidal Limit ( k i n )
~Paper Nill Discharges
Fig.4. The organic carbon and cellulose content of sediments from the Upper Medway Estuary.
Paper mill waste discharge to
Medway
353
most variable, demonstrate the mobility and changeable nature of the bottom deposits. By contrast, the organic carbon content of sediments from the outer estuary rarely exceeded 3 % and cellulose concentrations were less than 2/zg mgBenthos The distribution and abundance of the invertebrate macrofauna are presented in Fig. 5. Frequent and rapid salinity fluctuations present particular problems for animals inhabiting the upper estuary and the |
.SPRING
116
6 ? E
-
12-
AUTUMN
"16
"S 1
"14
-12 'i
"10 6 ~
"8 "6
'2 0
i
0
5
10
i
-
15
i
I
20
25
Distance Downstream from Tidal Limit (km.}
30
1]'Paper Nill Discharges
Fig. 5.
The number of taxa and abundance of benthic macroinvertebrates in the Upper Medway Estuary, spring and autumn 1981.
354
J. R. Wharfe, R. ,4. Dines, L. A. Bird
transport of high concentrations of suspended particulate material and the discharge of organic wastes further restricts the fauna. The longitudinal salinity profile for the upper estuary shows a marked change below Medway Bridge and as the channel widens and salinities increase, a largely oligochaete-dominated fauna is replaced by a mixed oligochaete-polychaete community. Few animals are able to adapt and thrive in the inhospitable environment of the upper estuary and species numbers are generally low. The invertebrate fauna was slightly more diverse in spring although, in both surveys, the lowest number of species was recorded at Snodland and Halling, immediately downstream from the paper mill discharges. Classically, the few surviving species were those most able to exploit the situation and the abundance of individuals was component 2 10 11
SPRING
"9 "8 2 1
"6 "5" "3
component 1
component 2 .10 9"'11
AUTUMN
"8 •
component
I
Fig. 6. Principal components analysis of sampling points in the Upper Medway Estuary based on the abundance and diversity of their benthic macrofauna: graphical plot of the first two components, for spring and autumn 1981. (l, Allington; 2, Aylesford; 3, New Hythe; 4, Burham; 5, Snodland; 6, Halling; 7, Medway Bridge; 8, Frindsbury; 9, Lower Upnor; 10, Qillingham; l l, Horrid Hill.)
Paper mill waste discharge to Medway
355
greatest at these sites with numbers of the dominant oligochaete, Limnodrilus hoffmeisteri, at Snodland exceeding 1.0 x 106 m-2 in spring and 0.5 x 106m-2 in autumn. Below Medway Bridge species numbers increased with the appearance of polychaetes such as Hediste diversicolor, Streblospio shrubsolii and Manayunkia aestuarina. There was also a notable change in the structure of the oligochaete fauna with the disappearance of Limnodrilus spp. and the appearance of the more saline-tolerant Tubifex costatus, Tubificoides benedeni, T. brownae, T. heterochaetus and T. pseudogaster. The increase in diversity downstream was accompanied by a dramatic decrease in the abundance of individuals. To investigate site groupings further, the data were subjected to principal components and average link cluster analysis. The first two principal components, which accounted for 74 ~o of the total variance in spring and 83 ~ in autumn, are plotted in Fig. 6. Sites upstream of Medway Bridge are separated from those downstream, reflecting the salinity differences, although consistently poor sediment conditions at all sites in the upper reaches prevented further division. The average link cluster analysis produced a similar pattern.
DISCUSSION The inhospitable soft sediment habitats of the Upper Medway Estuary support an impoverished fauna, although the few oligochaete species which survive are able to thrive in the absence of competition and predation. Numbers in excess of one million individuals per m 2 of sediment are often found (Table 3) and the tubifex worms are commercially exploited as a food source for tropical fish. Below Lower Upnor fewer oligochaetes survive and the soft sediment fauna is dominated by molluscs and polychaetes (Wharfe, 1977). The measurement of sediment organic content and redox potential profiles provides a rapid means of assessing the impact of organic waste discharges to estuaries and coastal waters where the bottom sediments are reasonably stable (Pearson & Stanley, 1979; Dines & Wharfe, 1985; Wharfe et al., 1985). In contrast to recent studies on the effects of paper mill discharges to the Swale where these methods have been deployed (Dines & Wharfe, 1985), the soft sediments of the Upper Medway Estuary are far less stable
356
J. R. Wharfe, R. A. Dines, L. A. Bird
TABLE 3 Number of Oligochaetes per m 2 of Sediment Species L. hoffmeisteri
T. tubifex
L. udekemianus L. profundicola
No. m 2
500000 280000 105 800 1 094 600 300 000 586217 127400 376 000 346 500 137 800
Location
Reference
Thames Birtwell & Arthur (1980) River Irwell Eyres et al. (1978) Upper Forth Estuary McLusky et aL (1980) Upper Medway Estuary Thames Birtwell & Arthur (1980) River Irwell Eyres et al. (1978) Upper Forth Estuary McLusky et al. (1980) Upper Medway Estuary Upper Medway Estuary Upper Medway Estuary
and make an assessment of the impact of organic waste discharges more difficult. Nevertheless, a combination of sediment organic carbon content and redox potential profiles clearly demonstrates the occurrence of reduced sediment conditions at depths below 6cm at some sites in the upper estuary and these effects are greatest at sites downstream of the paper mill discharges. Seasonal measurements show the same general pattern, although expected differences between summer and winter are less clear. More recent observations on the movement of sediments in the upper estuary suggest that the bottom deposits are less stable than those in the outer estuary and that a period of scour during high river flow conditions in the winter months is followed by a period of deposition during summer and autumn when river flows are reduced. At some sites the movement of sediments is dramatic, with up to 8 cm being removed or deposited in a month. As a consequence of the mobility of the sediments in the upper estuary, the effects of organic waste discharges are less severe than those described in Milton Creek (Dines & Wharfe, 1985) where, in the absence of any significant scour, large quantities of cellulose fibre settle out and combine with the stable bottom sediments. The effects of the recession on the paper industry have inadvertently reduced waste inputs to the Upper Medway Estuary and agreements on revised discharge consents are currently being formulated to lower them still further. A significant reduction in the amount of organic waste discharged to the upper estuary should increase faunal diversity, although high species richness seems unlikely given the naturally harsh conditions.
Paper mill waste discharge to Medway
357
A n improvement in water quality, particularly levels of dissolved oxygen during extended periods of warm weather and low flows, should encourage migratory fish.
ACKNOWLEDGEMENT The authors wish to thank the Southern Water Authority for permission to publish this paper. The opinions expressed are those of the authors and do not necessarily reflect the views of the Southern Water Authority. REFERENCES Birtwell, I. K. & Arthur, D. R. (1980). The ecology of tubificids in the Thames Estuary with particular reference to Tubifex costatus (Claparede). In Aquatic Oligochaete Biology, Proc. int. syrup. 1st, Sidney, BC, Canada, 1-4 May 1979, ed. by R. O. Brinkhurst, and D. G. Cook, 331-8. New York, Plenum Press. Dines, R. A. & Wharfe, J. R. (1985). The environmental impact of paper mill waste discharges to the Swale. Environ. Pollut., Ser. ,4, 38, 245-60. Eyres, J. P., Williams, N. V. & Pugh-Thomas, M. (1978). Ecological studies on Oligochaeta inhabiting depositing substrata in the Irwell, a polluted English river. Freshwater Biol., 8, 25-32. Gascoine, I. S. & Wildish, D. J. (1971). A chemical and biological study of the Medway Estuary. Wat. Pollut. Control Lond., 70, 1!-12. Institute of Offshore Engineering (1984). Hythe Long Sea Outfall: Environmental assessment of proposed area, August, 1983 survey. IOE/83/208, Heriot-Watt University, Edinburgh. McLusky, D. S., Teare, M. & Phizaklea, P. (1980). Effects of domestic and industrial pollution on the distribution and abundance of aquatic oligochaetes in the Forth Estuary. Helgoland. Meeresunters., 33, 384-92. Pearson, T. H. & Stanley, S. O. (1979). Comparative measurements of the redox potential of marine sediments as a rapid means of assessing the effect of organic pollution. Mar. Biol., 53, 371-9. Water Research Centre (1980). ,4 mathematical model for predicting water quality in the Medway Estuary: Report for the Southern Water Authority. SL478/20/01, Stevenage, Water Research Centre. Wharfe, J. R. (1977). An ecological survey of the benthic invertebrate macrofauna of the Lower Medway Estuary, Kent. J. ,4him. Ecol., 46, 93-113. Wharfe, J. R., Friend, K. & Dines, R. A. (1985). An evaluation of selected sediment parameters as a rapid means of assessing the impact of organic waste discharges to tidal waters. Environ. Pollut., Ser. B, 10, 159-72.
The Environmental Impact of Paper Mill Waste Discharges to the Upper Medway Estuary, Kent, England J. R . W h a r f e , R . A. D i n e s & L. A. Bird Southern Water Authority, Kent Division, Capstone Road, Chatham, Kent ME5 7QA, Great Britain
ABSTRACT The Upper Medway Estuary is considered as the narrow,funnel-shaped channel from the tidal limit at Allington downstream to Lower Upnor. Mathematical models are used to predict the depletion of dissolved oxygen resulting from organic waste inputs to the upper estuary. Surveys of the soft sediment fauna together with sediment redox potentials, organic carbon and cellulose content showed that conditions were generally poor and that faunal diversity was low, although afew surviving oligochaetes proliferated in the absence of predation and competition, with numbers in excess of 1.0 x 106 individuals per m 2 of sediment being recorded. The bottom sediments were relatively mobile, although a combination of sediment organic content and redox potential profiles clearly demonstrated the occurrence of reduced sediment conditions at depths below 6 cm at some sites in the upper estuary, with the greatest effect at sites downstream of the paper mill discharges. A significant reduction in the amount of organic waste discharged to the upper estuary should increase faunal diversity, although high species richness is considered unlikely given the naturally harsh conditions.
INTRODUCTION F o r regional water authorities whose resources are stretched by the pressures of recent legislation and international directives there is a need for techniques which allow the rapid and effective assessment of the impact of waste discharges to tidal waters. 345 Environ. Pollut. Ser. A. 0143-1471/86/$03-50 © Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Great Britain
346
J. R. Wharfe, R. A. Dines, L. A. Bird
The measurement of selected sediment parameters as a rapid means of assessing the effects of organic waste inputs has been evaluated (Pearson & Stanley, 1979; Wharfe et al., 1985) and successfully employed in environmental impact assessments of paper mill waste discharges to the Swale (Dines & Wharfe, 1985) and of partially treated domestic sewage released from a long sea outfall (Institute of Offshore Engineering, 1984). In estuaries where the bottom deposits are less stable, however, interpretation of the data requires greater care. This paper describes studies undertaken to assess the impact of paper mill discharges to the Upper Medway Estuary, Kent. The physiography and hydrography of the Medway Estuary have been described elsewhere (Gascoine & Wildish, 1971; Wharfe, 1977). For the purpose of this study the Upper Medway Estuary is considered to be the narrow, funnel-shaped channel from the tidal limit at Allington Sluice (TQ 745581) downstream to Lower Upnor (TQ 764713) (Fig. 1). The banks in the upper reaches are typically steep-sided and the horizontal area of intertidal sediment exposed at low water is small compared with that below Wouldham, where the channel widens. Here the banks become N
Lower Frindsbury Rochester ~ " ~ ~_ f
% H
Medway/--F/
I Wouldham 4aUing ~)S nodl,and
Paper Mill Discharge Pulp & Paper Mill @ Discharge • Sampling Points 1lkml
Burham ew
ythe ,,.Avlesford
Fig. 1. The Upper Medway Estuary showing the location of sampling points and paper mill discharges.
Paper mill waste discharge to Medway
347
more stable, though the gradient remains relatively steep and it is not until the outer estuary is reached below Lower Upnor that extensive intertidal mudflats appear. Fine deposits which characterise the upper reaches are deposited and resuspended by tidal action, creating a high suspended particulate load. At Allington the tide ebbs for three-quarters of the tidal cycle and salinities range from freshwater at the head of the estuary to 30%0 at Lower Upnor. The waters are generally well mixed, though point source organic inputs from two paper mills at Aylesford and Snodland cause severe, periodic, deoxygenation. The biochemical oxygen demand (BOD) of the paper mill discharges (average load of 5.35 tonnes day- 1 at Aylesford and 5.25 tonnes day- 1 at Snodland) added to that from the river (average load 6.5 tonnes day- 1) accounts for more than 90 % of the organic input to the upper estuary. The mill discharges contain large amounts of fibrous cellulose material and together they contribute more than 55 % of the total suspended particulate load entering the upper estuary. At some states of the tide these fibres settle out and combine with the sediments where their rate of decomposition is slow. Mathematical models of water quality in the Upper Medway Estuary, developed by the Water Research Centre (1980), are employed by the Southern Water Authority to predict distributions of chloride and dissolved oxygen. It is recognised, however, that conventional water quality data alone are inadequate for the sensible formulation of environmental quality objectives as anoxic sediment conditions are often not reflected by the overlying water. Therefore as part of the Southern Water Authority's pollution control programme a more detailed study of the effects of the mill discharges was initiated in 1981. The objectives of the programme were: (i)
to assess the environmental impact of paper mill discharges to the Upper Medway Estuary; (ii) to provide baseline data for monitoring changes resulting from anticipated improvements in effluent quality.
METHODS In order to determine the maximum impact of the mill discharges, samples of the intertidal sediments were taken close to the low water
348
J. R. Wharfe, R. A. Dines, L. A. Bird
mark. Eleven sites were selected of which nine were located in the upper estuary between Allington and Lower Upnor. Two sites in the outer estuary, at Gillingham and Horrid Hill, were included for comparative purposes (Fig. 1). The sites were sampled in March and September 1981, using sections of plastic pipe to obtain sediment cores 51 mm in diameter and 100ram deep. From each site, one core was taken for the measurement of sediment particle size distribution, redox potential profiles, organic carbon and cellulose content and five cores for identification and enumeration of the macrofauna. Techniques for sample preservation and subsequent analysis have been previously described (Dines & Wharfe, 1985). In addition, four sites in the upper estuary and one site in the outer estuary were sampled monthly during 1982, for sediment organic carbon content and redox potential measurements, to assess seasonal variability. Water quality data were obtained from routine surveys undertaken monthly by Southern Water Authority staff.
RESULTS
Water quality The principal feature of the paper mill effluent is a high particulate content and the suspended solids concentration of surface water samples from the vicinity of the paper mill discharges ranged from 25 to 190 mg litre -x during 1981. By contrast, the suspended solids concentration below Lower Upnor ranged from 10 to 40 mg litre-1 during the same period. The 5-day allylthiourea-inhibited BOD showed a similar contrast, ranging from 6 to 17 mg litre- 1 in the vicinity of the mill discharges and from 1 to 3 mg litre-x in the outer estuary. At Allington the freshwater flow ensures dissolved oxygen concentrations in excess of 80~o saturation at all states of the tide. Downstream in the vicinity of the mill discharges the dissolved oxygen concentration drops dramatically and values lower than 10 ~o saturation have been recorded under extreme conditions. Downstream of Rochester Bridge dissolved oxygen concentrations were generally in excess of 50 saturation at all states of the tide and above 75 ~o saturation in the outer estuary.
Paper mill waste discharge to Medway
349
The freshwater flow into the upper estuary maintains salinities below 1%o 5 km below Allington, and at Wouldham they rarely exceed 5%0. As the channel widens, the salinity regime changes rapidly in response to the tidal influence and at Medway Bridge salinities ranged from 1 to 20%0and at Lower Upnor from 15 to 30%0. Particle size distribution Particle size distributions are summarised in Table 1. Fine deposits are predominant in the upper estuary, though a high suspended particulate load, which is continuously deposited and resuspended, produces variable bottom types. This is particularly true between Allington and Burham where river scour is most noticeable. Here MD~b values varied TABLE 1 A Summary of the Particle Size Distribution of Sediments from the
Upper Medway Estuary Site~Season
Allington Aylesford New Hythe Burham Snodland Hailing Medway Bridge Frindsbury Lower Upnor Gillingham Horrid Hill
S A S A S A S A S A S A S A S A S A S A S A
MDO
Q 1(a-Q3 q~
3.00 8-25 1.10 1.30 4.00 7.00 2.95 2.10 8.10 7.80 8-30 7.90 6-05 7.60 7-75 7-35 8.75 4.60 11.00 8.50 7-65 7-25
1.85-6.30 6.60-9.60 -2.00-6.70 -0.10-2.60 -0.02-8.40 3.50-9.80 1.95-5.25 1.20-3.90 5.70-11.60 5.25-10.00 6.20-10.80 5.70-10.60 2.95-10.00 5.15-10.90 5.45-11.05 4.55-11.30 4-95-12.00 2-60-9.00 4-00-11.00 6-60-9.95 5-00-11.70 5.00-12-00
S, spring 1981; A, autumn 1981.
% < O"125 mm 48.26 93.55 33.51 21.36 55.23 77.12 47.81 30.77 92.41 86.12 94.36 93.88 72.78 91.77 98.80 96.72 98-04 65.10 80.26 90.85 96.05 95.15
350
J. R. Wharfe, R. A. Dines, L. A. Bird
from + 1 to + 8 with as little as 20 %, or as great as 90 %, of the particles being smaller than 0.125 mm. Downstream of Snodland the bottom types are more stable and MDq~ values ranged from + 6 to +8 (with the exception of a single result of + 11 at Gillingham in spring 1981). At these sites the proportion of particles of less than 0-125mm was generally greater than 90 %. Redox profiles Redox potential profiles are presented in Fig. 2. September 1981 sampling coincided with storm conditions and the data for some sites are atypical of
Eh(m~ -200
• :SPRING • :AUTUMN
o. ALLINGT'ON O"
0
AYLESFORD
+200
*400
_~__~~ --
~ S N O ~ #
HAL~~_
~
FR,N S URY
HORRIDHILL
~
-
r t Fig. 2. Sedimentredox potentials for the Upper Medway Estuary, spring and autumn 1981.
351
Paper mill waste discharge to Medway A
B
H
F
HH. .Jan / Mar
10
Jun
c
Sept
'~
-/.,00
0
/-i-I / Y .400
Oct
/ Dec
Eh (rn~)
Fig. 3. Quarterly average redox profiles for the Upper Medway Estuary, 1982. (A, Allington; B, Burham; H, Halling; F, Frindsbury; HH, Horrid Hill.) those recorded in succeeding years when samples collected during autumn corresponded more closely with those from the spring. The 1982 seasonal data presented in Fig. 3 show a typical pattern with sediment conditions generally more reduced in the late summer months due to high summer temperatures. Proceeding downstream from Allington there was a gradual decline in potentials and in the vicinity o f the mill discharges the sediments showed reducing conditions within a few millimetres of the surface. Negative potentials of lower than - 100 mV were regularly recorded at depths of TABLE 2
Percentage Organic Carbon Content (Quarterly Averages and Range) of Sediments from the Upper Medway Estuary 1982/Site
January March April-June July-September October-December
Aylesford
Burham
4.42 (2.9-5.8) 5.77 (4-2 8. l) 4.75 (1.9 7.4) 5.12 (2.9-7.0)
3.93 (2-5-6.3) 4.03 (2.7.-4.9) 3.96 (2.6-4.7) 5.19 (3.0-6.6)
Hailing
6.62 (6.1-7.0) 6.29 (5.9-6.6) 6. l l (5.8 6-5) 6.10 (5.5-6.6)
Frindsbury Horrid Hill
3.59 (3.2 3.8) 4.49 (4.2-4.8) 3.91 (3.5-4.6) 4.23 (3-7-4.9)
1.97 (1.3 2.3) 2.4 l (2.4-2.5) 2.42 (2.4-2-5) 2.50 (2.4-2.5)
352
J. R. Wharfe, R. A. Dines, L. A. Bird
6 cm and below from Burham to Hailing. There was a marked recovery below Medway Bridge, where positive values to a depth of 6 cm indicated well-oxidised sediments. Organic carbon and cellulose content Sediment cellulose and organic carbon content for auttlmn 1981 and spring 1982 are shown in Fig. 4 and the 1982 seasonal organic carbon data in Table 2. The associated distributions of both determinands showed elevated concentrations in the upper estuary. Values at Aylesford (organic carbon 1.9-7.4 %, cellulose 4.7-18.5/~g m g - 5 , where they were "~118-46
-lZ,
SPRING
~
6-
.12 "10
4~ I:1
~
2-
"8 "6 "4
d
o~
"2
cO
-0 --"
o,_
AUTUMN
10 E Cl.
U
O
6 2-
~
./, -2
o
g
~"
1'0
l's
~0
2's
o 30
Distance Downstream from Tidal Limit ( k i n )
~Paper Nill Discharges
Fig.4. The organic carbon and cellulose content of sediments from the Upper Medway Estuary.
Paper mill waste discharge to
Medway
353
most variable, demonstrate the mobility and changeable nature of the bottom deposits. By contrast, the organic carbon content of sediments from the outer estuary rarely exceeded 3 % and cellulose concentrations were less than 2/zg mgBenthos The distribution and abundance of the invertebrate macrofauna are presented in Fig. 5. Frequent and rapid salinity fluctuations present particular problems for animals inhabiting the upper estuary and the |
.SPRING
116
6 ? E
-
12-
AUTUMN
"16
"S 1
"14
-12 'i
"10 6 ~
"8 "6
'2 0
i
0
5
10
i
-
15
i
I
20
25
Distance Downstream from Tidal Limit (km.}
30
1]'Paper Nill Discharges
Fig. 5.
The number of taxa and abundance of benthic macroinvertebrates in the Upper Medway Estuary, spring and autumn 1981.
354
J. R. Wharfe, R. ,4. Dines, L. A. Bird
transport of high concentrations of suspended particulate material and the discharge of organic wastes further restricts the fauna. The longitudinal salinity profile for the upper estuary shows a marked change below Medway Bridge and as the channel widens and salinities increase, a largely oligochaete-dominated fauna is replaced by a mixed oligochaete-polychaete community. Few animals are able to adapt and thrive in the inhospitable environment of the upper estuary and species numbers are generally low. The invertebrate fauna was slightly more diverse in spring although, in both surveys, the lowest number of species was recorded at Snodland and Halling, immediately downstream from the paper mill discharges. Classically, the few surviving species were those most able to exploit the situation and the abundance of individuals was component 2 10 11
SPRING
"9 "8 2 1
"6 "5" "3
component 1
component 2 .10 9"'11
AUTUMN
"8 •
component
I
Fig. 6. Principal components analysis of sampling points in the Upper Medway Estuary based on the abundance and diversity of their benthic macrofauna: graphical plot of the first two components, for spring and autumn 1981. (l, Allington; 2, Aylesford; 3, New Hythe; 4, Burham; 5, Snodland; 6, Halling; 7, Medway Bridge; 8, Frindsbury; 9, Lower Upnor; 10, Qillingham; l l, Horrid Hill.)
Paper mill waste discharge to Medway
355
greatest at these sites with numbers of the dominant oligochaete, Limnodrilus hoffmeisteri, at Snodland exceeding 1.0 x 106 m-2 in spring and 0.5 x 106m-2 in autumn. Below Medway Bridge species numbers increased with the appearance of polychaetes such as Hediste diversicolor, Streblospio shrubsolii and Manayunkia aestuarina. There was also a notable change in the structure of the oligochaete fauna with the disappearance of Limnodrilus spp. and the appearance of the more saline-tolerant Tubifex costatus, Tubificoides benedeni, T. brownae, T. heterochaetus and T. pseudogaster. The increase in diversity downstream was accompanied by a dramatic decrease in the abundance of individuals. To investigate site groupings further, the data were subjected to principal components and average link cluster analysis. The first two principal components, which accounted for 74 ~o of the total variance in spring and 83 ~ in autumn, are plotted in Fig. 6. Sites upstream of Medway Bridge are separated from those downstream, reflecting the salinity differences, although consistently poor sediment conditions at all sites in the upper reaches prevented further division. The average link cluster analysis produced a similar pattern.
DISCUSSION The inhospitable soft sediment habitats of the Upper Medway Estuary support an impoverished fauna, although the few oligochaete species which survive are able to thrive in the absence of competition and predation. Numbers in excess of one million individuals per m 2 of sediment are often found (Table 3) and the tubifex worms are commercially exploited as a food source for tropical fish. Below Lower Upnor fewer oligochaetes survive and the soft sediment fauna is dominated by molluscs and polychaetes (Wharfe, 1977). The measurement of sediment organic content and redox potential profiles provides a rapid means of assessing the impact of organic waste discharges to estuaries and coastal waters where the bottom sediments are reasonably stable (Pearson & Stanley, 1979; Dines & Wharfe, 1985; Wharfe et al., 1985). In contrast to recent studies on the effects of paper mill discharges to the Swale where these methods have been deployed (Dines & Wharfe, 1985), the soft sediments of the Upper Medway Estuary are far less stable
356
J. R. Wharfe, R. A. Dines, L. A. Bird
TABLE 3 Number of Oligochaetes per m 2 of Sediment Species L. hoffmeisteri
T. tubifex
L. udekemianus L. profundicola
No. m 2
500000 280000 105 800 1 094 600 300 000 586217 127400 376 000 346 500 137 800
Location
Reference
Thames Birtwell & Arthur (1980) River Irwell Eyres et al. (1978) Upper Forth Estuary McLusky et aL (1980) Upper Medway Estuary Thames Birtwell & Arthur (1980) River Irwell Eyres et al. (1978) Upper Forth Estuary McLusky et al. (1980) Upper Medway Estuary Upper Medway Estuary Upper Medway Estuary
and make an assessment of the impact of organic waste discharges more difficult. Nevertheless, a combination of sediment organic carbon content and redox potential profiles clearly demonstrates the occurrence of reduced sediment conditions at depths below 6cm at some sites in the upper estuary and these effects are greatest at sites downstream of the paper mill discharges. Seasonal measurements show the same general pattern, although expected differences between summer and winter are less clear. More recent observations on the movement of sediments in the upper estuary suggest that the bottom deposits are less stable than those in the outer estuary and that a period of scour during high river flow conditions in the winter months is followed by a period of deposition during summer and autumn when river flows are reduced. At some sites the movement of sediments is dramatic, with up to 8 cm being removed or deposited in a month. As a consequence of the mobility of the sediments in the upper estuary, the effects of organic waste discharges are less severe than those described in Milton Creek (Dines & Wharfe, 1985) where, in the absence of any significant scour, large quantities of cellulose fibre settle out and combine with the stable bottom sediments. The effects of the recession on the paper industry have inadvertently reduced waste inputs to the Upper Medway Estuary and agreements on revised discharge consents are currently being formulated to lower them still further. A significant reduction in the amount of organic waste discharged to the upper estuary should increase faunal diversity, although high species richness seems unlikely given the naturally harsh conditions.
Paper mill waste discharge to Medway
357
A n improvement in water quality, particularly levels of dissolved oxygen during extended periods of warm weather and low flows, should encourage migratory fish.
ACKNOWLEDGEMENT The authors wish to thank the Southern Water Authority for permission to publish this paper. The opinions expressed are those of the authors and do not necessarily reflect the views of the Southern Water Authority. REFERENCES Birtwell, I. K. & Arthur, D. R. (1980). The ecology of tubificids in the Thames Estuary with particular reference to Tubifex costatus (Claparede). In Aquatic Oligochaete Biology, Proc. int. syrup. 1st, Sidney, BC, Canada, 1-4 May 1979, ed. by R. O. Brinkhurst, and D. G. Cook, 331-8. New York, Plenum Press. Dines, R. A. & Wharfe, J. R. (1985). The environmental impact of paper mill waste discharges to the Swale. Environ. Pollut., Ser. ,4, 38, 245-60. Eyres, J. P., Williams, N. V. & Pugh-Thomas, M. (1978). Ecological studies on Oligochaeta inhabiting depositing substrata in the Irwell, a polluted English river. Freshwater Biol., 8, 25-32. Gascoine, I. S. & Wildish, D. J. (1971). A chemical and biological study of the Medway Estuary. Wat. Pollut. Control Lond., 70, 1!-12. Institute of Offshore Engineering (1984). Hythe Long Sea Outfall: Environmental assessment of proposed area, August, 1983 survey. IOE/83/208, Heriot-Watt University, Edinburgh. McLusky, D. S., Teare, M. & Phizaklea, P. (1980). Effects of domestic and industrial pollution on the distribution and abundance of aquatic oligochaetes in the Forth Estuary. Helgoland. Meeresunters., 33, 384-92. Pearson, T. H. & Stanley, S. O. (1979). Comparative measurements of the redox potential of marine sediments as a rapid means of assessing the effect of organic pollution. Mar. Biol., 53, 371-9. Water Research Centre (1980). ,4 mathematical model for predicting water quality in the Medway Estuary: Report for the Southern Water Authority. SL478/20/01, Stevenage, Water Research Centre. Wharfe, J. R. (1977). An ecological survey of the benthic invertebrate macrofauna of the Lower Medway Estuary, Kent. J. ,4him. Ecol., 46, 93-113. Wharfe, J. R., Friend, K. & Dines, R. A. (1985). An evaluation of selected sediment parameters as a rapid means of assessing the impact of organic waste discharges to tidal waters. Environ. Pollut., Ser. B, 10, 159-72.