REGULATED RIVERS: RESEARCH & MANAGEMENT, VOL. 8, 83-94 (1993)

IMPACT OF COMBINED SEWER OVERFLOWS O N THE WATER QUALITY OF A N URBAN WATERCOURSE

ADRIAN REES AND KEITH N. WHITE Victoria University of Manchester, Department of Environmental Biology, Williamson Building, Oxford Road,

Manchester M i 3 9PL, UK

ABSTRACT

The lower Irwell/upper Manchester Ship Canal is a canalized waterway in the north-west of England with a catchment area of 700 km’. A 12 month study established a baseline of key chemical and physical parameters (dissolved oxygen, biological oxygen demand, temperature, pH, conductivity, NO;, NO;, PO:-, NH;, suspended solids) with which episodic deteriorations in water quality along the river could be compared. The relative importance of changes in flow, and the impact of the tributaries on water quality in the Irwell, was assessed by mass balance calculations and multivariant analysis. Particular attention has been given to the interactions between sediments and the chemistry of the water column. The main conclusions to emerge are that: (i) the lower Irwell/Ship Canal is subject to significant organic pollution from storm water overflows discharging directly to the river above the study area and via its major tributaries, the Irk and Medlock; (ii) the Irk and, to a lesser extent, the Medlock have an adverse effect on water quality in the Irwell-increases in mass flux below the confluences above that attributable to direct loading suggest that sediment interactions with the water column are partly responsible; (iii) high flow per se, rather than combined sewer overflows in storm events, may be causing deteriorations in water quality due to the release of pollutants from resuspended sediments; and (iv) limited storm data appear to show a readily-soluble ‘first flush’ of pollution to be followed by a delayed impact caused by degradation of newly introduced material. The feasibility of short-term restorative measures such as the installation of oxygen/air injection systems and the provision of weirs are examined in the context of river management; weirs do not appear to be promising solutions.

KEY WORDS Organic pollution Combined sewer overflows Sediments Pollutant flux Pollutant amelioration River management

INTRODUCTION

The Manchester Ship Canal, built between 1885 and 1894, was designed to allow the merchants and industrialists of Manchester, Salford and adjacent manufacturing centres to bypass the financial stranglehold of the Liverpool docks on trade (Figure 1). Tonnage carried by the canal reached a peak of 18.5 million tonnes in 1955, but has fallen steadily since then to around 10 million tonnes in 1990.

The canal was formed in part by the canalization of rivers of the Mersey catchment, the lower Irwell and the Mersey (Figure l), with a catchment area of 700 km’. Thus the canal retains a vital land drainage and sewerage role, serving the whole Metropolitan Manchester and Salford area (Hydraulics Research, 198 1).

When it opened, the Manchester Ship Canal’s feeder waters, the rivers Irwell and Mersey, were already badly polluted. In 1868 there were approximately 10 500 factories in the Irwell catchment alone (Department of the Environment, 1986), and the canal was seen as a convenient sink for industrial and domestic wastes.

Pollution in the region is still significant. The National Water Council (NWC) classification scheme that operates in the UK recognizes four main classes of water quality, class 1 being the best and class 4 the worst. The Mersey catchment comprises 4% of Britain’s total river length, while having 24% of the class 4 rivers (Harper, 1984), which includes the lower Irwell/upper Ship Canal. In this system recurrent organic pollution has brought about high biochemical oxygen demand (BOD) and low dissolved oxygen (DO), resulting in bubbling at the water surface from the release of reduced gases such as methane and elemental nitrogen (Hendry et al., 1992).

0886-9375/93/010083- 12%11 .OO 0 1993 by John Wiley & Sons, Ltd.

Received 10 October 1991 Accepted I7 July 1992

84 A. REES AND K. N. WHITE

Figure 1. Location of Irwell/Manchester Ship Canal in a national and regional context (adapted from Montgomery, 1988)

The revitalization of the docks at Liverpool, London, Preston and Salford by a strategy of land use change to retailing, water-related leisure, housing and ‘clean’ industry has pointed the way to a future use of the Ship Canal corridor, as indicated by the success of the Salford Quays redevelopment at the head of the Ship Canal, where such activities have been established in the disused docks (Hindle, 1992; Struthers, 1992). Isolation of the canal as in these dockland reclamation schemes is an option that is precluded by its drainage function within the region. Concomitant with this drainage role is episodic discharge from combined sewer overflows (CSOs) of poor quality effluent to the lower Irwell/Ship Canal and its tributaries. There are a large number of CSOs within the study area; Hydraulics Research found 33 authorized outfalls recorded upstream of Mode Wheel Locks (Figure 2), although their own survey established that there were 519 (Hydraulics Research, 1981). Water quality improvement in the Irwell/Ship Canal system would firstly require identification of inputs and necessitate the construction of a predictive model to quantify effects on receiving water before effective management of the system could be attempted.

Ongoing efforts to deal with this legacy of pollution in the Mersey Basin, arising from the commitment to raise the quality of the worst rivers to class 2 by ‘around 2010’ (Mersey Basin Campaign, unpublished report, 1991), can only be enhanced by a more detailed understanding of the responses to and processes occurring in pollution events.

A study is being undertaken of the Irwell/Ship Canal system from the lower Irwell above the point of canalization to the first of a series of locks that link the canal to the sea (Figure 2). This involved the monitoring of physicochemical parameters which target the impact of organic pollution, to establish its effects on the receiving waters. This paper gives an overview of the extent and dynamics of organic pollution in the lower Irwell and upper reaches of the Manchester Ship Canal.

The specific objectives are to: (i) examine the temporal changes in the physical and chemical hydrography at points along the Irwell/Ship Canal and its tributaries; (ii) to assess the frequency and magnitude of episodic deteriorations in water quality caused by CSOs; and (iii) examine the effects of inputs from the tributaries.

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N / Mode Wheel Locks - Figure 2. Boundaries of study area and locations of sites

The eventual aim of the study is to formulate a model of the system's responses to organic pollution that will assist in ameliorating these impacts, but this paper will be confined to an examination of the implications of these findings for short-term amelioration of water quality problems.

METHODS

A sampling strategy was formulated to incorporate parameters that targeted organic pollutants (see House, 1989). Those used were BOD, temperature, DO (percentage saturation and mg l-'), NHZ, PO:-, NO;, NO;, suspended solids (total, organic percentage), pH and conductivity. Sampling and analysis followed the recommendations of the UK Standing Committee of Analysts (Department of the Environment, 1978-88, ongoing series).

Samples of surface water were taken on a fortnightly basis between September 1989 and September 1990. Data exchange after an intercalibration exercise with the National Rivers Authority (NRA) augmented the data set so that weekly results were available for some sites for a 12 month period. During storms, sites were sampled at two-hourly intervals initially in the first storm; once it was established that it was a feasible work rate, storm water samples were taken at 45 minute intervals in subsequent storms.

Sample sites were chosen to assess the impact of the tributaries Irk and Medlock; confluences were thus bracketed (Figure 2). Flow gauges on the most upstream site (site 6, at which the NRA provided all data) and on the tributaries allowed estimates of mass flow or mass flux (expressed as kg day-'). Storm sampling sites were limited by logistic necessity to three, bracketing one confluence in each storm.

RESULTS

The ranges and means of parameters for the period from August 1989 to August 1990 for all sites are given in Figure 3. The Irwell falls into the NWC's category of a borderline class 3 (poor quality)/class 4 (grossly polluted) river, with the Irk, Medlock and Ship Canal being class 4, using 95% criteria (e.g. to qualify as class 3, BOD must be < 17 mg 1-' and 0, > 10% saturation). Figure 3 also shows that the Irwell is already

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Figure 3. (a) Mean values for the period August 1989 to August 1990 for NH, and NO; 0 at five sites on the Irwell(1-5) and on the Medlock (site 7) and Irk (site 8); ranges shown by bars. (b) Mean values for PO:- IS and NO; at sites on the Irwell and its

tributaries. (c) Mean values for oxygen saturation Ed, suspended solids % and BOD 0 at sites on the Irwell and its tributaries

polluted at the most upstream site (9, containing high levels of ammonium and phosphate, for example, and remains so on moving downstream to site 1.

Seasonal trends Oxygen and temperature follow the predictable seasonal patterns of a winter trough/summer peak for

temperature and a winter peak/summer trough for DO, with no evidence of thermal discharges. Neither pH or conductivity behaviour are suggestive of alkaline/acid or salt-rich inputs.

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Medltwk confluence 4 0 .

4)

There is a general trend for seasonality of parameters, but this is less marked in the tributaries. For example, at site 5 on the Irwell, nitrate ranges from 4.6 mg 1-' on 27 September 1989 to 1.3 mg 1-' on 14 February 1990 to 6.1 mg 1-' on 25 July 1990, showing a smooth pattern of summer peak/winter trough. Corresponding values for nitrate in the Medlock on these dates were 0.9, 2.6 and 1.26 mg 1-', respectively, with the Irk also presenting an unpredictable change in solute levels. In addition, nitrate chemistry in the tributaries appears to be influenced by different factors to those operating in the Irwell.

Impact of the tributaries on water quality Figure 4(a) and 4(b) shows the large impact that the Irk and Medlock can have on mass flux in the main

channel of the Irwell under varying flow regimes. Note also that there appear to be no significant point sources of pollution other than the tributaries.

By expressing mass flow at sites downstream of confluences (output) as percentages of (flux upstream + tributary flux) (input), some indication is made of changes in mass flux that are in excess of direct loading effects by the tributaries. Figures 5(a) and 5(b) and 6(a) and 6(b) show values of mass flux of some parameters at sites 2 and 4 as percentage ratios of (site 3 + Medlock flux) and (site 5 + Irk flux), respectively.

Generally, the mass flux of BOD, suspended solids, phosphate and nitrate below the Irk confluence is consistently increased by more than direct loading by the tributary; occasionally the ammonium flux is significantly increased. Less consistently, the BOD, suspended solids and phosphate flux below the Medlock confluence are subject to inputs additional to direct loading [e.g. on 14 February 1990, PO:- downstream of the confluence was 145% of the value predicted by summing inputs (Figure 5(a)].

Previous flow conditions complicate the picture, and can have a cumulative effect. For example, on 28 February 1990 the preceding two months of high flow may have resulted in the BOD and suspended solids ratios downstream of the Medlock confluence being only 35 and 46%, respectively, of the flux upstream (Figure 5(b)). However, the Irk's influence was greatest on that day; this may have been due to the Irk having its second highest flow of this period, whereas it was only the Irwell's fourth highest. Weighting of the flows in the main channel of the Irwell in relation to those in the Irk and Medlock is being looked at with respect to tributary impacts.

Sediments and resuspension Examination of Figures 5 and 6 reveals that the tributaries frequently increase the flux by more than can be

accounted for by direct, additive loading. This discrepancy appears to be linked to the resuspension of sediments at the confluences, where there is visible turbulence. There is a definite association between the flux of suspended solids and BOD (Figure 4(a) and 4(b)). Note also that flux in Figure 4(a) is in a low flux (<750 M1 day-') state, whereas in Figure 4(b) flux is for a high flow (> 1950 M1 day-') state: the BOD flux increases eight-fold, as has the flow multiplier, whereas suspended solids transport increases 20-fold.

kg BOD or s.s./day kg BOD or s.s.ldey 11,OOO'sl I 120, 6000 r 1

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0 ' ' . ' ' ' ' " " ' 0 ~ " ~ " " ' ~ " ' ' 0 0 2 0 4 0 6 0 8 1 1 2 1 4 1 6 1 8 2 2 2 2 4 2 6 0 0 2 0 4 0 6 0 8 1 1 2 1 4 1 6 1 8 2 2 2 2 4 2 6

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Figure 4. (a) Changes in mass flux of BOD -0- and suspended solids -A- moving downstream on the Irwell under low flow conditions, and (b) changes in mass flux of BOD and suspended solids in a high flow state. Note the marked change at each confluence

88 A. REES AND K. N. WHITE

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Figure 5. (a) Variation in ratio of mass flux downstream of the Medlock confluence to (mass flux upstream plus tributary flux) with flow for PO:- -A- and NH, -0-. (b) Variation in ratio of mass flux downstream of the Medlock confluence to (mass flux upstream plus

tributary flux) with flow for BOD * and suspended solids F

The occurrence of resuspension is verified by plotting the ratio of mass flux of suspended solids at site 3 : site 4 against flow. Site 3, the downstream site, is deeper than site 4, so a decrease in the mass flux of suspended solids between the two sites is predicted as particulates drop out of the water column. Figure 7 shows that, at low flows, this does occur, with the mass flux at the deeper site being about 50% of that at the upstream site. However, with increasing flow the ratio increases to 1 : 1, implicating either resuspension or reduced settling of suspended particulates; increasingly high flow leads to a decrease in the ratio, which may indicate that scouring in high flows brings about a depletion in the sediment sources of particulates.

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Figure 6. (a) Variation in ratio of mass flux downstream of the Irk confluence to (mass flux upstream plus tributary flux) with flow for PO:- -A- and NO; r: . (b) Variation in ratio of mass flux downstream of the Irk confluence to (mass flux upstream plus tributary flux)

with flow for BOD d- and suspended solids Z

Storm events Generally, this picture in response to storms seems to be one of greatly increased initial levels of suspended

solids (e.g. 119.2 mg 1-' at site 4 in spate compared with a mean of 24.6 mg 1-I) and BOD (e.g. 32 mg 0 , l - I at the same site compared with a peak value of 15.5 mg O2 1-' under non-spate conditions). There is a rapid decrease in suspended particulates after this 'first flush', but a delayed release of pollutants from the newly introduced material follows within about 24 hours, which then appears to tail off to values possibly lower than in a steady-state condition.

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Figure 7. Increasing ratio of mass flux of suspended solids at site 3 (downstream, deeper) to mass flux at site 4 (upstream, shallower) with flow indicates the occurrence of resuspension

Antagonistic relationships between ammonium and nitrate and oxygen were observed in the routine data set and were more pronounced in storms. The NHl-NO; correlation for a storm on the Irk was significantly negative (I = 0.851; p c 0.001; n = 14). This is also shown on the Medlock (Figure 8). In addition, the ammonium pulse/nitrate trough corresponds to a simultaneous BOD peak (Figure 9) and supports work by Hvitved-Jacobsen (1982) and Harremoes (1982) on the delayed oxygen demand of newly settled sediments.

The correlation of phosphate and ammonium (I = 0.682; n = 14) indicates that this oxygen sink is largely derived from organic sewage material. Again there is a delayed release of these solutes, from an initial pulse of 2 mg 1-' P0:-/7-1 mg I - ' NHZ, to a trough after three hours of 0.7 and 3.5 mg I - ' respectively, then climbing again to 1-5 and 8-3 mg l-', respectively, after about eight hours.

DISCUSSION

The flux of pollutants was seen to increase, sometimes dramatically, after each confluence. There appears to be no other significant point sources of pollution other than the tributaries, an assumption that may be useful

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Figure 8. Depletion of oxygen -0- and NO; -A- coinciding with a pulse of NH, -0- during a storm event on the Medlock. Nitrate is below limits of detection

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Figure 9. Peak of BOD + corresponds to the NH, pulse and oxygen (-O-) sag observed in Figure 8

when it comes to modelling the system. In addition, this suggests that the impact of CSOs on water quality is not direct, but via the tributaries. The consequence of this is that there may be a double pulse of deterioration of water quality, both from the first flush from accumulated detritus in the sewerage and by the delayed degradation of recently settled material causing a secondary oxygen sag (Harremoes, 1982).

Hvitved-Jacobsen (1982) found the delayed DO sag to last for about 12-24 hours after the pollution event, a time-scale compatible with that found in this study. Though not explicitly mentioned, the study of Kreutzberger et al. (1980) of CSO effects on DO also showed a DO sag to take place within, and last for, about 12-14 hours. In the latter study, the magnitude of the decrease in DO was from 4 mg I - ' to below the limits of detection. These two studies contrast with that reported here in finding this effect on oxygen; it may be that surface reaeration in the Medlock and Irk is sufficient to offset the demand of the degrading sediments and nitrification processes.

Nitrate chemistry appears to be influenced by different factors in the Irwell to those in the Irk. In the Irwell, a highly significant negative correlation occurs between 0, saturation and solutes (e.g. I = - 0.796; p = < 0.001; n = 22) between oxygen and NO; at site 2), which is reversed in the Medlock (I = 0-564; p = < 0.001; n = 38); the two seem to be independent in the Irk ( r = -0-139; p = 0.417; n = 38). This has two possible explanations.

One is that oxygen is the limiting factor for nitrification in the tributaries: if a high sediment oxygen demand exists, as in the Irk and Medlock, denitrification can occur at DO levels of 6-8 mg 1-' (Koike and Hattori, 1978). In the Irwell, high flows give rise to a higher oxygen saturation by surface reaeration while diluting solutes.

Alternatively, the high flows that bring about the high percentage of 0, are also resulting in nitrate inputs to the tributaries that outweigh the dilution effects. These inputs might be allochthonous (CSOs) or autochthonous (disturbed sediments); differentiating between the two is proving difficult. Several studies have implicated disturbed sediments as a source of increased solute levels (e.g. Casey and Farr, 1982).

The chemistry of nitrogen species in general in the system is likely to be complex. The exogenous sources may be in an oxidized state on entering the system (i.e. as NO; from fertiliser run-off), or as reduced species (NH, or organic forms of nitrogen). The sediments of the Manchester Ship Canal/Irwell are highly reducing and rich in organic material (up to 75%) due to the significant faecal input from untreated storm overflows; under these conditions, denitrification pathways are energetically less likely to be taken (Knowles, 1982), whereas organic-rich sediments have been found to be sources of NH: by several workers (e.g. Koike and Hattori, 1978).

92 A. REES AND K. N. WHITE

However, the water column overlying the sediments is sufficiently oxidized to allow both nitrification and denitrification to proceed, depending on the oxygen and microbiological regime existing at a given time. In addition, the correlation between the organic fraction of suspended solids and PO:- and N H t (Figure 10) indicates a water column source for ammonification. The importance of sediment-water column interactions for water chemistry (e.g. Teague et al., 1988; Cerco, 1985; Kuenzler, 1982; Kreutzberger et al., 1980) has to be emphasized if a truly accurate prediction of pollutant behaviour is to be made.

Disturbance of sediments may also have repercussions for nitrogen chemistry. Any communities of nitrifying bacteria at the surface of the sediments, as they are aerobes; this is also the mobile zone, however, which is most likely to be removed in high flows (Jobson and Carey, 1989), so that nitrogen transformations may be affected for some time after spates (Cooper, 1983).

The main impact of CSOs on water quality may be indirect, via resuspension of sewage-derived sediments. The model of Kreutzberger et al. (1980) attributed the observed rapid decline in DO primarily to scouring of the sediments, as rates of DO decline were in excess of rates that could be accounted for by direct loading. In one sense, then, CSOs are always having impacts, not just during storms, as high flows per se will provide instream sources of pollutants (Casey and Farr, 1982), as will the observed turbulence at the confluences. The change in solute levels that Casey and Farr (1982) found (e.g. NO; increased from 3.3 to 3.7 mg 1-I) is much smaller than that found in spates in the Irk or Medlock (e.g. a range of 2.6-6-5 mg 1-' in spate on the Irk), indicating the polluted nature of the sediments in this system and their potential for pollution. The highly significant correlation of BOD with suspended solids in the Irwell/Ship Canal system, an association also observed by Casey and Farr (1982), again points to the negative effects of resuspension on water quality.

Perhaps due to the practical difficulties involved, little work appears to have been carried out on the effects of the build-up of sediment sources of pollutants in dry periods, and their depletion by scouring in high flow states. Those workers who do incorporate this into predictions of water quality take an indirect approach by using the antecedent dry period as a parameter in their models (Kreutzberger et al., 1980). It is hoped that a more direct method could be used in this study, by analysing sediment cores for particle size distribution and physicochemical properties (e.g. redox potential, organic content).

With the relatively dry weather that has recently been prevalent in the north-west of England, the aspect of the study concerned with modelling of overflow impacts has too little data to draw firm conclusions. However, major trends in water quality in the Irwell system have been identified.

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Figure 10. The marked correlation between NH, +, PO:- X and the organic % of suspended solids -0- is strongly indicative of a water column source of ammonia

SEWER OVERFLOWS 93

Ameliorative measures The current investment in regenerating the Ship Canal corridor is likely to reduce the pollutant load over

the long term. However, the pressure from developers and local authorities is for more immediate measures. In the short term, amelioration of pollution in the Ship Canal must be based around satisfying the oxygen

sink of the sediments and water column, especially during the low flow summer period, when the apparent threshold for the onset of bubbling from sediment gases of below about 4 mg 0 , l - ’ depth-averaged through the water column (Webb, 1992) is often crossed. Meeting this oxygen demand should prevent the accumulation of reduced species in sediments and the eruption of sediment ‘rafts’ pushed to the water’s surface by the accumulation of trapped, reduced gases. The production of an oxic crust at the sediment-water column interface, which might reduce the sediment-water flux of some chemical species such as phosphate, would be an additional desirable result.

Presently, work at the University of Manchester has been centred on the use of a British Oxygen Company Vitox oxygen bubble injection unit. A survey of oxygen levels before and after oxygen injection resulted in a regression model which showed that the Vitox was increasing the saturation. However, the increase above the predicted oxygen level only persists for about 0.5-1 km downstream of the Vitox unit, depending on the flow conditions and rate of oxygen injection (Webb, 1992), necessitating the installation of several units. The capital costs and running of the necessary units is being examined to determine if this is a viable solution.

As there is still some traffic on the Ship Canal, weirs are not an alternative means of reaeration. Although a decline in traffic might make this feasible in the future, poor water quality in the Irwell may preclude this option. Work by Lakso (1989) on aeration at overflow weirs in combination with results from our investigations into the Irwell’s water quality suggests that the frequency and sufficient height (0.3-1-2 m drop in water level) of weirs necessary to increase oxygen levels to 4mg 1-’ (both Vitox units and weirs can increase DO by about 3 mg 1-’) would result in the canal being below sea level before reaching the sea.

In the light of this discussion, abatement of polluting inputs to the system must be the long-term goal. Currently, the monitoring of storm event dynamics seems to be opening up promising avenues of research into the processes that are of special relevance in determining water quality, and from there the construction of a model that may be of some help in ameliorating the impact of CSOs in the upper part of the Ship Canal corridor.

ACKNOWLEDGEMENTS

This research has been carried out under a Science and Engineering Research Council (SERC) grant. I thank the National Rivers Authority and North West Water plc for extra funding, and doubly thank the NRA for their data.

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Cerco, C. F. 1985. ‘Sedimentation column exchanges of nutrients and oxygen in the tidal James and Appomattox rivers’, Report Order

Cooper, A. B. (1983). ‘Effect of storm events on benthic nitrifying activity’, Appl. Enoiron. Microbiol., 46, 957-960. Department of the Environment 1977-88. Methods for the Examination of Waters and Associated Materials. HMSO, Southampton. Department of the Environment 1986. River Quality in England and Wales 1985. A Report of the 1985 Survey. HMSO, Southampton. Harper, R. 1984. ‘COPA I1 and the River Mersey; a regional Water Authority view’, Chem. Ind., 16/7/84, 502-505. Harremoes, P. 1982. ‘Immediate and delayed oxygen demand in rivers’, Waf. Res., 16, 1093-1098. Hendry, K., Webb, S. F., White, K. N., and Parsons, A. N. 1992. ‘Water quality and urban regeneration: a case study of the Mersey

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