MACROINVERTEBRATE BIOMONITORING AND WATER QUALITY MANAGEMENT WITHIN URBAN CATCHMENTS

Transcription

1 Hydrological Processes and Water Management in Urban Areas (Proceedings of the Duisberg Symposium, April 1988). IAHS Publ. no. 198, MACROINVERTEBRATE BIOMONITORING AND WATER QUALITY MANAGEMENT WITHIN URBAN CATCHMENTS A.D. Bascombe, J.B. Ellis, D.M. Revitt and R.B.E. Shutes Urban Pollution Research Centre, Middlesex Polytechnic Queensway, Enfield, EN3 4SF, UK ABSTRACT The management role of benthic macroinvertebrate monitoring as a means of evaluating the impacts of transient urban discharges is described for a catchment in N London, UK. Ecological variations and indices are identified and related to hydrochemical and toxic peturbations which prevent the establishment of a stable ecosystem. INTRODUCTION Macroinvertebrate species exhibit a wide variation of response to pollutants and have been extensively monitored in lotie water bodies to evaluate water quality and complement physico-chemical surveys (Hawkes, 1979; Shutes, 1985). Seasonal samples of the macroinvertebrate community can indicate the effects of pollutant sources which may not have been detected by either intermittent physico-chemical sampling or continuous monitoring of a restricted range of parameters. Receiving water impacts of urban discharges are conventionally stated in terms of hydraulic and hydrochemical criteria and the ecological dimension of environmental impact has been overlooked (Gujer and Krejci, 1987). Urban discharges entering receiving streams from both point and non-point sources contain elevated concentrations of both organic and inorganic toxic components. (Ellis et al., 1986). Although transient in nature, the complex and toxic composition as well as high storm discharge intensity of urban runoff will have marked effects on macroinvertebrate species present especially in the vicinity of the offending outfall. Inevitably this will result in a lower biological classification of the receiving stream than would be obtained by a chemical sampling regime. These intermittent effects may be more properly evaluated by applying hydrobiological indices or investigating bio-species uptake rates of toxins. The UK Water Authorities are adopting the BMWP Score System (National Water Council, 1981) or the ASPT modification for biological surface water classification. This system involves the identification of organisms to family level (but does not require any estimate of abundance) and assigns a score determined by organic pollution tolerance. By reviewing the score allocation it can be adopted for reliable management purposes on urban receiving waters. Integrated multidisciplinary biochemical surveys to determine tolerance limits of species to stresses can make important contributions to operational urban river basin management. CATCHMENT CHARACTERISTICS AND EXPERIMENTAL APPROACHES Seven sampling sites have been established along the Salmon's Brook which is a 15 km 209

2 A.D. Bascombe, J.B. Ellis, D.M. Revitt and R.B.E. Shutes tributary of the River Lee, in NE Londen (Figure 1). The sample locations represent sites of increasing urbanization from the rural upstream section (Site A) through to sites immediately downstream of road runoff (Site D), industrial and urban area runoff (Site E) and a combined or storm sewerage overflow (Site G). A monthly sampling programme, commencing in March 1986, has provided baseflow samples of water, sediment and macroinvertebrates from each of the seven sites as well as field measurements of flow, ph, conductivity and dissolved oxygen. Biological samples were collected by controlled kick sampling based on ten 30 second kicks at each site. The organisms were identified and BMWP scores estimated. Macroinvertebrates, both in bulk and as individual species (Asellus aquaticus, Gommants pulex and Limnaea peregra) were analyzed for their Cd, Cu, Pb and Zn content after starving for one day to eliminate errors due to any re-ingested gut material. For the detritivores, a fine nylon mesh was used to separate the organisms from their own faecal pellets prior to analysis. Metal levels in the dried animal and sediment (sieved to 1 mm) samples were determined by flame atomic absorption spectrophotometry after digestion with a 9:1 mixture of concentrated nitric and perchloric acids. The lower metal levels in the water samples were analyzed by anodic stripping voltammetry after performing a similar digestion procedure. Laboratory analyses of the water samples for total hardness, alkalinity, BOD and ammonia-n were also carried out. 210

3 Macroinvertebrate biomonitoring and water quality management catchnent boundary - major watercourses surface drainage A background site Figure 1. Metal distributions in water, sediment and macroinvertebrate tissue samples for sites on the Salmon's Brook, N London. 211

4 A.D. Bascombe, J.B. Ellis, DM. Revitt and R.B.E. Shutes BIOTIC IMPACT INDICES AND WATER MANAGEMENT The means and ranges of metal values determined for water, sediment and macroinvertebrate tissue at the seven sampling sites are presented in Figure 1. Sediment metal levels increase downstream by up to ten times their upstream concentration, with enhanced levels being particularly apparent at site D where highway runoff enters the river, and at site G adjacent to the sewerage effluent and combined sewer outfall. Metal concentrations in the water phase increase downstream by a similar order of magnitude to sediment levels, although cadmium remains at a fairly constant low level throughout the catchment. Macroinvertebrate tissue metal concentrations, although generally reflecting these established downstream trends, are subject to a much greater variation as individual species show different metal uptake or retention behaviours. This is clearly demonstrated by Asellus aquaticus for which average tissue metal concentrations remain relatively uniform in upstream organisms but increase dramatically below the sewerage outfall with zinc reaching an average concentration of 373 Mg g at site G; an increase of over three times compared to the upstream levels. The impact of urbanisation on the biotic community becomes evident below side C where Sialic sp. and Dytiscus, which are characteristic of marginal habitats, disappear altogether (Figure 2). Although the DO regime of the urban sites is compatible with the existence of a high species diversity, Gommants abundance is low and Baetis sp. are absent; both indications of the impact of transient urban discharges. Overall species richness falls from a total of fifteen at Site A to seven at site G. The observed variation in the BMWP hydrobiological index score against sampling site, illustrated in Figure 3a, shows a persistently decreasing trend downstream. Particularly low scores are obtained at site F where recent engineering works have increased localised siltation and only a limited number of species remain in the community. A subsequent increase of BMWP score at site G, despite the higher levels of organic pollution observed, occurs as a result of increased community diversity adjacent to the sewerage outfall. 212

5 Macroinvertebrate biomonitoring and water quality management toiftcephlté tf, Cerlm >p. 3E=aMK I m- i p - ' ru*l/«i ivtffnr Figure 2. Spatial variation of mean abundance for macroinvertebrate species for sites on the Salmon's Brook, N London. The BMWP score variability also decreases downstream, implying a more consistent community structure exhibiting less annual variation and which is also subject to less stress-related change. This community stability is probably related to the dominance of the treatment plant effluent over the flow regime which provides some measure of flow equilibrium during the inter-storm periods. The application of the BMWP score system to the pollution condition and regime of the urban river can be interpreted in terms of water quality management. The current water quality classification used by the Water Authorities of England and Wales is that developed 213

6 A.D. Bascombe, J.B. Ellis, D.M. Revitt and R.B.E. Shutes CLASS IA +201t CLASS IB CLASS II -20»J +20* % > CLASS III +20* ( "^4 0 V ** 7» "' '«" ^.rluriil Area C D EFO " 20 * Sampling slto +20* J -20* «D Sampling site E F G Figure 3. River classification and BMWP scores for the Salmon's Brook, N London by the National Water Council (NWC, 1977). This NWC classification is essentially based on chemical criteria although EIFAC and EEC Directives are also considered. Associated with the river classification are 95 percentile criteria expressed in terms of DO, BOD, NH 4 -N as well as of biota, fish life and use of water. Although the use of five classes in the NWC classification (Figure 3b) does increase the refinement of the method, much valuable management information remains unavailable. For example, Class II covers a 214

7 Macroinvertebrate biomonitoring and water quality management very wide range of water quality, yet the quality of individual rivers within this class is not distinguishable. Given that Water Authorities have stated River Quality Objectives (RQO's) for specific rivers and channel reaches as an essential element to achieve acceptable "levels of service", it is undoubtedly desirable to have flexible methods of identifying the location and movement of a river within a class band. The classification is particularly poorly suited to receiving waters persistently subject to some degree of pollution and to irregular, transient pollutant slugs typical of urban discharges. The application of a hydrobiological index to the NWC classification would provide an appropriate reinforcement to the management facility offered by the latter system and Figure 3b illustrates a proposed superimposition of the BMWP scores against the NWC classification with ±20% critical areas indicated either side of the class boundaries. It must be stressed that detailed calibration of chemical and biological classifications need to be undertaken on a regional basis as the score bands for the Class I conditions of Figure 3b apply to stable, productive baseflow rivers. The BMWP scores are also sensitive to sampling and physical site influences and therefore need to be verified by supporting ecological indicators. However, the structural methodology offered by such superimposition provides a very convenient framework for not only monitoring water quality of polluted and "grey-water" systems but also in providing a full and integrated operational management approach. Figure 3a shows the NWC classification scheme superimposed upon the BMWP score for the Salmon's Brook, indicating it to be a Class II river in its upstream rural sections although the average BMWP values place it firmly within the lower 20% critical area. In the urban reaches, the river biologically sags into Class III and even approaches the critical upper limits of a Class IV river. There are however discrepancies between specific macroinvertebrate metal uptake behaviour and the community hydrobiological index, suggesting a serious inadequacy of hydrobiological community score systems to assess the impact of toxic metal pollution. Any revision of the community score system must incorporate information on the relative sensitivity of different species in each group to toxic substances and therefore include reference to the bioavailability of toxic species. Figure 2 illustrates selected species found in the sampling programme and shows their mean abundance. Species with high BMWP score ratings tend to be found in greater abundance at the upstream rural sites, and there are distinct and obvious differences in species composition between the extreme sampling locations within the catchment as well as identifiable species associated with the sewer outlets. Near to such outlets, the species present are indicative of a polluted meso- or poly-saprobic state being dominated by diptera and worms. The distinct community structural differences that are observed amongst the macroinvertebrate populations, and which become particularly apparent below the outfall at site G, are not always reflected in the simplified community index score, which may be similar for different locations. Macroinvertebrate monitoring would therefore appear to offer many advantages for the assessment of urban runoff impacts and for a qualitative, time-integrated approach to water 215

8 A.D. Bascombe, J.B. Ellis, DM. Revitt and R.B.E. Shutes quality management of urban rivers. Ecological approaches can either complement existing chemical procedures or provide a reliable, inexpensive alternative method for surface water quality classification where chemical data are absent. Although biological monitoring can provide a practical and sensitive tool there is still a need to further modify the ecological scoring system for transient discharges as well as to identify an appropriate species for specifically monitoring toxic impacts. ACKNOWLEDGEMENTS The authors wish to thank the UK Science and Engineering Research Council as well as the Water Research Centre for their support for this study. REFERENCES Ellis, J.B., Hamilton, R.S., Revitt, D.M., and Shutes, R.B.E The effects of urbanization on receiving water quality: heavy metal toxicity. In: The effects of land use on freshwaters. J. Solbe (éd.), Ellis Horwood Ltd., Chichester, pp Gujer, W. and Krejci, V Urban storm drainage and receiving water ecology. In: Urban stormwater quality, planning and management. W. Gujer and V. Krejci (eds.), Proc. 4th Int. Conf. Urban Storm Drainage, Ecole Poly. Fed. Lausanne, Switzerland. pp Hawkes, H.A Invertebrates as indicators of water quality. In: Biological indicators of water quality. T.A. Evison (éd.), J. Wiley & Sons, London, pp National Water Council, Final report of the working party on consent conditions for effluent discharges to freshwater streams. NWC, London. National Water Council, River quality: the 1980 survey and future outlook. NWC, London. Shutes, R.B.E A comparison of benthic macroinvertebrate fauna of two North London streams. Envir. Tech. Letters, 6: