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Abstract Technical Note Estimating water pollution risks arising from road and railway accidents R.F. Lacey1 & J.A. Cole2 1White Cottage, Lower End, Great Milton, Oxford, OX44 7NL, UK 23 Grangefield Way, Aldwick, Bognor Regis, West Sussex, PO21 4EG, UK both formerly with WRc plc, Medmenham, Marlow, Bucks, SL7 2HD, UK A method is presented for estimating thefrequency of spillage of toxic liquids from roadtanker accidents. The calculation requiresinformation about the vehicular flow of tankers, their accident rate and the probability that an accident will result in a spill. The available sources of input data are discussed. Railway goods traffic, either involving liquids transported in tanker wagons, or hazardous freight packed in individual containers as part wagon loads, is amenable to similar analysis but relevant input data are much more difficult to obtain. For any type of spill, the estimation of frequency should be accompanied by surface and groundwater pathway analyses, to assess the consequential damage to the aquatic environment and any possible hazard to public water supplies. This paper serves as an outline of a subject calling for more thorough study, for which improved databases, geared to the needs of risk assessment, would be strongly desired. Keywords: accident, environment, pathways, railway freight, risk, road transport, spillage, tanker, toxic hazard, water pollution Introduction This note outlines a procedure for quantifying the risk of water pollution, particularly affecting surface waters and groundwaters used as a raw water source of potable supply, arising from accidental spillage caused by traffic accidents on roads or railways. The procedure was originally developed by RFL for the road transport case, as part of a Pollution Risk Management project, which WRc plc undertook for a consortium of water companies in 1994. Subsequently the procedure was extended to incorporate rail traffic accidents. At the time this project was begun, the authors were not aware of any previous work in this area. Our approach leant heavily on information contained in a report by the Health & Safety Commission, Advisory Committee on Dangerous Substances, 1991. Only one more recent example is known to us of environmental risk assessment applied to road and rail accidents, summarized in a report by Fedra (2000). This includes a case study of petrol and diesel fuel, transported by road from Lisbon to Aveiras, Portugal, and a study of various hazardous goods conveyed by rail through Alpine valleys in Switzerland. The report cited does not go into the detail of risk assessment, but shows how geographical information systems, telecommunications and Hazchem data bases could all help to cope with accidental spills. The question of pollution risk can be broken down into three components: (1) What is the risk of occurrence of a substantial spill of pollutant along a given length of highway or railway track? (2) What movement, dispersal and alteration of the pollutant will occur in its passage towards and through surface waters (including natural and arti- ficial drainage, streams and rivers, marshes, lakes, reservoirs) and groundwaters (including water in the unsaturated zone, and phreatic and confined waters)? (3) What hazard does the pollutant present to the environment, to riparian water users and to water sources used for private and public water supplies? Here we focus mainly on component 1, but go on to offer general advice on components 2 and 3, with reference to relevant published work. Explanation of terms In everyday parlance the terms ‘hazard’ and ‘risk’ are used imprecisely with variations of meaning. In quanti- tative risk assessment, however, it is helpful to reduce ambiguity by using these terms in a more standard way. In this paper we adopt usage similar to that of the Royal Society Study Group (Warner 1983): A hazard is a set of circumstances (such as the transport of a toxic liquid in a particular type of tanker over a given distance and road category) that could lead to a specified type of adverse event (for example a spill). The risk of an adverse event is conveniently quantified in terms of the frequency of occurrence (or rate of inci- dence) of that event. Here we need to deal with fre- quencies expressed in two different ways: Quarterly Journal of Engineering Geology and Hydrogeology, 36, 185–192 1470-9236/03 $15.00 � 2003 Geological Society of London f1 = accidents / million vehicle-km for a specified type of vehicle; f2 = accidents / km-year for a specified road and traffic loading. The connection between them is given by f2 = V f1 where V is the relevant flow of traffic (millions of vehicles per year). This relationship will be elaborated further below. Any spillage arising from a transport accident becomes the hazard governing any subsequent con- tamination of the water environment. In the reasoning that follows we shall focus on estimating the frequencies of potential pollution-producing events. Cases considered We restrict ourselves to spills arising from the com- mercial freight of liquids in tanker lorries or rail tank and container wagons in transit. It is necessary to specify: (i) the particular stretch of road or railway for which the frequency of spills is to be estimated; (ii) the type or types of vehicle from which spills may occur, and the classes of initiating event; (iii) the types of pollutant that are to be considered; (iv) the range of spill volumes that would be of concern. In the following discussion we concentrate on those types and sizes of spill that are thought to have the most damaging further consequences, either in a single accident or from the cumulation of accidents that are likely to take place over a period of time. For road transport we restrict attention to spills from tanker lorries of over 3.5 tonnes gross vehicle weight, caused by collision or roll-over. This would appear to cover most large-volume spills but excludes spills from the vehicles’ own fuel tanks or from drums or smaller vessels carried by lorries. For spills from rail tankers we shall concentrate on those associated with the standard types of tank wagon used for the transport of motor spirit. These have typical capacity of either 32 or 75 tonnes and nominal minimum shell thickness of 6 mm. It is important to make the distinction between these thinner walled tanks and those of higher specification (nominal shell thickness 11– 16 mm) used for more dangerous substances such as liquefied gases. (Health & Safety Commission, Advisory Committee on Dangerous Substances, 1991, appendix 8). In addition to spills from rail tankers we give some estimates that refer to spills from freight in containers. Our analysis excludes hazards associated with loading and unloading. Those operations ideally should, but do not always, take place at sites with special safeguards for containment of spills. Sites with and without such safe- guards merit separate risk assessments. Similarly we have not considered the risk of accidents in areas such as lorry parks or rail marshalling yards. It would, of course, be important to include all such areas in a risk audit of a water-supply catchment. General principles The logic and symbols applicable to estimating the frequency of spills are explained in Figure 1. Appropri- ate units are given in square brackets. The following sections explain how the input quantities: V, ptanker, f1, L and pspill can be dealt with. For L this is straightforward since a specific length of road or track will be considered. For pspill , the con- ditional probability that an accident gives rise to a spill, there is unlikely to be suitable local information and so this must be estimated from national data. However, for the other basic quantities, V, f1 and ptanker, decisions have to be made whether to estimate them at a local or national level. These decisions involve very different considerations for the two types of transport, road and rail, and so different approaches must be taken, although the general principles of the final calculation are the same. Fig. 1. Flow diagram of spillage risk assessment. LACEY & COLE186 Lengths of road and railway considered Figure 2 illustrates an imaginary area with a river and an aquifer, traversed by a railway. Any spill in the railway segment Xr (or indeed anywhere in the river catchment upstream of Z) will threaten the water quality at the pumped abstraction site Z. Any spill in the railway segment Xa (or anywhere across the aquifer’s exposure) will threaten the groundwater quality in the downflow direction, which will be drawn towards the pumped water well W. Note in this particular example the hatched zone where both the aquifer and the river are at risk, should a pollution incident occur there. Obviously real situations are liable to be far more complex, especially the interaction of surface and groundwaters, together with the phreatic and confined zones of the latter. Traffic flows Road traffic For estimating the vehicular flow of tanker lorries on a given stretch of road there are three possible approaches. The choice of which to take depends on the class of liquid to be considered and on the required accuracy of the estimate. Using national averages. From national traffic censuses the Department of Transport publishes annual statistics from which it is possible to estimate annual average traffic flows, classified by type of road and type of vehicle, as for example those given in Table 1, derived from tables 3.16 and 4.9 of Transport Statistics, Great Britain 1992 (Department of Transport, Scottish Devel- opment Department & Welsh Office, 1993). In this table a heavy goods vehicle (HGV) is a vehicle of over Fig. 2. Schematic plan of a river catchment and an aquifer traversed by a railway. Table 1. Average flows of HGVs and all motor vehicles, by class of road, 1991 data * Road category Flow of HGVs (million vehicles/year) Flow of all motor vehs (million vehicles/year) HGVs/ All vehs. (%) Motorway 2.8 19.7 14.0 Built-up major roads Trunk 0.51 6.8 7.5 Principal 0.27 5.5 4.9 Non-built-up major roads Trunk 0.60 5.5 11.0 Principal 0.17 2.5 6.8 Minor roads 0.019 0.50 3.8 * Derived from tables 3.1 and 4.9 of Transport Statistics Great Britain 1992 (Department of Transport, Scottish Development Department & Welsh Office 1993) ESTIMATING WATER POLLUTION RISKS ARISING FROM ROAD AND RAILWAY ACCIDENTS 187 3.5 tonnes gross vehicle weight. Note from Table 1, that flows of HGV not only vary by two orders of magni- tude, as between minor road and motorways, but that the HGV proportion increases too, in going from minor to major highways. Of the goods traffic in 1991 about 4% of the overall tonnage transported involved the transport of petrol and petroleum products, 6% involved ‘chemicals’ and 26% involved food, drink and tobacco (Department of Trans- port, Scottish Development Department & Welsh Office, 1993). These are the main categories of commodity to include liquids capable of giving polluting spills. Percent- ages such as these could be used in conjunction with Table 1 to give approximate estimates of tanker traffic for the respective types of load. The percentages for ‘chemicals’ and for ‘food, drink and tobacco’ would, however, over- estimate the transport of liquids because they do also include commodities in solid and gaseous forms. Using special surveys. If information is needed for a specific chemical or group, or if local circumstances are likely to be very different from the national average, there will be no alternative to carrying out a special survey or study. This could take the form of a tanker census on the particular road or a study of the numbers of loads delivered to or from key factories, stores or outlets. Using link specific flows. There is an intermediate approach between those outlined in the two previous paragraphs. The Department of Transport produces link-specific estimates of annual average daily flow for each section of the major road network, and these can be supplied on various media. Similar and additional infor- mation may also be available from county highways departments. If such estimates are not available for HGVs but only for all motor vehicles, they could be re-scaled for HGVs using the percentages in the final column of Table 1. The flow of, for example, tankers carrying petrol or petroleum products would then be calculated by applying the commodity percentage (4%) as in the last but one paragraph. Rail freight traffic While for road transport it is possible to estimate traffic flows from readily accessible national statistics, this does not appear to be possible for rail freight, for two reasons: (i) From the information that is available about specific chemicals (Health & Safety Commission, Advisory Committee on Dangerous Substances, 1991, appendix 8) it is very clear that rail traffic is very heterogeneous across the rail network. Route specific information is therefore essential to describe the types and volumes of loads conveyed. An extreme example would be the transport of chlorine, for which the route from Merseyside to Anglesey carries 60% of the national traffic in that substance. (ii) There does not appear to be a publicly accessible database for link-specific rail traffic loads. The only possible sources of information would appear to be the railway operators themselves. Accident and spillage frequencies For the estimation of spillage frequencies arising from transport accidents there is no national data collection system. For both the road and rail transport cases we are reliant on information collected for the special studies undertaken for or mentioned in the report of the Health & Safety Commission, Advisory Committee on Dangerous Substances, 1991. For spills from rail tankers this information (from appendix 8 of the Health and Safety Commission Report) permits only a combined estimate of the accident involvement rate and prob- ability of a specified spillage (see Fig. 1). For road tankers, however, there is an advantage in carrying out a more detailed treatment in which these quantities are estimated separately, thus allowing for the different rates of accidents that prevail on different classes of road. Road accident rates Typical accident involvement rates for HGVs and for all road vehicles are shown in Table 2, classified by type of road. This table was compiled from table 41 of Road Accidents Great Britain 1991 (Department of Transport, Scottish Development Department & Welsh Office, 1992). There is no information specific to tanker vehicles but it is very reasonable to assume that their accident involvement rate would be similar to the average for all HGVs. The application of one of these rates to a specific stretch of road involves the assumption that the road is typical of its class, not much safer nor much more dangerous. It might be thought that it would be prefer- able to obtain accident information for the particular road in question and hence compute a specific estimate Table 2. Vehicle accident involvement rates, by type of road, 1991 data ** Accidents per million vehicle-km (HGVs) Accidents per million vehicle-km (All motor vehicles) Motorway 0.24 0.23 Built-up roads 0.88 1.52 Non built-up roads 0.52 0.60 ** Derived from table 41 of Road Accidents Great Britain 1991 (Department of Transport, Scottish Development Department & Welsh Office 1992) LACEY & COLE188 of the accident rate. Local road-accident statistics for each particular stretch of road are usually obtainable from county highways departments. There are, however, two disadvantages associated with the use of this. (i) The available data may not distinguish tankers, nor even perhaps HGVs. (ii) Even if data are available for the relevant class or classes of vehicle, the number of events may be too small to enable a reliable estimate of frequency to be made. If local data are available, the recommended way of dealing with these difficulties would be, first, to use national rates (Table 2) to estimate the expected number ne of accidents for the same class of vehicle and window of time as those for which the observed number no is obtained. Then compare no with ne. Only if no and ne are statistically significantly different would it be worth making adjustment for this by multiplying the relevant rate from Table 2 by no/ne. It is important to note that, in this procedure, no and ne must refer to exactly the same class of vehicle and event. It is not, however, essential that this be ‘all severity’ involve- ments of tanker vehicles. Provided that it is reasonable to assume that the proportional excess no/ne would be roughly the same for all classes of vehicle, the value of this ratio could be estimated for any class for which data is conveniently available. The largest available class should be chosen. We would recommend using local accident information only after the incorporation of a local link- specific estimate of traffic flow. Road spillage frequencies For road tankers the Health & Safety Commission, Advisory Committee on Dangerous Substances, 1991 provides estimates of the average spill rate per tanker- km, for three different types of initiating event: rollover, collision and body material or equipment failure. Restricting attention to traffic accidents (rollover or collision) it is possible to use the HSC results to estimate the probability that a traffic accident involving a tanker will result in a spill (see Table 3). The estimates in this table were obtained by dividing the average spill-rate per tanker-km by the average accident involvement rate per tanker-km. The latter was taken to be 0.656 per million vehicle-km, an average for HGV for 1981–85, the time period for the HSC study. Rail accident frequencies In the early 1960s accident statistics showed freight derailments at about 0.2 per million train-km (i.e. 2 � 10�6 per train-km), which was comparable to the risks of HGVs on motorways. Tanker traffic in that decade was about 5 � 107 wagon-km/year. Some trains would have comprised mixed loads, with only a few loaded tankers, others would be all-tanker trains, full or empty. To convert the wagon-km/year to train-km/year one needs a figure for W, the average number of full tanker wagons in all goods trains. In the absence of published data on this, take a realistic estimate such as W = 6. The frequency of derailment of such trains would thus be estimated as: (0.2� 10�6)� (5� 107)/6 per year = 1.3 per year, involving all mixes of tanker traffic, ranging from all-tanker cargo to trains with only one tanker-wagon. Further information on freight trains is given by HM Railway Inspectorate (Health and Safety Executive 1994) who reported 110 incidents of rail freight col- lisions (12 incidents), derailments (78 incidents) and fire (29 incidents) on the then British Railways network during 1993/94, out of a total of 889 train accidents affecting all classes of BR rail traffic. (The 1993/94 freight train accident statistics showed an improvement over those for 1992/93, when freight train derailments were twice as common). However no data appear on the proportion of such incidents affecting full tanker wagon loads. If, at a rough guess, that proportion were 2%, the inference would be that 110 � 0.02 i.e. 2 train tanker train incidents could have occurred across the whole freight-carrying network in the year in question. This estimate is comparable to the 1.3 tanker-train derailments/year estimated in the previous paragraph. The above two paragraphs are based on rather sparse data, so are tentative. A more detailed study would undoubtedly recognise that the risks of freight train derailments are heavily dependent on the level of track maintenance and also depend on the quality of rolling stock, permitted running speeds and the complexity of the track (with greater risks at junctions and crossovers). Signalling error