Multiobjective risk index for the transport by

truck of flammable liquids.

M. Fehervari & P. LeonelliDipartimento di Ingegneria Chimica, Miner aria e delle Tecnologie Ambientali,Universita degli Studi di Bologna, Italy.

Abstract

Here are considered the risks both for people and for environment due toadverse effects of incidents that are caused from a release of flammable liquidtransported by road trucks. A particular presentation is reserved to the damagesto the soil and groundwater that can be potentially impacted, and is also traced acomparison with fatalities among population. A quantitative risk assessmentmethodology for soil volumes and groundwater areas impacted is proposed.Thus, as well as the construction of the cumulated frequency versus fatalities isthe well known societal risk index (the F/N&t curve), the occurrence frequencieswith the soil contaminated volumes or the groundwater impacted areas definetwo homologues environmental risk index, the F/N u and the F/Ng* curves.Starting from the a.m. risk index, through the human life value and the soil andgroundwater remediation costs, it is possible to determine a damagemonetization and a multiobjective risk index (the F/M curve) is defined. Finally,a hypothetical case study is presented and discussed.

1 Introduction

The incidents involving road tanks that transport hazardous materials (HazMat)could have serious consequences for population and environment. In the last 15years, the risk reduction policy [1] and the territory use planning, as well asrouting optimization models development [2], have stimulated the definition ofrisk analysis methodologies to identify and quantify the risks of such hazardoustransports, borrowing the concepts already developed in the Quantitative RiskAssessment (QRA) of process chemical industry [3]. Opportune variationsallows the treatment of hazard rising from linear sources, such are the different

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means of transport for chemicals [1,2,4]. Until now, the attention was drivensubstantially to the immediate and acute risk for human health of theindividuals exposed to the adverse effects (the loss of human lives like referencedamage), and the risk for the environment has been skipped. But, in somecircumstances, the pollution could engage a not negligible role. In fact, acomplete evaluation that investigates all the risks of hazardous materialstransport, should be composed from a crowd of voices [2]: immediate anddeferred risk for human health (death/injuries from acute toxic exposure,radiation from fires, overpressure, and chronic exposure to toxic and/orcarcinogenic substances); immediate and deferred risk for the environment(damages to natural resources and to ecosystems); immediate and deferred riskfor good, services and production (direct and indirect economic losses). Thisevaluation becomes as more complex as the number of parameters to be takeninto consideration rise: this makes difficult defining a comprehensivemethodology. Attempts to manage the complexity and the heterogeneity of amulti-discipline risk assessment use a traditional Cost-Benefit Analysis (CBA)and/or a newly Multi-Criteria Decision Analysis (MCDA) [5], which aredifferent in principle, but complemental if the CBA monetization defines one ormore MCDA criteria. For this scope a multiobjective risk index is defined,which is expressed by an F/M curve, being F the frequency of having damageswhose economic evaluation is equal or greater than a reference damage M(money).The aim of the research project, which this paper refers to, is to quantify therisk due to the transportation of flammable liquids by road trucks. At themoment, we have been considering the transport of fuels and combustibles, andwe have driven our attention to the potential impact of an accidental release onsoil and groundwater natural resources, that could have similar effects incomparison with damages for people. Then an evaluation of the soil andgroundwater remediation costs is made considering appropriate remediationtechnologies [6,7]. Putting together this monetization with Human Life Value(HLV) [2], it is possible to evaluate a composed damage estimation utilizing thea.m multiobjective risk index (the F/M curve). Therefore, a comparison betweenhuman fatality risk and soil and groundwater pollution risks is allowed.In the following sections we report the scenario characterization, the risk indexdefinitions used in this work and the methodology scheme to explain the riskindex calculation way. Finally a complete hypothetical case study of gasolinetransportation by truck is presented and discussed.

2 Overview

When a release of a flammable liquid, i.e. gasoline or oil, occurs, there can befatalities among on road (road traffic people) and off road (habitants in thesurroundings) population if fire or explosion outcome occurs, and the burningrate is so high that an eventual infiltration in the soil is negligible. On thecontrary, if the pool of hydrocarbons persists, there can be the contamination of

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soil and groundwater. As a matter of fact, in the major part of cases, thepetroleum product infiltration is faster than pool evaporation. Thus the spilledproduct migrates from the surface into the soil and, if the water table is at lowdepth and the permeability of vadose zone is high, it reaches the aquiferinterface (the capillarity fringe), where a so called Light Non-Aqueous LiquidPhase (LNAPL) lens is formed and the soluble fraction contaminates thesaturated zone. We show in Figure 1 the event tree and the schematic picture ofthe incident that can aid to explain the considerations previously mentioned.

3 F/N and F/M definitions

In order to obtain a global picture of the risk that a flammable liquidtransportation creates for humans and natural resources over a potentialimpacted area, it is important to evaluate the well-known 'societal risk' [1-4]and a similar 'environmental risk'. The most powerful representation of societalrisk is given by a F/N&t curve, where F is the cumulated frequency (y axis) thata number equal or greater than Nfat fatalities (x axis) occurs. In the samemanner we define the useful intermediate environmental risk index and themultiobjective F/M curve. In this paper we don't recall exhaustively the classic

POOL /%%"

MICE

Yes

FATALITIES

^ p Dispersion or Fire

POLLUTION

Vadose zone

LNAPL Lens - ™=Water table

V—i

Saturated zone

Aquifer flux

Figure 1: Event tree and road truck incident picture for flammable liquids.

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risk assessment methodology to estimate frequencies and consequencesmagnitude, but a complete QRA for transport of HazMat [1] should beperformed. Briefly we summarize herein the basic aspects of the HazMattransport societal risk calculation procedure [4] to obtain the F/N&* curve.Using the concept of scenario', defined by a combination of a point risk source,a final outcome, a meteorological condition and a seasonal situation, for eachscenario two items have to be evaluated: the number of 'scenario fatalities',written A ,/"", depending on and demographic distribution and humanvulnerability, and the 'scenario frequency' f™". For a selected set of fatalityvalues Nfat*, the frequency which refers to a scenario with fatalities A /™" equalor greater than A/ /* have to be summed together to give the cumulatedfrequency F(Nfat*). Considering that the place where accidents can occur couldchange , the further step should be to evaluate the line integral along theHazMat route of F(Nfat*). If no ignition occurs, only the soil and groundwaterpollution are considered, with the complement of ignition probability. Under thesame paradigm, we build the F/Nsou and F/N curves, being F the frequency ofhaving a contamination of a quantity of soil greater than NS<,U* (the total volumeof contaminated soil) and and groundwater eventually A^* (the areal extensionof contaminant plume in the aquifer). Obviously the definition of'scenario' fora pollution event is different: in this case we need to know the pedologic,lithologic and hydrogeologic characteristics along the road track and theemergency and remediation response times to determine effects of the releases.The F/Mfat curve for fatalities can be easily obtained by the societal risk curveconsidering the HLV. The translation from F/Nsoii and F/Ngw to P/M u andF/Mgw curves is just a little more complex. We assume that, referring to anatural resource like soil and groundwater, good parameters to express theenvironmental damage could be the final costs that spring when a siteremediation action is adopted. For our goal, we individualize three cost itemsthat have the greatest importance in the total site remediation cost: the surfacesoil layer treatment, the vadose zone remediation and the groundwaterremediation. Assuming at this approximation level no 'scale effects', if weknow a rough unit cost for each item, we simply multiply by the contaminatedvolumes and areas estimated and we obtain the M^n and M^, terms with thesame cumulated frequencies, in other words F/Msoii and F/Mg* curves. At thisstep it is possible to evaluate the environmental F/Menv summing the M // andMpv of each scenarios and calculating the relative frequencies. In this way, forevery monetized consequence of magnitude M* = A /* = Af *, the cumulatedfrequencies can be summed F(M*) = F(Mfa*) + F(Menv )- Then, for ourpollutant flammable liquids, the total F/M curve is given, collecting fatalities,soil and groundwater damages respectively.

4 Soil and groundwater pollution risk assessment methodology

The methodology applied to quantify the risk of pollution due to transport ofpetroleum products in terms of soil and groundwater remediation costs is a

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subroutine of a general QRA for transport of HazMat procedure [1,4], wellestablished for immediate health risk assessment: traffic accidents analysis;release probabilities analysis; accident consequences analysis; riskrecomposition procedure. In our case we have a typical flow-chart structurewhere the main blocks are reported in Figure 2. For travelling risk sources, i.e.road trucks transporting gasoline, following basic data and parameters have tobe known:• Substance physical and chemical properties.• Traffic statistics: trucks n? crossing the route yearly; incident frequency/^.• Incident characterization: number Nrel of 'rupture classes' equivalent to

released volumes Vrel chosen to describe all possible spills; releaseprobability of a Vrelt volume once an incident occurs Prel ignitionprobability of spilled product once a release occurs Pign

• Environmental data: containment or drain systems; surface soil roughnessHmin\ elevation data (from topographic map); soil data (from pedologicmap); subsoil data (from lithologic map); groundwater data (fromhydrogeologic map); annual recharge.

• Emergency and remediation parameters: timing of emergency andremediation actions (early excavation time //; plume containment time f,;remediation action time fj); remediation costs (soil excavation and treatmentunit cost Uexc\ vadose zone remediation unit cost U d\ groundwaterremediation unit cost U ).

For each route segment

Figure 2: Soil and groundwater pollution risk assessment methodology.

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Usually, the classic transport risk analysis foresees a route segmentation. Asmatter of fact, when we consider a route, this runs across a variety ofenvironmental situations, (i.e. building distribution that influences a clouddispersion or an explosion scenario). The same applies to our methodology:morphologic, pedologic, lithologic and hydrogeologic characteristics andsurrounding conditions can change drastically along the road track. In factcontaminant fate and transport models are strongly sensitive to environmentalparameters as well as consequences models in risk analysis. For this reason anopportune route segmentation in Nseg different tracts must be performedpreventively, where, for each segment, we can take constant properties for theenvironmental damages evaluation.We start calculations defining the 'source term', which is the pool formed bythe released substance. His shape, extension and height are determined takinginto account the instantaneous spilled volume Vrelj and the containments atground level. If containments are absent we assume conservatively a circularpool with maximum horizontal impacted extension (Rp radius), obtained by anaverage pool height equal to soil roughness Hmin. Conversely, for aconservative evaluation of the infiltration depth in the vadose zone, we need themaximum pool height, which is assumed equal to 2Hmin for free surfaces. Atthis time we concentrate our study on flat and unconfined land surfaces.The next step of procedure is the evaluation of the LNAPL infiltration using asimple model for the vadose zone [8,9]. For an optimal representation of theporous media behaviour, we should take into account the vertical heterogeneity,in other words the stratigraphy of soil. Unfortunately this information isunknown for vast areas and the available screening models for LNAPLinfiltration usually consider a uniform layer. For our goal a simple mono-layer(where the pedology is relaxed) could be sufficient to obtain an rough andconservative trend, but a two-layers system (soil and subsoil) allows to take intoaccount the passive protection of less permeable cover soil towards subsoil andgroundwater. In this case the porous media properties are opportunely averagedtaking into consideration the layer thickness in order to simulate an equivalentLNAPL monolayer infiltration. Running vadose zone infiltration model, weobtain the profile of vertical progressive movement of LNAPL front Zfr. If, atthe instant // (complete excavation of the impacted first layer) the LNAPL frontis below the water table, the effective front depth is equal to water table depthZwt. At this level we evaluate the excavated soil thickness Hexc= Zfr(ti). Thisvalue may be placed in a working range [Hmin:Hmax] imposed by excavatoroperability and economic efficiency considerations: if Zfr(tj)<Hmin then Hexc=Hmin; if Zfr(t])>Hmax than Hexc=Hmax. The latter case means that the earlyexcavation doesn't stop the LNAPL infiltration, which it could go on. A majorcontaminated depth Zfr(ts) will be reached before remediation actions time /j.As well as the effective front depth evaluation at //, we establish the maximumdepth of vadose zone contamination Hvad Zfrfts )-Hexc after comparisonbetween Zfr(ts ) and Zwt. In each case the excavated soil volume Vexc, andeventually the contaminated vadose zone volume Vvad are cylinders with Rp

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radius and Hexc and Hvad height respectively, and the total contaminated soilvolume Vsoil is their sum.Then, if the LNAPL reaches the water table, a floating lens is formed atcapillarity fringe and it expands their dimensions until when a maximum radiusRl is reached [8,9]. Through this source water soluble substances dissolves inthe groundwater. Therefore, we need a simple groundwater dispersion model[8,9] to simulate the transport and eventually their attenuation of a referencecontaminant (i.e. benzene for gasoline) in the aquifer, knowing saturated zoneporous media properties, aquifer configuration and concerning dissolvedconstituent properties. Generally, the basic information deriving fromgroundwater dispersion model is the spatial and time evolution of concentrationcontours in the aquifer flux direction: this is our starting data to evaluate areaextension of contaminant plume. Usually, when a groundwater contaminationoccurs, containment measures are adopted to limit the plume extension and topermit a safe remediation action. At time fj, physical or hydrogeologic barriersare posed straight ahead (in hydrogeological meaning) to the point of release.Their distance from release point will be equal or greater than the maximumlongitudinal extension x(c°(t2))> where c° is the contour of assumed concen-tration threshold value (i.e. imposed by Law). During the time betweencontainment measures adoption and groundwater remediation action f,, thecontaminant plume can extend his lateral extension. We empirically take theside widening by y(c°(ts))=x(c°(t2))a/ajc where <z* and ay are longitudinal andlateral hydrodynamic dispersion coefficients. Thus we evaluate the area portionAgw of contaminated groundwater by a trapezoidal shape with Rl and y(c°fts))half-basis and Rl+xfc ftJ) height.At this step the Vexc, Vvad and Agw quantities are available. Setting Uexc,Uvad and Ugw remediation cost units, chosen among the values written in costremediation analysis studies found in literature [6,7], we are able to calculatethe incident Msoil=Mexc+Mvad and Mgw remediation costs for each releasetype and road segment. We need to underline the importance of an accuratechoice among the multitude of soil and groundwater remediation technologies(using for example comprehensives software tools [7]), because each localsituation requires appropriate techniques in term of cleanup efficiency andeconomic cost. In our case, knowing pollutant substance and the soil andgroundwater characterization along the route, it is sufficient to roughly selectcleanup technologies and their typical costs, avoiding at this level a deep costeffectiveness analysis (capital and operating costs, scale factors, etc.).Finally F/N and F/M curves are performed according to their conceptualdefinition (section 3). Considering in sequence all the Nrel rupture classes, foreachy-segment with length Lj[hn], the calculated quantities Vsoilj, AgWj, Msoiljand MgWj, generally suitable like N^ are compared with fixed sets of magnitudedamages Nf; then the F(Nk*); overall the route length are calculated:

X , 0 - Pig», N,, > N',

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5 Case study

Risk Analysis II

We present herein a test case study (Figure 3), where 10000 tk yf* unleadedgasoline trucks travel near a small inhabited center on a 4.5 km road route withan 5-10"^ ev tk~* km~* incident frequency and 0.06 persons ni* on roadpopulation. This route passes through an alluvial plane with three different soiltypes and an unconfined aquifer, with minimum depth equal to 1 m and 0.005m/m piezometric gradient average. The annual recharge is 850 mm yf*.Containments are not present and the soil roughness is 2.5 cm. Assuming threerupture classes (fW,=5, 10, 20 /, with Preli =8.240 , 1.64-10'*, 1.6-10 andP/gfl=0.1), we applied our methodology, using the EPA "Hydrocarbon SpillScreening Model" [9] to evaluate the impact over soil and groundwater. Thisis composed by three modules that can run in sequence: vadose zone kinematicoily pollutant transport model; oil lens motion and chemical dissolution into theaquifer model; transient source gaussian plume model. Knowing the pollutioncontest, we select a superficial excavation (Hmin=4Q cm, Hmax=100 cm) andan on site bioremediation (total unit cost t/so,/=250 $/m\ a soil vapourextraction for vadose zone cleanup (C/w=40 $/m*\ and, where needed, an airsparging groundwater remediation ((7 =50 $/m*). We hypotize f/=5 d, =30 d,13=400 d remediation actions characteristic times.F/Nsoii and F/Ng* results are reported in Figure 4 (A; B). In this case impactedsoil volume varies from 100 to 2000 m? and groundwater plume extension isbetween 2000 and 40000 nf. Frequencies of groundwater pollution are threetimes smaller than soil pollution because LNAPL reach the water table only for1.5 km route segment length approximately. In Figure 4 (C) we report partialand final multiobjective F/M curve after damages monetization for this study.Taking for reference line the fatalities curve, we show a comparable"environmental weight" of soil damages at high frequencies and of groundwatercontamination at high magnitudes.

Legend

Population density\person s/m*]

a: sub-urban area 8.33-10"*b: rural area 2.27-10"c: uninhabited area 0.00

Soil hydraulic conductivity[m/d]

I: medium sand 20III coarse sand 100III] very coarse sand

and gravel 250

Figure 3: Hypotetic case study map.

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1.&02

277

1.E+OE

(A)

(B)

1.508

1.E-05-

(C)

Figure 4: Total and disjoined rupture classes P/N i (A) and F/Ngw (B) curves;multiobjective F/M curve (C) (assumed HLV=lO* $ person*).

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278 Risk Analysis II

6 Conclusions

In this paper we have extended the F/N societal risk concept to the risks for soiland groundwater natural resources. We used a simple monetization approach tocalculate F/M multiobjective risk index to compare the human life losses withenvironmental damages and, applying the methodology to petroleum productroad transport; we showed how these risks can be of the same order ofmagnitude.We highlight that proposed methodology is a basic framework to manage thedifferent nature of risks of adverse effects arising from HazMat accidentalreleases. Surely it can be consolidated, improved (i.e. with a sensitivity analysisand more sophisticated cost analysis) and extended (to other substances, otherreference damages, road/rail/pipeline networks). Nevertheless the obtainableresults could be useful answers in a wide range of multi-criterial decisionprocesses.

Acknowledgements

Autors thank S. Bonvicini (DICMA, University of Bologna - Italy), G. Crema,F. Ermolli and R. Polverari (Aquater S.p.A. - Italy) for useful discussions andGNDRIE-CNR (Italy) for their financial support.

References

[1] Centre for Chemical Process Safety of AIChE Guidelines for chemicaltransportation risk analysis, 1995.

[2] Bonvicini, S., Leonelli, P. & Spadoni, G. Risk analysis in hazardousmaterials transportation: a methodology for the selection of a multicriterionoptimal route. Proc. of the European Conf. on Safety and Reliability, eds.Lydersen, Hansen & Sandtorv, Balkema: Rotterdam, pp.1111-1117, 1998.

[3] Centre for Chemical Process Safety of AIChE Guidelines for chemicalprocess quantitative risk analysis, 1989.

[4] Leonelli, P., Bonvicini, S. & Spadoni, G. New detailed numericalprocedures for calculating risk measures in hazardous materialtransportation. J. Loss Prev. Proc. Ind, 12, pp. 507-515, 1999.

[5] Munda, G. Multicriteria evaluation in a fauzzy environment: theory andapplications in ecological economics. Physica-Verlag: Heidelberg, 1995

[6] Federal Remediation Technologies Roundtable, Remediation ScreeningMatrix, www.frtr.ROV/matrix2, 2000.

[7] Hyman, M. & Bagaasen, L. Select a site cleanup technology. Chem Eng.Progress, pp. 22-43, August 1997.

[8] ASTM, RBCA Fate and Transport Models: Compendium and SelectionGuidance, 1998.

[9] Chabernaud, R.J., Weaver J.W., Tauxe J.D., Lien B.K & Provost J.B. TheHydrocarbon Spill Screening Model, EPA/600/R-94/039a and 039b.

Risk Analysis II, C.A. Brebbia (Editor) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-830-9

Section Six:

Water resources modelling and

management

Risk Analysis II, C.A. Brebbia (Editor) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-830-9