PH: S0273-1223(98)00510-1

8) Pergamon Wat. Sci. Tech. Vol.38,No.4-~, pp. 131-138.1998.IAWQ

C 1998 Published byElsevier Science Ltd.Printed inGreatBritain. AU rightsreserved

0273-1223/98 S19'00+ 0'00

INTEGRATED ENVIRONMENTALRISK ASSESSMENT FORPETROLEUM-CONTAMINATED SITES - ANORTH AMERICAN CASE STUDY

Z. Chen, G. H. Huang andA. Chakma

Environmental Systems Engineering. Faculty of Engineering. University ofRegina.Regina. SKS4S OA2. Canada

ABSTRACT

Development of petroleum industries is associated witha number of environmental concerns. Among them,soil and groundwater contamination by petroleum products is of majorconcern. In this study, an integratedrisk assessment approach is proposed for evaluating environmental risks derived from petroleum­contaminated sites. The proposed approach is composed of (i) a hydrocarbon spill screening model (HSSM)which is usedfor simulating immisicible flowof released hydrocarbons in vadose zone, formation of lens incapillary fringe, dissolution of pollutants at watertable, and transport of the pollutants to receptors, and (ii) afuzzy relation analysis (FRA) model which is developed for comprehensively evaluating risks caused by anumber of pollutants with different impact characteristics, based on the HSSM results. This hybrid HSSM­FPAapproach wasapplied to a case studyfor a petroleum-contaminated site in western Canada, where soiland groundwater was contaminated by industrial wastes containing benzene, toluene, ethylbenzene andxylenes (BTEXs). The results suggest that the HSSM-FRA can provide insight into the potential risk to thereceptor of concern downward the acquifer and can serveas a basisfor further remediation-related decisionanalysis. © 1998 Published by Elsevier Science Ltd.All rights reserved

KEYWORDS

Groundwater, hydrocarbon, petroleum, riskanalysis, simulation, sitecontamination, soil.

INTRODUCTION

Industrial activities are often associated with various environmental concerns. Among them, threats fromhydrocarbon spilland leakage havebeenwidely anecdotal at almost everypetroleum industry site.A numberof studies for environmental risk assessment of petroleum contamination have been conducted. Rosenblatt(1994) proposed a method for evaluating health risks from a buried mass of diesel fuel before and afterbioremediation. Liptak (1996) exploited a set of chemical-specific, risk-basd soil cleanup guidelines, whichresulted in timely and cost-effective remediation. Koblis (1993) studied the impact of surrogate selection onrisk assessment for total petroleum hydrocarbons. Hallenbeck and Flowers (1992) undertook a studyof riskassessment forworker exposure to benzene.

Generally, most of the previous risk analysts argued that risk should be measured through probability(relative likelihood) of possible contamination and magnitude (seriousness) of consequences from thecontamination. Thus, risk could be expressed as a probability distribution over a number of adverseconsequences. However, when applied to diverse problems, probability theory often retains a fundamentalassumption about the subject area involved. Specifically, it assumes that there exists a historical run forobservation of events. In fact, when attempting to model behaviors of environmental processes, analystsoften suffer from a lack of data or imperfect knowledge about the processes. This may frustrate rigorousprobabilistic studies (Lein, 1992). Another problem withthe probability theoryis its lawof excluded middle

131

132 Z. CHENet al.

[P(AuAC) " I] and contradiction [p(Ar\Ac

) =0]. For instance, rotating a dice, the results will be 6, 5, 4, 3, 2or I, but never4.5 or 2.1. However, intuitively andcommonsensically, this is not true in otherproblems (e.g.a colorcan be red,not red, or reddish) (Laiand Hwang, 1992).

Another major approach for risk assessment is based on fuzzy set theory, which is suitable for situationswhen probabilistic information is not available (uncertainties present as fuzzy membership functions)(Bardossy et al., 1991). Therehas beena lackof previous studies using fuzzy risk assessment for petroleumwaste management In comparison, many applications to other areas have been reported. For example,Ganoulis et aI.(1995) proposed a fuzzy arithmetic forecological riskmanagement underuncertainty. Donaldand Ross(1996) usedfuzzy logicand similarity measures for riskmanagement of hazardous wastes.

Generally, the fuzzy risk analysis methods have advantages in their effectiveness in reflecting uncertaintiesand their applicability to practical problems. It is typically more difficult for planners/engineers to specifyprobability distributions than to define membership functions. Thus, extension of the methods to petroleumwaste management areawould be desirable forgenerating effective evaluations anddecisions.

The objective of this research is to develop an integrated environmental risk assessment approach and applyit to a casestudyfor a petroleum-contaminated site in Saskatchewan, Canada. A hydrocarbon spill screeningmodel (HSSM) is employed for simulating the process of contaminant transport in soil and groundwater.Based on the HSSM outputs, a fuzzy risk assessment model is then developed for comprehensivelyevaluating risksassociated withthe sitecontamination.

METHODS

Hydrocarbon SpiU Screening Model (HSSM)

Contamination from petroleum products is usually characterized by contaminant concentrations ingroundwater and soil. The petroleum-related contaminants under consideration include benzene, toluene,ethylbenzene, xylenes (BTEXs), and total petroleum hydrocarbons (TPH). In the HSSM, concentrations ofcontaminants at different temporal and spatial unitscan be obtained through the following transport equation(Weaver, 1996):

TJR (0cI8t) =V . DVc- q·Vc- A,TJRc + J(t) (I)

where TJ denotes porosity, R is retardation factor, c is contaminant concentration in groundwater (mg/rrr'), qis darcy velocity (m/sec), D is dispersion constant (m), A. is the first orderdecayconstant (sec"), J(t) is theamountof massperunitvolume of aquiferaddedper unittime(mg/m' sec).

The HSSM was recommended by the USEPA for simulating subsurface releases of light nonaqueous phaseliquid(LNAPL) and its subsequences. Especially, it is useful for evaluating impacts of hydrocarbon releaseson the subsurface acquifer (Weaver et al., 1994; Charbeneau et aI., 1995). Themodelcontains threemodulesfor simulating LNAPL flow through vadose zone, LNAPL spreading in capillary fringe, and LNAPLtransport alongwithgroundwater flow. Generally, hydrocarbons are released near the surface and transporteddownward the vadose zoneto watertable.At the watertable,a hydrocarbon lensforms and spreads laterally.Constituents from the hydrocarbon lensdissolve intogroundwater flowing beneath the lens,creating a plumewhich maycontaminate downgradient or otherexposure points.Thus, the HSSM maybe used for estimatingeffectsof LNAPL loading, partition coefficients, and groundwater flowvelocities on pollutant transport, andobtaining approximate concentrations of hydrocarbon constituents at receptors of concern.

The main modeling inputs include (i) parameters specifying the type, extent and magnitude of the LNAPLrelease, (ii) residual oil contents for the unsaturated and saturated zones,(iii) residual watercontent of the oillens, (iv) transport properties of waterand LNAPL (density, viscosity, and surface tension), (v) aquiferandsoil/water retention characteristics (vertical and horizontal hydraulic conductivity, porosity, irreducible water

Integrated environmental riskassessment 133

content, pore size distribution index, and air entry head), (vi) dissolved constituent characteristics (initialconcentrations within the LNAPL, aqueous solubility, and soil-water and oil-water partition coefficients).and (vi) aquifer transport characteristics (vertical, longitudinal and transverse dispersivities, hydraulicgradient, and half-lives of different constituents withinthe aquifer).

Fuzzy Relation Analysis (FRA)

Based on the HSSM simulation results, a fuzzy relation analysis (FRA) model is developed forcomprehensively evaluating environmental risks due to multiple pollutants with uncertain impactcharacteristics. The concept of fuzzy relation was firstly applied to medical diagnosis by Zadeh (1969). Itillustrates the link between the malfunctioning of a system and the possible symptom. In a very generalsetting, the process of fuzzy relation analysis can be conveniently described by pointing out relationshipsbetween a coIlection of pattern features and their class membership vectors. It is useful for multifactorialevaluation and riskassessment underimprecision and uncertainty (Asse, 1987; Pedrycz.1990). The axiomaticframework of fuzzy set operation provides a natural settingfor constructing multiattribute value functions inorder to sort a set of potential actions and make an effective assessment The method has been applied to anumberof practical problems. suchas assessment of environmental quality,evaluation of industrial products,and classification of acute toxicity from poisons (Jennings.1988; Ivanov and Ryvkin, 1991). This study wiIlprovide an additional extension of its application.

Fuzzy relations are fuzzy subsets of X x Y which present mappings for X ~ Y (Zadeh 1965 and 1971;Kaufmann 1975). Definitions of fuzzy relations and their compositions for risk assessment are described asfollows. Let X.Y c B be universal sets. ThenB={(x, y), J.ll(X, y» I (x, y) S; X x Y } is a fuzzy relation on

Xx Y. Let~ ={lit Ii =1,2, .. . .m} be a n-dimension fuzzy vector, and B={rij Ii =1,2, ... ,m;j =1,2, ... ,n] be a mxn fuzzy relation matrix. Then. a m-dimension fuzzy vectorBcanbe obtained as foIlows:

B" ~ 0 B={b, • b2 •. . .• b.} . (2)

The above processis named fuzzy transformation. According to the principle of fuzzy set operation. Dcanbedetermined by a max-min or max-ecomposition (Zadeh, 1971). For the max-min composition. we have:

bj = ~ (l!jMij) = max {min (a, • rlj), min (a2. rlj), . ..• min (am. rll\i) }.,.1j=I.2... .. n. (3)

For the max-e composition.we have:

bj ... ~ (a, x rij) = max {(a, x fJj ), (a2 x r2j ), •• • • (am x rll\i ) }.,.1j = 1,2, ... , n, (4)

Thus, for a system containing several poIlutants with high concentrations, we can analyze their integratedrisks by usingthe abovemodeling approach. Assume that 8j is the degreeof significance for pollutant i, andrij is the grade of membership for fuzzy relation between pollutant i and risk level j. Thus. bj , whichrepresents the membership gradefor integrated risk levelj to occur,can beobtained through model (2).

CASESTIJDY

Overviewor tbe Study System

The study site is located in western Canada(Figure I). It has been operated as an oil refinery industry formore than 50 years. Petroleum derived contam~tion has be~n .found sin~e the 1960sdue to undergroundpiping.storagetank leakage. and hydrocar~n spills.Twoprehm~ studies(Fletc~er and Munneke, 1994)IOdicated that soil and groundwater at the site werecontaminated With petroleum denved poIlutants.

134 ZCHENttal.

3 •

2 •

1 •

~

a ••. -0 D .ilB~aa C:"=::I

Figure I. Thestudy site

oo

• Contaminated site• Receptor point

Data Investigation

The study area has approximately 23,000 m3 of contaminated soil. The release of hydrocarbon wascontinuous from theearly 1960s to the 1990s. In the recent years, bioremediation andothercontrol activitieswere undertaken to recover the contaminated soil and groundwater (Fletcher and Munneke, 1994). Table Ipresents the results of laboratory analyses for hydrocarbon concentrations in the soil. The depth ofgroundwater table is 1.1 to 1.5 m. Therefore, the groundwater can be easilyaffected by the contaminatedsoil. The groundwater plume flows from the site towards a creek which is an important recreational waterbody for the nearby city.Thedistance from the contaminated siteto the receptor pointwhich is closest to thecreekis about450 m (Figure I). Thus, contaminants within the soil can be carried through the groundwaterinto the creek. Historical monitoring data indicated that the creek water has indeed been contaminated byhydrocarbon constituents. Consequently, it is desired that effective evaluation of the associated risks topublic waterusebe undertaken.

Results

Figure 2 presents the result of HSSM simulation modeling at the threereceptor points with distances to thecontaminated site being 250, 350 and 450 m, respectively. It is indicated that, before the remediationactivities are undertaken, highconcentrations of BTEXs existat the three points with littlevariation. This ismainly dueto the factthatrelease of LNAPL from the industry wascontinuous in most of the timeduring the1960s - t990s. Since the 1990s, some remediation activities, such as bioremedlation, soil regeneration,biobarrier and biofilter, have been undertaken. These activities lead to significant decline of BTEXsconcentrations in soilandgroundwater (Fletcher andMunneke, 1994).

Table2 shows the result of integrated risk assessment through theH~S~-FRA approach (for receptor point3), withthedetails foryear2001 provided in Figure 3. Therelated guidelines for hydrocarbon concentrationsin groundwater are given in Table 3. Generally, because remediation activities have been undertaken sinceearly I990s, the risk levels forthe receptor points havebeensignificantly reduced after 1995. After 2002, thethreatfrom this sitemaybecome insignificant.

Integrated environmental riskassessment

Table I. Hydrocarbon concentrations in thesoil(ppm)

Contaminated Site Composite 1 Composite 2 Composite 3 Average

Landfarm Area 5330

Loading Facility 4300

1100

1600

3500

1700

3310

2533

Table 2. Result of integrated riskassessment through HSSM·FRA approach

Unit 1985 1995 2001 2002

Benzene (mg/L) 11.7 10.3 0.25 0.01

Toluene (mg/L) 36.0 29.2 0 0

Ethylbenzene (mg/L) 62.0 57.3 0.375 0.009

Xylenes (mg/L) 48.6 14.8 0 0

Risklevel Highly risky Highly risky Slightly risky Clean

Table 3. Guidelines forhydrocarbon concentrations in groundwater

Area Media Chemicals Guideline Limit (ugIL)

Petroleum Water Benzene 300Industry Ethylbenzene 700Area Toluene 300

Xylenes 5,000

136 z. CHEN el al.

20101980 1990Time (yr)

1970

Benzene~ 15m

!~ 10.00~

i S.DOOGI

'"50.000 "---"-.wL._-'- --'- .Io.-__.-...->O- ........

1960

2010200019901980Time (yr)

1970

Toluenei 60.00

-; 4S.DOeE30.00

~ IS.DO...~ 0.000 L...J"o!.L.__...I.- ........ -.l._'"""'"'-..._........ .....U 1960

2010200019901980Time (yr)

1970

Ethylbenzenei 7S.DO.... 60.00~ 4S.DO~

~ 30m~ 15.00'"~ 0.000 l...L...:A__...L- .......--.-.-I. ""'"--...J....... --:.~.l.-. ....... .......

U 1960

Xylenes

.-- -- .__._­----------------

200019901980

Time (yr)

1970

250(m)

_._._.-. 350 (m)

--------- 450 (m)

Figure 2. Result ofHSSMsimulation modeling forBTEXs at thethree receptor points(with distances to thecontaminated sitebeing 250, 350, and450m,respectively)

Integrated environmental riskassessment

Slightly Risky

Clean

H· hI 0 k*.: Practically not~ ynsy okmy

Risky lightly risky

- max-min composition

Slightly Risky

Clean1

H' hl . k 0.' Practically not'. y ns y.\2~ risky

Risky ~lightly risky

- max-. composition

Figure 3. Result of integrated riskassessment through theHSSM-FRA approach

137

The HSSM-FRA approach is useful for comprehensively evaluating risks within a system containing manyfactors with complicated interrelationships. For the study problem under consideration, this method allowsmanagers to have a systematic and consistent method for assessing environment risks associated withdifferent source conditions. The results can thus provide useful bases for determining desirable siteremediation strategies.

CONCLUSIONS

An integrated risk assessment approach. HSSM-FRA. is proposed for environmental risk assessment ofpetroleum-contaminated sites. The HSSM is used for conservatively simulating (i) LNAPL transport fromnear the surface to the capillary fringe, (ii) radial spreading of a LNAPL lens through the capillary fringe,(iii) dissolution of LNAPL constituents intoa water tableaquifer, and (iv) transport of LNAPL through theflowing groundwater to a potential exposure location. The FRA is used for integrated assessment of effectsand risks caused by a number of contaminants. Thus, the HSSM-FRA can incorporate information ofemission source, pollutant transport, pollution impacts, and remediation techniques within a generalframework.

The HSSM-FRA approach is applied to the case studyfora petroleum-contaminated site in western Canada.The results are useful for risk analysis of LNAPL spill and/or leakage as well as their impacts on soil,groundwater, andpotential receptors.

138 z. CHEN et al.

Furtherexplorations basedon the proposed methodology wouldbe beneficial. Forexample, the HSSM-FRAcan also be usedfor analyzing othersite contamination problems, suchas leakage from underground storagetanks. Integration of the HSSM-FRA with other simulation tools for surface water quality will provide amoreeffective decision-support system for managing thesitecontamination problems.

ACKNOWLEDGMENT

This research hasbeensupported by the Natural Sciences andEngineering Research Council of Canada.

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