Environ. Sci. Technol. 2000, 34, 3843-3848
Target Cleanup Levels at the Site of a Former Manufactured Gas Plant in Northern Italy: Deterministic versus Probabilistic Results LUCA BONOMO, STEFANO CASERINI,* CRISTIANO POZZI, AND DANIELA ALBERTA UGUCCIONI Politecnico di Milano, D.I.I.A.R. Sez. Ambientale, Piazza Leonardo da Vinci 32, 20133 Milano, Italy
A quantitative analysis of allowable soil concentrations using deterministic and probabilistic risk analysis methods was carried out for a large and heavily contaminated industrial site where remediation costs to meet soil standards (not risk-based) issued by the regulatory agency were higher than the commercial value of the site. Deterministic cleanup targets, based on a risk-based corrective action procedure, allow a 5-fold reduction in the volume of soil to be decontaminated. The site-specific, probabilistic analysis shows that the deterministic cleanup standards for volatile organic compounds were lower than those required to meet the probabilistic criteria but were higher, and thus less conservative, for polycyclic aromatic hydrocarbons and inorganic substances. The probabilistic methods used show that deterministic standards may hide significantly different levels of conservatism in relation to the uncertainty and variability present in each exposure parameter. The expediency of probabilistic calculation to avoid over-remediation efforts is not certain as probabilistic results may be more stringent than deterministic ones; therefore, remediation costs are related not only to the allowable concentrations of single species but also to the mix of contaminants found on a specific site.
Introduction Until recently, remediation programs focused on the reduction of contaminants at a given site, with the ultimate goal of achieving background levels or meeting other very restrictive criteria. The philosophy of the risk-based corrective action (RBCA) approach (1) is totally different because RBCA decisions are based on reducing site risks to low and acceptable levels. The RBCA process incorporates a tiered approach so that the data collection effort will match the relative risk and complexity of each site. The purpose of this paper is to illustrate the implementation of a RBCA program at a former manufactured gas plant (MGP) in northern Italy, where this risk assessment procedure has recently been accepted by the regulatory authority when it is not technically or economically feasible to attain regional soil standards. Consistent with the RBCA principles presented by the American Society for Testing and Materials (ASTM), healthbased cleanup goals are developed through a tiered process: conservative or default assumptions in tier 1 are replaced by * Corresponding author e-mail:
[email protected]; telephone: +39-02-23996430; fax: +39-02-23996499. 10.1021/es990588d CCC: $19.00 Published on Web 07/29/2000
2000 American Chemical Society
more site-specific information and data in tiers 2 and 3. The regulatory authority agreed to the calculation of operative cleanup targets through the development of tiers 1 and 2. Subsequently, a probabilistic exposure assessment was performed for the tier 3 evaluation, by replacing the point values of a number of exposure variables with probability distributions to obtain a better understanding of calculated cleanup targets. The equations used in the deterministic and probabilistic approaches are the same; however, in the deterministic approach it is possible to directly back-calculate the allowable concentrations from the acceptable risk, while in the probabilistic approach this back-calculation requires the solution of a deconvolution problem (2). Probabilistic Cleanup Target in the Methodology section describes the method adopted to avoid the need for deconvolution and to lighten the burden of calculation. The probabilistic framework was selected because it overcomes certain limitations in the deterministic approach. In particular, the deterministic approach lacks any precise evaluation of the uncertainties and the conservative estimates implied in the results. Additionally, probabilistic cleanup targets are based on the probability of exceeding a defined level, and this explicit risk information is unavailable with a deterministic risk assessment.
Bovisa MGP Site The Bovisa MGP site is a 430 000 m2 industrial estate on the northwestern outskirts of Milan. The site has been utilized since the turn of the century for gas production initially by coal gasification and later by catalytic reforming of light petroleum hydrocarbons. Since the early 1980s, natural gas supply gradually replaced manufactured gas. In 1994, all gas production equipment was taken out of service. In the urban redevelopment plan, the Bovisa MGP site is scheduled to house a new branch of the Politecnico di Milano with residential buildings, offices, and a sports field. Due to the potentially serious environmental impact of the former chemical processes, several site assessment initiatives were carried out. A total of 160 bores were drilled to a depth of 22 m, and 535 samples were collected at various depths and analyzed. Groundwater analyses were carried out on 66 samples from 38 piezometers, and 12 lysimeters were used to evaluate the potential pollution of infiltration waters (3). Comparing detected concentrations with regulatory standards, it was revealed that surface soil (0.3-0.5 m) is heavily contaminated throughout the site. In three hot spots, contamination was found to a depth of 15-20 m. Water quality of the first aquifer, which is 22 m deep and not used for water supply, is slightly affected (4). No contamination was found in the second aquifer, which is at a depth of 40 m. Primary contaminants of concern were PAH (polyaromatic hydrocarbons), heavy metals, VOC (volatile organic compounds), and cyanide whose detected concentrations exceeded the regulatory soil triggering values in more than 75% of the samples taken. It was calculated that, if the cleanup targets were set at a level to meet regulatory standards, the cost of the land reclamation would reach the commercial value of the site. At this cost, the cleanup project was not considered economically feasible, thus compromising the future redevelopment plan of the entire area.
Methodology Chemicals of Concern and Exposure Pathways. As requested by the regulatory authority, chemicals of concern were VOL. 34, NO. 18, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Equations To Estimate Probability Distributions of Incremental Lifetime Cancer Risk and Hazard Quotienta exposure pathway
vapor inhalation
soil ingestion
carcinogenic chemicals
R)
R)
dermal contact
R)
(
Ca CPIRIS
(70BWkg) )IR 1/3
noncarcinogenic chemicals
air‚EF‚ED
HQ )
Ca‚IRair‚EF‚ED RfD‚BW‚ATnc
HQ )
Cs‚IRsoil‚RAFo‚EF‚ED × 10-6 RfD‚BW‚ATnc
HQ )
Cs‚SA‚M‚RAFd‚EF‚ED × 10-6 RfD‚BW‚ATnc
BW‚ATc BW 1/3 IRsoil‚EF‚ED 70 kg × 10-6 BW‚ATc
(
(
(
(70BWkg) )SA‚M‚RAF ‚EF‚ED × 10
Cs CPIRIS
Cs CPIRIS
) ) 1/3
d
BW‚ATc
-6
Abbreviations: Ca air), air concentration; Cs (mg/kg), soil concentration; CPIRIS (mg kg-1 day-1), cancer potency from IRIS database; RfD (mg kg-1 day-1), reference dose; IRair (m3/day), inhalation rate; IRsoil (mg/day), soil ingestion rate; BW (kg), body weight; SA (cm2/day), skin surface area per event; M (mg/cm2), soil-to-skin adherence factor; RAFo (unitless), oral adsorption factor; RAFd (unitless), dermal adsorption factor; EF (day/yr), exposure frequency; ED (yr), exposure duration; ATc (day), averaging time for carcinogens; ATnc (day), averaging time for noncarcinogens. a
(mg/m3
identified by comparing detected concentrations with U.S. EPA Region IX Preliminary Remediation Goals (PRGs) that give soil concentrations below which adverse effects to human health are unlikely to occur. Chemicals of concern are those whose detected concentrations exceed at least one of the PRGs for industrial and residential use and for groundwater protection. In this way the risk-based approach embraces 9 carcinogenic and 10 noncarcinogenic chemicals: benzene, benzo[a]pyrene, benz[a]anthracene, benzo[b]fluoranthene, benzo[k]fluoranthene, crysene, dibenz[a,h]anthracene, indeno[1,2,3-cd]pyrene, arsenic, toluene, xylenes, naphthalene, anthracene, acenaphthene, fluoranthene, fluorene, pyrene, dibenzofuran, and cyanide (as free cyanide). In relation to future land use, exposure pathways were considered for outdoor and indoor vapor inhalation, particulate inhalation, soil ingestion, and dermal contact. Contaminated water ingestion was excluded since the nearest wells at waterworks are located 1 km downgradient and draw water from the second aquifer. As the site redevelopment will take about 10 yr, some areas will be entirely completed while other surrounding areas will remain under development. Transient and future exposure scenarios were therefore evaluated: three transient scenarios considered construction workers, students/employees, and residents; three future scenarios evaluated exposure by residents, students/employees, and technical personnel. Cleanup goals have been calculated for surface and subsurface soil down to a level of 10 m, the depth that could be reached during decommissioning and construction works. A monitoring plan, scheduling analysis every 3 months, is deemed appropriate for groundwater protection. Deterministic Cleanup Targets. Tier 1 cleanup goals were calculated under default with very conservative assumptions, i.e., maximum values for the exposure parameters; tier 2 was then performed relaxing the most conservative assumptions, i.e., the most likely value for exposure parameters, and using site-specific data. Tier 1 and 2 cleanup goals are calculated through RBCA equations (1) using an acceptable incremental lifetime cancer risk (ILCR) equal to 10-5 for each carcinogen and an acceptable hazard quotient (HQ) value equal to 1 for each noncarcinogen. Lacking a RBCA methodology to follow when several chemicals of concern are present, allowable residual concentrations have been calculated independently for each chemical, thus neglecting the additivity of risk from carcinogens and noncarcinogens affecting the same organ. Probabilistic Cleanup Targets. In the probabilistic framework of tier 3, RBCA equations cannot be used directly to calculate cleanup targets as random variables do not follow the same rules of ordinary algebra (5, 6). Equations were rearranged in order to calculate the risk associated with a 3844
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hazardous chemical concentration in soil. The ILCRs and HQs were estimated by equations presented in Table 1 where boldface variables are considered random. The total risk is the sum of risks associated with each exposure route. The risk associated with the inhalation routes was evaluated by estimating the air concentration from the unit soil concentration through the volatilization factor used by the ASTM’s RBCA guide (1). The ILCR and the HQ distributions associated with a multiple-pathway exposure to a unit soil concentration (1 mg/kg) of each chemical of concern were simulated using Crystal Ball. The acceptable risk distribution was assigned by constraints on percentiles, and the allowable residual concentration for each chemical was calculated according to one of the methods suggested by Burmaster and Thompson (6) here briefly explained: the acceptable risk distribution is defined by a single constraint on the 95th percentile of the risk distribution that must be equal or lower than 10-5 for carcinogens and equal or lower than 1 for noncarcinogens. The ILCR and HQ distributions associated to a unit soil concentration of each chemical of concern were re-scaled so that the 95th percentile is 10-5 for carcinogens and 1 for noncarcinogens. The calculated cleanup target concentration is equal to f times the unit concentration where f has the following expression for the ith carcinogen and for the jth noncarcinogen:
fi )
10-5 R95 i
(1)
fi )
1 HQ95 j
(2)
with R95 being the 95th percentile of the ILCR distribution and HQ95 being the 95th percentile of the HQ distribution associated to a unit soil concentration. This way of determining the acceptable distribution of risk allows, by definition, an infinite number of risk distributions to be acceptable. To test the convergence and the stability of the numerical output, three independent runs at 2, 4, 8, 16, and 32 thousand iterations were performed. The relative error on calculated concentrations (the ratio of standard deviation to mean for each number of iterations) was computed, with the conclusion that 16 000 iterations are sufficient to ensure the stability of results. In this case, the numerical error on the 95th percentile is equal to 2% and significantly below the error associated with analytical methods for the determination of chemical concentrations in soil. Input Point Value Parameters and Input Probability Distributions. Exposure factor values employed in the tier
TABLE 2. Exposure Parameters Considered as Random Variables (Log-Normal Distribution) for Residential Scenarioa
exposure variable
unit
body weight, adult body weight, child inhalation rate for unit of body weight, adult inhalation rate for unit of body weight, child soil to skin adherence factor daily soil ingestion rate, child inhalation rate, adult; IRair ) IRBW_adult‚BW_adult inhalation rate, child; IRair ) IRBW_child‚BW_child skin surface area, adult; SA_adult ) a(BW_adult)b skin surface area, child; SA_child ) a(BW_child)b
parameters of the log-normal distributions employed
Independent Variables kg µ ) 67.6b kg µ ) 14.2b m3 kg-1 day-1 µ ) 0.225 m3 kg-1 day-1 µ ) 0.452 m3 kg-1 day-1 µ ) 0.52 mg/day µ ) 4.13 Dependent Variables m3/day µ ) 15.2 m3/day µ ) 6.4 cm2/day µ) 4550b cm2/day µ) 1550b
σ ) 11.9b σ ) 3.02b σ ) 0.0646 σ ) 0.0677 σ ) 0.99 σ ) 0.8
ref
tier 2 point value
tier 2 percentile
(7) (7) ( 9) ( 9) (12) (13)
71.8 15 0.263 1.15 0.5 200
66 61 76 >99.9 72 93
σ ) 5.2 σ ) 1.7 σ ) 550b σ ) 225b
( 9) ( 9) (8) (8)
18.9 17.3 5000 2000
79 >99.9 83 97
a The last two columns show the point values selected in tier 2, and the corresponding percentile on the probability distribution. b Parameters evaluated as a function of age.
TABLE 3. Probabilistic, Deterministic (Tier 2) Cleanup Targets and Regional Standards for Most Significant Carcinogenic and Noncarcinogenic Chemicals of Concern regional standards (mg/kg) recreational residential
tier 1 cleanup targets (mg/kg)
tier 2 cleanup targets (mg/kg) 1.13 0.84 4.1a
chemical group
chemical
VOC benzo-PAH inorganic
benzene benzo[a]pyrene arsenic
0.02 0.02 20
Carcinogens 0.625 0.03 1.25 0.5 30 2.4a
VOC PAH inorganic
toluene antracene cyanide
10 0.02b 5
Noncarcinogens 62.5 2.7 1.25b 286 10 588
103 482 1090
probabilistic (tier 3) cleanup targets (mg/kg) 1.28 0.44 2.1a
1.8 7.7 18.5 99%) if compared with the acceptable probability of exceeding a 10-5 ILCR or 1 HQ. From a regulatory point of view, this information justifies the extra effort of a probabilistic analysis, while from a strictly economic point of view this conclusion is more questionable. The possibility to perform a detailed and more precise sensitivity analysis is an advantage that the probabilistic method provides relative to the deterministic one. In terms of allowable concentrations, however, there is no guarantee that probabilistic methods will lead to less stringent targets. Remediation costs are dependent both on the single contaminant target concentration and on the contaminant mix at a given site, so that whether a lower target concentration raises remediation costs or not is site specific. Because of the contaminant mix at the Bovisa site, lower target concentrations for PAH would have implied higher remediation costs. Sensitivity analysis indicates that to increase the accuracy of the results efforts should focus on a better definition of probability distributions for inhalation rates, child soil ingestion, and soil-to-skin adherence factors. Given the scarcity of Italian data, most of the probability distributions were based on United States data, and this may be a limit to the validity of the case presented.
Acknowledgments This research has been partially funded by the Italian Ministry for the Scientific Research (MURST). We are grateful to AEM (Azienda Energetica Milanese) for providing site data.
Glossary
MGP
manufactured gas plant
PAH
polycyclic aromatic hydrocarbons
PRGs
preliminary remediation goals
RAFd
dermal adsorption factor
RAFo
oral adsorption factor
RfD
reference dose
SA
skin surface area per event
VOC
volatile organic compounds
Literature Cited (1) American Society for Testing and Materials. Standard Guide for Risk Based Corrective Action Applied at Petroleum Release Sites; Designation E 1739-95; ASTM: Philadelphia, 1995. (2) Ferson, S. Using approximate deconvolution to estimate cleanup targets in probabilistic risk analyses. Hydrocarbon Contaminated Soils, Vol. 5; Amherst Scientific Publishers: Boston, 1995; pp 245-254. (3) Vacca, M.; Ferruti, L.; Montuori, V.; Milani, A.; Cerretti, R. The reuse of a former MGP site in Milano (Italy); ConSoil ’98, May 17-21, EICC, Edinburgh, U.K., Sixth International Conference; F2k/TNO. (4) Electrical Power Research Institute. Site Investigation and Risk Assessment Report Bovisa MPG Site, Milan; EPRI: Palo Alto, CA, 1996. (5) Burmaster, D. E.; Thompson, K. M. Human Ecol. Risk Assess. 1995, 1 (1), 89-100. (6) Burmaster, D. E.; Thompson, K. M. Human Ecol. Risk Assess. 1995, 1 (1), 101-120. (7) Burmaster, D. E.; Crouch, E. A. C. Lognormal Distribution for Body Weight as a function of Age for Males and Females in the United States, 1976-1980; Alceon Corporation: Cambridge, December 1996. (8) Burmaster, D. E. Lognormal Distribution for Skin Area as a Function of Body Weight; Alceon Corporation: Cambridge, June 1997. (9) Office of Environmental Health Hazard Assessment-California Environmental Protection Agency. Air Toxics Hot Spot Program, Risk Assessment Guidelines, Technical support document, Exposure Assessment and Stochastic Analysis; Draft Report, December 1996. (10) U.S. Environmental Protection Agency, National Center for Environmental Assessment, Office of Research and Development. Exposure Factors Handbook; Draft Report, August 1996. (11) Lombardia Region. Standard di qualita` dei suoli per la bonifica dei terreni contaminati sul territorio lombardo (Soil quality standards for cleanup programs in Lombardia), August 1, 1996; VI/17252. (12) Finley, B. Risk Anal. 1994, 14 (4), 555-569. (13) Thompson, K. M.; Burmaster, D. E. Risk Anal. 1991, 11 (2).
ASTM
American Society for Testing and Materials
ATc
averaging time for carcinogens
ATnc
averaging time for noncarcinogens
BTEX
benzene, toluene, ethylbenzene, xylene
BW
body weight
Ca
air concentration
CPIRIS
cancer potency from IRIS database
Cs
soil concentration
ED
exposure duration
EF
exposure frequency
HQ
hazard quotient
ILCR
incremental lifetime cancer risk
IRair
inhalation rate
IRsoil
soil ingestion rate
Received for review May 24, 1999. Revised manuscript received May 1, 2000. Accepted June 8, 2000.
M
soil-to-skin adherence factor
ES990588D
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