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Utility of risk-time curves in selecting remediation alternatives
Waste Management & Research (1995) 13, 167-174
U T I L I T Y O F R I S K - T I M E C U R V E S IN S E L E C T I N G REMEDIATION ALTERNATIVES Edward A. M c B e a n I and Frank A. Rovers -~
tDepartment o.[ Civil Engineering, University (~J Wuterloo. Waterloo, Ontario. Canada N2L 3GI and ~-Conestoga-Rovers and Associates, Ltd.. Waterloo, Ontario, Canada N2V 1C2 (Received 27 July 1992. accepted Or revised.[brm 28 Februarr 1994)
The basis for risk-time curves associated with remediation of hazardous waste sites is developed. The utility of the risk-time curves as a part of a methodology for selecting between alternative remediation schemes is demonstrated when the time-frames for remediation are different for different alternatives. The potential for inclusion of uncertainty aspects in risk-time curves is considered. I. Introduction
Extensive societal pressures are being exerted, in general, to decrease the risks to which people are exposed during daily living and, in particular, to minimize the exposure risks in relation to hazardous wastes. Thus, in the remediation o r a hazardous waste site, or in the planning of a new hazardous waste disposal site, it would be inconceivable to consider only the economic costs associated with the remedial action selected (or the site design costs, for a new site); risks considerations must be reflected. The consideration of risk or, in a more formal sense, risk assessment, is a process that seeks to estimate the likelihood of occurrence of adverse effects due to exposures to chemical, physical and/or biological agents in humans and ecological impacts within an ecosystem. The quantification of the exposure levels is a substantial challenge usually necessitating the modelling of existing and/or anticipated future exposures. In a typical scenario in which there is an exposure, contaminants may be transported via one or more media (including air, soils/sediments, surface water and groundwater) to potential receptors (through, for example, inhalation, dermal contact and/or ingestion). The exposure assessment aspect of the risk assessment must then involve the characterization of the physical and exposure setting, including contaminant distributions leading from sources on a hazardous waste site to the points of exposure, the identification of significant migration and exposure pathways, the identification of potential receptors, the development of exposure scenarios (including the determination of current and future exposures) and the estimation of chemical intakes for all potential receptors and significant pathways of concern. Consider now the following example problem involving hazardous air contaminants being released from a series of inactive wastewater ponds. Assume the property is located within a residential and light industrial setting. Since the subject ponds have thick clay linings, it is not expected that the underlying unconfined water supply aquifer has been impacted: this is confirmed by results of the field investigations. The ponds have periodically been dredged and the resulting sludge stockpiled on the surrounding open land. Monitoring indicates the presence of several chemicals within the dredge spoil. To abate potential health risks, several remedial action alternatives to deal with the stockpiled waste are: (1) transport for off-site disposal: (2) landfilling on-site; and (3) 0734-242X/95/020167+ 08 $08.00/0
© 1995 ISWA
168
E. A. McBean & F. A. Rovers
stabilization by on-site treatment and subsequent transport for off-site disposal. All of these involve different degrees of excavation, loading, transportation and unloading of the wastes. Both short-term (subchronic) and long-term (chronic) effects of the chemicals of concern must be considered. Following preliminary examination, the critical and significant exposure pathway is determined for this hypothetical site to be due to wind erosion and mechanical resuspension of fugitive dust from the contaminated soils in the evaporation ponds and waste stockpiles. Using a Gaussian plume model, it is possible to quantify the risks for the chemicals of concern, based on the ground-level concentration. A part of the challenge in development of the risk assessment arises in that there is uncertainty in the mathematical models employed, there is uncertainty in the data assignments utilized in the models, and there is uncertainty in the impact ofdifferent exposure levels, (i.e. are the levels sufficient to initiate cancer?). These aspects of uncertainty in exposure assessment, in general, are important and will be considered later. However, of immediate interest is that the period of remediation, and thus the period of exposure risk for the nearby human subjects, is essentially the same for all of the remediation alternatives. Therefore, the selection between the remediation alternatives essentially involves the comparison of exposure levels with air quality standards along with the cost information, to in turn make a selection between the remediation alternatives. Consider now an alternative where the remediation alternatives do not entail the same duration of exposure risk. The identification of the most effective alternative is now more complicated. An effective tool for this additional complication involves use of risk-time curves.
2. Bases of risk-time curves
The use of cost-time curves in the selection between remediation alternatives has been standard practice for many years. Typically, the curves themselves have only been considered peripherally since, using a discount rate, the temporal variability of the costs is modified to a present value and/or translated into an equivalent annual cost. The equivalencing concept is utilized to allow comparisons between alternatives by removing the individual temporal variations, placing the comparisons between the alternatives onto an equal basis. Risk-time curves are of a similar nature, except that the curves indicate the temporal, changing levels in terms of risk, as a function of time, for each of the remediation alternatives. The general character of the curves is best demonstrated by example (purposely kept simple for ease of explanation). Consider the following: (1) Industrial wastes have been improperly disposed of by placement (i.e. dumping) of hazardous material (e.g. paint sludges) into trenches, and covering the material with soil. As a result of this poor disposal practice, the surrounding groundwater is being contaminated. (2) Remediation alternatives include the following: (i) Do-nothing alternative--This alternative involves no capital and operating expenditure in terms of dollars but will result in the contamination of the local groundwater supplies. At this site, there will be an increased incidence of cancer in the local population, as the chemicals spread and enter the groundwater and lakes in the immediate vicinity. As well, there will be emissions to the atmosphere.
169
Utility of risk-time curves
(ii) Excavation/incineration/relandfilling alternative--Excavation of the refuse will result in elevated emission levels from the waste as it is uncovered, transported, stored and handled at a staging area and finally rehandled for treatment at an incinerator. During excavation, the potential exists for explosions to occur, due to the mixing of incompatible wastes. In addition, incineration of the wastes and the adjacent contaminated soil removed with the wastes will result in flue gas emissions and will generate a significant quantity of fly ash and unburnable residue. Although good incineration practices will minimize any emissions from the incineration process, the incineration of thousands of tonnes of site material will result in release of some chemicals to the atmosphere. A schematic depiction of the activities and the consequent migration pathways is depicted in Fig. 1. (iii) Inplace containment, groundwater pumping and treatment alternative--Off-site contaminant migration can be attenuated using a physical barrier (a grout wall, bentonite trench and sheet piles), a groundwater pumping system and a treatment system. In this situation, there will be air emissions as the groundwater is treated by air stripping, in addition to disturbance of the waste and contaminated soil during construction. A schematic depiction of the activities and the consequent migration pathways associated with this remediation alternative are depicted in Fig. 2. Volatilization Volatilization Flue Volatilization and particulate v->gas and particulate ~ migration ~ migration and particulate [V-->migration emissions
Excavation and stockpiling
t
Vehicular transport to incinerator
l/ Stockpiling "1 at incinerator
I Incinerator
~'1Relandfilling
Fig. I. Materialshandlingactivitiesand primaryreleasemechanismsfor excavation/incineration/relandfilling alternative.
Volatilization
l
and particulate migration
Excavation and stockpiling
r---> VOCsrelease
Air stripping
Fig. 2. Sourceopportunitiesand primaryreleasemechanismsfor inplacecontainment,groundwaterpumping and treatmentalternative.
170
E. A. McBean & F. A. Rovers
Each of the remediation alternatives (i)-(iii), have associated capital and operating costs (measured in U.S. dollars) and different elements of risk. Note that the assumption in the comments to follow is that the risk refers strictly to the risk to nearby residents; another level of risk is associated with on-site construction workers, but these concerns will be ignored in the interest of brevity and the ease of explanation of the principles of risk-time curves. To characterize the risk-time curves, pathways migration models for contaminants are employed. The models must describe the mass, physical state, and containment structure for each chemical involved. Although such models can assume many different forms, the essential feature is that the environmental transport and fate analyses of the chemicals must be quantified as the chemicals migrate to the receptors (the nearby residents). The primary transport mechanisms in this site assessment include the groundwater pathway in (i) and those noted in Figs 1 & 2. The mathematics for describing in detail the pathways assessment are beyond the scope to be addressed herein; suffice it to indicate that the range of sophistication of such models is considerable. The interested reader is referred to McTernan & Kaplan (1990) as an example, for additional discussion of mathematical models of migration pathways. Following the quantification of the remediation alternatives in terms of releases of the various chemicals as a function of time, and the utilization of the environmental pathways models to characterize the transfer of chemicals from the site to the receptors, the concentrations of the individual chemicals at the various receptors are developed. These determinations must reflect the temporal variations associated with the release mechanisms, since not all of the various release mechanisms are functioning at all times. The aggregation must then take place over the array of chemicals, from all the exposure pathways, resulting in the risk-time curves depicted in Fig. 3. Brief comments on the individual risk-time curves for the various remediation alternatives are as follows: 12 11 10 Do-nothing
9 8 7 ~D
6 Excavatiordincineration/relandfilling 5 4 /
3
Inplace c o n t a i n m e n t
.2 1 10
I
I
I
20
30
40
Years
Fig. 3. Risk-time curves for different remediation alternatives.
50
Utility of risk-time curves
171
(i) Do-nothing alternative--As the groundwater plume continues to migrate away from the buried refuse, the exposure risk continues to escalate, as additional sources of water supply for various residents are successively contaminated. (ii) Excavation/incineration/relandfilling alternative--This alternative has the potential to substantially reduce public risk in future generations but increases the health risk in the short-term (over the period of 10 years of construction and implementation). In this case, mean health risk reduction to future generations will be large, but there is a large risk transfer to the nearby residents during the 10year period of excavation/incineration and relandfilling activities due to such features as the increased opportunity for volatilization, the risk of explosions during excavation and the air emissions during incineration. The number of receptors exposed to elevated risks as a result of the excavation/incineration and relandfilling will be dependent on the locations of the various activities, and this information must be reflected in the analyses. (iii) Inplace containment and groundwater pumping--The health risk afforded by the proposed pumping and treatment alternative is elevated over the "do-nothing" alternative for the first few years due to the construction activity and the treatment emissions (volatilization) due to the air stripping and carbon adsorption processes for treatment of the pumped groundwater. The treatment emissions will continue throughout the concern with the site, projected as needing 100 years of operation. A significant benefit of the risk-time curves is the immediate indication associated with each alternative, of any periods of elevated risk. It is readily apparent, for example, that the levels of risk for the inplace containment/groundwater pumping do not reach the same magnitude as the excavation/incineration/relandfilling alternative, but of course, the risks continue over a much longer time-frame with inplace containment/ groundwater pumping.
3. Equivalent-value assignments Standard practice in dealing with cost-time curves utilizes the discounting of the various cost-time components back to present value, using a discount rate. A similar discounting procedure is not normal practice in risk-time curves. Instead, the normal practice usually equivalences the risk to a lifetime equivalent using no discounting. Therefore, the next task involves integration of the risk curves over the population-at-risk, and equivalencing back to present value. Given the information on cost-and risk-time curves, the resulting values for the three remediation alternatives plotted as expected cost vs. expected risk, are plotted on Fig. 4. If one was to accept the alternative with the lower expected value of risk, this would imply that the excavation/incineration/relandfilling alternative would be the most desirable alternative--it has a higher cost than the inplace containment, but it has onethird of the expected risk.
4. Incorporating aspects of uncertainty Although there is an inclination to select the remediation alternative with the lowest expected value, there is substantial uncertainty in risk assessment associated with many data assignments. This consideration is particularly relevant for this example since the waste disposal records for old landfill sites are typically quite poor. Thus, for example,
172
E. A. McBean & F. A. Rovers 450 400 350
Excavation/incineration/relandfilling X
300 Inplace
250
containment
X ~.~ 200 o r..) 150 100
Do-nothing
50
X I 10
I 20
I
I
I
30
40 Risk (x 10z)
50
I 60
I
70
80
Fig. 4. Societal risk-cost tradeoff for expected values.
knowledge of the quantities of various chemicals buried, the specific locations relevant to the concern of missing incompatible wastes during excavation and the resulting potential for an explosion occurring, and the ability to assign representative chemical release rates during stockpiling, incineration and relandfilling, all of these types of factors indicate that the uncertainty in the risk-time curves may be very relevant in terms of decisionmaking. Selection between remedial alternatives may not be adequate, based on expected values alone--the decision-maker may want to adopt a risk-averse strategy. Reflecting the uncertainty of the risk, the plot of societal risk vs. time curves become as depicted in Fig. 5; the solid lines depict the expected values of the risk-time and the associated dashed line indicates uncertainties as quantified by one standard deviation away from the mean. In turn, the risk-time curves (m the plural sense indicating the expected value and the uncertainty aspects), the resulting cost vs. risk tradeoffs became as depicted in Fig. 6. The solid lines surrounding the expected values indicate one standard deviation from the mean, for both the costs and the risks. Several points are noteworthy: (1) The cost uncertainty is of a smaller magnitude than the risk uncertainty. The elements of cost uncertainty relate to the projection of the future technological costs (e.g. operational costs of the treatment facility in 20 years). There is uncertainty associated with such cost projections but the magnitudes are expected to be relatively small. On the other hand, the uncertainties associated with risk are substantial. (2) The cost uncertainty is depicted as Gaussian or normally-distributed as the bounds are symmetrical about the mean. Conversely, the risk uncertainty is log-Gaussian or log normal. This is to be expected since the projections of the future costs could just as easily be on the high side, as the low side. (The Central Limit Theorem
173
Utility of risk-time curves
! I
I i
~
I
Do-nothing / / " /
i
!
"d 0.1
llllllllltlllll#ll
? ......... i:
-I
I
Ii
"$
,
I 0.01 ~.
/# J
/f .
~
Excavation/ineineration/relandfilling
1
••sS•
I
I ~/ . .
.4
I
/ ~
'
//"
. . . . . . . . . . .
~
0 0011 0
~
•
'
Inplace containment
..................................... ................ -_/_-- . . . . . . . . . . . . . . . . . . . . . . . . . . .
s
I
lO
20
I
I
30
40
50
Years Fig. 5. Log risk-time curves for societywith different remediation alternatives includingmargins for uncertainty.
450 400 350
Excavation/ineineration/relandfilling X
300 Inplace containment 250 o
~,~200 ~ lgO
100 Do-nothing
50 I
10
I
20
I
30
40 Risk (x 102)
I
50
I
60
Fig. 6. Societalrisk-costtradeoff reflecting uncertainty.
I
70
80
174
E. A. McBean & F. A. Rovers
indicates that such uncertainty can be described as a Gaussian distribution. For further reading see, for example, Bethea et al. 1985). Conversely, the distribution of the risk is frequently log normal since it is bounded on the low side by zero and unbounded on the high side; the distribution is skewed to the right. Note that both components of uncertainty (in costs and in risks) appear symmetrical around the expected value in Fig. 5 but the characterization of the risk is by a logarithmic axis, and thus, it is symmetrical in a log-transformed sense. As a result of including the uncertainty aspects of the risk, the selection between the alternatives is not so obvious. The excavation/incineration/relandfilling alternative has the lower expected value of risk but also possesses the very real possibility (probability) that the risk impact is larger. In other words, the excavation/incineration/relandfilling alternative has the lower expected risk but has a much greater probability that risks associated with this alternative are much higher. Consequently, the greater costs and the potential for greater risks might well argue for selection of the inplace containment/ groundwater pumping alternative as opposed to the excavation/incineration/relandfilling alternative. 5. Conclusions
More formal requirements for consideration of risk management are arguing for increased sophistication in risk assessments. Risk-time curves assist in this regard by identifying the temporal variabilities which may be very relevant in selecting the best remediation alternative(s). The risk-time curves also possess the capability to reflect uncertainty aspects in assignment of risk features. References
Bethea, R., Duran, B. & Boullion, T. (1985). Statistical Methods for Engineers and Scientists. Marcel Dekker Inc., N.Y., U.S.A. McTernan, W. F. & Kaplan, E., eds. (1990). Risk Assessment for Groundwater Pollution Control, Monograph of the American Society of Civil Engineers, New York, N.Y., U.S.A.
U T I L I T Y O F R I S K - T I M E C U R V E S IN S E L E C T I N G REMEDIATION ALTERNATIVES Edward A. M c B e a n I and Frank A. Rovers -~
tDepartment o.[ Civil Engineering, University (~J Wuterloo. Waterloo, Ontario. Canada N2L 3GI and ~-Conestoga-Rovers and Associates, Ltd.. Waterloo, Ontario, Canada N2V 1C2 (Received 27 July 1992. accepted Or revised.[brm 28 Februarr 1994)
The basis for risk-time curves associated with remediation of hazardous waste sites is developed. The utility of the risk-time curves as a part of a methodology for selecting between alternative remediation schemes is demonstrated when the time-frames for remediation are different for different alternatives. The potential for inclusion of uncertainty aspects in risk-time curves is considered. I. Introduction
Extensive societal pressures are being exerted, in general, to decrease the risks to which people are exposed during daily living and, in particular, to minimize the exposure risks in relation to hazardous wastes. Thus, in the remediation o r a hazardous waste site, or in the planning of a new hazardous waste disposal site, it would be inconceivable to consider only the economic costs associated with the remedial action selected (or the site design costs, for a new site); risks considerations must be reflected. The consideration of risk or, in a more formal sense, risk assessment, is a process that seeks to estimate the likelihood of occurrence of adverse effects due to exposures to chemical, physical and/or biological agents in humans and ecological impacts within an ecosystem. The quantification of the exposure levels is a substantial challenge usually necessitating the modelling of existing and/or anticipated future exposures. In a typical scenario in which there is an exposure, contaminants may be transported via one or more media (including air, soils/sediments, surface water and groundwater) to potential receptors (through, for example, inhalation, dermal contact and/or ingestion). The exposure assessment aspect of the risk assessment must then involve the characterization of the physical and exposure setting, including contaminant distributions leading from sources on a hazardous waste site to the points of exposure, the identification of significant migration and exposure pathways, the identification of potential receptors, the development of exposure scenarios (including the determination of current and future exposures) and the estimation of chemical intakes for all potential receptors and significant pathways of concern. Consider now the following example problem involving hazardous air contaminants being released from a series of inactive wastewater ponds. Assume the property is located within a residential and light industrial setting. Since the subject ponds have thick clay linings, it is not expected that the underlying unconfined water supply aquifer has been impacted: this is confirmed by results of the field investigations. The ponds have periodically been dredged and the resulting sludge stockpiled on the surrounding open land. Monitoring indicates the presence of several chemicals within the dredge spoil. To abate potential health risks, several remedial action alternatives to deal with the stockpiled waste are: (1) transport for off-site disposal: (2) landfilling on-site; and (3) 0734-242X/95/020167+ 08 $08.00/0
© 1995 ISWA
168
E. A. McBean & F. A. Rovers
stabilization by on-site treatment and subsequent transport for off-site disposal. All of these involve different degrees of excavation, loading, transportation and unloading of the wastes. Both short-term (subchronic) and long-term (chronic) effects of the chemicals of concern must be considered. Following preliminary examination, the critical and significant exposure pathway is determined for this hypothetical site to be due to wind erosion and mechanical resuspension of fugitive dust from the contaminated soils in the evaporation ponds and waste stockpiles. Using a Gaussian plume model, it is possible to quantify the risks for the chemicals of concern, based on the ground-level concentration. A part of the challenge in development of the risk assessment arises in that there is uncertainty in the mathematical models employed, there is uncertainty in the data assignments utilized in the models, and there is uncertainty in the impact ofdifferent exposure levels, (i.e. are the levels sufficient to initiate cancer?). These aspects of uncertainty in exposure assessment, in general, are important and will be considered later. However, of immediate interest is that the period of remediation, and thus the period of exposure risk for the nearby human subjects, is essentially the same for all of the remediation alternatives. Therefore, the selection between the remediation alternatives essentially involves the comparison of exposure levels with air quality standards along with the cost information, to in turn make a selection between the remediation alternatives. Consider now an alternative where the remediation alternatives do not entail the same duration of exposure risk. The identification of the most effective alternative is now more complicated. An effective tool for this additional complication involves use of risk-time curves.
2. Bases of risk-time curves
The use of cost-time curves in the selection between remediation alternatives has been standard practice for many years. Typically, the curves themselves have only been considered peripherally since, using a discount rate, the temporal variability of the costs is modified to a present value and/or translated into an equivalent annual cost. The equivalencing concept is utilized to allow comparisons between alternatives by removing the individual temporal variations, placing the comparisons between the alternatives onto an equal basis. Risk-time curves are of a similar nature, except that the curves indicate the temporal, changing levels in terms of risk, as a function of time, for each of the remediation alternatives. The general character of the curves is best demonstrated by example (purposely kept simple for ease of explanation). Consider the following: (1) Industrial wastes have been improperly disposed of by placement (i.e. dumping) of hazardous material (e.g. paint sludges) into trenches, and covering the material with soil. As a result of this poor disposal practice, the surrounding groundwater is being contaminated. (2) Remediation alternatives include the following: (i) Do-nothing alternative--This alternative involves no capital and operating expenditure in terms of dollars but will result in the contamination of the local groundwater supplies. At this site, there will be an increased incidence of cancer in the local population, as the chemicals spread and enter the groundwater and lakes in the immediate vicinity. As well, there will be emissions to the atmosphere.
169
Utility of risk-time curves
(ii) Excavation/incineration/relandfilling alternative--Excavation of the refuse will result in elevated emission levels from the waste as it is uncovered, transported, stored and handled at a staging area and finally rehandled for treatment at an incinerator. During excavation, the potential exists for explosions to occur, due to the mixing of incompatible wastes. In addition, incineration of the wastes and the adjacent contaminated soil removed with the wastes will result in flue gas emissions and will generate a significant quantity of fly ash and unburnable residue. Although good incineration practices will minimize any emissions from the incineration process, the incineration of thousands of tonnes of site material will result in release of some chemicals to the atmosphere. A schematic depiction of the activities and the consequent migration pathways is depicted in Fig. 1. (iii) Inplace containment, groundwater pumping and treatment alternative--Off-site contaminant migration can be attenuated using a physical barrier (a grout wall, bentonite trench and sheet piles), a groundwater pumping system and a treatment system. In this situation, there will be air emissions as the groundwater is treated by air stripping, in addition to disturbance of the waste and contaminated soil during construction. A schematic depiction of the activities and the consequent migration pathways associated with this remediation alternative are depicted in Fig. 2. Volatilization Volatilization Flue Volatilization and particulate v->gas and particulate ~ migration ~ migration and particulate [V-->migration emissions
Excavation and stockpiling
t
Vehicular transport to incinerator
l/ Stockpiling "1 at incinerator
I Incinerator
~'1Relandfilling
Fig. I. Materialshandlingactivitiesand primaryreleasemechanismsfor excavation/incineration/relandfilling alternative.
Volatilization
l
and particulate migration
Excavation and stockpiling
r---> VOCsrelease
Air stripping
Fig. 2. Sourceopportunitiesand primaryreleasemechanismsfor inplacecontainment,groundwaterpumping and treatmentalternative.
170
E. A. McBean & F. A. Rovers
Each of the remediation alternatives (i)-(iii), have associated capital and operating costs (measured in U.S. dollars) and different elements of risk. Note that the assumption in the comments to follow is that the risk refers strictly to the risk to nearby residents; another level of risk is associated with on-site construction workers, but these concerns will be ignored in the interest of brevity and the ease of explanation of the principles of risk-time curves. To characterize the risk-time curves, pathways migration models for contaminants are employed. The models must describe the mass, physical state, and containment structure for each chemical involved. Although such models can assume many different forms, the essential feature is that the environmental transport and fate analyses of the chemicals must be quantified as the chemicals migrate to the receptors (the nearby residents). The primary transport mechanisms in this site assessment include the groundwater pathway in (i) and those noted in Figs 1 & 2. The mathematics for describing in detail the pathways assessment are beyond the scope to be addressed herein; suffice it to indicate that the range of sophistication of such models is considerable. The interested reader is referred to McTernan & Kaplan (1990) as an example, for additional discussion of mathematical models of migration pathways. Following the quantification of the remediation alternatives in terms of releases of the various chemicals as a function of time, and the utilization of the environmental pathways models to characterize the transfer of chemicals from the site to the receptors, the concentrations of the individual chemicals at the various receptors are developed. These determinations must reflect the temporal variations associated with the release mechanisms, since not all of the various release mechanisms are functioning at all times. The aggregation must then take place over the array of chemicals, from all the exposure pathways, resulting in the risk-time curves depicted in Fig. 3. Brief comments on the individual risk-time curves for the various remediation alternatives are as follows: 12 11 10 Do-nothing
9 8 7 ~D
6 Excavatiordincineration/relandfilling 5 4 /
3
Inplace c o n t a i n m e n t
.2 1 10
I
I
I
20
30
40
Years
Fig. 3. Risk-time curves for different remediation alternatives.
50
Utility of risk-time curves
171
(i) Do-nothing alternative--As the groundwater plume continues to migrate away from the buried refuse, the exposure risk continues to escalate, as additional sources of water supply for various residents are successively contaminated. (ii) Excavation/incineration/relandfilling alternative--This alternative has the potential to substantially reduce public risk in future generations but increases the health risk in the short-term (over the period of 10 years of construction and implementation). In this case, mean health risk reduction to future generations will be large, but there is a large risk transfer to the nearby residents during the 10year period of excavation/incineration and relandfilling activities due to such features as the increased opportunity for volatilization, the risk of explosions during excavation and the air emissions during incineration. The number of receptors exposed to elevated risks as a result of the excavation/incineration and relandfilling will be dependent on the locations of the various activities, and this information must be reflected in the analyses. (iii) Inplace containment and groundwater pumping--The health risk afforded by the proposed pumping and treatment alternative is elevated over the "do-nothing" alternative for the first few years due to the construction activity and the treatment emissions (volatilization) due to the air stripping and carbon adsorption processes for treatment of the pumped groundwater. The treatment emissions will continue throughout the concern with the site, projected as needing 100 years of operation. A significant benefit of the risk-time curves is the immediate indication associated with each alternative, of any periods of elevated risk. It is readily apparent, for example, that the levels of risk for the inplace containment/groundwater pumping do not reach the same magnitude as the excavation/incineration/relandfilling alternative, but of course, the risks continue over a much longer time-frame with inplace containment/ groundwater pumping.
3. Equivalent-value assignments Standard practice in dealing with cost-time curves utilizes the discounting of the various cost-time components back to present value, using a discount rate. A similar discounting procedure is not normal practice in risk-time curves. Instead, the normal practice usually equivalences the risk to a lifetime equivalent using no discounting. Therefore, the next task involves integration of the risk curves over the population-at-risk, and equivalencing back to present value. Given the information on cost-and risk-time curves, the resulting values for the three remediation alternatives plotted as expected cost vs. expected risk, are plotted on Fig. 4. If one was to accept the alternative with the lower expected value of risk, this would imply that the excavation/incineration/relandfilling alternative would be the most desirable alternative--it has a higher cost than the inplace containment, but it has onethird of the expected risk.
4. Incorporating aspects of uncertainty Although there is an inclination to select the remediation alternative with the lowest expected value, there is substantial uncertainty in risk assessment associated with many data assignments. This consideration is particularly relevant for this example since the waste disposal records for old landfill sites are typically quite poor. Thus, for example,
172
E. A. McBean & F. A. Rovers 450 400 350
Excavation/incineration/relandfilling X
300 Inplace
250
containment
X ~.~ 200 o r..) 150 100
Do-nothing
50
X I 10
I 20
I
I
I
30
40 Risk (x 10z)
50
I 60
I
70
80
Fig. 4. Societal risk-cost tradeoff for expected values.
knowledge of the quantities of various chemicals buried, the specific locations relevant to the concern of missing incompatible wastes during excavation and the resulting potential for an explosion occurring, and the ability to assign representative chemical release rates during stockpiling, incineration and relandfilling, all of these types of factors indicate that the uncertainty in the risk-time curves may be very relevant in terms of decisionmaking. Selection between remedial alternatives may not be adequate, based on expected values alone--the decision-maker may want to adopt a risk-averse strategy. Reflecting the uncertainty of the risk, the plot of societal risk vs. time curves become as depicted in Fig. 5; the solid lines depict the expected values of the risk-time and the associated dashed line indicates uncertainties as quantified by one standard deviation away from the mean. In turn, the risk-time curves (m the plural sense indicating the expected value and the uncertainty aspects), the resulting cost vs. risk tradeoffs became as depicted in Fig. 6. The solid lines surrounding the expected values indicate one standard deviation from the mean, for both the costs and the risks. Several points are noteworthy: (1) The cost uncertainty is of a smaller magnitude than the risk uncertainty. The elements of cost uncertainty relate to the projection of the future technological costs (e.g. operational costs of the treatment facility in 20 years). There is uncertainty associated with such cost projections but the magnitudes are expected to be relatively small. On the other hand, the uncertainties associated with risk are substantial. (2) The cost uncertainty is depicted as Gaussian or normally-distributed as the bounds are symmetrical about the mean. Conversely, the risk uncertainty is log-Gaussian or log normal. This is to be expected since the projections of the future costs could just as easily be on the high side, as the low side. (The Central Limit Theorem
173
Utility of risk-time curves
! I
I i
~
I
Do-nothing / / " /
i
!
"d 0.1
llllllllltlllll#ll
? ......... i:
-I
I
Ii
"$
,
I 0.01 ~.
/# J
/f .
~
Excavation/ineineration/relandfilling
1
••sS•
I
I ~/ . .
.4
I
/ ~
'
//"
. . . . . . . . . . .
~
0 0011 0
~
•
'
Inplace containment
..................................... ................ -_/_-- . . . . . . . . . . . . . . . . . . . . . . . . . . .
s
I
lO
20
I
I
30
40
50
Years Fig. 5. Log risk-time curves for societywith different remediation alternatives includingmargins for uncertainty.
450 400 350
Excavation/ineineration/relandfilling X
300 Inplace containment 250 o
~,~200 ~ lgO
100 Do-nothing
50 I
10
I
20
I
30
40 Risk (x 102)
I
50
I
60
Fig. 6. Societalrisk-costtradeoff reflecting uncertainty.
I
70
80
174
E. A. McBean & F. A. Rovers
indicates that such uncertainty can be described as a Gaussian distribution. For further reading see, for example, Bethea et al. 1985). Conversely, the distribution of the risk is frequently log normal since it is bounded on the low side by zero and unbounded on the high side; the distribution is skewed to the right. Note that both components of uncertainty (in costs and in risks) appear symmetrical around the expected value in Fig. 5 but the characterization of the risk is by a logarithmic axis, and thus, it is symmetrical in a log-transformed sense. As a result of including the uncertainty aspects of the risk, the selection between the alternatives is not so obvious. The excavation/incineration/relandfilling alternative has the lower expected value of risk but also possesses the very real possibility (probability) that the risk impact is larger. In other words, the excavation/incineration/relandfilling alternative has the lower expected risk but has a much greater probability that risks associated with this alternative are much higher. Consequently, the greater costs and the potential for greater risks might well argue for selection of the inplace containment/ groundwater pumping alternative as opposed to the excavation/incineration/relandfilling alternative. 5. Conclusions
More formal requirements for consideration of risk management are arguing for increased sophistication in risk assessments. Risk-time curves assist in this regard by identifying the temporal variabilities which may be very relevant in selecting the best remediation alternative(s). The risk-time curves also possess the capability to reflect uncertainty aspects in assignment of risk features. References
Bethea, R., Duran, B. & Boullion, T. (1985). Statistical Methods for Engineers and Scientists. Marcel Dekker Inc., N.Y., U.S.A. McTernan, W. F. & Kaplan, E., eds. (1990). Risk Assessment for Groundwater Pollution Control, Monograph of the American Society of Civil Engineers, New York, N.Y., U.S.A.