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Assessing effects of site characteristicson remediation secondary life cycleimpact with a generalised frameworkDeyi Houa, Abir Al-Tabbaaa & Jian Luob

a Department of Engineering, University of Cambridge,Trumpington Street, Cambridge CB2 1PZ, UKb School of Civil & Environment Engineering, Georgia Instituteof Technology, Mason 226, 790 Atlantic Drive, Atlanta, GA, USA30332-0355Published online: 16 Jan 2014.

To cite this article: Deyi Hou, Abir Al-Tabbaa & Jian Luo (2014) Assessing effects of sitecharacteristics on remediation secondary life cycle impact with a generalised framework, Journalof Environmental Planning and Management, 57:7, 1083-1100, DOI: 10.1080/09640568.2013.863754

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Assessing effects of site characteristics on remediation secondary life

cycle impact with a generalised framework

Deyi Houa*, Abir Al-Tabbaaa and Jian Luob

aDepartment of Engineering, University of Cambridge, Trumpington Street, Cambridge CB2 1PZ,UK; bSchool of Civil & Environment Engineering, Georgia Institute of Technology, Mason 226,

790 Atlantic Drive, Atlanta, GA, USA 30332-0355

(Received 13 July 2013; final version received 5 November 2013)

The ‘sustainable remediation’ concept has been broadly embraced by industry andgovernments in recent years in both the US and Europe. However, there is a strongneed for more research to enhance its ‘practicability’. In an attempt to fill this researchgap, this study developed a generalised framework for selecting the mostenvironmentally sustainable remedial technology under various site conditions. Fourremediation technologies were evaluated: pump and treat (P&T), enhanced in situbioremediation (EIB), permeable reactive barrier (PRB), and in situ chemicalreduction (ISCR). Within the developed framework and examined site conditionranges, our results indicate that site characteristics have a profound effect on the lifecycle impact of various remedial alternatives, thus providing insights and valuableinformation for determining what is considered the most desired remedy from anenvironmental sustainability perspective.

Keywords: sustainable remediation; life cycle assessment; groundwater remediation;contaminated land

1. Introduction

Land contamination is a major challenge to modern society; with an estimated 294,000

contaminated sites in the US (USEPA 2004) and over 300,000 ha of potential

contaminated land in the UK (EA 2005). The land contamination issue can deteriorate

due to other environmental-friendly practices such as recycling (Hou 2011; Hou et al.

2012). Land remediation requires the investment of a significant amount of resources.

The US Superfund clean-up programme alone is spending approximately $1.2 billion

annually in the federal budget; and the requested reinstating of the Superfund tax would

add over $20 billion in funding over the next decade (USEPA 2013). Historically,

remediation was considered to be an inherently sustainable practice because it restores

contaminated land and reduces urban sprawl and greenfield development. Modern green

design standards, such as the Leadership in Energy and Environmental Design (LEED)

programme, recognise brownfield remediation as a major credit towards sustainable

development (USGBC 2011). However, remediation operations are also associated with

adverse environmental effects throughout its life cycle. It is only in recent years that

researchers and industrial practitioners have started to examine these secondary

environmental impacts from a ‘sustainability’ perspective, and using a life-cycle

approach (Ellis and Hadley 2009).

*Corresponding author. Email: deyi.hou@gmail.com

� 2014 University of Newcastle upon Tyne

Journal of Environmental Planning and Management, 2014

Vol. 57, No. 7, 1083–1100, http://dx.doi.org/10.1080/09640568.2013.863754

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Sustainable remediation is an emerging field, with its concept first adopted by several

organisations which are sponsored primarily by the industry. These organisations include

the Network for Industrially Contaminated Land in Europe (NICOLE, founded in 1995 in

Europe), the Contaminated Land: Applications in Real Environments (CLAIRE, founded

in 1999 in the UK), and the Sustainable Remediation Forum (SURF, founded in 2006 in

the US). Sustainable remediation, or its variant ‘green remediation’, is also increasingly

supported by governments (CLARINET 2002; ITRC 2011; USEPA 2010a). Various US

federal agencies, e.g. USACE (USACE 2010), AFCEE (AFCEE 2010), NAVFAC

(NAVFAC 2011) and state governments, e.g. California-DTSC (DTSC 2009), have

developed guidance, protocols and software packages in this field in the past few years.

A number of research studies have been conducted relating to this field, and they have

used life cycle assessment (LCA) (Lemming, Hauschild, and Bjerg 2010; Morais and

Delerue-Matos 2010), multi-criterion analysis (Harbottle, Al-Tabbaa, and Evans 2008;

Sparrevik et al. 2012) and footprint calculators (USEPA 2010b; Lubrecht 2012) in

sustainability evaluation for remedy selection. Several early studies laid the foundations

for conducting life cycle assessment (LCA) in the field of remediation, primarily to

account for secondary impacts in order to support informed decision making (Diamond

et al. 1999; Volkwein, Hurtig, and Kl€opffer 1999; Blanc et al. 2004). Two review papers

published in 2010 summarised the findings and challenges based on a review of over 10

remediation LCA studies (Lemming, Hauschild, and Bjerg 2010; Morais and Delerue-

Matos 2010). Since 2010, over a dozen more LCA studies have been published

(Owsianiak et al. 2013). These studies have rendered a wide range of findings and

implications to assist researchers and industrial practitioners in taking a holistic view in

remediation decision-making processes.

As an International Organization for Standardization (ISO) standardised method,

LCA can be used with other factors, such as cost and performance data, to select the most

appropriate remedial alternative. However, performing an LCA can be resource and time-

consuming (USEPA 2006), and may not be affordable by most remediation projects.

Moreover, the incorporation of sustainability in remediation decision making is a

continuous and iterative process (Holland 2011). As traditional LCA is a relatively static

process (i.e. requiring solid definition of the system), it may need adaptation to

accommodate the unique challenges in sustainable remediation decision making in a

project life cycle where sigificant changes may occur in the system. One of the

challenges is that most existing studies have used highly site specific data, which make it

difficult to extend the implications from these studies to other sites with different site

conditions.

This study aims to explore the feasibility of using a generalised LCA model to derive

knowledge that can be more broadly applicable than what traditional LCA renders. Four

remediation technologies are assessed in this study: pump and treat (P&T), enhanced in

situ bioremediation (EIB), permeable reactive barrier (PRB), and in situ chemical

reduction (ISCR). This LCA has chosen chlorinated ethylene as the study subject because

chlorinated solvents are the most prevalent organic contaminants in soil and groundwater

(ATSDR 2007), due to their past extensive industrial use as cleaning and degreasing

solvents, adhesives, coating solvents and as feedstock for other chemicals (Stroo and

Ward 2010). Unlike most existing remediation LCA studies that are limited by highly site

specific conditions and thus rendering mixed results (see Table S1 in the Supplementary

Material available via the article website), the present generalised remediation life cycle

assessment (GRLCA) studies the influence of broad-range varying site-specific

parameters, including site location, plume dimension, hydrology, chemistry and

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geochemistry. The present study also represents the first LCA of ISCR with in situ soil

mixing, and one of only a few LCA studies on EIB.

2. Materials and methods

2.1. Generalised site conceptual model

A generalised site conceptual model (GSCM) was established based on a review of

remediation activities at four typical chlorinated solvent contaminated sites (see

Supplementary Material available via the article website, section B). These sites are

located in California, USA, with site size ranging from a few hectares to over 100 ha. The

plume dimensions and hydrogeological properties show moderate to strong variation

among these sites. A broad range of remediation activities have been conducted at these

sites to treat contaminated groundwater, ranging from conventional P&T to innovative

ISCR. The GSCM, as shown in Figure 1, was developed to encompass the site conditions

encountered at these sites. The system parameter ranges were expanded to account for a

wider range of site conditions (see Table 1).

In the GSCM, the contaminated area consisted of a treatment zone that was the target

of active remediation (e.g. chemical or biological treatment), and a monitored natural

attenuation (MNA) zone, where there was no active remediation, where instead risks

were managed through regularly monitoring the natural attenuation of the plume by

dilution and natural degradation. A series of parameters depicted the location of the site,

hydrogeology, extent and migration of contaminants, etc. The typical parameters

observed at these four sites were used to generate a range of parameters that were used in

the GSCM study. Section B of the Supplementary Material (available via the article

website) includes a diagram of the GSCM and simulated parameter range. In this study,

to simplify the analysis, tetrachloroethene (PCE) and its daughter products were selected

Figure 1. A diagram of the generalised site conceptual model (Denotation: W1 ¼ widthof treatment zone [40 m]; W2 ¼ width of plume area for monitored natural attenuation [270 m];L1 ¼ length of treatment zone [60 m]; L2 ¼ length of MNA zone [260 m]; D ¼ depth to upperlimit of contaminated aquifer [4 m]; B ¼ thickness of contaminated aquifer [6 m]).

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as the modelled contaminants in the GSCM. The present GRLCA framework is a

parameterised LCA based on the GSCM.

2.2. LCA functional unit and LCA boundaries

The goal of the LCA was to compare the secondary life cycle impact from four

remediation options for the GSCM. The functional unit was to reduce the levels of PCE

and its daughter products (i.e. TCE, DCE and vinyl chloride) within the treatment zone of

the GSCM (see Figure 1) to levels below the California Maximum Contaminant Level

Table 1. Range of system parameters in the generalised site conceptual model.

Key system parametersRange of parameter

values Default values Basis

Plume dimensionWidth of treatment zone 4 m to 200 m 40 m A factor of 1/2 or 2 was applied to

the minimum/maximumvalues in Table S1 (in theSupplementary Materialavailable via the articlewebsite) to encompass andexpand the range of parametervalues being simulated in thisstudy, with the exception oftreatment zone width, forwhich the minimum value wasreduced to 4 m to encompass awider range of possible siteconditions. The default valueswere set at or near thegeometric mean of theminimum and maximumvalues.

Length of treatment zone 15 m to 300 m 60 mAquifer starting depth 1 m to 18 m 4 mAquifer thickness 2 m to 18 m 6 mMNA plume width 60 m to 1200 m 270 mMNA plume length 50 m to 1400 m 260 m

HydrogeologyHydraulic conductivity 0.5 to 100 m/day 7 m/day As aboveHydraulic gradient 0.0001 to 0.02 0.003 Reflect a wide range of

possibilities based onhistorical data at the typicalsites (i.e. accounting fortemporal variation).

Chemistry andgeochemistry

Half-lives of PCE anddaughter products inZVI treatment

0.1 hour to 10 hour 2.4 hour (USEPA 2000; Rabideau,Suribhatla and Craig 2005;Muegge 2008).

ZVI longevity in PRB 5, 10, 15, and30 years

15 years

Native Electron AcceptorDemand (EIB)

0.002 to 0.2 kgH2/m

3 water0.02 kg

H2/m3 water

Information from typical sites.

Native demand of ZVIdosage in soil mixing

30 to 3,000 mgZVI/kg soil

280 mg ZVI/kgsoil

Information from typical sitesand Bozzini 2006; Sale andOlson n.d.).

Site locationSite worker travel

distance20 km to 500 km 100 km Information from typical sites.

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(Cal-MCL) for drinking water (CDPH 2011), with a 30-year timeframe. The Cal-MCL is

5 mg/L for PCE, 5 mg/L for TCE, 6 mg/L for 1.1-DCE and cis-1.2-DCE, and 0.5 mg/L for

vinyl chloride. Groundwater monitoring and possibly supplemental remediation activities

are expected to continue after 30 years, but there are high uncertainties as to what end-of-

life (EOL) processes may occur. The impact of EOL activities may be minimal with the

exception of its addition to waste generation. The LCA system boundary is depicted in

Figure S1 in the Supplementary Material available via the article website. For P&T, the

system encompassed the installation of extraction wells and monitoring wells, the

construction of the treatment system and conveyance pipe, the provision of a pump and

electricity, the maintenance of the system, replacement of GAC, as well as groundwater

monitoring. For EIB, the system encompassed the installation of an injection well and

monitoring wells, the provision of substrate, injection operation and groundwater

monitoring. For PRB, the system encompassed the provision of ZVI, construction of the

PRB wall, installation of monitoring wells and groundwater monitoring. For ISCR, the

system encompassed the provision of ZVI, in situ mixing operation and groundwater

monitoring.

The LCA method generally included all processes used in the life cycle of

remediation processes, with some exceptions noted in Section C of the Supplementary

Material available via the article website. A theoretically complete LCA should be

‘cradle to grave’, including all raw materials and energy sources started in the earth and

ended back in the earth. However, such a complete LCA is rarely done due to limits in

time and resources. The LCA method employed in this study has excluded end-of-life

processes of remedial systems, emissions from waste disposal and lifestyle impacts of

remediation workers.

2.3. LCA inventory

The LCA in this study consisted of two parts. In the first part, the processes of each

remedial alternative were examined; the use of consumables, energy, equipment, etc.

dependent on the varying parameters of the GSCM were estimated based on the operation

experiences at the four sites described above, as well as published literature. Detailed

methods and assumptions used in these estimates are listed in Section C of the

Supplementary Material available via the article website. In the second part of the LCA,

existing databases were used to obtain resource input and emission output of

consumables, energy, equipment, etc. that were used in the remedial processes. The

primary LCA inventory data source was the ecoinvent database (Hischier and Weidema

2010). A complete listing of LCA inventories, sources and associated assumptions are

provided in Section C of the Supplementary Material available via the article website.

Given the complexity of remediation projects, certain details were not considered

significant contributors in differentiating the environmental impact of various remedial

alternatives. For example, the manufacture of minor capital equipment (e.g. a crane used

in setting up GAC units), minor materials used in the project (e.g. metals other than steel,

additives in a water/vegetable oil mixture) are omitted in the present study.

2.4. Life cycle impact assessment method

This study used the Tool for the Reduction and Assessment of Chemical and Other

Environmental Impacts (TRACI) developed by the USEPA (Bare et al. 2002). It is a

midpoint impact analysis method that translates categorical impact into equivalent

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quantities of a selected chemical/substance. All impact categories in TRACI are included

in this study: ozone depletion, global warming, acidification, eutrophication, smog

formation, criteria air pollutant (particulates), human health risk-cancer, human health

risk-non-cancer, ecotoxicity and fossil fuel depletion. The LCIA results were normalised

by background impact to assist in meaningful interpretation. The normalisation factors

for all TRACI impact categories were based on a USEPA database (Bare, Gloria, and

Norris 2006). These factors represent a national average in the year of 1999.

2.5. Weighting and interpretation

This GRLCA was conducted in general accordance with ISO 14040 and ISO 14044 (ISO

2006a, 2006b). The impact scores for each TRACI category were weighted and

combined in a single environmental impact score. This score is calculated using the

following equation:

S ¼X

viSi ¼X Si

Ti

where S is the overall impact score for the studied alternative, wi is the weighing factor

for the ith impact category, Si is the normalised impact in the ith impact category for the

studied alternative, and Ti is the highest impact of all studied remediation alternatives in

the ith impact category. Based on this definition, the alternative with the lowest impact

score is the most desirable alternative. While the normalised impact for each category

was used to evaluate the effect of site characteristics on individual impact, the weighted

score for all impacts was used to evaluate the effect of site characteristics on the

desirability of each remedial alternative. As weighting factors are largely a value choice,

ultimately it is a choice of stakeholder preference. The present study assumes that

stakeholders would focus on the relative advantage of remedial alternatives rather than

the relative importance of impact categories, partly because most stakeholders lack the

expertise to judge which impact category is more important. The weighting function

expressed by the above equation gives a maximum score of 1 and a minimum score of 0

to each category, thus offering straightforward interpretation to stakeholders (note: one

exception is for ecotoxicity, which has negative impact values; consequently the

weighted score could be negative). In order to better understand the effect of this

weighting process, three times weight was given to global warming, human health-cancer

and eutrophication as a sensitivity test.

3. Results and discussion

3.1. Effect of site characteristics on secondary life cycle impacts

3.1.1. Effect of plume dimension

The effects of plume dimension on secondary life cycle impact were examined regarding

six parameters: the width of treatment zone, the length of treatment zone, the depth of

plume aquifer (starting depth), plume thickness, MNA zone width, and MNA zone

length. Figures 2(a) through to 2(f) present how plume dimension affects the life cycle

global warming potential of the four remedial alternatives, and Figures 2(a’) through to

2(f’) show how plume dimension affects the overall impact score of all impact categories.

Global warming was selected to illustrate the effect of site characteristics on individual

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impact categories, because global warming is commonly viewed as one of the most

important impact categories (Bare, Gloria, and Norris 2006).

Our results show that PRB is the most sensitive to treatment zone width and aquifer

thickness because the life cycle impact of PRB is mainly due to the use of iron ZVI (see

Section 3.2), which in turn is mainly controlled by treatment zone width and aquifer

thickness. It is evident that PRB loses its competitive advantage to P&T as treatment

zone width and aquifer thickness increase; therefore, conclusions regarding the relative

advantage of PRB in existing studies (Bayer and Finkel 2006; Higgins and Olson 2009)

Figure 2. Effect of plume dimensional parameters on secondary life cycle impact: global warmingpotential and overall impact score.

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must be carefully placed in the context of the specific plumes’ dimension before applying

such knowledge to sites with different plume sizes. This study found that EIB is

moderately sensitive to all treatment zone dimensions and relatively strongly sensitive to

MNA zone dimension. This is because EIB, as an active treatment technology (vs. P&T

and PRB as passive treatment technology), is dependent on the ‘area’ of treatment rather

than interception width of treatment. Moreover, EIB has a strong demand for long-term

monitoring in both the treatment zone and MNA zone; therefore it is more sensitive to

MNA dimension. ISCR exhibits similar trends as EIB, but it is more sensitive to

treatment zone dimension and less sensitive to MNA dimension as compared to EIB.

P&T exhibits a similar trend to PRB, but its reliance on treatment zone width and aquifer

depth/thickness is stepwise rather than continuous due to the deployment of discrete

number of extraction wells and pumps. Generally, the overall impact score of P&T

improves when treatment zone dimension increases. This is because P&T has relatively

significant capital investment which leads to reduced marginal cost when the system

scales up. PRB becomes relatively more desirable when treatment zone length and

aquifer depth increases, and it becomes less desirable when treatment zone width and

aquifer thickness increases. EIB becomes less desirable when the MNA zone dimension

increases.

3.1.2. Effect of hydrogeology

Two hydrogeological parameters, hydraulic gradient and hydraulic conductivity, were

examined for their effects on the secondary life cycle impact of the four remedial

alternatives. Figures 3(a) and 3(b) show that PRB is most sensitive to these

hydrogeological parameters. Its life cycle global warming potential impact varies by

over one order of magnitude within the parameter ranges. In comparison, ISCR is not

sensitive to these parameters at all because the ISCR design is only dependent on

treatment zone size and geochemical conditions. In PRB design, areas of higher

hydraulic conductivity may be preferred because it results in a larger capture zone

(Gavaskar et al. 2000). However, the present study suggests that this preference must

be balanced with the potential increase in secondary impact, and under high hydraulic

conductivity in situ chemical treatment with soil mixing can be more desirable than the

passive interception technology. EIB and P&T are also sensitive to the hydrogeological

conditions because the amount of vegetable oil used in EIB is dependent on

groundwater flux through the treatment zone, and the number of extraction wells in

P&T is dependent on hydraulic gradient and hydraulic conductivity. Figures 3(a’) and 3

(b’) show that PRB is the most desired remedial alternative when hydraulic gradient or

hydraulic conductivity are low; EIB becomes the most desired remedy when hydraulic

gradient or hydraulic conductivity are in the middle range; and ISCR becomes the most

desired remedy when hydraulic gradient or hydraulic conductivity are high. It is also

interesting to note that P&T becomes relatively more desirable when hydraulic

conductivity is very high.

3.1.3. Effect of chemistry and geochemistry

The effects of chemistry and geochemistry factors on the secondary life cycle impact are

shown on Figures 3(c) through to 3(f) and 3(c’) through to 3(f’). As shown on Figure 3(c),

PRB has the lowest global warming potential as the contaminant half-life in PRB was

less than 1 hour, but it has the highest global warming potential as the contaminant half-

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life was above 5 hours. In a similar way the global warming potential of EIB is affected

by the native electron acceptor demand and that of ISCR is affected by native dosage

demand. Even though each of these parameters only affect the secondary impact of one

remedial alternative, as shown on Figures 3(e’) through to 3(f’), the overall impact score

of all remedial alternatives is affected. Existing studies have shown that groundwater

constituents can significantly affect the secondary impact of PRB (Mak and Lo 2011),

and our study further shows that the geochemical condition of the native groundwater can

affect the relative desirability of PRB, as well as the other three remedial alternatives.

Figure 3. Effect of hydrogeology and chemistry characteristics on secondary life cycle impact:global warming potential and overall impact score.

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3.1.4. Effect of site location

Site location can play a critical role in determining the secondary impact of remediation

activities, especially if the remedial alternative involves large quantities of material

transport in and out of the site (Harbottle, Al-Tabbaa, and Evans 2007; Gallagher,

Spatari, and Cucura 2013). For remedies with more intensive site activities, travelling

associated with monitoring and sampling had been omitted by some existing LCA studies

(Bayer and Finkel 2006; Higgins and Olson 2009); however, our study found this to be a

significant contributor to impact (see Section 3.2). Our study shows that as the travelling

distance increases, life cycle impacts of all remedial alternatives increase, but relatively

speaking, P&T and EIB become significantly less desirable while PRB becomes

significantly more desirable, and ISCR remains in a similar position. This was primarily

because a significant portion of EIB secondary impact was associated with groundwater

monitoring, which required a large number of field trips during 30 years of groundwater

monitoring. It was also because EIB had a higher demand for groundwater monitoring;

EIB tends to be less effective than the other remedial alternatives, and it can potentially

generate toxic daughter products, thus requiring more intensive groundwater monitoring.

With regard to P&T, it not only requires groundwater monitoring trips but also O&M

trips, thus rendering a high effect of travelling distance.

3.1.5. Sensitivity of weighting factors

The weighting process can integrate results for various impact categories to render a single

final score, thus allowing for a straightforward interpretation in a management and

decision-making oriented setting. In the present study, the weighted final score plays an

important role in that it was used to directly compare different remedial technologies in

evaluating the effect of site characteristics. In the sensitivity analysis, three times weight

was given to global warming, human health-cancer and eutrophication, respectively.

Results showed that under different weighting settings, the shape and relative position of

each remedial alternative’s dependence on each site characteristic had only very minor

changes and may be considered generally unchanged (for example, see Figures S4-1 and

S4-2 in the Supplementary Material available via the article website). If more dramatic

adjustment is made to the weighting factors, more significant variation is expected, but it is

unlikely the overall conclusions about the effect of site characteristics would be changed.

3.2. Material and energy consumption analysis

The four remedial alternatives were also evaluated in terms of material and energy

consumption in the construction and operation processes. Figure 4 shows the relative

contribution of the usage of various materials, construction/operation energy, site worker

travelling, excavation/drilling and chemical analysis to the secondary life cycle impact of

all remedial alternatives in the default GSCM. For P&T, construction and operation

energy accounted for 50–70% of impacts in all impact categories, while materials such as

cement and PVC pipes used in facility construction had very small contributions. This is

consistent with previous findings that energy demand is the primary driver of P&T’s

secondary impact while facilities is a minor contributor (Bayer and Finkel 2006; Higgins

and Olson 2009). On the other hand, some existing LCA studies have omitted long-term

monitoring and sampling by assuming all remedial alternatives have similar monitoring

requirements (Higgins and Olson 2009), or assuming long-term monitoring sampling

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represent only a low contribution to the LCA results (Bayer and Finkel 2006). However,

our study found that the travelling associated with operation and maintenance (O&M)

visits and groundwater monitoring over 30 years, as well as laboratory analysis in this

period, accounted for a significant portion of secondary impact. Such monitoring

requirements are also very different for different remedial alternatives. Therefore, our

study suggests that long-term monitoring and sampling is an important component to be

included in LCA of P&T systems.

For EIB, there were more varieties regarding which material/energy component was

contributing to each type of secondary impact. For example, vegetable oil accounted for

the majority of eutrophication impact, while chemical analysis accounted for the majority

of acidification impact, and site worker travelling accounted for the majority of smog air

impact. This lack of a predominant process in EIB was also found by other studies

Figure 4. Material and energy consumption of four remedial alternatives. (See online colourversion for full interpretation.) P&T: pump and treat, EIB: enhanced in situ remediation withsoybean oil as substrate, PRB: permeable reactive barrier with zero-valent iron (ZVI) as reactivemedia, ISCR: in situ soil mixing with ZVI to achieve in situ chemical reduction. Impact categoriesinclude global warming (GW), acidification (Ac), smog air (SA), eutrophication (Eu), humanhealth: cancer (HHC), human health: non-cancer (HHNC), criteria air pollutants (CAP), ozonedepletion (OD), waste (Wa), and fossil fuel depletion (FFD).

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(Lemming et al. 2010). This suggests that to mitigate the overall environmental impact of

EIB, efforts must be made in more than one component. For PRB, iron ZVI accounts for

a predominant portion of secondary impacts in all categories. In comparison, in an

existing LCA study on a funnel and gate type PRB (Higgins and Olson 2009), even

though iron ZVI still accounted for 40–70% of the secondary impact, it was much lower

than the 60–90% observed in the present study. Therefore, to reduce the secondary

impact of PRB, especially for straight wall type PRB, researchers and practitioners might

focus on increasing the longevity of ZVI media (i.e. reduce refill frequency) and

enhancing the degradation kinetics (i.e. reducing fill amount). For ISCR, excavation/

drilling (i.e. in situ soil mixing) accounts for a major portion of most secondary impacts,

while worker travelling, Iron-ZVI and chemical analysis still account for a significant

portion of some types of secondary impact. Figure 5 further shows that for P&T, the

contribution of construction/operation energy to global warming potential becomes even

more dominant when treatment zone width increases; for EIB, the contribution of

vegetable oil increases significantly when treatment zone width increases while the

contribution of other components decreases proportionally; for PRB, the contribution of

iron-ZVI increases significantly when treatment zone width increases while the

Figure 5. Material and energy consumption of four remedial alternatives: the effect of treatmentzone width on global warming potential. (See online colour version for full interpretation.)

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contribution of other components decreases proportionally; for ISCR, the contribution of

both excavation/drilling (i.e. in situ soil mixing) and iron-ZVI increase significantly when

treatment zone width increases while the contribution of other components decreases

proportionally.

4. Conclusions and implications

The GRLCA results indicate that site-specific parameters can have profound effects on

the secondary life cycle impact of remediation alternatives at chlorinated solvent sites. In

evaluating four remediation methods, P&T, PRB, EIB and ISCR, plume dimension

parameters and hydrogeological parameters were found to have the most extensive and

significant effects. Overall, the GRLCA supports the premise that site characteristics can

largely determine what type of remedial alternative has the least amount of secondary life

cycle impact. Such secondary impact is mainly due to the use of energy (e.g. fuel and

electricity) and materials (e.g. iron-ZVI in PRB and vegetable oil in EIB) during

remediation operation; but post-remediation long-term monitoring can also contribute to

a significant portion of secondary impact, especially for technologies with less certainty

in effectiveness and thus stronger demand for long-term monitoring (e.g. EIB).

Therefore, it is important to account for secondary impact using a life cycle approach,

especially for the monitored natural attenuation technology which tends to require

enhanced long-term groundwater monitoring (Clement, Truex, and Lee 2002; R€ugneret al. 2006). The cost as well as such secondary life cycle impact of groundwater

monitoring can also be substantially reduced by performing remedial process

optimisation (Hou and Leu 2009).

The results of this study support its primary goal of exploring the feasibility of using a

generalised LCA model to derive knowledge that can be more broadly applicable than

what traditional LCA renders. For example, an existing study found that PRB was superior

to P&T in all impact categories when the longevity of reactive media exceeds 10 years

(Higgins and Olson 2009). On the other hand, it is known that ZVI PRBs can last 10 to

30 years (ITRC 2005). This seems to be good news for PRB technology developers,

suggesting that PRB should gain much popularity over P&T. However, in reality, P&T still

has predominant popularity over PRB. Among other factors such as technological

uncertainty, our study also suggests that site specific characteristics may explain this

observed discrepancy. While existing studies show that PRB is more desirable than P&T

under specific site conditions (Bayer and Finkel 2006; Higgins and Olson 2009), our study

shows that P&T can have less environmental impact than PRB under many different site

conditions: wide treatment zone, thick aquifer, high hydraulic gradient, high hydraulic

conductivity and long contaminant half-life. Therefore, the GRLCA presented in this study

can be used to derive knowledge that is more broadly applicable than traditional LCA has

rendered, offering important implications for high level decision makers.

This study found that in general source zone treatment technologies (i.e. EIB and

ISCR) tend to have less life cycle impact than containment technologies (i.e. P&T and

PRB). Even though in situ source zone treatment technologies such as EIB and ISCR

tend to be less effective than P&T or PRB in preventing contaminant migration, they can

provide a more environmental-friendly solution when combined with monitored natural

attenuation. The GRLCA also identified a number of methods to improve the

environmental sustainability of existing remediation technologies. For P&T, the major

contributor of life cycle impact is operation energy; therefore, its environmental impact

can be effectively reduced by using more sustainable energy sources, as well as the use of

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energy saving measures such as variable frequency drive. For PRB, the major contributor

is reactive media; therefore, technological developers could focus on developing reactive

media with better reaction kinetics, thus reducing the amount of media required, and

reactive media with longer life time, thus reducing the frequency of media replenishment.

For EIB, the environmental impact is mainly from long-term groundwater monitoring;

therefore, continuous optimisation of groundwater monitoring strategies becomes critical.

For ISCR, the major impact contributor is in situ mixing; therefore, technology developers

may focus on increasing auger’s mixing efficiency to improve the competiveness of this

technology.

4.1. Limitations and future study

This study aimed at assessing the secondary impacts that are associated with the actual

remediation activities. In environmental assessment typology, on par with ‘secondary

impacts’ are ‘primary impacts’ which are associated with the state of the site (e.g. site

contamination), and ‘tertiary impacts’ which are associated with the effects of the post-

rehabilitation fate of the site (Lesage et al. 2007). These different types of impacts reflect

different perspectives and each assessment may have its unique applications; land

developers and site neighbours would be most interested in the primary impact,

environmentally-conscious site owners and remediation practitioners would be interested

in both primary and secondary impacts, and urban planners would be mainly interested in

tertiary impacts (Lesage, Deschenes, and Samson 2007). The present study focuses on

secondary impacts because primary impacts can often be addressed through a site specific

risk assessment which is of higher quality than LCA, and also because comparing

primary impact with secondary impact increases uncertainty (Lesage, Deschenes, and

Samson 2007). However, when evaluating the overall sustainability of the various

remedial technologies, the primary impact associated with the contaminants and their

daughter products must be taken into consideration. For example, in EIB and ISCR, PCE

may be degraded to more toxic and persistent daughter products such as vinyl chloride.

The residual contaminants can contribute significant primary impact if they were

eventually captured by groundwater users (Lemming et al. 2010). Therefore, to maintain

the overall sustainability of EIB and ISCR, it is essential to ensure the complete

degradation and/or to control the potential migration of these daughter products.

This study is not a comprehensive sustainability assessment; it focuses on the effect of

site characteristics on the secondary life cycle impact of remediation operations.

Moreover, while this study can provide implications for decision makers at various

contaminated sites, we do not intend to replace site-specific evaluations with such

generalised evaluation. Nevertheless, the GRLCA method can provide many valuable

implications to a broad range of audiences. A major contribution of this study is to have

identified the effect of some key site-specific parameters, based on which many existing

LCA studies can be extrapolated in a semi-quantitative manner. Considering that many

remediation projects are too small to afford a full-scale LCA, such an expansion of the

existing scientific knowledge base can be very meaningful to remediation practitioners.

Future studies may be conducted using the GRLCA method to explore the secondary

impact of remediation in various media (e.g. vadose zone soil or sediment) and different

types of contaminants (e.g. heavy metals).

The present study is also limited in that it sets a fixed 30-year remediation time

horizon. It is consistent with some existing remediation LCA studies (Page et al. 1999;

Higgins and Olson 2009); however, it may not reflect what can exactly occur at any

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specific site. Closure of chlorinated solvent sites are dependent on complex

geophysichemical processes and social decision-making processes, which often render

high uncertainty. It is expected that supplemental remediation activities may continue

after 30 years. There is high uncertainty associated with such EOL activities, thus

limiting the added value of modelling such activities. The present study chose to use a

fixed time horizon largely due to the uncertainty associated with modelling remediation

time. Moreover, in practice many site owners choose to switch to monitored natural

attenuation rather than continuous operation of expensive P&T systems for an elongated

period or replenish PRB systems. The present study sets a 30-year time period for post-

remediation monitoring, which is consistent with the general post-closure care requirement

for RCRA hazardous waste management units (Code of Federal Regulations. Part 264—

Standards for Owners And Operators Of Hazardous Waste Treatment, Storage, And

Disposal Facilities. x264.117 Post-Closure Care and Use of Property). As some

researchers and practitioners may be interested in another scenario in which a remediation

system would be operated for up to hundreds of years in order to reach a certain remedial

goal (Cadotte, Deschenes, and Samson 2007), variable remediation time frames may be

used in future studies using this general framework. Such future studies may be conducted

either through testing a time horizon as an uncertainty parameter, like some existing

studies have been conducted (Bayer and Finkel 2006), or by constructing a quantitative

relationship between remediation timeframe and system parameters.

Acknowledgement

The first author’s research is funded through the Cambridge International Scholarships Schemewhich is gratefully acknowledged.

Supplemental data

Due to the limit of paper length, more details about modelling sites, remediation activities, life cycleinventory and impact assessment results are provided as supplementary materials available via thearticle website.

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