Environ. Sci. Technol. 2010, 44, 9163–9169
Environmental Impacts of Remediation of a Trichloroethene-Contaminated Site: Life Cycle Assessment of Remediation Alternatives G I T T E L E M M I N G , * ,† MICHAEL Z. HAUSCHILD,⊥ JULIE CHAMBON,† PHILIP J. BINNING,† ´ CILE BULLE,‡ MANUELE MARGNI,† CE AND POUL L. BJERG† Department of Environmental Engineering, Technical University of Denmark (DTU), DK-2800 Lyngby, Denmark, Department of Management Engineering, Technical University of Denmark (DTU), DK-2800 Kgs. Lyngby, Denmark, and The Interuniversity Research Centre for the Life Cycle of Products, Processes and Services (CIRAIG), E´cole Polytechnique de Montre´al., P.O. Box 6079, Montre´al, Que´bec H3C 3A7, Canada
Received June 14, 2010. Revised manuscript received October 20, 2010. Accepted October 25, 2010.
The environmental impacts of remediation of a chloroethenecontaminated site were evaluated using life cycle assessment (LCA). The compared remediation options are (i) in situ bioremediation by enhanced reductive dechlorination (ERD), (ii) in situ thermal desorption (ISTD), and (iii) excavation of the contaminated soil followed by off-site treatment and disposal. The results showed that choosing the ERD option will reduce the life-cycle impacts of remediation remarkably compared to choosing either ISTD or excavation, which are more energydemanding. In addition to the secondary impacts of remediation, this study includes assessment of local toxic impacts (the primary impact) related to the on-site contaminant leaching to groundwater and subsequent human exposure via drinking water. The primary human toxic impacts were high for ERD due to the formation and leaching of chlorinated degradation products, especially vinyl chloride during remediation. However, the secondary human toxic impacts of ISTD and excavation are likely to be even higher, particularly due to upstream impacts from steel production. The newly launched model, USEtox, was applied for characterization of primary and secondary toxic impacts and combined with a site-dependent fate model of the leaching of chlorinated ethenes from the fractured clay till site.
Introduction Chlorinated ethenes, such as perchloroethene (PCE) and trichloroethene (TCE), are among the most frequent contaminants found in soil and groundwater due to their * Corresponding author phone: (+45) 4525 1595; fax: (+45) 4593 2850; e-mail:
[email protected]. † DTU Environment. ⊥ DTU Management Engineering. ‡ CIRAIG. 10.1021/es102007s
2010 American Chemical Society
Published on Web 11/05/2010
extensive and widespread use as cleaning agents and metal degreasers (1). In the 2004 U.S. National Priority List chlorinated ethenes were by far the most common group of organic contaminants at sites prioritized for remediation (2). Remediation methods for chlorinated ethenes can be either ex situ methods, where contaminated soil/groundwater is excavated/pumped to the surface and treated on- or off-site, or they can be in situ methods that remove contaminants via mass transfer or mass removal by targeting them at their actual location in the subsurface. Life cycle assessment (LCA) is an ISO standardized and widely used method for environmental assessment of products and services. It has also been applied in the field of soil and groundwater remediation to compare the environmental impacts of remediation alternatives as reported in two recent literature reviews (3, 4). Existing studies have, however, focused mainly on ex situ remediation and contaminants such as metals, PAHs, and hydrocarbons (3). LCA studies addressing chlorinated solvent remediation (5, 6) focus on the comparison of groundwater plume remediation techniques e.g. in situ permeable reactive barriers and conventional pump-and-treat systems, whereas in situ methods for source zone remediation of chlorinated ethenes have not yet been a focus of published LCA studies. Environmental impacts from remediation can be divided into primary and secondary impacts (see e.g. refs 7 and 8). Primary impacts are the local toxic impacts from the residual site contamination, whereas secondary impacts are impacts on the local, regional, and global scale generated by the remediation activities. In addition, a study introduced the term tertiary impacts to describe impacts associated with the postremediation fate of a brownfield (9), but these are not considered here. Existing studies that include primary impacts use generic characterization factors that do not take the site-specific contaminant fate and exposure into consideration and furthermore focus on impacts in surface water or soil (7-9). Although highly relevant for remedial actions, primary impacts in groundwater have not been targeted in existing studies. This may be because of the fact that the groundwater compartment is not included as emission or impact compartment in the applied models for life cycle inventory (LCI) and life cycle impact assessment (LCIA). The aim of this study is to use LCA for a comparison of primary and secondary environmental impacts of three alternative technologies for remediating a TCE-contaminated source zone. Site-generic characterization factors for toxic emissions do not adequately represent the fate of chlorinated ethenes at contaminated sites because they disregard deeper soil layers and groundwater causing the main part of the contamination to end up in the atmosphere (10). Furthermore, they do not include the formation of metabolites during biodegradation of chlorinated ethenes, of which particularly vinyl chloride is problematic due to its toxic and carcinogenic effects (10). Therefore a site-dependent assessment is used here, taking into account the site-specificity of the fate of the contaminant including formation of metabolites. In addition, site-dependent exposure parameters are used for calculation of exposure concentrations and the exposed number of people. In this case the primary impacts are related to human exposure via ingestion of groundwater due to on-site leaching of contaminants and are evaluated using a site-specific leaching model. Primary ecotoxic impacts in groundwater are neglected, and no discharge to surface water is included because the groundwater plume is assumed to be fully abstracted by the downstream drinking water well. The compared remediation technologies are as follows: (i) VOL. 44, NO. 23, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Conceptual model of the contaminated site and placement of the microbial degradation zones used in the numerical model of natural (no action) and enhanced reductive dechlorination (ERD). The sand layer embedded in the clay till is neglected in the numerical modeling of the site because of its limited extent. Model results of mass discharge of trichloroethene (TCE), cis-dichloroethene (cis-DCE), and vinyl chloride (VC) to the regional aquifer is shown together with the accumulated emissions of each chlorinated ethene to the aquifer. enhanced reductive dechlorination (ERD) which is an in situ bioremediation method employing bioaugmentation and biostimulation; (ii) in situ thermal desorption (ISTD) which is a physical remediation method deploying thermal conductive heating of the contaminated soil; and (iii) excavation of the contaminated soil followed by off-site treatment and final disposal in a landfill. Based on the LCA results the environmental impacts of the three remediation alternatives are compared. Environmental hotspots of each technology are identified and improvement options suggested.
Materials and Methods Case study. A TCE-contaminated site in central Copenhagen is situated within the catchment of a water supply that extracts drinking water for approximately 44,000 people located 2000 m down gradient from the site. The site was used as a metal shop, boilermaker shop, woodcutting shop, etc. until 1986. A conceptual sketch of the contaminated site is shown in Figure 1. The contamination is located mainly in a fractured clay till and partly in a sand layer embedded in the clay till. The contaminated source zone extends vertically from 3 to 8 m below ground surface and has a horizontal cross-sectional area of 140 m2. The estimated mass of TCE in the approximately 700 m3 of contaminated soil is 40 kg. The clay till overlies a regional limestone aquifer used for drinking water extraction. The goal of the LCA is to compare three options for remediating the contaminated source zone. In addition to the three remediation scenarios the assessment includes a no action scenario, in which no active remediation takes 9164
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places and monitoring of the naturally occurring degradation is the only activity included. The no action scenario is included as a reference scenario only and cannot be seen as a viable management scenario comparable to the three remediation scenarios. The functional unit provided by all options compared in this study is the treatment of the 700 m3 of contaminated soil with a 98% removal of the contaminant mass within this volume. This remediation target was chosen as it reduces the mass discharge to the groundwater sufficiently to ensure that the Danish groundwater quality criterion (1 µg/L) for chlorinated ethenes is not exceeded. The timeframe for the LCI is unrestricted in order to capture both short- and longterm impacts. The scope of the LCA is to include all important activities on site and off site (see overview in Table 1 and a more complete listing in the Supporting Information (SI) Table S1) of each scenario, i.e. covering raw materials acquisition, materials production, use stages, and end-of-life processes. Primary data regarding energy and material consumption for the different remedial activities is collected from the actual consultants, contractors, or producers likely to undertake the work. Inventory data for the background system (production of steel, electricity, plastic, asphalt, lorry transport, etc.) were based on average technology data from the ecoinvent life cycle unit process database version 2.0 (11). These generic data are supported by a collection of specific data for remediation-related processes for which no data are found in ecoinvent or other general LCA databases (production of bioculture and activated carbon, laboratory analyses of soil and groundwater). Electricity used in the operation
TABLE 1. Overview of Main Activities Included in Each Scenario
no action (NoA)
enhanced reductive dechlorination (ERD)
monitoring of groundwater installation of additional (person transportation, water monitoring wells sampling and analysis)
in situ thermal desorption (ISTD)
excavation with off-site treatment and disposal (EXC)
installation of heater and installation of sheet pile wall extraction wells, heaters, collection pipes and top cover
pumping and injection of molasses and bioculture
heating of soil
excavation and backfilling
monitoring of soil and groundwater
ventilation of soil and pumping of water
soil transportation
transportation of materials, activated carbon treatment equipment and people
off-site soil treatment
monitoring of soil
soil disposal
transportation of materials, equipment and people
monitoring of soil removal of sheet pile wall and asphalting transportation of materials, equipment and people
timeframe: 1200 years
timeframe: 38 years
phases is assumed produced as the average Danish electricity grid mix including imports (11). All steel (unalloyed steel and stainless steel) is modeled as secondary steel produced from scrap, and a recycling rate of 90% is assumed for all steel products. Based on experience from contractors, heaters, and temperature sensors for ISTD are assumed to be reused directly on four projects each, and the sheet pile wall for the excavation scenario is reused directly on two projects before the steel is recycled. For all scenarios, including the longterm ones (ERD and NoA) current technology is assumed for the entire period. Additional assumptions and the applied ecoinvent processes for the background system are summarized in Tables S2-S4 in the SI. A number of sensitivity scenarios have been analyzed in order to test the importance of main input parameters to each of the remediation methods. The substrate amount (molasses), the electricity source, and the soil transportation distance are the parameters tested in the sensitivity analysis of the ERD, ISTD, and excavation scenario, respectively. The life cycle impact assessment method applied is EDIP2003 (12) for the categories global warming, ozone formation, acidification, and eutrophication. Respiratory impacts associated with particulate matter (PM2.5-10 µm, PM
