1 Department of Environmental Engineering, Technical University of Denmark2 Quantitative Sustainability Assessment, Department of Management Engineering, Technical University of Denmark3 Department of Management Engineering, Technical University of Denmark
Many contaminated sites worldwide constitute a hazard to their surroundings and must undergo remediation. Chloroethenes such as trichloroethene (TCE) and perchloroethene (PCE) are among the most frequently encountered contaminants in the subsurface due to their widespread use as solvents in dry-cleaning and industries. Chloroethenes are dense non-aqueous phase liquids (DNAPLs) with high density and viscosity and low solubility in water. These characteristics allow a spill to migrate deep into the subsurface, where it can act as long-term source of dissolved-phase groundwater contamination. Due to the longevity of chloroethene source zones, conventional pump-andtreat technologies are inefficient and may require operation for centuries. Excavation of the contaminated soil and subsequent treatment and disposal of the soil is another ex situ option, however most suitable for contaminant source zones located close to the surface. As an alternative to these ex situ remediation methods, in situ remediation methods for chloroethenes have been developed to target the contaminants in their subsurface location. These technologies cover chemical, biological and physical methods of which the latter can be enhanced by heating the subsurface. This PhD project investigated the applicability of life cycle assessment as a tool for environmental assessment of remediation of contaminated sites. This was done focusing specifically on chloroethene-contaminated sites and remediation technologies relevant for this type of contaminant. LCA is an environmental assessment tool that compiles a very wide array of environmental exchanges (emissions to air, water, and soil, and resource consumption) associated with the life cycle of a product or service .and translates them to impacts (global warming, acidification, human toxicity, ecotoxicity, etc.). A literature survey showed that although a number of studies of LCA and remediation had been published during the recent 11-year period only two of them included assessment of chloroethene remediation. However, these studies focused on ex situ remediation or groundwater plume remediation using a reactive barrier. Thus, the majority of innovative in situ remediation methods for chloroethene source zone remediation were not covered in the literature. Within the project, life cycle assessments of remediation alternatives for source zone remediation of two chloroethene-contaminated sites were performed. These studies covered the assessment of in situ techniques soil vapor extraction (SVE), in situ thermal desorption (ISTD) and enhanced reductive dechlorination (ERD) and the ex situ technique of excavation followed by off-site treatment. The results from the first case study, which compared SVE, ISTD and excavation with off-site treatment, showed that SVE had the lowest environmental impacts when a timeframe of 30 years was used, but became less preferable than ISTD and excavation if a more realistic timeframe of 100 years was used. In the other case study, ERD, ISTD and excavation with off-site treatment were compared. The study showed that ERD is a promising low-impact technology for this type of site as it had significantly lower impacts than ISTD and excavation in all impact categories and performed only slightly worse than the no action scenario, where only monitoring was carried out. ISTD had the highest potential impact on global warming due to the large electricity use, but for the remaining impact categories excavation had comparable or larger impact scores than ISTD. The above mentioned results cannot be seen as to apply universally. LCAs of contaminated site remediation are inherently site-specific as many inputs to the LCA depend on the location of the site, e.g. transportation distances for excavated soil and clean refill and the countryspecific electricity production. The depth, water content and contaminant levels of the remediated soil volume are other sources of variation between sites. In addition, system and time boundaries and the type of LCA conducted (attributional or consequential) has an impact on the final results. Life cycle assessments aim to compare environmental burdens associated with different ways of obtaining the same function or service denoted the functional unit. Most studies define the functional unit as the volume of contaminated soil or groundwater to be treated and combine it with a remedial target for the contaminant concentration. However, although two remediation methods reach the same remedial target with time, their timeframes can be substantially different. This quality difference can be included in the LCA by assessing the so-called primary impacts. Primary impacts are local toxic impacts related to the contamination at the site as opposed to the secondary impacts stemming from the remedial actions. Primary impacts have typically been assessed using site-generic characterization models representing a continental scale and excluding the groundwater compartment. Soil contaminants have therefore generally been assigned as emissions to surface soil or surface water compartments. However, such site-generic assessments poorly reflect the fate of chloroethenes at contaminated sites as they exclude the groundwater compartment and assume that the main part escapes to the atmosphere. In the two case studies, the primary impacts were assessed using sitedependent procedures, where the contaminant emissions to groundwater over time were estimated based on site-specific contaminant fate and transport models. This made it possible to account for important processes, such as the formation of chlorinated degradation products and to include the site-specific exposure of humans via ingestion of groundwater used for drinking water. The inclusion of primary impacts in the environmental assessment of remediation alternatives gave a more complete basis for comparison of technologies with substantially different timeframes and efficiencies. LCA was concluded to be a useful tool for environmental assessment of remediation of contaminated sites although unresolved issues remain. Among the obstacles identified for the use of LCA as decision support for remedy selection is the fact that conducting an LCA is very data and time consuming. Furthermore, the multi-indicator result may be difficult to interpret especially given the higher uncertainty of the toxicity-related impact categories. Thus, improvements of characterization methods for toxic impacts as well as expansion of remediation-relevant LCI databases were among issues identified for future attention in order to enhance the applicability of LCA. Moreover, further development of methods for monetization of life cycle impacts may enhance the use of LCA within this field as it makes it easier to integrate the result of the environmental assessment with other decision criteria such as remediation cost.