Enhanced reductive dechlorination in clay till contaminated with chlorinated solvents

Chlorinated solvents are among the most frequently found contaminants in groundwater. In fractured media, chlorinated ethenes and ethanes are transported downwards through preferential pathways with subsequent diffusion into the sediment matrix. Due to slow back diffusion it can serve as a long term secondary source that can leach to the underlying aquifer. As some of the chlorinated solvents and their degradation products are toxic and carcinogenic, remediation technologies applicable in low permeability settings are needed. Enhanced reductive dechlorination (ERD) has been proven efficient in high permeability aquifers and has also been applied at a number of low permeability clay till sites. This thesis presents the results of an investigation of chlorinated ethenes (and ethanes) degradation in clay till with the objective of obtaining knowledge of degradation processes in clay till and to evaluate ERD as remediation technology. The development of degradation in clay till was investigated at two sites: one where natural attenuation processes (transport, sorption, diffusion and degradation) had been on-going for four decades (Vadsbyvej) and another which had been undergoing ERD for four years (direct push delivery, Gl. Kongevej). Degradation of chlorinated ethenes (and ethanes) in the clay till matrix and in embedded high permeability features was investigated by high resolution sampling of intact cores combined with groundwater sampling. An integrated approach using chemical analysis, molecular microbial tools and compound specific isotope analysis (CSIA) was used. The results from the full scale investigation were compiled with another full scale ERD remediation in clay till (gravitational injection, Sortebrovej, Denmark). The study of on-going natural degradation of chlorinated ethenes and ethanes in clay till (Vadsbyvej) revealed a very complex system where diffusion, biotic and abiotic degradation processes occurred simultaneously. High resolution sub sampling with combined use of chemical analysis, molecular microbial tools and CSIA was necessary to identify both biotic and abiotic degradation zones. Reductive dechlorination of TCE to cis-DCE had developed in entire clay till sections of up to 3 m, whereas sub-sections with partial degradation to VC and ethene were more sporadically distributed due to scarcity of Dehalococcoides with the functional gene vcrA. The study shows the potential for development of degradation throughout the entire clay matrix. When ERD is applied in a low permeability settings one of the major constraints is to obtain the necessary contact between electron donor, bacteria and contaminants to achieve reasonable remediation timeframes. Two injection methods (hydraulic fracturing with gravitational injection and direct push delivery) were therefore tested in clay till by injection of amendment-comparable tracers to investigate the possibility to overcome diffusion limitations in the low permeability matrix. The study of hydraulic fracturing demonstrated that it was only possible to create a horizontal fracture in 3 m b.s., whereas it was not successful between 6-9.5 m b.s. at the test site. Closely spaced (10 and 25 cm) horizontal delivery of amendment-comparable tracers was achieved by direct push delivery using a GeoProbe® from 2.5 to 9.5 m b.s. Contrary to these results, fractures were not in all cases observed for every 25 cm (the injection interval) after injection of electron donor and bacteria by direct push delivery. The primary propagation path for organic substrate and bacteria was natural sand stringers and sand lenses. However, by direct push delivery organic substrate was also spread in natural or induced fractures. After four years of ERD in clay till, reductive dechlorination of chlorinated ethenes had developed very heterogeneously in the clay till matrix after both gravitational injection (Sortebrovej) and direct push delivery (Gl. Kongevej). In some areas degradation was restricted to narrow zones around soft clay till, sand stringers and sand lenses, and in other sections degradation had developed through entire sections of the clay till matrix (up to 1.8 m at Gl. Kongevej). Only minor or no degradation developed in the untreated intervals. Reductive dechlorination in the clay till matrix at Gl. Kongevej was documented by enriched isotope fractionations of TCE and cis-DCE and the presence of Dehalococcoides with the vcrA gene in the clay till matrix. The degradation of chlorinated ethenes in the clay till matrix was not as advanced as in the high permeability features indicating that sediment analysis is needed to evaluate the performance of ERD in clay till. The shortest remediation timeframes were found in areas where degradation had developed more extensively in the clay till matrix (approximately 20 years), whereas longer remediation timeframes were found when degradation was restricted to narrow reaction zones around sand stringers and sand lenses (up to 170 years). This illustrates the necessity of developing degradation in the entire clay till matrix to obtain reasonable timeframes of the remediation.