At a low permeability clay till site contaminated with chlorinated ethenes (Gl. Kongevej, Denmark), enhanced reductive dechlorination (ERD) was applied by direct push injection of molasses and dechlorinating bacteria. The performance was investigated by long‐term groundwater monitoring, and after 4 years of remediation, the development of degradation in the clay till matrix was investigated by high‐resolution subsampling of intact cores. The formation of degradation products, the presence of specific degraders Dehalococcoides spp. with the vinyl chloride (VC) reductase gene vcrA, and the isotope fractionation of trichloroethene, cis‐dichloroethene (cis‐DCE), and VC showed that degradation of chlorinated ethenes occurred in the clay till matrix as well as in sand lenses, sand stringers, and fractures. Bioactive sections of up to 1.8 m had developed in the clay till matrix, but sections, where degradation was restricted to narrow zones around sand lenses and stringers, were also observed. After 4 years of remediation, an average mass reduction of 24% was estimated. Comparison of the results with model simulation scenarios indicate that a mass reduction of 85% can be obtained within approximately 50 years without further increase in the narrow reaction zones if no donor limitations occur at the site. Long‐term monitoring of the concentration of chlorinated ethenes in the underlying chalk aquifer revealed that the aquifer was affected by the more mobile degradation products cis‐DCE and VC generated during the remediation by ERD.

The degradation of chlorinated ethenes and ethanes in clay till was investigated at a contaminated site (Vadsby, Denmark) by high resolution sampling of intact cores combined with groundwater sampling. Over decades of contamination, bioactive zones with degradation of trichloroethene (TCE) and 1,1,1-trichloroethane (1,1,1-TCA) to 1,2-cis-dichloroethene (cis-DCE) and 1,1-dichloroethane, respectively, had developed in most of the clay till matrix. Dehalobacter dominated over Dehalococcoides (Dhc) in the clay till matrix corresponding with stagnation of sequential dechlorination at cis-DCE. Sporadically distributed bioactive zones with partial degradation to ethene were identified in the clay till matrix (thickness from 0.10 to 0.22m). In one sub-section profile the presence of Dhc with the vcrA gene supported the occurrence of degradation of cis-DCE and VC, and in another enriched δ13C for TCE, cis-DCE and VC documented degradation. Highly enriched δ13C for 1,1,1-TCA (25‰) and cis-DCE (−4‰) suggested the occurrence of abiotic degradation in a third sub-section profile. Due to fine scale heterogeneity the identification of active degradation zones in the clay till matrix depended on high resolution subsampling of the clay till cores. The study demonstrates that an integrated approach combining chemical analysis, molecular microbial tools and compound specific isotope analysis (CSIA) was required in order to document biotic and abiotic degradations in the clay till system.

Enhanced Reductive Dechlorination (ERD) has been successfully used in high permeability media, such as sand aquifers, and is considered to be a promising technology for low permeability settings. One of the main challenges in low-permeability settings is to obtain good contact between the injected bacteria and electron donor and the contaminants trapped in the matrix. The objective of this study is to assess whether the degradation processes take place only in higher permeability zones in the clay till such as sand lenses or stringers, or if degradation processes also develop within the clay till matrix. Isotope fractionation data, biogeochemical data and numerical modelling is used to identify the zones in the clay till where natural dechlorination has occurred. The results can be further combined with other data (molecular biological tools, pore size constrains, etc...) in order to identify the controlling parameters that allow degradation in the matrix. Reactive transport models coupled with isotope fractionation are widely used to document natural attenua-tion along flowpathes in high permeability porous media. Isotope fractionation due to diffusion is often negligible in advection dominating systems, but diffusion is the dominant transport process in the clay matrix, and the heavier isotopes are expected to diffuse slower than the lighter ones. But it can be difficult to distinguish whether isotope fractionation is caused by diffusion or degradation. In the present work, modelling was used to investigate several conceptual models regarding dechlorination locations (in high permeability zones, in the whole

Methods for effective delivery of remediation amendments for in situ remediation of contaminated clay till sites are sought. The capabilities of direct-push delivery are promising but not yet scientifically documented. Therefore, a field study of direct-push delivery was carried out at an uncontaminated, naturally fractured, basal clay till site (K similar to 10(-7)-10(-10) m/s) in 2008-2009. A mixture of tracers (brilliant blue, fluorescein, and Rhodamine WT), the characteristics of which are comparable to several current remediation amendments, was delivered in aqueous solution at pressures of similar to 5-10 bar at several locations and depth intervals [2.5-9.5 m below surface (b.s.)], representing both the vadose and saturated zones. Extensive coring to 12 m b.s. and excavation to 5 m b.s. were carried out to identify the lateral and vertical extent of tracer distribution. A tracer distribution radius of minimum 1 m was achieved at all depths. Close vertical spacing of delivery points (10-25 cm) provided good vertical distribution without significant merging of individual delivery propagation paths. The results are promising with regard to achieving adequate distribution of remediation amendments in clay till. DOI: 10.1061/(ASCE)EE.1943-7870.0000451. (C) 2012 American Society of Civil Engineers.

Three methods for enhanced delivery of in situ remediation amendments in low‐permeability deposits have been tested at a site in Denmark: pneumatic fracturing, direct‐push delivery, and hydraulic fracturing. The testing was carried out at an uncontaminated part of a farm site, previously used for storage of chlorinated solvents, underlain by basal clay till with hydraulic conductivity ranging from 7.1× 10 –11 to 3.5 × 10 –7 m/s at testing depths 2.5 to 9.5 m b.s. Fluorescent tracers fluorescein and rhodamine WT were delivered. Tests of all three delivery methods have not been carried out at a single site before, and thus, this study provides unique data for comparison of enhanced delivery methods in both the vadose and saturated zone. Results show that pneumatic fracturing with nitrogen gas and propagation pressures of 1 to 9 bar had a distribution radius of less than 2 m, and produced dense networks of tracer‐filled natural fractures above the redox boundary (0 to 3 m b.s.) and widely spaced, discrete, induced, tracer‐filled subhorizontal fractures at depth (>3 m b.s.). Direct‐push delivery at pressures of 8 to 30 bar had a distribution radius of approximately 1 m, distributed tracer primarily in natural fractures above the redox boundary and in discrete, closely spaced (but not merging) induced fractures below the redox boundary. Hydraulic fracturing with a sand‐guar mixture at pressures of 0 to 6 bar produced an elliptical, asymmetrical, bowl‐shaped fracture with a physical radius of approximately 3.5 m at 3 m b.s. The geometry of hydraulic fractures attempted emplaced at 6.5 and 9.5 m b.s. is uncertain, but clearly not horizontal as desired. The direct‐push delivery method is robust and efficient for enhanced delivery at the clay till site in question, which based on thorough geological characterization is deemed a geologically representative basal clay till site.

Contamination, constituting a threat to groundwater resources, often occurs in low permeability sediments such as clayey tills overlying aquifers. Clayey tills are mostly fractured, and contamination migrating through the fractures has spread to the low permeability matrix by diffusion. This results in a long term source of contamination due to back-diffusion. The challenge in remediation of the low permeability matrix by in-situ technologies is to bring the reactant (e.g. donor) and the contaminant and for biological methods the bacteria in contact. Remediation of the matrix is diffusion limited, as contaminants has to diffuse through the matrix to natural or induced fractures and/or sand stringers or lenses, where reactant and e.g. specific degraders can be supplied, or reactants has to diffuse into the matrix. Enhancement techniques, such as environmental fracturing offers assistance to remediation efforts at contaminated, low-permeability sites via creation of active fracture networks, and hence, reduction of mass transport limitations set by diffusion in low-permeability matrices. Pilot studies of pneumatic fracturing, hydraulic fracturing and direct push injections, focusing on direct documentation of fracture propagation patterns and spacing and tracer distribution, were performed at a typical basal clay till site. The studies applied a novel package of documentation methods, including injection of 3-5 tracers with different characteristics (bromide, uvitex, fluorescein, rhodamine WT, and brilliant blue), subsequent tracer-filled fracture documentation via direct and indirect methods, and geological characterization of the fractured site. The direct documentation methods consisted of Geoprobe coring, augering, and excavation. Conceptual models and mass balances Abstract chlorinated ethene contamination in clayey till, including the sites Rugårdsvej and Sortebrovej in/near Odense in Denmark. Model calculations reveal that the treatment period for significant reduction of the cis-dichloroethylene (c-DCE) level in the matrix at Rugårdsvej for a distance of 0.3 m to 1 m between fractures or sand stringers will likely be on the order of 10 to 100 years if biodegradation is restricted to the fractures or sand stringers. At by pulsed addition to donor was in test-areas injection The biodegradation of c-DCE and vinylchloride in from the induced and sand stringers. degradation of c-DCE VC ethene in induced 5 ethene was to ethane, only degradation in the fracture after 18 months. In the sand stringer test site reduction of sulphate in the sand stringers observed as only clear evidence of an effect of the biostimulation. After 5 and 18 months the effect of the stimulated degradation in the clayey till matrix was documented through detailed profiles of sediment samples from intact cores of the clayey till. Profiles from the induced fracture test-area revealed a reaction zone in the clay in contact with the induced fracture, where biodegradation of c-DCE and VC in the low permeability matrix was documented. The degradation of the chlorinated ethenes occurred in the presence of relatively high concentrations of sulphate in the matrix. full scale in clayey injection a release pulsed