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Comparison of Delivery Methods for Enhanced In Situ Remediation in Clay Till

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Abstract

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.

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