A DIRECT-PUSH FREEZING DRIVE SHOE FOR COLLECTING SEDIMENT CORE SAMPLES WITH INTACT PORE FLUID, MICROBIAL, AND SEDIMENT DISTRIBUTIONS

Abiotic and biological reactions in shallow groundwater and bottom sediments are central to understanding groundwater contaminant attenuation and biogeochemical cycles. The laminar flow regime in unconsolidated surficial aquifers creates narrow reaction zones. Studying these reaction zones requires fine-scale sampling of water together with adjacent sediment in a manner that preserves in situ redox conditions. Collecting representative samples of these narrow zones with traditional subsurface sampling equipment is challenging. For example, use of a basket type core catcher for saturated, non-cohesive sediments results in loss of fluid and sediments during retrieval.

A sample-freezing drive shoe designed for a wire line piston core sampler allowed collection of cores with intact sediment, microbial, and pore fluid distributions and has been the basis for studies documenting centimeter-scale variations in aquifer microbial populations. However, this freezing drive shoe design is not compatible with modern-day direct push sampling rigs.

A re-designed sample-freezing drive shoe compatible with a direct-push dual-tube coring system was developed and field-tested. The freezing drive shoe retained sediments and fluid distributions in saturated sediment core samples by freezing a 10-centimeter plug below the core sample with liquid CO­₂. Core samples collected across the smear zone at a crude oil spill site near Bemidji, Minnesota, were successfully extracted without loss of fluid or sediment. Multiple core sections in the aquifer were retrieved from a single hole. This new design makes an effective sampling technology available on modern-day direct push sampling equipment to inform myriad questions about subsurface biogeochemistry processes.