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Modern contaminant hydrology has brought us to the realization that decisions regarding management of subsurface contamination at Department of Defense (DoD) facilities need to be based on an understanding of all contaminant phases (i.e., aqueous, non-aqueous-liquid, sorbed, and vapor) and the biogeochemical conditions in which the contaminants are present. The practical approach for collection and analysis of frozen-cores presented here represents an important new tool for improving that understanding. Uniquely, core samples frozen in situ before recovery can preserve pore fluids, volatile compounds, dissolved gases, redox conditions, mineralogy, microbial ecology, and pore structure. Furthermore, in situ freezing improves quality of recovered core by preventing materials from dropping out of sample liners during recovery to ground surface. Collectively, steps followed for collecting frozen cores are referred to here as cryogenic core collection (C3).
Because freezing provides effective field preservation, frozen cores can be processed efficiently under controlled laboratory conditions to resolve a broad spectrum of chemical, physical and biological characteristics. Critically, processing core in the laboratory simplifies field work and improves the resources (e.g., anaerobic chambers) that can be utilized in preparation of samples for analysis. Furthermore, laboratory processing frozen cores allows “production line” processing and analysis of large quantities of samples, referred to here as high-throughput core analysis.
Building on these ideas, the overarching objectives of this research were to:
Cryogenic Core Collection - C3 has been considered by others, including most recently Johnson et al. (2012) as part of SERDP- ER-1559. One of the key limitations of previous cryogenic methods is their ability to collect large amounts of core in often-challenging media in support of site characterization. Necessary elements for large-scale site characterization using cryogenic core collection include fast freezing, operating procedures and equipment that are compatible with standard drilling techniques, the ability to penetrate hard formations and an efficient process for removal of frozen cores from core barrels. At the same time, cryogenic core collection has several potential advantages over conventional core collection in that it minimizes sample loss during the recovery process, facilitates high-throughput sampling, and can potentially help control problematic flowing sands.
Herein, standard hollow stem auger (HSA) drilling equipment (e.g., CME-55/75) and sampling systems (CME Continuous Sample Tube Systems) have been adapted to collect cryogenic core. Specifically, either a copper coil or dual-wall cooling cylinder are placed in a CME Continuous
Sample Tube System. Liquid nitrogen (LN) is circulated through the coil or dual-wall cooling cylinder to affect in situ freezing of core in polyvinyl chloride (PVC) sample liners.
Critical element of the freezing system include: a) using insulation in the sample barrel to maximize delivery of coolant to the core versus the drill pipe/formation, b) increasing back pressure on the LN line once freezing temperatures reach the LN discharge at grade to hold LN in the cooling coil/barrel, c) limiting sample drives to 2.5 feet to maximize recovery, d) using insulation on all LN lines to focus cooling capacities onto the sample, and e) using sample coring, versus driving, techniques to minimize sample disturbance.
The described methods were iteratively developed through seven field efforts, including work at the Drilling Engineers Inc. facility in Fort Collins, CO; Colorado State University’s (CSU’s) Agricultural Research Development& Education Center (ARDEC) facility near Fort Collins, CO; F.E. Warren (FEW) AFB in Cheyenne, WY; and a former refinery near Casper, WY. Work at the refinery was funded by Chevron.
High-Throughput Core Analysis (HTCA). High-throughput core analysis focused on the cores collected from FEW AFB. Cores were kept frozen from the time of collection until processing in the lab at CSU. Sub-samples were obtained by chopping the frozen cores into 1-inch thick disks, referred to as “hockey pucks”. One sub-sample was recovered for every four inches of frozen core. Each 1-inch-thick frozen sub-sample was further divided into quarters for subsequent extractions and/or analyses. Sub-samples were analyzed for target contaminants, dissolved gases, inorganic ions (chloride, nitrate, and sulfate), pH, oxidation/reduction potential (ORP), and water content. Select subsamples were characterized for biological community, hydraulic conductivity and particle size distribution. In addition, funds provided by GE were used to explore the use of medical Computer Tomography (CT) and Magnetic Resonance Imaging (MRI) scanning equipment to characterize non-aqueous phase liquids (NAPLs) in the intact frozen core.
Cryogenic core collection. Over the course of the field demonstrations, the efficiency of cryogenic core collection greatly improved. The time required to freeze a section of core was reduced from ~50 minutes to ~5 minutes. At FEW AFB, 52 feet of frozen-core were collected over two days of field effort. At the former refinery site, 36 feet of frozen-core were collected in one day. With the exception of caliche beds at FEW and large cobbles at the refinery, frozen cores with recoveries near 100 % were obtained.
High-throughput core analysis. High-throughput core analysis methods provided high- resolution definition of aqueous-, sorbed-, and gas-phase contaminants with detection limits as low as (10 μg/kg). In addition, frozen core was used to resolve geology, microbial ecology, mineralogy, and permeability. An intriguing result is that methane in the core was detected as well. In locations where both TCE and methane were present, inverse correlations were observed. With respect to use of medical scanning equipment funded by GE, CT scans were not useful for NAPL detection. However, MRI of frozen core was able to identify NAPLs when present in the cores. A key result was the realization that freezing water suppresses the MRI signal from the protons in the frozen water, while the signal from the NAPL (which remained a liquid at -20C) was not attenuated.
The combination of cryogenic core collection and high-throughput sampling yielded high quality samples suitable for a wide range of chemical, physical and biological analyses. The protocols for sample collection and processing are sufficiently robust that they can now be used routinely at field sites.