James M. Phelan

Development of a mechanistic model for the movement of chemical signatures from buried land mines/UXO

The detection and removal of buried landmines and unexploded ordnance (UXO) is one of the most important problems facing the world today. Numerous detection strategies are being developed, including IR, electrical conductivity, ground- penetrating radar, and chemical sensor. Chemical sensor rely on the detection of explosive chemical molecules, which are transported from buried UXO/landmines by advection and diffusion in the soil. As part of this effort, numerical models are being developed to predict explosive chemical signature transport in soils. Modifications have been made to TOUGH2, a general-purpose porous media flow simulator, for application to the chemical sensing problem resulting in the T2TNT code. Understanding the fate and transport of explosive signature compounds in the solid will affect the design, performance, timing and operation of chemical sensing campaigns by indicating preferred sensing strategies.

Chemical sensing thresholds for mine detection dogs

Mine detection dogs have been found to be an effective method to locate buried landmines. The capabilities of the canine olfaction method are from a complex combination of training and inherent capacity of the dog for odor detection. The purpose of this effort was to explore the detection thresholds of a limited group of dogs that were trained specifically for landmine detection. Soils were contaminated with TNT and 2,4-DNT to develop chemical vapor standards to present to the dogs. Soils contained ultra trace levels of TNT and DNT, which produce extremely low vapor levels. Three groups of dogs were presented the headspace vapors from the contaminated soils in work environments for each dog group. One positive sample was placed among several that contained clean soils and, the location and vapor source (strength, type) was frequently changed. The detection thresholds for the dogs were determined from measured and extrapolated dilution of soil chemical residues and, estimated soil vapor values using phase partitioning relationships. The results showed significant variances in dog sensing thresholds, where some dogs could sense the lowest levels and others had trouble with even the highest source. The remarkable ultra-trace levels detectable by the dogs are consistent with the ultra-trace chemical residues derived from buried landmines; however, poor performance may go unnoticed without periodic challenge tests at levels consistent with performance requirements.

Effect of diurnal and seasonal weather variations on the chemical signatures from buried land mines/UXO

The chemical signature form buried landmines/UXO is affected by a number of environmental fate and transport processes in the soil such as vapor-solid and liquid-solid sorption, diffusion, biodegradation, and water movement. For shallow burial depths, land surface processes, such as wind, solar and long-wave radiation, and precipitation play an important role. The impact of these land surface processes has been evaluated for a landmine/UXO buried 5 cm below the surface using actual weather data for an entire year using the T2TNT computer code. The gas-phase concentration of the chemical signatures, which is used by most chemical sensors currently being developed, shows appreciable diurnal variation and minimum seasonal changes due to the change in the weather. The most dramatic variation in the gas-phase concentration occurs immediately after a rainfall following a long dry period. This information will impact the use of chemical sensors by indicating the best times of the day and best times of the year to sense these signatures.

Laboratory data and model comparisons of the transport of chemical signatures from buried land mines/UXO

Sensing the chemical signature emitted from the main charge explosives from buried landmines and unexploded ordnance (UXO) is being considered for field applications with advanced sensors of increased sensitivity and specificity. The chemical signature, however, may undergo many interactions with the soil system, altering the signal strength at the ground surface by many orders of magnitude. The chemidynamic processes are fairly well understood from many years of agricultural and industrial pollution soil physics research. Due to the unique aspects of the surface soil environment, computational simulation is being used to examen the breadth of conditions that impact chemical signature transport, from the buried location to a ground surface release. To provide confidence in the information provided by simulation modeling, laboratory experiments have been conducted to provide validation of the model under well-constrained laboratory testing conditions. A soil column was constructed with soil moisture monitoring ports, a bottom porous plate to regulate the soil moisture content, and a top plenum to collect the surface flux of explosive chemicals. The humidity of the air flowing through the plenum was set at about 50 percent RH to generate an upward flux of soil moisture. A regulated flux of aqueous phase 2,4-DNT was injected into the soil at about ten percent of the upward water flux. Chemical flux was measured by sampling with solid phase microextraction devices and analysis by gas chromatography/electron capture detection. Data was compared to model results from the T2TNT code, specifically developed to evaluate the buried landmine chemical transport issues. Data and model results compare exceptionally well providing additional confidence in the simulation tool.

Effect of soil layering on NAPL removal behavior in soil-heated vapor extraction

Abstract Soil heating has been proposed as a method to enhance the vapor extraction of NAPLs from contaminated soils. Three-dimensional fluid flow and heat transfer simulations have been performed for soil-heated vapor extraction to determine the transient system performance for a hypothetical configuration. Soil layering has been considered in evaluation of the initial non-aqueous phase liquid (NAPL) distribution and in evaporation and transport to the vapor extraction location. Results from this layered model are compared with results for a homogeneous system with an initially uniform NAPL, indicating the influence of layering, the initial NAPL distribution, the type of NAPL, and the possibility of enhanced vapor diffusion. Not only is the NAPL removal time reduced significantly with the addition of heat, but the uncertainty in the removal time owing to a number of difficult to characterize in situ factors, such as layering and the initial NAPL distribution, is much less than for standard soil vapor extraction without heating, owing to the rise in temperature and increase in NAPL vapor pressure with time.

Phase Partitioning of TNT and DNT in Soils

Detecting the presence of a burred landmine or unexploded ordnance using explosive chemical vapors has been considered possible with advances in sensitivity and selectivity of emerging chemical sensing technologies. Sampling and analysis of explosive vapors emanating from soils is a significant challenge due to the low vapor concentrations produced through this process; Understanding the environmental impacts to this chemical signature is also important, as environmental factors have a dramatic effect on the transfer of the chemical mass between soil solid, liquid and vapor phases. This work was completed to assess the phase partitioning phenomena of two explosive chemical materials (2,4,6-m and 2,4-DNT) typically found in landmines and ordnance. Laboratory measurements of water solubility, soil-water partitioning, and soil-vapor partitioning were completed in the absence of literature values. An integrated soil system partitioning analysis was completed using soil physics phase partitioning theory for use in evaluation of the impact of environmental factors. Using estimates of soil residues from samples adjacent to burred landmines and ordnance, performance requirements for vapor sensing buried landmines and unexploded ordnance have been estimated.

Simulation of the environmental fate and transport of chemical signatures from buried land mines

The fate and transport of chemical signature molecules that emanate from buried landmines is strongly influenced by physical chemical properties and by environmental conditions of the specific chemical compounds. Published data have been evaluated as the input parameters that are used in the simulation of the fate and transport processes. A one- dimensional model developed for screening agricultural pesticides was modified and used to simulate the appearance of a surface flux above a buried landmine and estimate the subsurface total concentration. The physical chemical properties of TNT cause a majority of the mass released to the soil system to be bound to the solid phase soil particles. The majority of the transport occurs in the liquid phase with diffusion and evaporation driven advection of soil water as the primary mechanisms for the flux to the ground surface. The simulations provided herein should only be used for initial conceptual designs of chemical pre-concentration subsystems or complete detection systems. The physical processes modeled required necessary simplifying assumptions to allow for analytical solutions. Emerging numerical simulation tools will soon be available that should provide more realistic estimates that can be used to predict the success of landmine chemical detection surveys based on knowledge of the chemical and soil properties, and environmental conditions where the mines are buried. Additional measurements of the chemical properties in soils are also needed before a fully predictive approach can be confidently applied.

Data-model comparison of field landmine soil chemical signatures at Ft. Leonard Wood.

Chemical signatures from buried landmines vary widely due to landmine and environmental conditions. The simulation model T2TNT was developed to evaluate the nature of chemical transport in the soil surrounding a buried landmine. This model uses landmine chemical emission, soil physics, soil-chemical interaction, and surface weather data to estimate surface and subsurface concentrations to help understand the phenomenology of landmine trace chemical detection. While T2TNT compares favorably to controlled laboratory experiments for a buried source of DNT, field data-model comparisons are needed to further increase confidence in T2TNT predictions. The only multi-season landmine soil residue data are from a long-term monitoring project at the DARPA-developed Ft. Leonard Wood Site in Missouri, USA. About 1000 soil residue samples had been taken over six sampling events spanning 21 months since landmine burial. This effort compares the soil residue data from two landmine types to T2TNT model predictions. A one-dimensional model was used to represent the situation and used actual weather data from the site during this period, landmine flux data specific for the mines buried, and temperature and moisture-content dependent degradation rates. Spatial and temporal predictions of chemical concentrations in the soil compare favorably with the soil residue data from Ft. Leonard Wood, increasing confidence in the utility of T2TNT estimates of landmine signature chemicals for other locations.

Explosive chemical emissions from landmines

Chemical sensing for buried landmines is a complex phenomenon that includes mine chemical emissions, soil chemical transport/degradation, and detection at the ground surface. The technology to assess soil chemical transport has evolved and now provides a complex systems analysis capability using high fidelity computational simulation tools. Data requirements to evaluate a chemical sensing scenario include soil chemodynamic properties, micrometeorological conditions, and mine chemical emissions. Mine chemical emission tests were performed on four antipersonnel landmines using whole landmines in soil flux chambers. Soil flux chambers are simple containers that surround landmines with dry soil that act as an adsorbent. After a certain soak time, residue analysis of the soil provides the total chemical emission - a combination of both permeation and leakage. An evaluation of permeation differences into wet soil versus dry soil was also completed using thin polymer coupon sections.

Effect of weather on landmine chemical signatures for different climates

Buried landmines are often detected through their chemical signature in the thin air layer, or boundary layer, right above the soil surface by sensors or animals. Environmental processes play a significant role in the available chemical signature. Due to the shallow burial depth of landmines, the weather also influences the release of chemicals from the landmine, transport through the soil to the surface, and degradation processes in the soil. The effect of weather on the landmine chemical signature from a PMN landmine was evaluated with the T2TNT code for three different climates: Kabul, Afghanistan, Ft. Leonard Wood, Missouri, USA, and Napacala, Mozambique. Results for TNT gas-phase and solid-phase concentrations are presented as a function of time of the year.