KEYWORDS: Land mines, Fourier transforms, Climatology, Solar radiation, Solar radiation models, Liquids, Diffusion, Soil science, Temperature metrology, Soil contamination
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.
KEYWORDS: Land mines, Data modeling, Chemical analysis, Mining, Soil science, Chemical detection, Fourier transforms, Chemical elements, 3D modeling, Statistical modeling
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.
KEYWORDS: Soil science, Data modeling, Land mines, Humidity, Solids, Biological and chemical sensing, Mathematical modeling, Seaborgium, Explosives, Sensors
Sensing the chemical signature emitted from the main charge explosives from buried landmines 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. A simulation code named T2TNT was developed specifically to evaluate buried landmine chemical transport issues. A vapor-solid partitioning parameter that is strongly dependent on soil moisture content is included in T2TNT. Laboratory soil vapor flux experiments were conducted to provide data to validate the T2TNT model under well-constrained laboratory testing conditions. The landmine source release, soil transport and surface flux was simulated by aqueous phase injection of DNT, evaporation induced upward water flux and solid phase microextraction sampling of headspace vapor in an air flowing plenum. The surface soil moisture content was reduced by suction removal of soil water followed by artificial rain to evaluate the soil-vapor partitioning function in T2TNT. The data showed the dramatic decline in DNT vapor concentrations expected as the surface soil moisture declined; and, then rebounded upon wetting. This phenomenon was modeled with T2TNT and showed excellent correlation.
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.
KEYWORDS: Soil science, Data modeling, Soil contamination, Land mines, Humidity, Solids, Biological and chemical sensing, Sensors, Chemical analysis, Particles
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.
The purpose of the Explosives Fate and Transport (EF and T) experiments is to define in detail the accessible trace chemical signature produced by the explosives contained in buried landmines. We intend to determine the partitioning, composition, and quantity of explosive related chemicals which emanate form different kinds of landmines buried in multiple soil types and exposed to various climatic events. We are also developing a computer model that will enable us to predict the composition and quantity of ERC under a much wider range of environmental conditions than we are able to measure experimentally.
KEYWORDS: Liquids, Explosives, Diffusion, Solids, Capillaries, Soil science, Land mines, Biological and chemical sensing, Chemical fiber sensors, Solar radiation
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.
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.
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