A small-scale field column experiment was set up to assess the impact of a native tropical grass (Fimbrystilis Cymosa)
on the transport and distribution of high explosives (TNT and DNT). Explosives powders in a membrane were embedded
as a point source below 2 inches from the column surface. Three different surfaces were layered on top of the explosives
layer: one column with sand, two columns with Fimbrystilis Cymosa, and one column with a mixture of (sand+clay) soil.
Hydraulic differences due to surface vegetation which would affect explosives transport were monitored by measuring
the amount of infiltrated rain water. For the biogeochemical parameters, explosives concentrations in the infiltrated water
were quantified. At the end of the experiment, each column was sacrificed by multiple layers and distribution of
explosives concentrations, soil pH, and soil dehydrogenase concentration was quantified from the layers. Plants were
also analyzed for explosives concentrations in their leaves and roots.
KEYWORDS: 3D modeling, Solar radiation models, Solar radiation, Explosives, Sensors, Process control, Diffusion, Visible radiation, Ultraviolet radiation, Data modeling
Chemical, biological, and canine detection of buried explosive devices (BEDs) rely on the presence of explosive related
compounds (ERCs) near the soil-atmospheric surface. ERC distribution near this surface and its relation to the location
of BEDs is controlled by fate and transport processes. Experimental work was conducted in a 3D laboratory-scale
SoilBed system to determine the effect of cyclic rainfall, evaporation, temperature, and solar radiation on on the fate,
transport and detection of ERCs near soil surfaces. Experiments were conducted by burying a TNT/DNT source under
the soil surface, and applying different rainfall and light radiation cycles while monitoring salt tracers and TNT solute
concentrations temporally and spatially within the SoilBed. Transport of non-reactive solutes was highly influenced by
the cyclic variations on water flux, water content, evaporation, and influx concentrations. Concentrations of TNT and
other ERCs were further affected by vapor transport and sorptive and degradation processes.
Detection of explosive-related chemicals (ERCs) derived from landmines sources is influenced by fate and transport
processes. Characterization and quantification of the effects of environmental factors on the fate and transport
behavior ERCs near soil surface environments requires the development of physical models that can mimic the
conditions found in the field. The development of the scalable systems and methods involves proper reproduction of
soil composition, lithology and structures, appropriate placement of boundary conditions, and suitable simulation of
representative environmental conditions. This paper evaluates the ability of different packing methods for clayey
soils to attain physical and transport properties representative of field conditions, and which can yield reproducible
results across different scales and dimensions. Characteristics and reproducibility of packing properties is evaluated
in terms of soil bulk density, porosity, flow capacity and particle size distribution. The packing methods were tested
under different water content conditions and they are described as infiltration packing, saturation packing, plastic
limit packing, inverse infiltration packing, induced settling packing, and vibration packing. The systems were
evaluated for consistent bulk density, porosity, flow capacity and particle size distribution with depth. Preliminary
results exhibit satisfactory bulk density and porosity values for the vibration packing method under field water
content conditions, ranging from 1.15 to 1.31 g cm-3 and from 42 to 44%, respectively. This method also shows
acceptable flow capacity and the particle size distribution that is found in the field.
Widespread contamination of underground environments with dense non-aqueous phase liquids (DNAPLs) is of
great concern to the public, military, and industrial sectors. Proper management of contaminated sites requires
detection and monitoring of the contaminants, and accurate knowledge of their transport behavior in underground
environments. Over the last years we have done great efforts to develop and integrate technologies that serve to
locate contamination and monitor transport mechanism underground. In this paper, we describe a two-dimensional
multiphase flow experiment to develop and evaluate two modes of concurrent detection and monitoring
technologies: Cross Well Radar (CWR) and Image Analysis (IA). Loop antennas preset at specific locations in the
tank are used to evaluate wave scattering properties of the soil under different conditions, while color images are
acquired. The electromagnetic response in the CWR antennas and IA are used to establish the relation between
electrical soil properties variations and changes spatial and temporal mass of water and contaminants. The
technologies used in this research are both in development, but they can be successful tools for the detection,
monitoring and imagining of underground contaminants and process. Once develop, the technology may be applied
for detecting and monitoring other buried objects.
The existence of explosive related chemicals (ERCs) near the soil-atmospheric and other surfaces depend
on their fate and transport characteristics within the environmental settings. Consequently, detection of
ERC in environmental matrices is influenced by conditions that affect their fate and transport. Experimental
work to study the fate and transport behavior of ERCs relies on proper temporal and spatial sampling
techniques. Because the low vapor pressure of these chemicals and their susceptibility to adsorption and
degradation, vapor concentrations in environmental matrices are very low. Depending on the environmental
conditions, the amount of samples that can be withdrawn for analysis is also limited. It is, therefore,
necessary to develop sampling technologies that can provide quantitative measures of ERC concentrations
in limited sampling environments.
This paper presents experimental work conducted to develop a sampling technique to quantify DNT and
TNT vapor concentrations of low vapor-pressure ERCs in environmental setting having limited sampling
volumes and large sample numbers. Two potential vapor sampling techniques, Solid phase Microextraction
(SPME) and Solid Phase Extraction (SPE), were developed and evaluated. SPME sampling techniques are
excellent to quantify for DNT and TNT at very low concentrations. Its passive sampling capabilities meet
the requirement for low-volume environmental sampling, but measured concentrations may be lagged in
time. SPMEs' requirements for immediate analysis after sampling limit the technique for continuous vapor
sampling.
SPE showed to be a sensitive and reproducible technique to determine vapor concentrations of TNT and
DNT in atmospheric and soil setting having limited sampling volumes and large sample numbers. Smallvolume
(600&mgr;L) air samples provide measurements in the &mgr;gL-1 concentration range using isoamyl acetate
and acetonitrile as the solvents. Small extraction volumes make this technique cost efficient and attractive.
Issues with extraction inefficiencies, however, were observed and are being investigated.
Chemical detection of buried explosives devices (BEDs) through chemical sensing is influenced by factors affecting the transport
of chemical components associated with the devices. Explosive-related chemicals, such as 2,4-dinitrotolune (DNT), are
somewhat volatile and their overall transport is influenced by vapor-phase diffusion. Gaseous diffusion depends on
environmental and soil conditions. The significance of this mechanism is greater for unsaturated soil, and increases as water
content decreases. Other mechanisms, such as sorption and degradation, which affect the overall fate and transport, may be more
significant under diffusion transport due to the higher residence time of ERCs in the soil system. Gaseous diffusion in soil was
measured using a one-dimensional physical model (1-D column) to simulate the diffusion flux through soil under various
environmental conditions. Samples are obtained from the column using solid phase microextraction (SPME) and analyzed with a
gas chromatography. Results suggest that DNT overall diffusion is influenced by diffusive and retention processes, water content,
source characteristics, and temperature. DNT effective gas phase diffusion in the soil decreases with increasing soil water
content. Vapor transport retardation was more dominant at low water contents. Most of the retardation is associated to the
partition of the vapor to the soil-water. DNT vapor flux is higher near the explosive source (mine) than at the soil surface. This
flux also increases with higher soil water content and temperature. Results also suggest non-equilibrium transport attributed to
mass transfer limitations and non-linear sorption.
Landmines and other buried explosive devices pose in an immense threat in many places of the world, requiring large efforts on detection and neutralization of these objects. Many of the available detection techniques require the presence of chemicals near the soil-atmospheric surface. The presence of explosive related chemicals (ERCs) near this surface and their relation to the location of landmines, however, depends on the source characteristics and on fate and transport processes that affect their movement in soils. Fate and transport processes of ERC is soils may be interrelated with each other and are influenced by chemical characteristics and interrelated soil and environmental factors. Accurate detection of ERCs near the soil surface must, therefore, take into the variability of ERC concentration distributions near the soil surface as affected by fate and transport processes controlled interrelated environmental factors. To effectively predict the concentration distributions of ERCs in soils and near soil surfaces, it is necessary to have good understanding of parameters values that control these processes. To address this need, field lysimeters have been designed and developed at the University of Puerto Rico, Mayaguez .This paper presents the design of two field lysimeter used to study the fate and transport behavior of ERC in the field subjected to varying uncontrolled subtropical environmental conditions in two different soils. Both lysimeters incorporate pressure and concentration sampling ports, thermocouples, and a drainage system. Hydrus-2D was used to simulate soil moisture and drainage in the lysimeter for average environmental conditions in the study for the two soils used. The field lysimeters allow collection and monitoring of spatial and temporal ERC concentrations under variable, uncontrolled environmental conditions.
Detection of Explosive Related Chemicals (ERCs) emanating from landmines is strongly influenced by fate and transport processes in variably saturated soils, which are affected by many environmental factors. To study the fate and transport behavior of ERCs in soils, it is necessary to conduct experiments is physical models designed for this purpose. Sampling and analysis of the chemicals in soil water and air is one of the most important components of this design.
In this project, air and water sampling devices and methods for sampling TNT and DNT in a tropical sandy soil were studied. Different stainless steel porous samplers were evaluated to determine sampling volumes and efficiencies. Results show that they can be used with proper extraction vacuum and pore size depending on soil water content. The stainless steel porous samplers show no or little effect of the sampling efficiencies of TNT and DNT.
The development of the scalable systems and methods involves proper reproduction of soil composition, lithology and structures, appropriate placement of boundary conditions, suitable simulation of representative environmental conditions, and the use of representative sampling systems. This paper evaluates the effect of different packing methods with a tropical sandy soil for obtaining a uniform and homogeneous packing so that these characteristics are comparable across all scales and dimensions. The packing methods used include piston-driven dry packing, piston-driven wet packing, and gravity-driven sedimentation packing. For dry and wet packing, the procedure consisted on the iterative addition of soil layers, mixing and compaction. Sedimentation packing involved the preparation of soil slurry and allowing its deposition under gravity. The systems were evaluated for consistent bulk density, porosity, homogeneity, and soil dispersivity. Preliminary results exhibit satisfactory bulk density and porosity values for the piston-driven methods, ranging from 1.59 to 1.64 g cm-3 and from 42 to 44%, respectively. Sedimentation packing results in fair homogeneity and gradation, while dry packing develops heterogeneous layering. Transport parameters were also evaluated resulting in consistent dispersivity values for wet piston-driven packing ranging from 0.09-0.19 cm. Wet piston-driven packing is recommended as they yielded the most reproducible results for tropical sandy soils. The reproducibility of the recommended method is tested and proven in other physical models of different scales and dimensions. The method herein developed are, therefore, applicable for the development of representative multidimensional physical models designed to simulate soil and environmental fate and transport processes occurring in field conditions where landmines and other explosive devices are present.
Understanding sorption mechanisms of Explosive Related Chemicals (ERCs) in subsurface environments is essential in
predicting their fate and transport, since sorption onto the soil may reduce the ERC concentration in the liquid and gas
phase, thus affecting its subsequent detection. This project is studying the equilibrium and non-equilibrium sorption of
2,4-Dinitrotoluene (DNT) onto soils under different conditions such as temperature and soil type (sandy soil, clayey
soil). The sorption behavior of DNT in tropical soils samples from Isabela, P.R. under saturated conditions is currently
being studied at 25±2°C. Kinetic sorption experiments showed that equilibrium was achieved after approximately 30
hours for sand and approximately 12 hours for clay. Equilibrium studies in the range of 2-14 mg/L of DNT followed a
Freundlich isotherm. These results demonstrate it is not appropriate to assume a linear relationship between the amount
of solute sorbed onto the solid and the concentration of the solute as may not always be the case for concentrations
outside a certain range that will be dependent on the type of soil and organic matter content. This might result in under
or over estimation of adsorption, and thus the fate and transport and eventual detection of DNT. At low DNT
concentrations (less than 6 mg/L for sand and less than 8 mg/L for clay), the isotherms were quasilinear. In this range, a
linear adsorption constant, Kd value of 0.0172 L/Kg and 1.46 L/Kg for sand and clay, respectively, showed there was
more affinity of the DNT for clay than for sand.
KEYWORDS: Head, Humidity, Visible radiation, Control systems, 3D modeling, Ultraviolet radiation, Temperature metrology, Lamps, Systems modeling, Soil science
This paper presents the development and testing of a three-dimensional laboratory-scale soil tank system for modeling ERC fate and transport under controlled, but variable environmental conditions in partially saturated soil. The system incorporates a rainfall simulator, variable light (visible and UV), temperature and relative humidity components, and a 3D SoilBed capable of simulating several boundary and initial conditions. Experimental work indicate that water and solute transport is highly influenced by interrelated environmental and boundary conditions. The presence of light and higher system temperatures induces greater water drainage and solute fluxes. During infiltration, hydraulic heads increase at faster rates under no light exposure suggesting greater water and solute retention. The spatial and temporal distribution of hydraulic heads during rainfall events is not uniform and flow patterns reflect preferential paths. Transport of conservative solutes closely follows water flow patterns, and reflects the influence of variable and interrelated environmental factors on spatial and temporal concentration distribution. These experiments show that interrelated environmental factors must be taken into account to accurately predict the distribution of chemicals near the soil-atmosphere surface. They demonstrate that non-reactive solutes are highly influenced by variation in hydraulic, advective, and dispersive processes induced by changes in environmental conditions. Greater impacts are expected for reactive and semi-volatile solutes such as ERCs. In such case, fate and transport will also be affected by variations in soptive, gas transport, and degradation processes.
Traditional approaches for locating and characterizing contaminated sites rely on invasive techniques which require drilling, testing, and sampling. These techniques provide the most direct access to the
subsurface, but they are generally expensive and only provide measurements at points in a three dimensional surface. Furthermore, invasive techniques in polluted areas can promote further spread of
contaminants. Development of non-invasive techniques that offer rapid and relatively inexpensive characterization is, therefore, necessary to detect and monitor plumes and sources of contaminants. Non-invasive techniques are also required for locating buried objects, such as landmines and unexploded ordnances. The use of cross well radar (CWR) as a non-invasive technique that has proven to be a reliable technology for detection of target objects that exhibit significant contrast of dielectric properties in saturated soils. Its application to detection of heterogeneously distributed phases in unsaturated soils under variable flow conditions has yet to be developed. This paper addresses the development of 2D flow and electromagnetic (EM) soilBed setup to further assess and enhance CWR technology for the detection of Dense Non-Aqueous Phase Liquids (DNAPLs) contamination and other target elements in variably-saturated soils subjected to transient flow conditions. Loop antennas have been developed and tested for this purpose. Transmission and reflection measurements are evaluated to determine the antenna's reliability and optimize their performance in the 2D electromagnetic field. The measurements indicate that a 2D EM boundary condition may be imposed by placing two parallel perfectly-reflecting metal plates along one of the dimensions of the soilBed setup. Transmission and reflection characteristics of the antennas vary with their method of construction. Results show a reliable and reproducible response from the loop antennas, but suggest some wave leakage and indicate that their design must be optimized. Measured variations in the transmission, reflection and impedance in the presence and absence of a buried object suggest that the 2D EM soilBed setup using loop antennas can be aplied to detect target elements in subsurface environments subjected to flow conditions. Future work addresses the assessment of CWR technology as a non-invasive method for detection and monitoring of heterogeneously-distributed target objects in subsurface environments.
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