Vadose Zone Monitoring with the Neutron Moisture Probe

The neutron moisture probe is widely applicable to vadose zone monitoring problems which require measuring variable moisture contents. Neutron data are proportional to hydrogen density (modified by local chemistry) and sensitive to wetting fronts as well as changing volumes of hydrocarbon liquids. They cannot, however, be used to confirm contaminant chemistry, nor to detect steady-state flow. Neutron data are amenable to statistical analysis, providing a measure of the significance of data variations. Detection of incipient moisture changes at numerous monitoring locations is more practical using raw neutron data than data calibrated for moisture content because calibrations suffer from uncertainties associated with soil heterogeneities. When properly applied, the neutron probe is an effective monitoring tool as illustrated by three example applications described in this paper: (1) neutron moisture logs are used to detect subtle lithologic changes and identify monitoring horizons; (2) sequential neutron data are used to track induced saturation at a soil flushing pilot study; and (3) neutron logs from a horizontal access tube beneath a waste facility are used to pinpoint moisture anomalies.     

低渗砂岩储层渗透率各向异性规律的实验研究

应用1摩尔/升的盐水作为孔隙介质,采用液体压力脉冲法对采自鄂尔多斯盆地某油田三叠系延长组低渗砂岩样品进行了渗透率随有效应力的变化规律实验研究.在围压0～100MPa,孔隙压力0～12MPa范围内,样品的渗透率变化范围在0～60×10-18 m2之间,实验结果表明样品的渗透率随有效应力的增加而减少.通过拟合实验结果,得到渗透率随有效应力的变化规律较好的符合幂函数关系,相关系数介于0.903～0.984之间.同时对同一样品的X,Y,Z相互垂直三个方向的渗透率随有效应力的变化进行了比较,结果表明渗透率的各向异性同样是压力的函数,且随着有效应力的增加,不同平面内的渗透率各向异性表现出了不同的变化规律.但在实验压力范围内,渗透率各向异性皆为正值,表明在有效应力为100MPa范围内孔隙内流体的流动方向未发生改变.研究结果为鄂尔多斯盆地低渗(超低渗)油气田开发,特别是对深部油气田开发过程中开发方式的选择与设计,充分利用渗透率各向异性的特点,提高采收率提供了新的岩石物性资料.     

Location and Characterization of Subsurface Anomalies Using a Soil Conductivity Probe

An electrical conductivity probe, designed for use with "direct push" technology, has been successfully used to locate buried drums, contaminant plumes, and to precisely locate and characterize a previously installed permeable reactive iron wall. The conductivity probe was designed to characterize various soil and sediment types as it was driven through the dry and saturated soil matrix; however, its ability to locate and characterize subsurface anomalies may also prove to be a valuable asset. The probe uses an electrical field that works like and mimics the results from an oilfield engineering tool called the Wenner array. This electrical field array penetrates into the soil matrix to measure the electrical conductivity of the soil matrix surrounding the probe to a radius of about two to four inches. This tool has allowed operating personnel to verify the location of manmade or natural subsurface anomalies with precision.     

On modelling installation designs for soil water instrumentation,with particular reference to casagrande type piezometer systems

Appropriate installation design of soil water monitoring equipment is necessary to ensure the meaningful interpretation of the resultant data. For example, although piezometers are widely used in monitoring positive pore water pressures, the limits on the shape factor of the system in terms of the ceramic tip and filter design have not been fully studied. The filter design boundary conditions are investigated here using a finite element analysis, in which threshold ceramic tip and filter characteristics (permeability and dimensions) are identified. From the modelling results, it is possible to select the most suitable design installation.     

Hydraulic Properties of Stony Vadose Zonesa

Hydraulic properties of saturated and unsaturated stony soils were studied on a 3.35 m long column, 1.24 m in diameter, filled with alternating sand and boulder layers. The boulders averaged 6.2 × 15 × 20 cm in size and were laid down on their flat side. Tensiometers and a neutron probe access tube were placed in the column for measuring pressure heads and water contents, respectively. Saturated conditions were obtained by ponding the column. The resulting hydraulic conductivity K was 5.1 m/day. This value could also be calculated from the measured K for the sand alone on separate samples, using a simple equation that takes into account the void ratio of the sand alone and that of the boulder-sand mixture. Unsaturated K was determined by applying water at less than ponded infiltration rates. Resulting relations between the unsaturated K and water content or negative pressure head could also be estimated from the relation between unsaturated K and pressure head for the sand alone and the calculated saturated K of the boulder-sand mixture. The method of Millington and Quirk for calculating the relation between unsaturated K and water content also gave reasonable results. The dispersivity of the boulder-sand column was 18 times that of the sand alone. Pore velocity was accurately estimated as the Darcy velocity divided by the volumetric water content. Hydraulic properties of stony vadose zones are difficult to determine. This work shows that they can be estimated from K relations measured in the laboratory on samples of the soil between the rocks. Knowledge of hydraulic properties of vadose zones is important in predicting movement of water and pollutants to the underlying ground water.     

Dynamic response of partially sealed circular tunnel in viscoelastic saturated soil

Based on Biot's wave equation, dynamic response of a circular tunnel with partially sealed liner in viscoelastic saturated soil is investigated. By introducing two scalar potential functions, the analytical solutions of stresses, displacements and pore pressure induced by axisymmetric gradually applied step load are derived in Laplace transform domain. Numerical results are obtained by inverting Laplace transform presented by Durbin and used to analyze the influences of partial permeable property of boundary and viscoelastic damping coefficient of soil on dynamic response of the tunnel. It is shown that the attenuation of radial displacement appeared with the increase of viscoelastic damping coefficient of soil, and relative rigidity of liner and soil, and the influence of partial sealing property of boundary on stresses, displacements and pore pressure is remarkable. The available solutions of permeable and impermeable boundary conditions are only two extreme cases of this paper.     

Instrumentation Design and Installation for Monitoring Air Injection Ground Water Remediation Technologies

An in situ instrumentation bundle was designed for inclusion in monitoring wells that were installed at the Wasatch Trailer Sales site in Layton, Utah, to evaluate in situ air sparging (IAS) and in-well aeration (IWA). Sensors for the bundle were selected based on laboratory evaluation of accuracy and precision, as well as consideration of size and cost. SenSym pressure transducers, Campbell Scientific Inc. (CSI) T-type thermocouples, and dissolved oxygen (DO) probes manufactured by Technalithics Inc. (Waco, Texas), were selected for each of the 27 saturated zone bundles. Each saturated zone bundle also included a stirring blade to mix water near the DO probe. A Figaro oxygen sensor was included in the vadose zone bundle. The monitoring wells were installed by direct push technique to minimize soil disruption and to ensure intimate contact between the 18 inch (46 cm) long screens and the soil. A data acquisition system, comprised of a CSI 21X data logger and four CSI AM416 multiplexers, was used to control the stirring blades and record signals from more than 70 in situ sensors. The instrumentation performed well during evaluation of IAS and IWA at the site. However, the SenSym pressure transducers were not adequately temperature compensated and will need to be replaced.     

High-Resolution Characterization of a Chlorinated Solvent Impacted Aquifer Using a Passive Profiler

A direct-drive high-resolution passive profiler (HRPP) was developed to quantify and delineate concentrations of chlorinated volatile organic compounds (CVOCs), geochemical indicators and CVOC-degrading microorganisms/genes, as well as to perform compound-specific stable isotope analysis (CSIA) of CVOCs and estimate interstitial velocity at <30-cm resolution. The profilers can be coupled together to provide a continuous sample interval and advanced to depths up to approximately 9 m below-ground surface (bgs) within saturated media where direct-push techniques are feasible. The HRPP was field tested in a previous dense nonaqueous phase liquid (DNAPL) source zone at the former Naval Air Station in Alameda, CA. HRPP data sets were compared to the following traditional groundwater data sets: CVOC and anion concentrations in standard and multilevel monitoring well water samples, CVOC concentrations in soil core samples, qualitative contaminant profiles delineated with a membrane interface probe (MIP), microbial community and CSIA profiles from Bio-Traps® deployed in wells, groundwater velocity from passive flux meters (PFMs), lithologic profiles correlated with MIP electrical conductivity (EC), and velocity estimates based on permeability profiles measured with a Geoprobe hydraulic profiling tool (HPT). In some cases, the HRPP data were equivalent to traditional techniques and, in other cases, the HRPP data were more representative of local variability rather than bulk aquifer conditions. Overall the results support the use of the HRPP to provide high-resolution data on concentrations, velocity, and microbial activity in temporary direct-push deployments without well installation, providing a new tool to better assess source zones and contaminated groundwater plumes, even in low permeability media, and to increase the fidelity of site transport models.     

Experimental studies in natural groundwater recharge dynamics: Assessment of recent advances in instrumentation

To quantify and model the natural groundwater-recharge process, two sites in south-central Kansas, U.S.A., were instrumented with various modern sensors and data microloggers. The atmospheric-boundary layer and the unsaturated and saturated soil zones were monitored as a unified regime. Data from the various sensors were collected using microloggers in combination with magnetic-cassette tape, graphical and digital recorders, analog paper-tape recorders, and direct observations to evaluate and automate data collection and processing.

Atmospheric sensors included an anemometer, a tipping-bucket raingage, an air-temperature thermistor, a relative-humidity probe, a net radiometer, and a barometric-pressure transducer. Sensors in the unsaturated zone consisted of soil-temperature thermocouples, tensiometers coupled with pressure transducers and dial gages, gypsum blocks, and a neutron moisture probe operated by an observer. The saturated-zone sensors consisted of a water-level pressure transducer, a conventional float gage connected to a variable potentiometer, soil thermocouples, and a number of multiple-depth piezometers.

Evaluation of the operation of these sensors and recorders indicated that certain types of equipment such as pressure transducers are very sensitive to environmental conditions. Extraordinary steps had to be taken to protect some of the equipment, whereas other equipment seemed to be reliable under all conditions. Based on such experiences, a number of suggestions aimed at improving such investigations are outlined.     

Monitoring in the Vadose and Saturated Zones Utilizing Fluoroplastic

A ground water monitoring program should include an investigation of all possible areas of concern. To be completely effective, the program should include soil sampling, soil analysis and water-quality examination of both the saturated and unsaturated zones. A well-tooled drill rig can take all the proper soil samples, perform all necessary tests and install a functional monitoring well. With the introduction of the fluoropolymer (Teflon(r)) sleeve lysimeter, a single monitoring well can be constructed to monitor both the saturated and unsaturated zones in one installation. The monitoring well screen and casing may also be completely constructed of fluoropolymer.
The sleeve lysimeter is designed with a threaded hollow inner diameter, allowing it to be attached between the joints of a casing string. This hollow I.D. acts as an extension of the casing; the lysimeter surrounds the casing. This creates an isolated vessel for sampling the vadose zone. Access to the screened monitoring well below is unaffected. Tests have shown that when properly installed, these porous fluoropolymer filter units can collect samples with no interaction between the filter and collected fluids.     

Improving watershed management practices in humid regions

In many parts of the world, watershed management practices have been extremely effective. However, implementation of soil and water conservation technologies in the humid African highlands, while beneficial in the short term, were remarkably unsuccessful in the long term. Insights from community knowledge perspectives have revealed that alternative methods are needed. Although conservation practices are designed to conserve water in semi‐arid areas, safely draining excess water is needed in humid areas. The objective of this paper is to review current watershed management approaches used in humid regions as exemplified by those used in Ethiopian highlands and then based on these findings propose more effective practices. Although current government sponsored practices primarily protect the hillsides, direct run‐off is generated from areas that become saturated on valley bottoms near rivers and on specific parts of the hillsides with degraded soils (or with highly permeable surface soils) and with perched water tables on slowly permeable horizons at shallow depths. In these areas, direct run‐off is increasing with deforestation and the soil degradation, demanding additional drainage ways that evolve in the form of gullies. Therefore, watershed management interventions for erosion control should prioritize revegetation of degraded areas, increasing sustainable infiltration, and rehabilitating gullies situated at saturated bottomlands.     

Subsurface Venting of Vapors Emanating from Hydrocarbon Product on Ground Water

The results of an API-sponsored pilot-scale subsurface venting system study are presented. The purpose of this study was to evaluate the effectiveness of forced venting techniques in controlling and removing hydrocarbon vapors from a subsurface formation. Both qualitative and quantitative sampling and analytical procedures were developed to measure hydrocarbon vapors extracted from the soil. Vapor recovery and equivalent liquid product recovery rates were measured at each test cell evacuation rate.
Two identical test cells were installed. Each cell contained 16 vapor monitoring probes spaced at distances from 4 to 44 feet from a vapor extraction (vacuum) well. Each cell was also configured with two air inlet wells to allow atmospheric air to enter the subsurface formation. The vapor monitoring probes were installed at three discrete elevations above the capillary zone. In situ vapor samples were obtained periodically from these probes to measure changes in vapor concentration and composition while extracting vapors from the vacuum well at three different flow rates (18.5 scfm, 22.5 scfm and 39.8 scfm). In situ vapor samples were analyzed using a portable gas chromatograph to quantify and speciate the vapors. Vacuum levels were also measured at each vapor sampling probe and at the vacuum well.
The soil venting techniques evaluated during this study offer an alternative approach for controlling and eliminating spilled or leaked hydrocarbons from sand or gravel formations of high porosity and moderate permeability. These techniques may also be used to augment conventional liquid recovery methods. The data collected during this study will be useful in optimizing subsurface venting systems for removing and controlling hydrocarbon vapors in soil. Study results indicate pulsed venting techniques may offer a cost-effective means of controlling or eliminating hydrocarbon vapors in soil.     

Evaluation of Subsurface Oxygen Sensors for Remediation Monitoring

Continuous remediation monitoring using sensors is potentially a more effective and inexpensive alternative to current methods of sample collection and analysis. Gaseous components of a system are the most mobile and easiest to monitor. Continuous monitoring of soil gases such as oxygen, carbon dioxide, and contaminant vapors can provide important quantitative information regarding the progress of bioremediation efforts and the area of influence of air sparging or soil venting. Laboratory and field tests of a commercially available oxygen sensor show that the subsurface oxygen sensor provides rapid and accurate data on vapor phase oxygen concentrations. The sensor is well suited for monitoring gas flow and oxygen consumption in the vadose zone during air sparging and bioventing. The sensor performs well in permeable, unsaturated soil environments and recovers completely after being submerged during temporary saturated conditions. Calibrations of the in situ oxygen sensors were found to be stable after one year of continuous subsurface operation. However, application of the sensor in saturated soil conditions is limited. The three major advantages of this sensor for in situ monitoring arc as follows: (1) it allows data acquisition at any specified time interval; (2) it provides potentially more accurate data by minimizing disturbance of subsurface conditions; and (3) it minimizes the cost of field and laboratory procedures involved in sample retrieval and analysis.     

A Full-Scale Porous Reactive Wall for Prevention of Acid Mine Drainage

The generation and release of acidic drainage containing high concentrations of dissolved metals from decommissioned mine wastes is an environmental problem of international scale. A potential solution to many acid drainage problems is the installation of permeable reactive walls into aquifers affected by drainage water derived from mine waste materials. A permeable reactive wall installed into an aquifer impacted by low-quality mine drainage waters was installed in August 1995 at the Nickel Rim mine site near Sudbury, Ontario. The reactive mixture, containing organic matter, was designed to promote bacterially mediated sulfate reduction and subsequent metal sulfide precipitation. The reactive wall is installed to an average depth of 12 feet (3.6 m) and is 49 feet (15 m) long perpendicular to ground water flow. The wall thickness (flow path length) is 13 feet (4 m). Initial results, collected nine months after installation, indicate that sulfate reduction and metal sulfide precipitation is occurring. Comparing water entering the wall to treated water exiting the wall, sulfate concentrations decrease from 2400 to 4600 mg/L to 200 to 3600 mg/L; Fe concentrations decrease from 250 to 1300 mg/L to 1.0 to 40 mg/L; pH increases from 5.8 to 7.0; and alkalinity (as CaCO₃) increases from 0 to 50 mg/L to 600 to 2000 mg/L. The reactive wall has effectively removed the capacity of the ground water to generate acidity on discharge to the surface. Calculations based on comparison to previously run laboratory column experiments indicate that the reactive wall has potential to remain effective for at least 15 years.     

Tide-induced airflow in a two-layered coastal land with atmospheric pressure fluctuations

Tide-induced airflow is commonly seen in coastal lands and affects ground stability especially with a less permeable pavement on the ground surface. A tide-induced airflow model in a two-layered unsaturated zone consisting of a highly permeable layer underneath a less permeable layer was established by Li and Jiao [Li HL, JJ Jiao. One-dimensional airflow in unsaturated zone induced by periodic water table fluctuation. Water Resour Res 2005;41:W04007. doi:10.1029/2004WR003916] to describe the one-dimensional airflow with constant atmospheric pressure at the ground surface. In this study, we expand the Li and Jiao model by considering the realistic atmospheric pressure fluctuations and the initial condition. A new transient solution to the airflow model is developed for an initial boundary value problem (IBVP). The transient solution can be used not only to calculate the subsurface air pressure at a future time with a known initial condition, but also to evaluate the asymptotic air pressure variations when time becomes long. The amplitude ratio and phase lag of the subsurface air pressure relative to the tide-induced hydraulic head variations inside the unconfined aquifer below the unsaturated zone are investigated. The results reveal that effect on the subsurface pressure due to changes of atmospheric pressure amplitude depends on the configurations of air resistance in the less permeable layer and the air-filled porosity difference in the two layers. The introduction of atmospheric pressure fluctuations into the airflow model leads to insignificant influence on water table level. A field application of the new solution at Hong Kong International Airport in Hong Kong, China is demonstrated. It indicates that the new transient solution can be conveniently used to evaluate the subsurface air pressure with discrete atmospheric pressure data at the ground surface.     

Design of Lysimeter Leak Detector Networks for Surface Impoundments and Landfills

Sampling of soil pore moisture in the vadose zone underneath land disposal facilities (landfills and surface impoundments) for hazardous waste has been suggested as an "early warning system" to detect leakage from these facilities. Some states require vadose zone moisture sampling at such sites. Given a leak of a particular size, mathematical models can estimate the necessary moisture sample volume collection times and lysimeter spacings to guarantee detection of the leak in a homogeneous medium. Examination of 47 hazardous waste sites existing in 1984 indicated the most were located in areas with water tables too shallow to permit vadose zone detection monitoring. Several of the 47 sites had soils that could be described as loamy sand, silt loam or silty clay. Using these three soils as examples, the process of lysimeter leak-detector network design has been illustrated. For a particular loamy sand with a saturates hydraulic conductivity of 10^-6 cm/ sec, the maximum ceramic lysimeter spacing is 15.5 feet at a depth of 30 feet to collec a moisture sample of 10 mL in one week from a 1 ft2 leak. For a silt loam, maximum lysimeter spacing would be 17 feet at depth of 15 feet. For silty clays, the maximum lysimeter spacing is 7 feet at a depth of 2 feet; maximum emplacement depth is about 9 feet. Calculations show that in some soils, suction lysimeters will not be able to collect usable moisture samples. Since soil properties vary widely and lysimeter spacing is strongly dependent on soil-moisture characteristics appropriate soil measurements and modeling must be performed at each disposal facility to estimate lysimete performance and to select locations for emplacement.     

Field Demonstration of the Horizontal Treatment Well (HRX Well®) for Passive In Situ Remediation

The horizontal reactive media treatment well (HRX Well®) uses directionally drilled horizontal wells filled with a treatment media to induce flow-focusing behavior created by the well-to-aquifer permeability contrast to passively capture proportionally large volumes of groundwater. Groundwater is treated in situ as it flows through the HRX Well and downgradient portions of the aquifer are cleaned via elution as these zones are flushed with clean water discharging from the HRX Well. The HRX Well concept is particularly well suited for sites where long-term mass discharge control is a primary performance objective. This concept is appropriate for recalcitrant and difficult-to-treat constituents, including chlorinated solvents, per- and polyfluoroalkyl substances (PFAS), 1,4-dioxane, and metals. A full-scale HRX Well was installed and operated to treat trichloroethene (TCE) with zero valent iron (ZVI). The model-predicted enhanced flow through the HRX Well (compared to the flow in and equivalent cross-sectional area orthogonal to flow in the natural formation before HRX Well installation) and treatment zone width was consistent with flows and widths estimated independently by point velocity probe (PVP) testing, HRX Well tracer testing, and observed treatment in downgradient monitoring wells. The actual average capture zone width was estimated to be between 45 and 69 feet. Total TCE mass discharge reduction was maintained through the duration of the performance monitoring period and exceeded 99.99% (%). Decreases in TCE concentrations were observed at all four downgradient monitoring wells within the treatment zone (ranging from 50 to 74% at day 436), and the first arrival of treated water was consistent with model predictions. The field demonstration confirmed the HRX Well technology is best suited for long-term mass discharge control, can be installed under active infrastructure, requires limited ongoing operation and maintenance, and has low life cycle energy and water requirements.     

Predicting Soil-Water and Soil-Air Transport Properties and Their Effects on Soil-Vapor Extraction Efficiency

Accurate prediction of water and air Iran sport parameters in variably saturated soil is necessary for modeling of soil-vapor extraction (SVE) at soil sites contaminated with volatile organic chemicals (VOCs). An expression for predicting saturated water permeability (k_l,s) in undisturbed soils from the soil total porosity and the field capacity soil-water content was developed by fitting a tortuous-tube fluid flow model to measured water permeability and gas diffusivity data. The new k_l,s expression gave accurate predictions when tested against independent k_l,s data. The k_l,s expression was implemented in the Campbell relative water permeability model to yield a predictive model for water permeability in variably saturated, undisturbed soil. The water permeability model, together with recently developed predictive equations for gas permeability and gas diffusivity, was used in a two-dimensional numerical SVE model that also included non-equilibrium mass transfer of VOC from a separate phase (nonaqueous phase liquid [NAPL]) to the air phase. SVE: calculations showed that gas permeability is likely the most important factor controlling VOC migration and vapor extraction efficiency. Water permeability and gas diffusivity effects became significant at water contents near and above field capacity. The NAPL-air mass transfer coefficient also had large impacts on simulated vapor extraction efficiency. The calculations suggest that realistic SVE models need to include predictive expressions for both conveciive, diffusive. and phase-partitioning processes in natural, undisturbed soils.     

A Flux Detection Probe to Quantify Dynamic Groundwater-Surface Water Exchange in the Hyporheic Zone

A new probe was designed to quantify groundwater-surface water exchange in the hyporheic zone under dynamic stage condition. Current methods focus on either vertical pore water velocity or Darcy flux measurements. Both parameters must be understood to evaluate residence time and mass flux of constituents. Furthermore, most instruments are not well suited for monitoring instantaneous velocity or flux under dynamic exchange conditions. For this reason, the flux detection probe (FDP) was designed that employs electrogeophysical measurements to estimate in situ sediment porosity, which can be used to convert pore water velocity to Darcy flux. Dynamic pore water velocity is obtained by monitoring fluid conductivity and temperature along the FDP probe. Pressure sensors deployed at the top and bottom of the probe provide the additional information necessary to estimate vertical permeability. This study focuses on the use of a geophysical method to estimate pore water velocity, porosity, and permeability within a controlled soil column where simulated river water displaces simulated groundwater. The difference between probe derived and theoretical pore water velocity using natural tracers such as electrical conductivity and temperature was −4.9 and 3.9% for downward flow and 1.1 and 12.8% for upward flow, respectively. The difference in porosity calculated from mass and volume packed in the soil column and probe measure porosity ranged between −3.2% and 1.5%. Also, the calculated hydraulic conductivity differed from probe derived values by −8.9%.     

Dynamic effects in a saturated layered soil deposit: centrifuge modeling

The dynamic response of a saturated layered soil deposit was modeled on the Princeton University geotechnical centrifuge using various centrifugal acceleration levels. The layered soil deposit consists of a saturated Nevada sand layer overlaid by a silt layer of low permeability. Measured acceleration and pore-water pressure time histories are used to validate the scaling laws used in interpreting dynamic centrifugal modeling test results. Careful measurements of the settlements at the silt surface are performed using a non-contact displacement transducer, and comparisons are made with measurements obtained with a standard linear voltage displacement transducer. Finally, the experimental results are used to verify the validity of the numerical procedures encompassed in the computer code DYNAFLOW.