Showing papers in "Vadose Zone Journal in 2003"

Measuring Soil Water Content with Ground Penetrating Radar: A Review

Abstract: We present a comprehensive review of methods to measure soil water content with ground penetrating radar (GPR) We distinguish four methodologies: soil water content determined from reflected wave velocity, soil water content determined from ground wave velocity, soil water content determined from transmitted wave velocity between boreholes, and soil water content determined from the surface reflection coefficient For each of these four methodologies, we discuss the basic principles, illustrate the quality of the data with field examples, discuss the possibilities and limitations, and identify areas where future research is required We hope that this review will further stimulate the community to consider ground penetrating radar as one of the possible tools to measure soil water content

A Review of Advances in Dielectric and Electrical Conductivity Measurement in Soils Using Time Domain Reflectometry

Abstract: Substantial advances in the measurement of water content and bulk soil electrical conductivity (EC) using time domain reflectometry (TDR) have been made in the last two decades. The key to TDR's success is its ability to accurately measure the permittivity of a material and the fact that there is a good relationship between the permittivity of a material and its water content. A further advantage is the ability to estimate water content and measure bulk soil EC simultaneously using TDR. The aim of this review is to summarize and examine advances that have been made in terms of measuring permittivity and bulk EC. The review examines issues such as the effective frequency of the TDR measurement and waveform analysis in dispersive dielectrics. The growing importance of both waveform simulation and inverse analysis of waveforms is highlighted. Such methods hold great potential for obtaining far more information from TDR waveform analysis. Probe design is considered in some detail and practical guidance is given for probe construction. The importance of TDR measurement sampling volume is considered and the relative energy storage density is modeled for a range of probe designs. Tables are provided that compare some of the different aspects of commercial TDR equipment, and the units are discussed in terms of their performance and their advantages and disadvantages. It is hoped that the review will provide an informative guide to the more technical aspects of permittivity and EC measurement using TDR for the novice and expert alike.

Measuring Soil Water Content with Ground Penetrating Radar

Abstract: We present a comprehensive review of methods to measure soil water content with ground penetrating radar (GPR). We distinguish four methodologies: soil water content determined from reflected wave velocity, soil water content determined from ground wave velocity, soil water content determined from transmitted wave velocity between boreholes, and soil water content determined from the surface reflection coefficient. For each of these four methodologies, we discuss the basic principles, illustrate the quality of the data with field examples, discuss the possibilities and limitations, and identify areas where future research is required. We hope that this review will further stimulate the community to consider ground penetrating radar as one of the possible tools to measure soil water content.

Hydropedology: Bridging Disciplines, Scales, and Data

Abstract: There is a growing recognition that synergy could be generated by bridging traditional pedology with soil physics and hydrology to enhance integrated studies of soil–water relationships across spatial and temporal scales. Hydropedology is suggested as such a bridge to address: (i) knowledge gaps between pedology, soil physics, and hydrology; (ii) multiscale bridging from microscopic to mesoscopic and macroscopic levels; and (iii) data translations from soil survey databases into soil hydraulic information. Knowledge gaps include flow and transport in the structured unsaturated zone, soil structure quantification, preferential flow modeling, landscape hydrology, soil spatial and temporal variability, quantitative use of field soil morphology for inferring soil hydrology, mechanisms controlling individual and interactive soil–water processes at multiple scales, pedotransfer functions (PTFs), and others. Hydropedology integrates the pedon and landscape paradigms to link phenomena occurring at microscopic (e.g., pores and aggregates), mesoscopic (e.g., pedons and catenas), and macroscopic (e.g., watersheds, regional, and global) scales. Through approaches such as PTFs, hydropedology also facilitates the bridging of data between soil survey databases and soil hydraulic information needed in simulation models. The bridging of disciplines, scales, and data represents potentially unique contributions of hydropedology to integrated soil and water sciences. It is hoped that hydropedology would contribute to our enhanced understanding of a variety of environmental, ecological, agricultural, and natural resource issues of societal importance. These include water quality, soil quality, landscape processes, watershed management, nutrient cycling, contaminant fate, waste disposal, precision agriculture, climate change, and ecosystem functions.

On leakage and seepage from geologic carbon sequestration sites: Unsaturated Zone Attenuation

Abstract: Geologic carbon sequestration is the direct injection of CO 2 into deep geologic formations for permanent disposal. Although numerous trapping mechanisms exist in the subsurface, it is possible that CO 2 will leak from the primary sequestration target and seep out of the ground. The unsaturated zone has the potential to attenuate leaking CO 2 and decrease seepage and near-surface CO 2 concentrations. Attenuation processes include permeability trapping, ponding as dense CO 2 spreads out on the water table, solubility trapping by infiltrating or residual water, and dilution through mixing with ambient soil gas. Numerical simulations of CO 2 flowing upward through a thick model unsaturated zone were performed to investigate the sensitivity of various unsaturated zone properties on CO 2 seepage flux and near-surface CO 2 gas concentrations. These two quantities are considered drivers for health and environmental risk due to exposure to CO 2 . For the conceptual model considered, seepage flux and near-surface CO 2 gas concentrations are most strongly controlled by the leakage rate at the water table, followed by the source zone radius. Permeability and permeability anisotropy, as well as porosity and infiltration rate are also important, although to a lesser degree. Barometric pumping causes local maxima in seepage flux and near-surface CO 2 concentrations, but has negligible effect in a time-averaged sense. When the leakage source is turned off, the CO 2 plume attentuates through dissolution into infiltrating water. For the case of a constant leakage rate, the unsaturated zone can attenuate low leakage fluxes but should not be expected to attenuate large CO 2 leakage fluxes.

Multi-Functional Heat Pulse Probe for the Simultaneous Measurement of Soil Water Content, Solute Concentration, and Heat Transport Parameters

Abstract: Water, solute, and heat transport processes in soils are mutually interdependent as each includes convective water flow and each transport mechanism is partly controlled by fluid saturation, pore geometry, temperature, and other soil environmental conditions. Therefore, their measurement in approximately identical measurement locations and volume is essential for understanding transport phenomena in soils. We introduce a 2.7-cm-diameter multi-functional heat pulse probe (MFHPP), which consists of a single central heater, four thermistors, and four electrodes (Wenner array) that together are incorporated in six 1.27-mm-o.d. stainless-steel tubes. The bulk soil thermal properties and volumetric water content of Tottori Dune sand were determined from the measurement of the temperature response of all four thermistor sensors after application of an 8-s heat pulse by the heater sensor. Simultaneously with the temperature measurements, the bulk soil electrical conductivity (ECb) was measured using the Wenner array, from which soil solution concentration (ECw) can be obtained after calibration. All measurements were taken during multistep outflow experiments, which also allowed estimation of the soil's hydraulic properties. We demonstrated that the MFHPP can effectively measure volumetric water content, thermal properties, and ECb, and can be used to indirectly estimate soil water fluxes at rates larger than 0.7 m d−1 in the sand.

Temperature-Profile Methods for Estimating Percolation Rates in Arid Environments

Abstract: Percolation rates are estimated using vertical temperature profiles from sequentially deeper vadose environments, progressing from sediments beneath stream channels, to expansive basin-fill materials, and finally to deep fractured bedrock underlying mountainous terrain. Beneath stream channels, vertical temperature profiles vary over time in response to downward heat transport, which is generally controlled by conductive heat transport during dry periods, or by advective transport during channel infiltration. During periods of stream-channel infiltration, two relatively simple approaches are possible: a heat-pulse technique, or a heat and liquid-water transport simulation code. Focused percolation rates beneath stream channels are examined for perennial, seasonal, and ephemeral channels in central New Mexico, with estimated percolation rates ranging from 100 to 2100 mm d−1. Deep within basin-fill and underlying mountainous terrain, vertical temperature gradients are dominated by the local geothermal gradient, which creates a profile with decreasing temperatures toward the surface. If simplifying assumptions are employed regarding stratigraphy and vapor fluxes, an analytical solution to the heat transport problem can be used to generate temperature profiles at specified percolation rates for comparison to the observed geothermal gradient. Comparisons to an observed temperature profile in the basin-fill sediments beneath Frenchman Flat, Nevada, yielded water fluxes near zero, with absolute values <10 mm yr−1. For the deep vadose environment beneath Yucca Mountain, Nevada, the complexities of stratigraphy and vapor movement are incorporated into a more elaborate heat and water transport model to compare simulated and observed temperature profiles for a pair of deep boreholes. Best matches resulted in a percolation rate near zero for one borehole and 11 mm yr−1 for the second borehole.

Laboratory Evaluation of the Dual-Probe Heat-Pulse Method for Measuring Soil Water Content

Abstract: The dual-probe heat-pulse (DPHP) method provides a means of estimating volumetric soil water content (θ) and change in volumetric water content (Δθ) from measurements of volumetric heat capacity. The purpose of this investigation was to characterize the accuracy and precision that can be achieved in measuring θ and Δθ with the DPHP method. Tempe pressure cells fitted with DPHP sensors were used to conduct desorption experiments in which DPHP-based estimates of θ and Δθ were compared with values estimated by the gravimetric method. For water contents corresponding to soil water pressure potentials below −100 kPa, comparisons were made by packing the pressure cells with soil wetted to known water contents. The investigation was conducted with seven soil materials representing a wide range of physical properties for mineral soils. The DPHP sensors slightly overestimated θ at low water contents, but it was shown that the bias could be removed by using an empirical calibration equation, θ = 1.09 θ DPHP − 0.045. This relationship appears to be general inasmuch as it was shown to be applicable for all seven soil materials and for water contents ranging from 0.02 to 0.59 m 3 m −3 . The general calibration equation was also shown to be effective in removing bias in Δθ estimates. Pooled regression analysis (all soil materials) showed that θ can be measured with a root mean square error (RMSE) of 0.022 m 3 m −3 . Greater precision can be achieved with Δθ measurements (RMSE = 0.012 m 3 m −3 ); however, the results indicated a decrease in precision with increasing magnitude of Δθ.

Review and Analysis of Chlorinated Solvent Dense Nonaqueous Phase Liquid Distributions in Five Sandy Aquifers

Abstract: To select and design effective remedial measures for dense, nonaqueous phase liquid (DNAPL) source zones, better understanding of the architecture of these zones is needed. In this study, a suite of investigative techniques was applied to perform detailed vertical delineation of chlorinated-solvent source zones in sand aquifers at five contaminated industrial sites (two in Connecticut, and one each in Florida, New Hampshire, and Ontario). The DNAPL occurs in the middle of the aquifers at three of the sites and at or near the bottom at the other two. The DNAPL entered the subsurface at these sites decades ago, and therefore the DNAPL zones have aged due to groundwater dissolution. The suite of investigative techniques was used to perform profile sampling using direct-push methods, in which depth-discrete soil and groundwater samples were taken with extremely close vertical spacing. The sampling included methods to distinguish between free-product and residual DNAPL at two of the sites. At each location where DNAPL was found, the DNAPL occurred in one or a few thin layers, generally between 1 and 30 cm thick. These layers were positioned within distinct grain-size zones, or at contacts between sedimentological layers. In some cases, the DNAPL layers have no apparent textural association. For any particular sampling hole to have a high probability of finding such layers, continuous cores must be collected and sampling of these cores must be done at very close vertical spacing (5 cm or less). Free-product DNAPL occurrences in conventional wells at three of the sites indicated, misleadingly, much greater DNAPL layer thicknesses than actual, and in one case, the conventional well may have caused short-circuiting of DNAPL from the middle to the bottom of the aquifer. Although all of the DNAPL source zones are comprised of only sporadic, thin DNAPL layers representing little total mass, these source zones are the cause of high-concentration dissolved plumes down gradient.

A Depth Control Stand for Improved Accuracy with the Neutron Probe

Abstract: The neutron thermalization method for soil water content measurement is well established as being accurate for deep soil profile measurements. However, the method has been criticized as inaccurate for shallow measurements ( 0.98 and RMSE values of calibration <0.01 m3 m−3. The stand is also useful for elevating the gauge high enough above the surface so that standard counts are not influenced by the water content or nature of the surface, thus enhancing accuracy of both the calibration and subsequent water content readings, both of which depend on standard count values. Also, the stand serves to prevent repetitive strain injuries to backs and knees caused by bending and kneeling to place the gauge on top of access tubes, but without additional occupational exposure to radiation.

Development of Thermo-Time Domain Reflectometry for Vadose Zone Measurements

Abstract: Soil and environmental studies in the vadose zone are often restricted by the lack of equipment that can measure the variation of soil physical parameters in space and time. In this paper we demonstrate that thermo-time domain reflectometry (thermo-TDR) can be used for a wide range of soil physical measurements. The thermo-TDR probe combines TDR and heat-pulse technologies into a single probe. It allows measurements of volumetric soil water content (theta), temperature (T), electrical conductivity (EC), thermal conductivity (lambda), thermal diffusivity (alpha), and volumetric heat capacity (rho c) at the same sampling positions simultaneously. Furthermore, other soil physical parameters, such as bulk density (rho(b)), air-filled porosity (n(a)), and degree of saturation (S), can be determined from their relationships with rho c and theta. We examined the performance of the thermo-TDR using both published data and laboratory measurements on packed columns and intact cores from six soils of varying texture. The results show that the thermo-TDR provides reliable measurements of theta, EC, rho c, lambda, n(a), and S, but relatively large errors exist in rho(b). The average standard error between thermo-TDR measurements and gravimetric measurements is 0.026 m(3) m(-3) for theta, 0.050 m(3) m(-3) for n(a), 0.069 for S, and 0.134 Mg m(-3) for rho(b). The standard error between thermo-TDR measurements and theoretical predictions of rho c is 0.134 MJ m(-3) K(-1). These promising findings coupled with the characteristics of small probe size and easy automation make the thermo-TDR sensor an ideal tool for studying coupled flow processes in the vadose zone. Further improvements in probe design, waveform interpretation, and determination of effective probe length are also noted as steps toward improving accuracy and precision of future thermo-TDR measurements.

A Sensitivity Analysis of Electrical Resistivity Tomography Array Types Using Analytical Element Modeling

Abstract: The analytic element method is used to investigate the spatial sensitivity of different electrical resistivity tomography (ERT) arrays. By defining the sensitivity of an array to a subsurface location we were able to generate maps showing the distribution of the sensitivity throughout the subsurface. This allows us to define regions of the subsurface where different ERT arrays are most and least sensitive. We compared the different arrays using the absolute value of the sensitivity and using its spatial distribution. Comparison is presented for three commonly used arrays (Wenner, Schlumberger, and double dipole) and for one atypical array (partially overlapping). Most common monitoring techniques use a single measurement to measure a property at a single location. The spatial distribution of the property is determined by interpolation of these measurements. In contrast, ERT is unique in that multiple measurements are interpreted simultaneously to create maps of spatially distributed soil properties. We define the spatial sensitivity of an ERT survey to each location on the basis of the sum of the sensitivities of the single arrays composing the survey to that location. With the goal of applying ERT for time-lapse measurements, we compared the spatial sensitivities of different surveys on a per measurement basis. Compared are three surveys based on the typical Wenner, Schlumberger, and double dipole arrays, one atypical survey based on the partially overlapping array, and one mixed survey built of arrays that have been shown to be optimal for a series of single perturbations. Results show the inferiority of the double dipole survey compared with other surveys. On a per measurement basis, there was almost no difference between the Wenner and the Schlumberger surveys. The atypical partially overlapping survey is superior to the typical arrays. Finally, we show that a survey composed of a mixture of array types is superior to all of the single array type surveys. By analyzing the spatial sensitivity of the single array, and most significantly the sensitivity of the ERT survey, we set the basis for quantitative measurement of subsurface properties using ERT, with applications to both static and transient hydrologic processes.

Field Evaluation of the Dual-Probe Heat-Pulse Method for Measuring Soil Water Content

Abstract: The dual-probe heat-pulse (DPHP) method is useful for measuring water content (θ) and change in water content (Δθ) near the soil surface. The method has been evaluated in laboratory and greenhouse experiments, but not in a field setting. Our objective was to test the DPHP method under field conditions and for a range of soil properties. Twenty-five DPHP sensors and five monitoring stations were constructed and installed at five locations in northeastern Kansas to measure θ and Δθ at 3-h intervals for 3 mo. In addition, θ was estimated by coupling Δθ measurements with independent measurements of θ obtained by soil sampling at sensor installation. Additional soil samples were collected from each location during the monitoring period to provide independent measurements of θ. Regression of DPHP and independent θ measurements revealed slight bias but substantial offset error (about 0.1 m3 m−3) in the DPHP method. The offset error could not be fully attributed to bias in any single input parameter, but could have been caused by a combination of biased parameters. Estimates of θ from Δθ measurements also revealed slight bias, but offset error was considerably smaller. Use of a published empirical calibration for DPHP sensors almost completely eliminated this bias and further reduced the offset error to approximately 0.01 m3 m−3. Thus, the Δθ approach combined with use of the empirical calibration appears to have practical utility.

Use of the Dual-Probe Heat-Pulse Technique to Monitor Soil Water Content in the Vadose Zone

Abstract: The dual-probe heat-pulse (DPHP) technique is emerging as a useful technique for measuring soil volumetric water content (theta). However, few published data are available regarding the performance of the DPHP technique under field conditions. The objective of this study is to evaluate the effectiveness of the DPHP technique for measuring theta under field conditions. We used 24 DPHP sensors to monitor theta in a soybean [Glycine max (L.) Merr.] field during the 2001 and 2002 growing seasons. The DPHP sensors demonstrated durability in field conditions and clear sensitivity to temporal and spatial variations of theta at the scale of measurement. The mean theta measured by the DPHP sensors (theta(DPHP)) was on average 0.040 m(3) m(-3) larger than the mean theta measured by soil sampling (theta(SS)). The response of the DPHP sensors was linear. Regressions of theta(DPHP) vs. theta(SS) yielded r(2) values of 0.949 and 0.843 at depths of 7.5 and 37.5 cm. The DPHP technique showed good resolution with RMSE values for the regression of 0.009 and 0.011 m(3) m(-3) at the two measurement depths. The slopes of the regressions were 0.75 rather than 1.0. Errors in theta(SS) are a likely cause of this low slope. We shifted all the theta values for each sensor up or down by a constant value to make the first theta measurement from each sensor equal theta determined from soil sampling near that sensor at the time of installation. This simple matching point procedure improved the accuracy of the DPHP technique, resulting in a -0.024 m(3) m(-3) average difference between theta(DPHP) and theta(SS). Also, the matching point procedure markedly reduced the variability between sensors, reducing the average SD from 0.063 to 0.026 m(3) m(-3). This procedure requires no additional soil sampling and is recommended for field applications of the DPHP technique.

Effects of pH on Metals Precipitation and Sorption: Field Bioremediation and Geochemical Modeling Approaches

Abstract: At the Sanders Lead car-battery recycling plant, near Troy, AL, groundwater is highly acidic (pH varies from 3 to 35) and carries high concentrations of Pb, Cd, Zn, Cu, and Fe Pilot field experiments conducted at the site show that in situ metabolism of sulfate reducing bacteria (SRB) can produce desired geochemical effects to remove heavy metal Pb, Cd, Zn, and Cu from groundwater A reaction path model of sulfate reduction shows the redox potential (Eh) effects on mineral precipitation and pH controls on the sorption of different metals Lead strongly adsorbs to hydrous ferric oxide (HFO) present in the aquifer over a wide pH range Both sorption (due to pH increase) and solid sulfide formation are important for removing Pb Although theoretical modeling shows that the sorption of most cations is promoted as pH increases, HFO can only scavenge Zn, Cd, Co, and Ni at relatively neutral pH conditions Thus concentrations of our primary contaminants Zn and Cd attenuate in acidic conditions primarily via precipitation or coprecipitation of solid sulfide phase as Eh drops The modeling result explains why the Pb plume is retarded in migration with respect to the Cd plume under the low-pH conditions at the site For As, arsenate sorbs strongly onto the protonated weak sites of ferric hydroxide for the pH range of calculation Arsenite sorption is also favored by increasing pH, however, arsenite desorbs and becomes mobilized at very low oxidation state as it reacts with dissolved sulfide to form AsS complexes In addition, intermittent rainfall events could cause short-term Eh increases, potentially leading to oxidation of sulfide solids and subsequent pH decrease, and the remobilization of metals This study argues for the importance of accounting for pH changes when evaluating the fate, transport, and long-term stability of metals at shallow contaminated sites

Errors in Heat Flux Measurement by Flux Plates of Contrasting Design and Thermal Conductivity

Abstract: The thermal conductivity (lambda) of soils may vary by a factor of about 4 for a range of field soil water contents. Measurement of soil heat flux (G) using a heat flux plate with a fixed lambda distorts heat flow through the plates and in the adjacent soil. The objectives of this research were to quantify heat flow distortion errors for soil heat flux plates of widely contrasting designs and to evaluate the accuracy of a previously reported correction. Six types of commercially available heat flux plates with varying thickness, face area, and thermal conductivity (lambda(m)) were evaluated. Steady-state laboratory experiments at flux densities from 20 to 175 W m-2 were completed in a large box filled with dry or saturated sand having lambda of 0.36 and 2.25 W m-1 K-1. A field experiment compared G measured with pairs of four plate types buried at 6 cm in a clay soil with G determined using the gradient technique. The flux plates underestimated G in the dry sand by 2.4 to 38.5% and by 13.1 to 73.2% in saturated sand while in moist clay plate performance ranged from a 6.2% overestimate to a 71.4% underestimate. Application of the correction generally improved agreement between plate estimates and independent G measurements, especially when lambda > lambda(m), although most plate estimates were still significantly lower than the actual G. Limitations of the correction procedure indicate that renewed effort should be placed on innovative sensor designs that avoid or minimize heat flow distortion and/or provide direct, in situ calibration capability.

Unstable Flow during Redistribution in Homogeneous Soil

Abstract: Two- and three-dimensional laboratory experiments were conducted in a large 1 by 1 m 2 Hele–Shaw cell and a 10-cm cylinder column to study the stability of the redistribution process in a uniformly packed porous medium of coarse sand. Our results demonstrate that fingers form and propagate rapidly as soon as infiltration stops when the soil is initially dry, but form more slowly and are larger when the soil is wet. Finger widths ranged from about 4.5 cm when the soil was dry to 17 cm when the soil was very wet, and the fraction of the cross-sectional area occupied by fingers also increased (from 36 to 66%). Finger velocities declined with time in all studies, with averages ranging from 3.6 to 9.6 cm min −1 . We demonstrated that ponded infiltration was stable and uniform in our system, so that the fingering we observed was not due to soil heterogeneity. The porous medium retained a memory of the fingers formed in the first experiment, so fingers formed in subsequent redistribution cycles followed the old finger paths, even after 28 d had elapsed. Fingers provide channels for rapid drainage of previously infiltrated water, especially when the soil ahead of the front is dry. Our findings clearly contradict predictions made by the Richards equation, which calculates stable flow during redistribution in homogeneous soil.

Estimating Soil Hydraulic Parameters of a Field Drainage Experiment Using Inverse Techniques

Abstract: Accurate assessment of water flow and contaminant transport in unsaturated porous media at the field scale is often hindered by difficulties associated with obtaining reliable estimates of soil hydraulic properties. The unsteady drainage-flux method is one of the commonly used methods to measure in situ unsaturated hydraulic properties of soils. However, the properties obtained by this method using instantaneous profile data analysis may not be the best estimation of actual values of hydraulic properties. We present an improved analysis of the data from drainage experiments using inverse modeling, which uses nonlinear regression methods to estimate hydraulic parameters. Parameter identifiability is evaluated through sensitivity and uniqueness analyses. We used the combination of the inverse modeling program, UCODE, with the flow simulator, STOMP, for inverse modeling. Applying the inverse method to a field drainage experiment in sandy soil showed that all the van Genuchten (1980) hydraulic parameters could be estimated uniquely when both water content (theta) and pressure head (h) data were used. The parameter estimates by inverse technique using both theta and h data simulated the flow better than the parameter values obtained by the conventional instantaneous-profile analysis method. After the spatial and temporal sensitivities were analyzed, a more rational experimental design was recommended.

Soil Aggregate Structure Effects on Dielectric Permittivity of an Andisol Measured by Time Domain Reflectometry

Abstract: Various types of soil physical properties are affected by texture and structure. Our objective was to determine aggregate structure effect on the soil dielectric property of an Andisol measured by time domain reflectometry (TDR). The relationships between volumetric water content (θ) and dielectric permittivity (e) for both a wet-sieved aggregate and its crushed sample were examined and compared. In the θ–e relationship for 0.1- to 2.0-mm-diam. wet-sieved aggregates, the gradient of the θ–e curve moderately changed at a volumetric water content (critical water content), although this property disappeared when we crushed the aggregate structure. Furthermore, the critical value corresponded to the water content of the plateau in a bimodal-type water retention curve. We suggest that effects of aggregate structure on a soil9s dielectric property are involved in the aggregate sizes, the configuration of water in aggregates, the processes of water filling in intra- and interaggregate pores, and the low e value of bound water adsorbed on soil surfaces.

A Modified Vadose Zone Fluxmeter with Solution Collection Capability

Abstract: To assess contaminant fluxes in the vadose zone water flux and solute concentrations must be known but they are seldom measured simultaneously at the same location. A water fluxmeter (WFM) with divergence control was modified to measure solute concentrations by adding a funnel and collection vial to the bottom of the meter. Laboratory experiments using coarse and fine sands showed that measured solute concentrations and known water fluxes can be combined to provide estimates of solute flux. Water containing a NO⁻ ₃ tracer was applied at a rate of 1.97 × 10⁻⁸ m s⁻¹ (621 mm yr⁻¹), and water flux was simultaneously measured along with NO⁻ ₃ concentrations in the outflow water. The general agreement in fitted and measured pore-water velocities suggests that the breakthrough curves of NO⁻ ₃ measured using the drainage through the WFM can be used to estimate the pore-water velocity of the soil. Solute travel-time through the 60-cm-long wick was <10% of the travel time through the sands and could be neglected. Flow divergence was examined by measuring the soil water content and pressure head at different positions and by measuring the water flux passing through and around the WFM. Divergence was controlled by a 15-cm-high barrier such that more than 80% of the flow passed through the fluxmeter in both soils. Results show that the modified SFM can provide a convenient method for long-term monitoring of contaminant flux.

Physical Processes Affecting Natural Depletion of Volatile Chemicals in Soil and Groundwater

Abstract: A Fickian model is described for dispersive vapor transport due to “pumping” induced by barometric pressure fluctuations and periodic water table fluctuations. The approach is appropriate for time scales that are large relative to the period of induced airflow variations. Comparisons of the magnitude of dispersive fluxes with those due solely to molecular diffusion indicated that dispersive vapor transport becomes increasingly important as soil porosity decreases and as the depth to groundwater and the contaminant source increases. For soils with low air-filled porosity, barometric pumping is likely to dominate transport even for shallow soils. Barometric pumping may predominate for soils with moderate to high air-filled porosity with deeper groundwater ( >5–15 m). Water table pumping is predicted to predominate over diffusion only for high-frequency fluctuations, such as tidal conditions. A steady-state model for contaminant volatilization from groundwater is presented that considers diffusive and dispersive vapor transport, unsaturated zone aqueous phase advection, and dispersive mixing in groundwater, yielding an apparent first-order decay coefficient with respect to groundwater. Predicted volatilization coefficients for perchloroethene (PCE) range from 0.02 d −1 for various soil conditions and groundwater depths. Highest values are predicted for the most permeable soils. Volatilization rates are predicted to decrease with depth up to a point at which dispersive fluxes dominate over diffusion and then to increase to the extent that barometric pressure fluctuations propagate to greater depths. Vertical mixing within the saturated zone has a significant influence on volatilization from groundwater. Simple moving front and mixing cell models are presented to estimate depletion rates of soil contamination due to volatilization and leaching. Results indicate that natural depletion of residual soil NAPL may take many decades and is markedly influenced by soil conditions, hydraulic flux, and contaminant properties.

Toward Improved Identifiability of Soil Hydraulic Parameters: On the Selection of a Suitable Parametric Model

Abstract: We present a thorough identifiability analysis of the soil hydraulic parameters in the parametric models of Brooks and Corey (BC; Brooks and Corey, 1964), Mualem-van Genuchten (VG; van Genuchten, 1980), and Kosugi (KC; Kosugi, 1996, 1999) using the recently developed Shuffled Complex Evolution Metropolis (SCEM-UA) algorithm (Vrugt et al., 2002b, and unpublished data). Because the SCEM-UA algorithm globally thoroughly exploits the parameter space and therefore explicitly accounts for parameter interdependence and nonlinearity of the employed parametric models, the algorithm is suited to generate a useful description of parameter uncertainty and its antithesis, parameter identifiability. A set of measured water retention characteristics of the UNSODA database (Leij et al., 1996) spanning a wide range of soil textures and three transient laboratory outflow experiments with decreasing flow rates were used to illustrate that a parameter identifiability analysis facilitates the selection of an adequate parametric model structure and provides useful information about the limitations of a model. Moreover, results suggest that one should be especially careful in establishing pedotransfer functions without knowledge of the underlying posterior uncertainty associated with the soil hydraulic parameters using direct estimation methods.

Near-Surface Soil Water Content Measurements Using Horn Antenna Radar: Methodology and Overview

Abstract: Ground-penetrating radar (GPR) with a suspended 1-GHz horn antenna was deployed over bare and vegetated soil surfaces using surface reflection (SR) magnitudes and propagation times (PT) to calculate bulk soil dielectric permittivity and soil water contents Concurrent radar measurements over sand, Millville silt loam (coarse-silty, carbonatic, mesic Typic Haploxerolls), and sand–bentonite surfaces showed rapid drainage from sand and slower drainage from higher-surface-area textured soils Soil texture and temperature affected diurnal variations in measured water content (occurrence of minima and maxima) for both SR and 2-cm time-domain reflectometry (TDR) water content data Measurements over wheat canopy showed that while SR values were strongly altered by canopy biomass, PT measurements remained unaffected Wheat canopy influence on SR gradually intensified during the growth season until the canopy was removed and SR-based measurements rejoined with PT data Hornantenna radar measurements over natural surfaces offer a promise for remote mapping of soil texture and truthing of radar data collected from air- and spaceborne platforms, and they may be used in the field for water content and vegetation biomass measurements

Moisture characteristics of Hanford gravels: Bulk, grain-surface, and intragranular components

Abstract: Although gravels comprise large portions of some vadose zones, their unsaturated hydraulic properties have received relatively little attention. This study examines moisture retention relations in the 2- and 6-mm size fractions of gravels from the Hanford formation vadose zone (Washington State). Understanding flow and transport within this formation is important because parts of it have become contaminated by leakage of radioactive wastes at the Hanford Site. Moisture retention relations were obtained for a very wide energy range, with attention to water retained in intragranular pores and along grain surfaces. External surfaces of these gravels have root mean-squared roughnesses (rmsr) in the micrometer range, with sparsely distributed deep (hundreds of micrometers) pits. Water films on these external surfaces are volumetrically insignificant at matric potentials less than about −2 kPa. Residual water in these gravels occurs in intragranular pores, accounts for about 10% of the total porosity, and is effectively hydraulically immobile. The intragranular domain in Hanford gravels also has a large specific surface area of about 11 m 2 g −1 . Thus, exchanges of solutes (including contaminants) between the intragranular domain of Hanford gravels and their immediate surrounding are significant and diffusion limited.

Field Application of a Portable Air Permeameter to Characterize Spatial Variability in Air and Water Permeability

Abstract: The saturated hydraulic conductivity ( K s) is an essential parameter for modeling water and chemical transport in the vadose zone. Since in situ measurements of K s are complex and time-consuming, indirect methods that are dependable, fast, and inexpensive with regard to assessing magnitude and spatial variability in K s at the field scale are needed. In situ measurements of air permeability ( k a,in situ) may fulfill these criteria. In this study, a portable insertion-type air permeameter was used to measure k a,in situ in the Ap and B horizons at five agricultural field sites in Denmark with soil types ranging from sand to sandy loam. Around 100 k a,in situ measurements were performed within 2 d at each field site. The data showed spatial correlation in k a,in situ at three out of five sites, with correlation distances between 30 and >120 m. On the basis of additional laboratory measurements on large, undisturbed soil samples (6280 cm3), a log-log linear relationship between air permeability ( k a) measured at the actual soil-water content (close to field capacity) and K s was found. The K s– k a relation was in agreement with an earlier predictive relationship based on undisturbed 100-cm3 samples from nine other field sites. Using pedotransfer functions for K s based only on soil texture yielded an unrealistic narrow range in predicted K s values whereas pedotransfer functions based on k a,in situ yielded a more realistic prediction range. Measurements of k a,in situ constitute a promising indirect method for assessing spatial variability in K s at the field scale.

Numerical Simulations of One-Dimensional Infiltration into Layered Soils with the Richards Equation Using Different Estimates of the Interlayer Conductivity

Abstract: Transient water flow processes in unsaturated soils are usually modeled using the Richards equation. This paper compares several numerical approximations to this equation for vertical infiltration in layered soil profiles. Three formulations of the governing flow equation (i.e., the h -based, θ-based, and mixed forms) are compared for the critical test problem of infiltration into a layered soil profile with initially relatively low soil water contents. An efficient, yet relatively simple weighting algorithm is employed that improves the estimation of the interlayer hydraulic conductivity in an h -based finite-difference formulation. Results highlight improvements in the mass conservative properties of this model, which is termed the H-IL model . A comparison is then performed between the finite-difference H-IL model and a finite-element model for infiltration toward a water table. The H-IL model was found to be computationally very efficient for this test problem. For both illustrative flow examples, the different numerical models were evaluated in terms of their ability to reproduce an exact solution developed by Srivastava and Yeh (1991) in their work on one-dimensional, transient infiltration toward the water table in homogeneous and layered soils.

Monitoring Temporal Development of Spatial Soil Water Content Variation: Comparison of Ground Penetrating Radar and Time Domain Reflectometry

Abstract: We compare the capability of ground penetrating radar (GPR) and time domain reflectometry (TDR) to assess the temporal development of spatial variation of surface volumetric water content. In the case of GPR, we measured surface water content with the ground wave, which is a direct wave between the sender and receiver through the upper centimeters of the soil. Spatial water content variation was measured on 18 d with GPR and TDR during a 30-d monitoring period. To ensure large fluctuations in the spatial water content variation, we created a heterogeneous pattern of water content by irrigation on 2 d. The temporal development of the spatial variation was studied by means of the variogram and interpolated water content maps. To compare GPR and TDR variograms, we estimated confidence intervals of the experimental variograms and the variogram model parameters with a jackknife approach and a first-order approximation of model parameter uncertainty. The results showed that the 95% confidence intervals of the GPR experimental variogram were one to two orders of magnitude smaller than the 95% confidence intervals of the TDR experimental variogram because of the larger number of GPR measurements. Consequently, the uncertainty in the variogram model parameters was also much lower for GPR, which meant that the temporal development of the fitted GPR variogram model parameters was easier to interpret. Furthermore, the larger GPR measurement volume resulted in a low spatial nugget variance of 1 × 10-6 to 1 × 10-9 (m3 m-3)2 because short distance variation was averaged. This meant that GPR accurately measured spatial correlation lengths, even in the case of low water content variation. Interpolated maps showing the increase of water content due to irrigation and the subsequent gradual drying of the soil were more accurate and reproducible for GPR. It was concluded that the noninvasive GPR measurements provide the means to accurately and consistently monitor the development of spatial water content variation in time.

Soil Genesis and Classification, Fifth Edition.

Abstract: STANLEY W. BUOL, RANDAL J. SOUTHARD, ROBERT C. GRAHAM, and PAUL A. McDANIEL. Iowa State Press, 2121 South State Avenue, Ames, Iowa 50014. 2003. Hardback, 494 p. $79.99. ISBN 0-8138-2873-2. This book contains the most current information for the scientific field of pedology, the study of soil

Flexible Time Domain Reflectometry Probe for Deep Vadose Zone Monitoring

Abstract: Accurate determination of water content is an important aspect of most vadose zone monitoring programs. Real-time, continuous, in situ measurements of water content in relatively undisturbed conditions are usually limited to shallow soil horizons. We present a new methodology using time domain reflectometry (TDR) for water content monitoring in deep vadose zone horizons. The method uses flat, flexible, waveguides pressed against the wall of a borehole. The flexible TDR waveguides are attached to the outer side of a flexible sleeve filled with a liquid resin. The resin (e.g., a two-component urethane) generates hydrostatic pressure that forces the flexible waveguides against the borehole wall, ensuring a close fit to the irregular shape of the borehole walls. The probe can be used with either a standard TDR technique, which uses a cable tester (e.g., Tektronix 1520, Tektronix, Beaverton, OR) for collecting waveforms, or a water content reflectometer (e.g., model CS505, Campbell Scientific, Inc., Logan, UT), which provides a direct electrical output, which may be sampled using a data logger. Laboratory calibration experiments and a full-scale field experiment showed that the method is reliable and capable of providing accurate water content measurements in deep vadose zone horizons.