Modeling Evapotranspiration of Two Land Covers Using Integrated Hydrologic Model

Modeling evapotranspiration (ET) distribution in shallow water table environments is of great importance for understanding and reproducing other hydrologic fluxes such as runoff and recharge. Unfortunately, ET distribution can be the most difficult hydrologic process to analyze. The partitioning of ET into upper zone ET, lower zone ET, and groundwater ET is complex because it depends on land cover and subsurface characteristics. One comprehensive distributed parameter model, integrated hydrologic model (IHM), builds on an improved understanding and characterization of ET partitioning between surface storages, vadose zone storage, and saturated groundwater storage. It provides a smooth transition to satisfy ET demand between the vadose zone and the deeper saturated groundwater. In this paper, the IHM was used to analyze ET contribution from different regions of the vadose zone and saturated zone. Rigorous testing was done on two distinct land covers, grass land and forest land, at a study site in West-Central Florida. Sensitivity analysis on the key parameters was investigated and influence of parameters on ET behavior was also discussed. Statistics with the root mean square error and mean bias error for forest total ET were about 1.46 and 0.04 mm/day, respectively, and 1.61 and 1.07 mm/day for grass total ET. Modeling results further proved that ET distributions from the upper and lower soil and water table, while incorporating field-scale variability of soil and land cover properties, can be predicted reasonably well using IHM model.     

Analytical Modeling of Contaminant Transport through Vadose and Saturated Soil Zones

Screening level analyses for risk-based corrective actions and preliminary remediation investigations and feasibility studies for contaminated sites require simple and conservative models for the transport of contaminants to potential receptor locations. A quasi-analytical method is presented to analyze the transport of contaminants originating at a source located near the ground surface separated from the water table by a relatively thick vadose zone. The method includes one segment to compute source decay rate, a second to simulate vertical transport through the vadose zone, and a third to simulate saturated zone transport to the receptor. Practical application of the method is demonstrated by an illustrative example. The method may be useful and convenient for situations where the objectives of the analysis do not warrant the use of sophisticated numerical models.     

Sustainable Root Zone Salinity and Shallow Water Table in the Context of Land Retirement

This study uses five years of field data from the Land Retirement Demonstration Project located in western Fresno County of California to develop a comprehensive theoretical and numerical modeling framework to evaluate the specific site conditions required for a sustainable land retirement outcome based on natural drainage. Using field data, principles of mass balance in a control volume, the HYDRUS-1D software package for simulating one-dimensional movement of water, heat, and multiple solutes in variably-saturated media, and a model-independent parameter optimizer, the processes of soil water and solute movement in root zone and deep vadose zone were investigated. The optimization of unsaturated soil hydraulic parameters and downward flux (natural drainage) from the control volume against observed vadose zone salinity levels and shallow groundwater levels yield difficult to obtain natural drainage rate as a function of water table height within the control volume. The results show that the unsaturated soil hydraulic properties and the downward flux from the soil profile are the critical parameters. A “natural drainage approach” to sustainable land management for drainage-impaired land is proposed. With this approach it is feasible to design a sustainable land use regimen for drainage-impaired lands in general and retired lands in particular.     

Centrifuge Modeling of Nonaqueous Phase Liquid Movement and Entrapment in Unsaturated Layered Soils

Four geotechnical centrifuge tests with different soil layered systems were performed to investigate the movement and entrapment of water and of light nonaqueous phase liquids (LNAPLs) in unsaturated layered soil deposits. The tests were performed at 20 g and a vadose zone condition was created during the centrifuge tests by lowering the water table from the initially water saturated condition. During the water drainage stage, the water distribution within the models and the dynamic air–water capillary pressure saturation relationships of the three sands were obtained using tensiometers and resistivity probes. After achieving the unsaturated condition, a model LNAPL (Soltrol 220? or silicon oil) was injected near the soil surface and the movement and entrapment were monitored during the redistribution stage until the LNAPL reached the top of the water table. Complex LNAPL preferential flow and entrapment patterns were observed in the layered models with different textural interfaces due to the relative movement of all three phases [water, nonaqueous phase liquid (NAPL), and air]. The centrifuge tests data coupled with the numerical analyses show that NAPL properties, subsurface soil structures, initial water saturation, and NAPL infiltration rate affect the variation in entrapment conditions in heterogeneous unsaturated soil deposits.     

Vadose Zone Flow and Transport in Cracked Heavy Textured Soil

Investigated were physical processes governing vadose zone water flow and solute transport in a heavy-textured crack-prone soil of the semiarid Canadian Prairies. Soil moisture, soil temperature, and relevant meteorological observations were recorded over a period of about two years. Environmental chloride and tritium concentrations in the soil water were determined. Analysis of the data indicates that during snow melt, water and solutes are flushed down rapidly via cracks and fissures in the root zone. The soil at these depths is still frozen during snow melt. In late spring after the entire soil profile thaws, water and solutes move downward by a diffusion dominant advective-diffusive flow mechanism. The chloride and tritium profiles obtained within the vadose zone support this argument. This conceptual model of the flow and transport processes is supported by calculations of advective and diffusive soil water flux from chloride profiles. Simulation of the tritium profile using a simple analytical model also gives very good agreement with measured data.     

Flow of Gasoline through Composite Liners

A composite liner composed of a soil/clay liner and a flexible membrane is widely used for waste containment facilities. In this research, an organically modified clay (organoclay BB-40) liner and a high-density polyethylene (HDPE) membrane were studied for preventing the leakage and migration of gasoline from underground storage tanks into the surrounding environment. The equivalent hydraulic conductivity of intact HDPE to gasoline was determined using a specially built system, and the conventional hydraulic conductivity testing method was employed to determine the hydraulic conductivity of compacted organoclays and the permeation rate of gasoline through composite liners. The equivalent hydraulic conductivity of intact HDPE to gasoline was about 10–13 cm/s, and the hydraulic conductivity of the organoclay liner was approximately 6.0×10?9?cm/s, which is nearly 4 orders of magnitude lower than that obtained for unmodified clay. These results show that both organoclay and HDPE are effective in reducing the release of gasoline by advective flow, especially the intact HDPE. The flow of gasoline through the composite liners under the worst condition, was of the same magnitude as that through a single organoclay liner, independent of the flow shape. It can be anticipated that under good contact conditions, the defective HDPE would still be beneficial in reducing the permeation of gasoline due to the decrease of the wetted area of the underlying layer exposed to gasoline leakage.     

Difluoromethane as Partitioning Tracer to Estimate Vadose Water Saturations

Water saturation in the vadose zone is an important parameter for many nonaqueous phase liquid (NAPL) remediation technologies. Conventional soil boring analyses may not provide accurate average water saturation data. Previous studies have shown that a partitioning interwell tracer test (PITT) can provide an accurate estimate of average subsurface NAPL saturations. The PITT is proposed as a suitable technology for estimating average subsurface water saturations. In this research, difluoromethane was evaluated as a suitable tracer compound to partition among air, water, and NAPL phases. Method of moments analyses were used to develop equations necessary to determine water saturations from difluoromethane PITT results. One-dimensional column experiments provided air-water and air-NAPL partition coefficient data for difluoromethane. Difluoromethane is shown to be a suitable tracer for use in field PITTs to predict water saturations in the vadose zone.     

Case Study of Water Table Evaporation at Ichkeul Marshes (Tunisia)

The method of Gardner was used to estimate the evaporation rate from bare soils under high water table conditions, at Ichkeul marshes, of northern Tunisia. For this purpose, water content, soil-water suction, and water table depth were measured at three sites. Other common approaches that provide estimates of water table evaporation such as the Averianov and Coudrain-Ribstein et al. empirical formulas and the approach used by the U.S. Geological Survey's groundwater flow model MODFLOW were also evaluated. Next, a 2D groundwater simulation model, using the Gardner equations for computing evaporation rate, was developed to quantify the aquifer budget and groundwater losses via evaporation at the total area of the marshes. The model was calibrated in a steady-state condition. The results indicated significant groundwater losses by water table evaporation. The sensitivity of the groundwater model to the Averianov and MODFLOW approaches was analyzed. The best results (piezometric head and the area distribution of the evaporation discharge) were obtained with the Averianov formula.     

Water Flow through Cover Soils Using Modeling and Experimental Methods

The vertical flow of water in cover soils is simulated using published analytical and finite-element methods. The two methods gave virtually identical pressure head and water content profiles during steady infiltration of water in a multilayer soil cover and transient infiltration in a single-layer cover. The finite-element model was then used to simulate flow in two laboratory columns packed with multilayer soils and subjected to downward drainage and conditions of evaporation and no evaporation. The model adequately predicted transient pressure heads and water contents for the first 7.5 h of drainage in a till-sand layer without evaporation. Predictions at times equal to and greater than 3 days were not as good, probably due to the formation of discontinuous water pockets in the draining sand around the residual water content, which apparently produced “locked-in” or “static” nonequilibrium pressures. These pressures are not captured by existing methods used for estimating the unsaturated hydraulic conductivity–pressure function of soils. Further modeling showed that at times greater than 8 days, the flux from the column with evaporation was all in the vapor phase.     

Groundwater Reaeration and Hydrocarbon Plume Length: A Modeling Analysis

A series of three-dimensional numerical modeling experiments was conducted to determine the impacts of reaeration on the bioattenuation of petroleum hydrocarbon contaminants in groundwater systems. The influence of reaeration on plume length and mass loss was affected by four mechanisms including the groundwater mass transfer coefficient KL,GW, the concentration on partial pressure of O2 in the overlying soil gas PO2, the aquifer vertical dispersivity αv, and the reaeration zone Z or region of mixing between contaminated ground water and O2 supplied from the vadose zone. Results from this study showed that reaeration may reduce steady-state hydrocarbon plume length and mass by up to 87% as compared to the case in which reaeration is not considered. Among the four mechanisms, αv exerted the greatest influence on plume length followed by KL,GW, PO2, and Z. Comparisons between plume length and the ratio of reaeration transport KL,GW to hydraulic conductivity K indicated that appreciable reductions in steady-state plume length were possible when KL,GW/K ≥ 10?3.     

Modeling of Clay Liner Desiccation

The potential for the desiccation of clay liner component of composite liners due to temperature field generated by breakdown of organic matter in municipal solid waste landfills is examined using a model proposed by Zhou and Rowe. In these analyses, a set of fully coupled governing equations expressed in terms of displacement, capillary pressure, air pressure, and temperature increase are used, and numerical results are solved by using finite element method with a mass-conservative numerical scheme. The model results are shown to be in encouraging agreement with experimental data for a problem involving heating of a landfill liner. The fully coupled transient fields (temperature, horizontal stress change, suction head, and volumetric water content) are then examined for two types of composite liner system, one involving a geomembrane over a compacted clay liner (CCL) and the other involving a geomembrane over a geosynthetic clay liner (GCL). It is shown that there can be significant water loss and horizontal stress change in both the CCL and GCL liner even with a temperature increase as small as 20°C. The time to reach steady state decreases as boundary temperature increases. Under a 30°C temperature increase, it takes 5 years to reach the steady state water content with a GCL liner but 50 years with a CCL liner. The effects of various parameters, such as hydraulic conductivity and thickness of the liner, on the performance of the liner are discussed.     

Foundation Isolation for Seismic Protection Using a Smooth Synthetic Liner

Smooth synthetic materials placed underneath foundations of structures can provide seismic protection by absorbing energy through sliding. Cyclic and shaking table tests were conducted on a variety of synthetic interfaces to identify a suitable liner for use as foundation isolation. It was concluded that a high strength, nonwoven geotextile placed over an ultrahigh molecular weight polyethylene, UHMWPE (geotextile/UHMWPE) constitutes a liner that is well suited for this application. The static friction coefficient of the interface (between the geotextile and the UHMWPE) is about 0.1. The dynamic coefficient is about 0.07 and is insensitive to changes in slip rate and normal stress. A single-story structural model with and without foundation isolation was tested using a shaker table. The results demonstrate the role of foundation isolation in substantially reducing the seismic shear forces in the model. Accompanying this reduction in shear forces are slip displacements along the isolation liner. Permanent slip (final location of the structure relative to its initial position) can be reduced through the use of a small restoring force that could be provided through passive soil resistance. Peak-to-peak slip (maximum slip during shaking) needs to be permitted for foundation isolation to be effective. The experimental and analytical research results demonstrate the technical feasibility of using a smooth synthetic liner in earthquake hazard mitigation.     

Olympics in Atlanta: a fight against physics

Environmental stress can become so severe that athletes, in spite of proper training, heat acclimation, and hydration level, are unable to maintain thermal balance. Such incompensable conditions occur when air temperature exceeds 35 degrees C and relative humidity becomes higher than 60%. At these high environmental temperatures, the heat liberated during exercise can only be lost by evaporation of sweat, and therefore water vapor pressure sets limits on the possible rate of evaporation. Calculations are presented for the required and the maximal possible sweat evaporation rate for high-intensity, long-duration events, using marathon racing as an example. The consequence of the environmental heat stress is that the athlete must reduce the speed of running considerably to prevent potential heat injury. In certain extreme environmental conditions, sporting events should be canceled or postponed.     

诱导巷道的围岩松动破坏区数值研究

当前多数矿山已进入深部开采,而深部硬岩具有高储能、受扰动易破坏的特征,根据这些特点,在矿体中布置巷道诱导岩体破裂,为后续机械开采提供可能。以贵州开阳磷矿为工程背景,利用FLAC^3D软件模拟深部巷道开挖,研究断面形状、地应力水平和侧压系数对巷道开挖的影响,从而得出岩体的破裂规律。数值模拟结果表明：巷道断面形状对围岩变形影响较大,而矩形巷道周围岩体产生的位移和塑性破坏区均最大;巷道位移和塑性破坏区随应力水平的增加而增大;围岩破坏和位移随侧压系数的增大表现出先减小后增大的趋势。在高应力水平和高侧压系数下,采用矩形断面方式开挖巷道,围岩产生的位移和塑性破坏区最大,致裂效果最好,有利于机械切割回采。     

Evaluation of Multidimensional Transport through a Field-Scale Compacted Soil Liner

A field-scale compacted soil liner was constructed at the University of Illinois at Urbana-Champaign by the U.S. Environmental Protection Agency (USEPA) and Illinois State Geological Survey in 1988 to investigate chemical transport rates through low permeability compacted clay liners (CCLs). Four tracers (bromide and three benzoic acid tracers) were each added to one of four large ring infiltrometers (LRIs) while tritium was added to the pond water (excluding the infiltrometers). Results from the long-term transport of Br? from the localized source zone of LRI are presented in this paper. Core samples were taken radially outward from the center of the Br? LRI and concentration depth profiles were obtained. Transport properties were evaluated using an axially symmetric transport model. Results indicate that (1) transport was diffusion controlled; (2) transport due to advection was negligible and well within the regulatory limits of ksat ? 1×10?7?cm/s; (3) diffusion rates in the horizontal and vertical directions were the same; and (4) small positioning errors due to compression during soil sampling did not affect the best fit advection and diffusion values. The best-fit diffusion coefficient for bromide was equal to the molecular diffusion coefficient multiplied by a tortuosity factor of 0.27, which is within 8% of the tortuosity factor (0.25) found in a related study where tritium transport through the same liner was evaluated. This suggests that the governing mechanisms for the transport of tritium and bromide through the CCL were similar. These results are significant because they address transport through a composite liner from a localized source zone which occurs when defects or punctures in the geomembrane of a composite system are present.     

Measurement of Subsurface Unsaturated Hydraulic Conductivity

Measurement of unsaturated hydraulic conductivity is needed for precise control of water and solutes in the vadose zone. Because of the spatial variation of soils, a large number of surface and subsurface measurements are needed to characterize a field. In this work, permeameters were developed and tested for estimating subsurface unsaturated hydraulic conductivity. The permeameters apply water under tension; they are easy to use and have adequate accuracy. Unsaturated hydraulic conductivity was determined by measuring the steady flow rates for various values of negative pressure. Tests using a soil of known hydraulic conductivity showed that the permeameters provided valid measurements. Two types were used, a porous cloth model that was inflated against the soil and a porous ceramic cup that was rigid. The field testing determined that a rigid design using a ceramic cup coupled to the soil by a layer of fine sand was easier to use, was reliable, and provided good results.     

Contaminated Vadose Zone Characterization Using Partitioning Gas Tracers

This paper describes laboratory research conducted to investigate the performance of partitioning tracers for the detection of nonaqueous-phase liquids (NAPLs) in vadose zones. Once evaluated, the tracers may be used for volume estimation of NAPLs and remediation performance assessment of vadose zones. These laboratory studies used glass chromatography columns packed with: (1) Ottawa sand; and then (2) in a separate experiment, soil extracted from the Chemical Waste Landfill at Sandia National Laboratories. The columns were prepared in a manner that resulted in a three-phase system of air, water, and NAPL in the columns. Conservative and partitioning gas tracers were injected into the columns, and their elution concentrations were analyzed. The method of moments was used to estimate partition coefficients between the air and NAPL phases for each of the tracers. The partition coefficients and retardation factors, also estimated during the study, are used to select appropriate tracers for NAPL detection. This research identified several suitable perfluorocarbon tracers and demonstrated the feasibility of using partitioning tracers as a tool for NAPL detection in the vadose zone.     

Soil Salinity Modeling over Shallow Water Tables. II: Application of LEACHC

Root zone salinity is one of the major factors adversely affecting crop production. A saline shallow water table can contribute significantly to salinity increases in the root zone. A soil salinity model (LEACHC) was used to simulate the effects of various management alternatives and initial conditions on root zone salinity, given a consistently high water table. The impact of water table salinity levels, irrigation management strategies, soil types, and crop types on the accumulation of salts in the root zone and on crop yields was evaluated. There were clear differences in soil salinity accumulations depending upon the depth and salinity of the water table. In general, increasing water table depth reduced average soil profile salinity, as did having lower salinity in the water table. Among the four irrigation strategies that were compared, the 14-day irrigation interval with replenishment of 75% of evapotranspiration (ET) resulted in the lowest soil salinity. With a 4-day interval and 50% ET replenishment, a wheat yield reduction of nearly 40% was predicted after three years of salt accumulation. Soil type and crop type had minimal or no impact on soil salinity accumulation. Under all conditions, soil water average electrical conductivity increased during the 3-year simulation period. This trend continued when the simulation period was extended to 6 years. Under the conditions shown to develop the highest average soil salinity (high water table, low irrigation), an annual presowing irrigation of 125 mm caused a nearly 50% reduction in soil salinity at the end of the 6-year simulation period, as compared with the soil salinity given no presowing irrigation.     

Evaluating Ozone as a Remedial Treatment for Removing RDX from Unsaturated Soils

Years of wastewater discharge at the Department of Energy’s Pantex Plant have contaminated the vadose zone and underlying perched aquifer with hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). Because the vadose zone is acting as a continual source of groundwater contamination, removing RDX from the unsaturated zone is paramount to prevent further contamination. We determined the efficacy of ozone to degrade and mineralize RDX. Solution experiments showed that ozone (27?mg?L?1; 150?mL?min?1) was effective in mineralizing 80% of the RDX (30?mg?RDX?L?1) provided that some Pantex soil was present to buffer the solution pH. Soil columns treated with ozone produced 50% RDX mineralization within 1 day and >80% within 7 day. Experiments designed to evaluate aerobic biodegradation following partial ozonation of a RDX solution showed that ozone-generated RDX products were much more biodegradable than untreated controls in aerobic microcosms (35 versus <0.3% cumulative mineralization). These results support the use of ozone as a remedial treatment for the contaminated vadose zone at the Pantex facility.