Unsaturated Soil Zone Research Articles

The Case for Direct Measures of Soil-to-Groundwater Contaminant Mass Discharge at AFFF-Impacted Sites.

Many entities around the world are initiating massive field campaigns to characterize the environmental distribution of per- and polyfluoroalkyl substances (PFAS), particularly at aqueous film-forming foam (AFFF) impacted sites where historic point-source discharges occurred at the ground surface. Concurrently, many regulatory agencies are publishing criteria used in practice to define the "nature and extent" of PFAS-impacted environmental media. Specific to the soil-to-groundwater transport pathway protective of the groundwater ingestion end point, these soil criteria (or screening values) are to date exclusively based on the traditional approach used for hydrophobic organics with a number of simplifying assumptions. Research has clearly contradicted these assumptions, yet alternative methodologies have yet to emerge from the literature. This Perspective provides a rationale for why alternative approaches are critically necessary and proposes the application of lysimetry as a practical solution to accurately assess PFAS transport within unsaturated vadose zone soils. Ultimately, there is an urgent need to justify soil remediation on the basis of groundwater protection and to prioritize remedial efforts in order to optimize limited fiscal resources.

Verification of tree induced suction with numerical model

This study discusses the validation between explorations of transpiration on the unsaturated soil zone with unsaturated flow equation of tree water uptake model. Numerical simulation was performed to the model of the slope in relation to moisture migration patterns in the unsaturated zone within the vicinity of mature tree. The results of numerical simulation conducted on typical slope geometry model with tree at the toe of the slope of induced transpiration were later presented in this study. The results of numerical simulation and field monitoring of suction against depth are in acceptable condition. It is important to note that, several of the differences between simulated numerical and field measurement are related to inconsistence effect of root density.

Analysis of The Migration and Accumulation Process of Nitrate-nitrogen Pollutants in The Unsaturated Zone of Soil

In order to understand the migration process of nitrate-nitrogen pollutants in the shallow unsaturated zone of the soil, the Tianjin coastal dredger and typical fluvo-aquic soils in North China were used as the research objects. The Hydrus-1D software was used to establish a numerical model to simulate nitrate. The pollution process of nitrogen pollutants in the unsaturated zone of soil, the results show that: during the migration process of the two kinds of shallow vadose zones of soil, the nitrate-nitrogen pollutants migrate downward at the maximum concentration before reaching the maximum concentration. After the maximum concentration, the concentration of nitrate nitrogen in the soil no longer increases; due to the difference in the average particle size of the soil, the migration rate of nitrate nitrogen in fluvo-aquic soil is significantly greater than that in artificial dredger soil. Nitrate nitrogen is in the simulation period completely passing through the simulated soil layer, the fluvo-aquic soil is completely passed through by nitrate nitrogen at 2d, and the artificial dredge fill is completely passed through by nitrate nitrogen at 2.5d.

Detection and quantification of underground CO2 leakage into the soil using a fiber-optic sensor

• The fiber-optic sensor was implemented in the field for underground CO 2 gas leakage monitoring. • Hollow-core photonic crystal fiber was used to improve the sensitivity of the sensor. • The integrated sensor package was used to cancel complex crosstalk due to the temperature and humidity changes. • The proposed fiber-optic sensor has the potential to monitor the gas leakage continuously and remotely. This study presents a field demonstration of underground CO 2 leakage detection using a fiber-optic interferometric sensor at an artificial leakage experiment site. CO 2 gas was injected for a couple of days into the unsaturated soil zone and CO 2 leakage monitoring was continuously performed for nine days using the fiber-optic sensor buried in the soil, using a hollow core photonic crystal fiber (HC-PCF) based sensor. Temperature and relative humidity in the soil were simultaneously measured by the commercial sensors. The measurement results of the HC-PCF sensor were evaluated by comparing with those by the commercial non-dispersive infrared sensor. This comparison illustrated that the proposed HC–PCF sensor was able to in-situ monitor CO 2 gas concentration with a range of 0–100%. Accordingly, the proposed fiber-optic sensor has the potentials to provide a reliable assessment of CO 2 gas leakage in geologic carbon storage sites and other gas leaking sites.

Purification Efficiency for Treated Waste Water in Case of Joint Infiltration with Water Originating from Precipitation

The decentralized treatment of wastewater and its on-site infiltration is common practice, especially in rural areas. However, uncertainties exist on the degradation potential of treated wastewater constituents mixed with additional infiltration of large quantities of water originating from precipitation. The intermixture of these waters is causing a reduction of residence times of the treated wastewater as well as an increased water saturation within the unsaturated soil zone. This can lead to a negative influence on the purification efficiency. Laboratory-scale 1D column experiments, accompanied by numerical simulations of water flow using the software code HYDRUS 1D, have been performed to evaluate the risks for a reduction of the degradation efficiency. Water content measurements and analysis of relevant organic substances in samples taken at different depths were the basis for evaluating the influence of the joint infiltration on the purification efficiency. The results highlight that a joint infiltration of treated wastewater and additional water originating from precipitation is not affecting the degradation efficiency for treated wastewater constituents. Degradation rates under these conditions were similar to the infiltration of treated wastewater alone. Timeframes with high water saturation were limited to the duration of the precipitation event.

Anaerobic Respiration in the Unsaturated Zone of Agricultural Soil Mobilizes Phosphorus and Manganese.

Anaerobic conditions mobilize phosphorus (P) in soils and sediments. The role of anaerobic microsites in well-drained soil on P migration is unknown. This study aimed to identify mechanisms that control field-scale vertical P mobility as affected by organic fertilizers that may trigger variable redox conditions. Soils were sampled at different depths in a well-drained Luvisol after 19 years of application of organic fertilizers. The concentrations of P and manganese (Mn) in 0.45-μm-filtered extracts (10-3 M CaCl2) of field-moist soil samples were strongly correlated (r = + 0.95), and both peaked in and below the compacted plough pan, suggesting that reductive processes mobilize P. Waterlogged soil incubations confirmed that anaerobic respiration comobilizes Mn and P and that this leads to the release of colloidal P and iron (Fe). The long-term applications of farmyard manure and immature compost enhanced the concentrations of Mn, Fe, and aluminum (Al) in the soil solution of subsurface samples, whereas less such effect was found under the application of more stable organic fertilizers. Farmyard manure application significantly enhanced soil P stocks below the plough layer despite a small P input. Overall, multiple lines of evidence confirm that anaerobic respiration, sparked by labile organic matter, mobilizes P in this seemingly well-drained soil.

Numerical Investigation of Propagation of BTEX Compounds in Soil

BTEX compounds are among the light hydrocarbons of gasoline which have been noticed due to their toxicity and long term damaging effects in the environment. Understanding and prediction of how contaminations propagation are of great importance for designing an appropriate remediation method. In this paper, propagation pattern of BTEX compounds in saturated and unsaturated zones of soil is investigated by the finite element method. To investigate the accuracy and reliability of flow and transport modules of MOFAT software two different numerical analyses have conducted based on experimental and in-situ data and the obtained results have compared to results from previous studies. In the following, effect of physical factors of soil and contamination on the propagation pattern is studied based on flow module. The obtained results show that as soil hydraulic conductivity increases effect of NAPL’s physical factors in propagation rate are become more tangible. It is also observed that propagation depth of BTEX compounds is very sensitive to van Genuchten parameter α. The propagation rate of soluble components of BTEX compounds in aqueous, gas and NAPL phases have investigated by a larger model and by Simultaneous utilization of flow and transport modules.

Simulating Unsaturated Zone of Soil for Estimating the Recharge Rate and Flow Exchange Between a River and an Aquifer

Coupling surface water and groundwater models dynamically based on a simultaneous simulation of saturated and unsaturated zones of soil is a useful method for determining the recharge rate and flow exchange between a river and an aquifer as well as simultaneous operation of water resources systems. Thus, the main objectives of this study are to investigate the effects of surface water and groundwater interactions through their systematic simulation and to create a dynamic coupling between surface water and groundwater resources of the area by relevant mathematical models. Accordingly, hydrologic soil moisture method and MODFLOW model were employed to simulate the unsaturated and saturated zones, respectively. The results revealed that simultaneous simulation of the saturated and unsaturated zones of the soil can illustrate the interaction between surface water and groundwater at any spatial and temporal intervals well through using complete hydroclimatological balance components in the form of a coupled model. The application of this method in the Loor-Andimeshk Plain, located in the southwest of Iran, showed that aquifer recharge through the plain area from November to March is due to precipitation. On the other hand, in the warm months (June to September), the plain is merely fed through irrigation water penetration. As the level of river water in both Dez and Balarood rivers is higher than the Loor-Andimeshk aquifer level, hence the exchange occurs as a leakage from the river to the aquifer. The highest and lowest values of average exchangeable water in Balarood River occur in March and April and in Dez River are from June to September.

Numerical Evaluation and Management Suggestions for Heavy Metal Pollution Risks in a Sludge Landfill: A Case Study from Fuyong Landfill, Shenzhen, China

Despite the extensive attention paid to the transport of heavy metals in sludge landfills, the processes of transporting these pollutants from a landfill to the underground environment are quite complicated and subject to significant uncertainty. In this study, the transport of typical heavy metal pollutants in a sludge landfill through saturated and unsaturated soil zones during rainfall was investigated via numerical modeling. The objectives of the study are to evaluate the heavy metal pollution risk from a sludge landfill under rainfall infiltration conditions and to propose several management suggestions. The results indicate that, during rainfall, heavy metal concentrations at the top of the unsaturated sludge layer decrease rapidly, but they decrease more gradually at the bottom of the layer. The maximum concentration appears in vertical distribution and decreases gradually through the saturated zone. Nickel is the first heavy metal pollutant to break through the low‐permeability natural silt barrier. The transport parameters not only influence the simulated time for heavy metal pollutants to break through the silt layer and cause underground environmental pollution but also affect the extent to which the heavy metal pollutants in pore water exceed the guidelines. On the basis of these results, for dredged sludge with heavy metal concentrations significantly exceeding the standard, the concentration of heavy metals in pore water should be reduced before the sludge is landfilled, and a covering layer should be established on the sludge surface to control rainfall infiltration.

Fundamental Study on Transport Model for Radionuclides under Unsaturated Condition around Near-Surface Underground

ABSTRACTThe low-level nuclear wastes such as decontamination waste from Fukushima are disposed in near-surface underground, where the intermittent recharge of rain and groundwater causes spatial distribution of water content. Therefore, pores of soils are not filled with water, that is, an unsaturated zone will be formed. In such a condition, since the water flow path are detoured by clogged gas in pores of soil in the unsaturated zone, the migration path of radionuclide would be different from the saturated zone. So far, the one-dimensional advection-dispersion equation (ADE) model has been widely used in order to explain experimental results under an unsaturated condition. However, the detouring of local flow-paths remarkably affects the mass transfer. The one-dimensional ADE evaluates such a detouring effect by using Peclet number and retardation coefficient as fitting parameters. In other words, the one-dimensional ADE model is difficult to explain mass transfer under an unsaturated condition. Therefore, the purpose of this study is explaining such complicated transport of radionuclides using a multi-path model based on phenomena in underground. The proposed multi-path model considering both water saturation and permeability distributions showed good agreement with the experimental data under an unsaturated condition.

Effective saturation-based weighting for interblock hydraulic conductivity in unsaturated zone soil water flow modelling using one-dimensional vertical finite-difference model

Abstract Richards equation is solved for soil water flow modeling in the unsaturated zone continuum. Interblock hydraulic conductivities, while solving for Richards equation, are estimated by some sort of averaging process based on upstream and downstream nodes hydraulic conductivity values. The accuracy of the interblock hydraulic conductivity estimation methods mainly depends on the distance between two adjacent discretized nodes. In general, the accuracy of the numerical solution of the Richards equation decreases as nodal grid discretization increases. Conventional interblock hydraulic conductivity estimation methods are mostly mere approximation approaches while the Darcian-based interblock hydraulic conductivities involve complex calculations and require intensive computation under different flow regimes. Therefore, in this study, we proposed an effective saturation-based weighting approach in the soil hydraulic curve functions for estimating interblock hydraulic conductivity using a one-dimensional vertical finite-difference model which provides a parametric basis for interblock hydraulic conductivity estimation while reducing complexity in the calculation and computational processes. Furthermore, we compared four test case simulation results from different interblock hydraulic conductivity methods with the reference solutions. The comparison results show that the proposed method performance in terms of percentage reduction in root mean square and mean absolute error over other methods compared in this study were 59.5 and 60%, respectively.

Using Water Stable Isotopes for Identifying Groundwater Recharge Sources of the Unconfined Alluvial Zagreb Aquifer (Croatia)

The main purpose of this study was to understand the interactions between precipitation, surface water, and groundwater in the Zagreb aquifer system using water stable isotopes. The Zagreb aquifer is of the unconfined type and strongly hydraulically connected to the Sava River. As the groundwater is the main source of drinking water for one million inhabitants, it is essential to investigate each detail of the recharge processes of the aquifer to ensure adequate protection of the groundwater. Measuring the content of water stable isotopes in surface waters and groundwater enabled the creation of two- and three-component mixing models based on the isotopic mass balance for the purpose of the quantification of each recharge component. The mixing models gave ambiguous results. Observation wells equally distant from the Sava River did not have the same recharge component ratio. This indicated that there were more factors (in addition to the distance from the river) that were affecting groundwater recharge, and the properties of the unsaturated zone and surface cover data were therefore also taken into consideration. The thickness of the unsaturated zone and the characteristics of different soil types were identified as important factors in the recharge of the Zagreb aquifer. The areas with high thickness of the unsaturated zone and well-permeable soil had a very similar recharge component ratio to the areas with small thickness of the unsaturated zone but low-permeable soil.

Using stable isotopes to quantify water sources for trees and shrubs in a riparian cottonwood ecosystem in flood and drought years

AbstractRiparian cottonwood forests in dry regions of western North America do not typically receive sufficient growing season precipitation to completely support their relatively high transpiration requirements. Water used in transpiration by riparian ecosystems must include alluvial groundwater or water stored in the potentially large reservoir of the unsaturated soil zone. We used the stable oxygen and hydrogen isotope composition of stem xylem water to evaluate water sources used by the dominant riparian cottonwood (Populus spp.) trees and shrubs (Shepherdia argentea and Symphoricarpos occidentalis) in Lethbridge, Alberta, during 3 years of contrasting environmental conditions. Cottonwoods did not exclusively take up alluvial groundwater but made extensive use of water sourced from the unsaturated soil zone. The oxygen and hydrogen isotope compositions of cottonwood stem water did not strongly overlap with those of alluvial groundwater, which were closely associated with the local meteoric water line. Instead, cottonwood stem water δ18O and δ2H values were located below the local meteoric water line, forming a line with a low slope that was indicative of water exposed to evaporative enrichment of heavy isotopes. In addition, cottonwood xylem water isotope compositions had negative values of deuterium excess (d‐excess) and line‐conditioned (deuterium) excess (lc‐excess), both of which provided evidence that water taken up by the cottonwoods had been exposed to fractionation during evaporation. The shrub species had lower values of d‐excess and lc‐excess than had the cottonwood trees due to shallower rooting depths, and the d‐excess values declined during the growing season, as shallow soil water that was taken up by the plants was exposed to increasing, cumulative evaporative enrichment. The apparent differences in functional rooting pattern between cottonwoods and the shrub species, strongly influenced the ratio of net photosynthesis to stomatal conductance (intrinsic water‐use efficiency), as shown by variation among species in the δ13C values of leaf tissue.

Determination of diffusion coefficient of 137Cs at unsaturated zone of DH-2 site soil under δ = 1.41 g.cm−3 condition

Radiocesium is one of the reference radionuclide on low-level radioactive waste disposal system. For the safety assessment of environment around the disposal facility, it needs to be studied their characters on the candidate site of radioactive waste disposal facility. Diffusion coefficient is important to be studied especially on the unsaturated zone lies between the foundation of facility and the saturated zone. In this experiment, soil samples were taken out from the logging result of DH-2 hole of SP-4 site on the 7 m depth from surface. Work was done in the diffusion column by gravitation method under density condition was 1.41 g.cm−3. The diffusion coefficient is determined in one dimension using non-steady diffusion method with variation of diffusion time and has obtained the smallest of Da value of 137Cs in the soil sample was 1.76 E-12 m2s−1 at the 6 months of diffusion time. The results show that the longer of length diffusion time, the smaller diffusion value was obtained.

Assessing Managed Aquifer Recharge Processes under Three Physical Model Concepts

Physical models such as surface infiltration experiments in the lab and field are an approach to understand processes in the unsaturated soil zone. In the case of mapping processes influencing the operation of real-world managed aquifer recharge schemes they are helpful tools to determine interactions between processes in the unsaturated soil zone, and site-specific as well as operational parameters. However, the multitude of assumptions and scale-related limitations of downscale investigations often lead to over- or underestimations, rendering their results useless when translated to field-like conditions. Various real-world managed aquifer recharge operational scenarios were simulated in three physical models, a 1D-lab column, a rectangular shaped stainless steel 3D-lab infiltration tank and a rectangular shaped 3D-field unit, to understand the impact of the experimental set-up on the assessment of processes and to identify the experimental set-up which is most-suitable to describe these processes. Results indicate that water flow velocity, water saturation and oxygen consumption are often overestimated in 1D-column experiments due to sidewall effects and no existing lateral flow. For precise analysis of infiltration processes in general as well as during operation of managed aquifer recharge, 3D experiments are recommended due to their more realistic representation of flow processes.

Experimental and numerical study on the leaching of pesticides into the groundwater through a porous medium: Effects of transport parameters

Abstract This study aims to investigate, experimentally and numerically, the pesticide transport through the soil in unsaturated zone. The movement of pesticide is described with one dimensional model. The transport equation incorporates terms accounting for diffusion –convection, adsorption and reaction. The first objective of this work was to identifies the effects of the parameters transport such us; the dimensions first order degradation coefficient ( μ ), the peclet number ( ρ e ) and the dimensionless mass transfer coefficient (w). The evolution of pesticide concentration in function of depth and time was found by using the non-equilibrium transport model. The data obtained show the importance of the adsorbed concentration. The second objective is to find the relation between the concentration and the transport parameters because there is a difficulty of the analytical solution. The solution of the problem was carried out by a numerical approach by using the finite difference method and a Fortran program.

An integrated approach for assessing the migration behavior of chlorpyrifos and carbaryl in the unsaturated soil zone

ABSTRACTChlorpyrifos (O, O-diethyl O-3,5,6-trichloropyridin-2-yl phosphorothioate) and carbaryl (1-naphthyl methylcarbamate) are often applied concurrently as insecticides in food production. The aim of this study was to research their migration behavior in a real environment. We researched the leaching of both pesticides by setting up field lysimeters on a farm with the typical soil used in fruit production today. In order to analyze the variables involved in this process, we performed complementary adsorption studies, we performed complementary adsorption studies using batches and undisturbed soil laboratory columns for both compounds. The results for pesticide transport through the lysimeters showed that less than 1% of chlorpyrifos was recovered in the leachates, while almost 17% was recovered for carbaryl. Having completed the experiment in undisturbed laboratory columns, soil analysis showed that chlorpyrifos mainly remained in the first 5 cm, while carbaryl moved down to the lower sections. These results can be explained in view of the sorption coefficient values (KD) obtained in horizons A and B for chlorpyrifos (393 and 184 L kg−1) and carbaryl (3.1 and 4.2 L kg−1), respectively. By integrating the results obtained in the different approaches, we were able to characterize the percolation modes of these pesticides in the soil matrix, thus contributing to the sustainable use of resources.

Numerical simulations of triaxial shearing-infiltration tests

The failure of steep slopes during rainfall is commonly associated with a decrease in matric suction in the unsaturated soil zone above the water table. The shear strength characteristics of residual soil under water infiltration have been studied in the laboratory using unsaturated triaxial tests. This paper presents a development of a numerical model for simulating a triaxial shearing-infiltration test to investigate the shear strength characteristics of a compacted kaolin under infiltration condition. Both the hydraulic and mechanical responses of the compacted kaolin are modeled using the commercial software SIGMA/W and in-house software YS-Slope. The numerical analyses result and their validation against laboratory test results are presented and discussed in this paper. The results from the numerical analyses show good agreements with those from the laboratory tests, indicating that the proposed numerical model can be used to simulate the triaxial shearing-infiltration tests in laboratory.

Utilizing unsaturated soil zone models for assessing managed aquifer recharge

Managed aquifer recharge (MAR) is increasingly used to balance out the divergence between natural groundwater recharge and overexploitation of groundwater resources. As testing and design of recharge facilities can be cost and time-intensive, modeling tools hold great potential to design field investigations as well as to augment and extrapolate from their results. Focusing on unsaturated soil zone models, this study first reviews 16 previous modeling studies showing their range of model types and applications. The review is accompanied by an example of the use of HYDRUS 2D/3D, an unsaturated zone model, to design a novel small-scale infiltration basin. The basin is going to be established as part of a field and laboratory research facility for MAR in Pirna, Germany. Modeling results assisted in determining the dimensions of the infiltration unit as well as the placement of measurement devices and experimental scenario planning. Finally, the strengths and constraints of this modeling approach for MAR assessment are discussed.

Determination of Matric Suction and Saturation Degree for Unsaturated Soils, Comparative Study - Numerical Method versus Analytical Method

Matric suction is a soil parameter which influences the behaviour of unsaturated soils in both terms of shear strength and permeability. It is a necessary aspect to know the variation of matric suction in unsaturated soil zone for solving geotechnical issues like unsaturated soil slopes stability or bearing capacity for unsaturated foundation ground. Mathematical expression of the dependency between soil moisture content and it’s matric suction (soil water characteristic curve) has a powerful character of nonlinearity. This paper presents two methods to determine the variation of matric suction along the depth included between groundwater level and soil level. First method is an analytical approach to emphasize one direction steady state unsaturated infiltration phenomenon that occurs between the groundwater level and the soil level. There were simulated three different situations in terms of border conditions: precipitations (inflow conditions on ground surface), evaporation (outflow conditions on ground surface), and perfect equilibrium (no flow on ground surface). Numerical method is finite element method used for steady state, two-dimensional, unsaturated infiltration calculus. Regarding boundary conditions there were simulated identical situations as in analytical approach. For both methods, was adopted the equation proposed by van Genuchten-Mualen (1980) for mathematical expression of soil water characteristic curve. Also for the unsaturated soil permeability prediction model was adopted the equation proposed by van Genuchten-Mualen. The fitting parameters of these models were adopted according to RETC 6.02 software in function of soil type. The analyses were performed in both methods for three major soil types: clay, silt and sand. For each soil type were concluded analyses for three situations in terms of border conditions applied on soil surface: inflow, outflow, and no flow. The obtained results are presented in order to highlight the differences/similarities between the methods and the advantages / disadvantages of each one.