Increase In Hydraulic Conductivity Research Articles

Controls on subsurface transport of sorbing contaminant

Subsurface transport of a sorbing contaminant is poorly understood and characterized. Here, a new semi-analytical saturated–unsaturated flow and transport model is coupled to a kinetic sorption algorithm to assess the impact of changes in the subsurface permeability architecture and flow rate on sorption characteristics. The model outputs reveal the pronounced effect of the rate of vertical decline in Ks on the frequency of occurrence and spatial distribution of subsurface sorption as well as the timing and rate of sorbing contaminants discharged into stream. Sorption potential is weakened with infiltration rate. The impact of infiltration rate on the decline in sorption potential becomes more accentuated as the degree of subsurface vertical heterogeneity in saturated hydraulic conductivity increases. Porosity pattern also impacts sorption characteristics; but its effects highly depend upon the degree of vertical heterogeneity in Ks. The results and methodology presented in this paper have potential implications for assessing water quality in integrated groundwater–surface water systems as well as designing remediation systems.

Characterization of stony soils' hydraulic conductivity using laboratory and numerical experiments

Abstract. Determining soil hydraulic properties is of major concern in various fields of study. Although stony soils are widespread across the globe, most studies deal with gravel-free soils, so that the literature describing the impact of stones on the hydraulic conductivity of a soil is still rather scarce. Most frequently, models characterizing the saturated hydraulic conductivity of stony soils assume that the only effect of rock fragments is to reduce the volume available for water flow, and therefore they predict a decrease in hydraulic conductivity with an increasing stoniness. The objective of this study is to assess the effect of rock fragments on the saturated and unsaturated hydraulic conductivity. This was done by means of laboratory experiments and numerical simulations involving different amounts and types of coarse fragments. We compared our results with values predicted by the aforementioned predictive models. Our study suggests that it might be ill-founded to consider that stones only reduce the volume available for water flow. We pointed out several factors of the saturated hydraulic conductivity of stony soils that are not considered by these models. On the one hand, the shape and the size of inclusions may substantially affect the hydraulic conductivity. On the other hand, laboratory experiments show that an increasing stone content can counteract and even overcome the effect of a reduced volume in some cases: we observed an increase in saturated hydraulic conductivity with volume of inclusions. These differences are mainly important near to saturation. However, comparison of results from predictive models and our experiments in unsaturated conditions shows that models and data agree on a decrease in hydraulic conductivity with stone content, even though the experimental conditions did not allow testing for stone contents higher than 20 %.

Implications of Using Thermal Desorption to Remediate Contaminated Agricultural Soil: Physical Characteristics and Hydraulic Processes.

Given the recent increase in crude oil production in regions with predominantly agricultural economies, the determination of methods that remediate oil contamination and allow for the land to return to crop production is increasingly relevant. Ex situ thermal desorption (TD) is a technique used to remediate crude oil pollution that allows for reuse of treated soil, but the properties of that treated soil are unknown. The objectives of this research were to characterize TD-treated soil and to describe implications in using TD to remediate agricultural soil. Native, noncontaminated topsoil and subsoil adjacent to an active remediation site were separately subjected to TD treatment at 350°C. Soil physical characteristics and hydraulic processes associated with agricultural productivity were assessed in the TD-treated samples and compared with untreated samples. Soil organic carbon decreased more than 25% in both the TD-treated topsoil and the subsoil, and total aggregation decreased by 20% in the topsoil but was unaffected in the subsoil. The alteration in these physical characteristics explains a 400% increase in saturated hydraulic conductivity in treated samples as well as a decrease in water retention at both field capacity and permanent wilting point. The changes in soil properties identified in this study suggest that TD-treated soils may still be suitable for sustaining vegetation, although likely at a slightly diminished capacity when directly compared with untreated soils.

Exogenous application of abscisic acid (ABA) increases root and cell hydraulic conductivity and abundance of some aquaporin isoforms in the ABA-deficient barley mutant Az34.

Background and Aims Regulation of water channel aquaporins (AQPs) provides another mechanism by which abscisic acid (ABA) may influence water flow through plants. To the best of our knowledge, no studies have addressed the changes in ABA levels, the abundance of AQPs and root cell hydraulic conductivity (LpCell) in the same tissues. Thus, we followed the mechanisms by which ABA affects root hydraulics in an ABA-deficient barley mutant Az34 and its parental line 'Steptoe'. We compared the abundance of AQPs and ABA in cells to determine spatial correlations between AQP abundance and local ABA concentrations in different root tissues. In addition, abundance of AQPs and ABA in cortex cells was related to LpCell. Methods Root hydraulic conductivity (LpRoot) was measured by means of root exudation analyses and LpCell using a cell pressure probe. The abundance of ABA and AQPs in root tissues was assessed through immunohistochemical analyses. Isoform-specific antibodies raised against HvPIP2;1, HvPIP2;2 and HvPIP2;5 were used. Key Results Immunolocalization revealed lower ABA levels in root tissues of Az34 compared with 'Steptoe'. Root hydraulic conductivity (LpRoot) was lower in Az34, yet the abundance of HvPIPs in root tissues was similar in the two genotypes. Root hair formation occurred closer to the tip, while the length of the root hair zone was shorter in Az34 than in 'Steptoe'. Application of external ABA to the root medium of Az34 and 'Steptoe' increased the immunostaining of root cells for ABA and for HvPIP2;1 and HvPIP2;2 especially in root epidermal cells and the cortical cell layer located beneath, parallel to an increase in LpRoot and LpCell. Treatment of roots with Fenton reagent, which inhibits AQP activity, prevented the ABA-induced increase in root hydraulic conductivity. Conclusion Shortly after (<2 h) ABA application to the roots of ABA-deficient barley, increased tissue ABA concentrations and AQP abundance (especially the plasma-membrane localized isoforms HvPIP2;1 and HvPIP2;2) were spatially correlated in root epidermal cells and the cortical cell layer located beneath, in conjunction with increased LpCell of the cortical cells. In contrast, long-term ABA deficiency throughout seedling development affects root hydraulics through other mechanisms, in particular the developmental timing of the formation of root hairs closer to the root tip and the length of the root hair zone.

Is it getting hot in here? Adjustment of hydraulic parameters in six boreal and temperate tree species after 5years of warming.

Global temperatures (T) are rising, and for many plant species, their physiological response to this change has not been well characterized. In particular, how hydraulic parameters may change has only been examined experimentally for a few species. To address this, we measured characteristics of the hydraulic architecture of six species growing in ambient T and ambient +3.4°C T plots in two experimentally warmed forest sites in Minnesota. These sites are at the temperate-boreal ecotone, and we measured three species from each forest type. We hypothesized that relative to boreal species, temperate species near their northern range border would increase xylem conduit diameters when grown under elevated T. We also predicted a continuum of responses among wood types, with conduit diameter increases correlating with increases in the complexity of wood structure. Finally, we predicted that increases in conduit diameter and specific hydraulic conductivity would positively affect photosynthetic rates and growth. Our results generally supported our hypotheses, and conduit diameter increased under elevated T across all species, although this pattern was driven predominantly by three species. Two of these species were temperate angiosperms, but one was a boreal conifer, contrary to predictions. We observed positive relationships between the change in specific hydraulic conductivity and both photosynthetic rate (P=0.080) and growth (P=0.012). Our results indicate that species differ in their ability to adjust hydraulically to increases in T. Specifically, species with more complex xylem anatomy, particularly those individuals growing near the cooler edge of their range, appeared to be better able to increase conduit diameters and specific hydraulic conductivity, which permitted increases in photosynthesis and growth. Our data support results that indicate individual's ability to physiologically adjust is related to their location within their species range, and highlight that some wood types may adjust more easily than others.

Numerical investigation of the spatial scale and time dependency of tile drainage contribution to stream flow

Summary Tile drainage systems are pervasive in the Central U.S., significantly altering the hydrologic system. The purpose of this study was to assess the effects of tile drainage systems on streamflow. A physically based coupled hydrologic model was applied to a 45 km 2 agricultural Iowa watershed. Tile drainage was incorporated though an equivalent porous medium approach, calibrated though numerical experimentation. Experimental results indicated that a significant increase in hydraulic conductivity of the equivalent medium layer was needed to achieve agreement in total outflow with an explicit numerical representation of a tiled system. Watershed scale analysis derived the tile drainage contribution to stream flow ( Q T / Q ) from a numerical tracer driven analysis of instream surface water. During precipitation events tile drainage represented 30% of stream flow, whereas during intervals between precipitations events, 61% of stream flow originated from the tile system. A division of event and non-event periods produced strong correlations between Q T / Q and drainage area, positive for events, and negative for non-events. The addition of precipitation into the system acted to saturate near surface soils, increase lateral soil water movement, and dilute the relatively stable instream tile flow. Increased intensity precipitation translated the Q T / Q relationship downward in a consistent manner. In non-event durations, flat upland areas contributed large contributions of tile flow, diluted by larger groundwater (non-tile) contribution to stream flow in the downstream steeper portion of the watershed. Study results provide new insights on the spatiotemporal response of tile drainage to precipitation and contributions of tile drainage to streamflow at a watershed scale, with results having important implications for nitrate transport.

USING DIFFERENT TYPES OF SUBSOILER SHARES TO IMPROVE THE PRODUCTIVITY OF COMPACTED SOILS

Soil compaction is an environmental problem and has been recognized as the main form of soil degradation in some Mediterranean areas. Soil compaction may increase soil strength and compacted soil layers can affect root growth and crop productivity. The aim of the present work was to investigate the effect of using different types of sub-soilers shanks and shares on physical soil properties, seed germination and crop productivity of faba bean (vicia faba). The experiment was carried out at El- Hamam district area- Matrouh in sandy loam soil. The soil was characterizes by hard pan compaction down to 40 cm. Parameters such as changes in soil penetration resistance, bulk density, hydraulic conductivity and soil moisture content were measured. The results revealed that the highest increase of soil penetration resistance and lowest hydraulic conductivity due to the soil compaction occurred in non treated soil with sub-soilers. The seed germination and yield of faba bean increased with decreasing soil penetration resistance. A positive action was detected between using fit equipped mole behind different shanks and shares of the sub-soilers used on both soil penetration resistance, and hydraulic conductivity. The ability to eliminate soil compaction could be useful on agricultural field in the South district of El Hamam canal area by using sub-soiler with mole fit equipped behind shank as well as decreased the effect of the hard pans and improve soil properties such as soil penetration resistance and hydraulic conductivity. The deep tillage gained that the lowest energy requirements were recorded either without using the fit mole equipped or with using single point share with straight shank. However using the curved shank - winged share with mole achieved the highest seed germination values and crop yield as compared with other treatments.

Soil hydraulic conductivity as affected by vegetation restoration age on the Loess Plateau, China

The Loess Plateau of China has experienced extensive vegetation restoration in the past several decades, which leads to great changes in soil properties such as soil bulk, porosity, and organic matter with the vegetation restoration age. And these soil properties have great effect on the soil infiltration and soil hydraulic conductivity. However, the potential changes in soil hydraulic conductivity caused by vegetation restoration age have not been well understood. This study was conducted to investigate the changes in soil hydraulic conductivity under five grasslands with different vegetation restoration ages (3, 10, 18, 28 and 37 years) compared to a slope farmland, and further to identify the factors responsible for these changes on the Loess Plateau of China. At each site, accumulative infiltration amount and soil hydraulic conductivity were determined using a disc permeameter with a water supply pressure of –20 mm. Soil properties were measured for analyzing their potential factors influencing soil hydraulic conductivity. The results showed that the soil bulk had no significant changes over the initial 20 years of restoration (P>0.05); the total porosity, capillary porosity and field capacity decreased significantly in the grass land with 28 and 37 restoration ages compared to the slope farmland; accumulative infiltration amount and soil hydraulic conductivity were significantly enhanced after 18 years of vegetation restoration. However, accumulative infiltration amount and soil hydraulic conductivity fluctuated over the initial 10 years of restoration. The increase in soil hydraulic conductivity with vegetation restoration was closely related to the changes in soil texture and structure. Soil sand and clay contents were the most influential factors on soil hydraulic conductivity, followed by bulk density, soil porosity, root density and crust thickness. The Pearson correlation coefficients indicated that the soil hydraulic conductivity was affected by multiply factors. These results are helpful to understand the changes in hydrological and erosion processes response to vegetation succession on the Loess Plateau.

Rice husk biochar application to paddy soil and its effects on soil physical properties, plant growth, and methane emission

The objective of this study was to investigate the effects of the application of rice husk biochar on selected soil physical properties, rice growth, including root extension, and methane (CH4) emissions from paddy field soil. Three replication experiments were conducted using outdoor pot experiments utilizing commercial rice husk biochar mixed with paddy soil at a rate of 0 (control), 2, and 4 % (weight biochar/weight soil) in which the rice was cultivated for 100 days under a continuously flooded condition. The physical properties of soils were analyzed before and after the growing periods. Some parameters of rice growth and CH4 emissions of paddy soils were monitored weekly during the experiment. Root extension was also analyzed after harvesting. The experiments showed that the application of rice husk biochar improved the physical properties of paddy soils. It led to a decrease in bulk density and an increase in saturated hydraulic conductivity, including the total pore volume as well as the available soil water content. The shoot height of rice plants was significantly higher in soil amended with 4 % biochar than that in the control soil. However, other plant growth parameters and root extension were only slightly affected by the application. It was also found that amending soil with biochar led to a reduction of the total CH4 emissions by 45.2 and 54.9 % for an application rate of 2 and 4 %, respectively, compared with the control. Our results showed that the higher the application rate, the stronger the effect of biochar was observed. More research is still necessary for a better understanding of the underlying mechanisms.

Influence of structure and water pressure on the hydraulic conductivity of the rock mass around underground excavations

In this study, in situ water injection tests and numerical modeling in conjunction with core logs and television image measurements were applied for an investigation of the hydraulic properties of the rock mass around underground excavations. The aim of our work was to investigate the influence of rock structure and water pressure on the hydraulic conductivity of the rock mass around underground excavations. Thus, water injection tests were conducted in four boreholes that were drilled in the rock mass in underground tunnels in China. Seepage connection initiation and steady state seepage were proposed to identify the seepage evolution course within the rock mass by combining the results of pressure and hydraulic conductivity. Generally, hydraulic conductivity is strongly related to rock structure and water pressure. The results also revealed that a rearrangement of rock structure would occur in the damage zone, resulting in an increase in hydraulic conductivity of nearly one to several orders of magnitude. The results from the modified numerical model showed that the pore water distribution and the groundwater inflow rate are affected by both the hydraulic conductivity and the thickness of the damage zone and they are influenced more by the thickness of this zone.

Abscisic Acid Regulation of Root Hydraulic Conductivity and Aquaporin Gene Expression Is Crucial to the Plant Shoot Growth Enhancement Caused by Rhizosphere Humic Acids.

The physiological and metabolic mechanisms behind the humic acid-mediated plant growth enhancement are discussed in detail. Experiments using cucumber (Cucumis sativus) plants show that the shoot growth enhancement caused by a structurally well-characterized humic acid with sedimentary origin is functionally associated with significant increases in abscisic acid (ABA) root concentration and root hydraulic conductivity. Complementary experiments involving a blocking agent of cell wall pores and water root transport (polyethylenglycol) show that increases in root hydraulic conductivity are essential in the shoot growth-promoting action of the model humic acid. Further experiments involving an inhibitor of ABA biosynthesis in root and shoot (fluridone) show that the humic acid-mediated enhancement of both root hydraulic conductivity and shoot growth depended on ABA signaling pathways. These experiments also show that a significant increase in the gene expression of the main root plasma membrane aquaporins is associated with the increase of root hydraulic conductivity caused by the model humic acid. Finally, experimental data suggest that all of these actions of model humic acid on root functionality, which are linked to its beneficial action on plant shoot growth, are likely related to the conformational structure of humic acid in solution and its interaction with the cell wall at the root surface.

Effect of freeze–thaw cycles on the hydraulic conductivity of a compacted clayey silt and influence of the compaction energy

Compacted clay layers are often used in impervious barrier systems to prevent the migration of water and pollutants. Environmental factors, acting during or after the clay deposition, may affect the layer integrity and induce a variation of hydraulic conductivity over time. The aim of the present research is to assess this variation when induced by freeze–thaw cycles. The paper summarizes some results of tests performed on a series of clayey silt samples, reconstituted at various levels of compaction energy and subjected to cyclic freezing according to a controlled and repeatable procedure, set to reproduce the natural environmental conditions. The hydraulic conductivity is evaluated directly from a flexible wall permeameter and indirectly from oedometric tests. The results show the consequences of cyclic freezing in relation to the compaction level and lead to insights into the development of fracture networks responsible for the increase in hydraulic conductivity.

Hybrid Finite-Discrete Element Simulation of the EDZ Formation and Mechanical Sealing Process Around a Microtunnel in Opalinus Clay

The analysis and prediction of the rock mass disturbance around underground excavations are critical components of the performance and safety assessment of deep geological repositories for nuclear waste. In the short term, an excavation damaged zone (EDZ) tends to develop due to the redistribution of stresses around the underground openings. The EDZ is associated with an increase in hydraulic conductivity of several orders of magnitude. In argillaceous rocks, sealing mechanisms ultimately lead to a partial reduction in the effective hydraulic conductivity of the EDZ with time. The goal of this study is to strengthen the understanding of the phenomena involved in the EDZ formation and sealing in Opalinus Clay, an indurated claystone currently being assessed as a host rock for a geological repository in Switzerland. To achieve this goal, hybrid finite-discrete element method (FDEM) simulations are performed. With its explicit consideration of fracturing processes, FDEM modeling is applied to the HG-A experiment, an in situ test carried out at the Mont Terri underground rock laboratory to investigate the hydro-mechanical response of a backfilled and sealed microtunnel. A quantitative simulation of the EDZ formation process around the microtunnel is first carried out, and the numerical results are compared with field observations. Then, the re-compression of the EDZ under the effect of a purely mechanical loading, capturing the increase of swelling pressure from the backfill onto the rock, is considered. The simulation results highlight distinctive rock failure kinematics due to the bedded structure of the rock mass. Also, fracture termination is simulated at the intersection with a pre-existing discontinuity, representing a fault plane oblique to the bedding orientation. Simulation of the EDZ re-compression indicates an overall reduction of the total fracture area as a function of the applied pressure, with locations of ineffective sealing associated with self-propping of fractures. These results are consistent with hydraulic testing data revealing a negative correlation between pressure values and an increase in the EDZ transmissivity.

Numerical and Centrifuge Modeling of Seismic Soil–Foundation–Structure Interaction on Liquefiable Ground

AbstractThe effective mitigation of the liquefaction hazard requires an improved understanding of the consequences of liquefaction in terms of ground shaking, permanent displacement, and building performance. In this paper, results from centrifuge experiments of a shallow-founded structure on liquefiable sand are used to evaluate the predictive capabilities of a state-of-the-art numerical tool. Solid-fluid, fully-coupled 3D nonlinear numerical simulations were performed using the PDMY02 soil model implemented in a software modeling domain. The numerical model captured excess pore pressures and accelerations well in the free-field, but largely underestimated volumetric settlements due to loss of water during shaking. This was associated with the drastic increase in soil hydraulic conductivity when approaching liquefaction, which was not taken into account numerically, as well as the underestimation of soil volumetric compressibility. The contribution of volumetric strains to total building settlement was, ...

Shrink-swell potential, hydraulic conductivity and geotechnical properties of clay materials for landfill liner construction

Abstract This paper presents studies concerning the applicability of two clay materials for the construction of a sustainable landfill liner. The studies consisted in determination of basic characteristics of the materials, eg particle size distribution, bulk density, particle density, total porosity, pore size, mineralogy, specific surface area, nanoparticle size, and Atterberg limits, as well as measurements of their geotechnical and hydraulic parameters, such as in situ saturated hydraulic conductivity, modules of primary and secondary compression, cohesion, and angle of internal friction. Furthermore, the effects of compaction performed by the Proctor method at various water contents on swelling and shrinkage characteristics and saturated hydraulic conductivity were investigated in order to determine the compliance with the national requirements for selection of material for landfill liner construction. The determined characteristics and geotechnical parameters of the tested clay materials allowed qualifying them as suitable for municipal landfill construction. The shrinkage potential of the tested clays observed was rated as moderate to very high. The cyclic drying and rewetting of the clay materials performed resulted in a significant increase in saturated hydraulic conductivity. Thus, the clay sealing layers, as part of a multilayer liner, should be very carefully operated, preventing the drying out of the clay sealing and assuring the possibility of its constant saturation.

Effects of di-ammonium phosphate on hydraulic, compaction, and shear strength characteristic of sand and clay soils

The large amounts of phosphate fertilizers which are used in agriculture activities in Egypt lead to the contamination of large quantities of soil surrounding the agricultural fields. Also, the use of traditional flood irrigation method makes the problem more worse. An extensive laboratory testing program is carried out to determine the Atterberg limits, compaction characteristics, hydraulic conductivity, and shear strength parameters of clean and contaminated soils. Three types of soils are used including coarse sand, fine to medium sand, and silty clay. Batch tests are used to study the short-term effect of di-ammonium phosphate (DAP) contamination on the soil properties. Contaminated specimens are prepared by mixing the dried soil samples with different di-ammonium phosphate concentrations of 5, 10, and 20 %. The results indicate a significant reduction in Atterberg limits and increase in hydraulic conductivity for silty clay soil with the increase of phosphate concentration. The change that is induced by chemical reaction in the micro-structure of the soil is studied by laser scanning microscope before and after soil contamination. For sand soil, γ dmax is significantly increased with the increase of DAP concentrations up to 5 % and slightly increased with concentrations from 5 to 20 % which indicates excess phosphate in the soil. However, the general trend of hydraulic conductivity is towards reduction. In addition, the friction angle of two sand samples decreases about 10 % due to the increase of DAP concentration up to 20 %. For silty clay, the reduction in cohesion is about 20 % and the increase of friction angle is about 10 % with the increase of DAP up to 20 %.

Effects of phosphate dispersants on the liquid limit, sediment volume and apparent viscosity of clayey soil/calcium-bentonite slurry wall backfills

Soil-bentonite slurry-trench wall, usually consisting of sandy soil and Na-bentonite, is used extensively as in-situ engineered barriers for contaminant containment. Clayey soil/Ca-bentonite may be considered as an alternative backfill when Na-bentonite is scarce at some sites. Adding trace amount of phosphate dispersant to clayey soil/Ca-bentonite backfills may be advantageous to maintain the deflocculated structure of bentonite, which is benefit to minimizing increases in hydraulic conductivity of the backfills when attacked by contaminants. However, studies on the application of phosphate dispersants to the clayey soil/Ca-bentonite backfills are very limited. A series of laboratory tests are conducted for the measurements of liquid limit, sediment volume and apparent viscosity of the phosphate dispersant-amended backfills. The phosphate dispersants tested are sodium hexametaphosphate, sodium tripolyphosphate and sodium pyrophosphate with contents ranging from 0 to 2%. The results demonstrate that the values of liquid limit, sediment volume and apparent viscosity of the backfills decrease sharply when the dispersant content is relatively low (≤0.1 to 0.5%), while they change slightly at relatively high dispersant content (> 0.1 to 0.5%). The type and content of the dispersant as well as Ca-bentonite content have significant effects on the liquid limit, sediment volume and apparent viscosity of the backfill. The sodium hexametaphosphate is shown to have higher dispersibility compared with the others in terms of greater reduction in apparent viscosity, and its optimum content is suggested in a range of 0.1% to 0.5%. A power function is proposed which well quantifies the relationship between the measured apparent viscosity and liquid limit.

Impacts of presence of lead contamination in clayey soil–calcium bentonite cutoff wall backfills

Previous studies have shown that clayey soil admixed with Ca-bentonite can be considered as an alternative backfill for slurry-trench cutoff walls. However, the effects of contaminant exposure on the engineering properties of the clayey soil/Ca-bentonite backfills, such as compressibility and hydraulic conductivity, have not been investigated. This study presents several series of oedometer tests conducted to evaluate the compressibility and hydraulic conductivity of clayey soil/Ca-bentonite backfills exposed to different levels of lead (Pb) contamination. Kaolin is used as the base clayey soil, and it is amended with Ca-bentonite at three content levels (5, 10 and 15% by dry weight) to prepare the backfill samples. Pb is a representative metal contaminant commonly found at contaminated sites. The backfills are prepared by thoroughly mixing the kaolin–bentonite mixture with Pb concentrations of 60, 120 and 600mmol/L. The results reveal that the level of Pb concentration and bentonite content are the critical factors that affect the compressibility of the backfill. The ratio of the compression index of the Pb-contaminated backfill to that of the clean backfill decreases with an increase in the Pb concentration and bentonite content. The void ratio at an effective vertical compression stress of 1kPa is a useful characteristic parameter to correlate with the compression index for all the backfills tested here. The Pb-contaminated backfills exhibit a maximum fifty-fold increase in hydraulic conductivity when compared to clean backfills. The hydraulic conductivity of the clean backfills and those contaminated at low concentration of Pb (60mmol/L) is generally less than the typical regulatory limit of 10−9m/s; whereas the hydraulic conductivity exceeds the regulatory limit when the Pb concentration is higher than 120mmol/L. The ratio of hydraulic conductivity for Pb-contaminated backfills to that for clean backfill decreases with an increase in the effective vertical compression stress. This indicates that under relatively higher effective vertical compression stress conditions the detrimental effect of Pb on hydraulic conductivity can be insignificant. The hydraulic conductivity of the Pb-contaminated backfills decreases slightly when there is an increase in the bentonite content for a given Pb concentration, while the hydraulic conductivity ratio increases significantly with a rise in the bentonite content. A proposed empirical method is assessed to predict the hydraulic conductivity of Pb-contaminated clayey soil/Ca-bentonite backfills as well as clean ones subjected to vertical compressive stress lower than 1600kPa. Using this method, the predicted hydraulic conductivity values fall in the range of 1/3 to 3 times those obtained from the oedometer tests.

Analytical Study of Unsteady Nested Groundwater Flow Systems

Two analytical solutions using segregation variable method to calculate the hydraulic head under steady and unsteady flow conditions based on Tóth’s classical model were developed. The impacts of anisotropy ratio, hydraulic conductivity (K), and specific yield (μs) on the flow patterns were analyzed. It was found that the area of the equal velocity region increases and the penetrating depth of the flow system decreases at steady state with anisotropy ratio increases, which is defined asε=Kx/Kz. In addition, stagnant zones can be found in the flow field where the streamlines have opposite directions. These stagnant zones move toward the surface as the horizontal hydraulic conductivity increases. The results of the study on transient flow indicate that a relative increase in hydraulic conductivity produces a positive impact on hydraulic head and a relative enhancement in specific yield produces a negative effect on hydraulic head at early times.

The Adsorption Capacity and Geotechnical Properties of Modified Clay Containing SSA Used as Landfill Liner-Soil Materials

The potential of clay containing 0~5% sewage sludge ash (SSA) is assessed for use as a landfill liner-soil material. Low temperature N2adsorption, batch adsorption, permeability, and unconfined compressive strength tests are performed to evaluate pore structure, adsorption capacity, hydraulic conductivity, and unconfined compressive strength of the clays. The pore size distribution of the modified clay containing SSA is mainly composed of micropores (&lt;2 nm) and mesopores (2~7 nm). With the increasing of SSA from 0% to 5%, the adsorption capacity of Zn(II) and Cu(II) to the clay increases 37% and 273%, respectively. The hydraulic conductivity of modified clay is from 3.62 × 10−8to 2.17 × 10−8 cm/s. At SSA = 3%, the unconfined compressive strength of the clay reaches the maximum value of 601.1 kPa. After the clay containing SSA is contaminated by acid and alkali chemical solutions, the amount of mesopores and hydraulic conductivity increase. The adsorption capacity and unconfined compressive strength of contaminated clay decrease about 2∼44% and 25.7∼38.2%, respectively. The modified clay containing SSA can meet the adsorption and geotechnical requirement of landfill liners.