Clayey Sand Soil Research Articles

Site-Specific Erodibility in Claypan Soils: Dependence on Subsoil Characteristics

Abstract. Soil erosion is a primary factor limiting the productive capacity of many crop production fields and contributing to sediment and nutrient impairments of water bodies. Loss of topsoil is especially critical for areas of limited topsoil depth, such as the claypan area of the central United States. More than a century of conventional agricultural practices have eroded the topsoil and, in places, exposed the unproductive clay layer. This clay layer is impervious, limiting water infiltration and root penetration, and severely restricting agricultural productivity. Previous studies have documented changes in topsoil thickness using apparent electrical conductivity (ECa). However, that methodology is limited by its shallow depth of measurement within the soil profile, and as such cannot adequately explore factors within the soil profile that potentially contribute to topsoil erosion. In this study, we identified areas of limited topsoil depth using crop yields and ECa. Two areas within the production field varying in crop production and ECa were selected for detailed measurements using Electrical Resistivity Tomography. This methodology allowed delineation of soil stratigraphy to a depth of 5.3 m. The erodibility of undisturbed soil samples from the two areas were measured in an Erosion Function Apparatus to obtain the critical shear stress, or the applied stress at which soil begins to erode. Based on resistivity measurement, the highly productive region of the field had a thick (1.0-2.0 m) soil layer of saturated clayey sand soil over a uniform sandy material, with minimal clay layer. This soil had a critical shear stress of 12 Pa. The extent of historical erosion was evident in the poorly-producing area, as only a thin band of topsoil material remained over a thicker clay layer. The unproductive area with exposed clay layer had a critical shear stress of 128 Pa, indicating it was more resistant to erosion than the highly productive region. The clay layer was found to extend to 1.3-1.5 m in depth in the soil profile in the poorly producing area. Below this layer was a layer with similar resistivity to the high-producing region. The data reveal the extent of historical erosion within the crop production field and highlight significant variability in measured soil properties within a field of identical production practices. While spatial variations in topsoil have long been considered in developing management practices to improve soil health and productive capacity, our results indicate the importance of identifying variability of subsoil characteristics to address long-term impacts on soil erosion and productivity. Keywords: Soil erosion, Soil electrical conductivity, Claypan soil, Productive capacity, Electrical resistivity tomography.

Chemical potential of a two-component liquid in porous media: The case of unsaturated soil

The aim of this paper is to propose a method to determine the chemical potential of two miscible components of a solution in porous media, particularly in the case of an unsaturated soil. It is limited to the capillary state; the equilibrium of the liquid phase in the soil is determined by gas–liquid interfaces whose geometry obeys Laplace’s law. Deduction of the chemical potential of the two components from the chemical potential of pure water is based on the assumption that, for the same volume occupied by the fluid phase in the porous medium, the geometry of the fluid phase is independent of the composition of the solution.The expressions of the chemical potentials of the two components were established. They show the superposition of two effects: the effect of interfacial forces and the effect of the presence of the other component. A study of the water–alcohol solution in a clayey silty sand soil was then conducted to alcohol mole fractions between 0 and 0.15 and water content in the 5%–15% range. In this field, the chemical potential of the water was seen to be affected by the capillary effects below a water content of 10%. Above that, its variation as a function of the mole fraction of alcohol differed little from that of the potential of the water in a free solution. The chemical potential of the alcohol was little affected by the capillary effects.

Development of Correlation Between Standard Penetration Test and Pressuremeter Test for Clayey Sand and Sandy Soil

The standard penetration test (SPT) and Menard’s pressuremeter test (PMT) are among the available in-situ techniques. This research has been carried out with the aim to develop correlations among the in-situ parameters obtained by both methods.

Investigation of expansive soils in Obhor Sabkha, Jeddah-Saudi Arabia

Expansion or swelling of soil is a worldwide geotechnical problem that occurs in arid and semiarid regions where sabkha soils may occur as well. Expansive soil is dominated by the presence of active clay minerals. The expansive and sabkha soils are characterized by a large seasonal variation in soil moisture content leading to a large change in the volume and the consistency of the soil and, thus, causing serious damages to buildings and infrastructure. Although sabkha soil covers large and strategically important areas along the Red Sea and Arabian Gulf coasts in Saudi Arabia, no one paid proper attention to the type of clay minerals in those soils or to their expansion potential, which is a crucial step prior to any construction. The geotechnical properties, active clay mineral types, and the degree expansion potential of soils were investigated in Obhor area at the north of Jeddah City. Twenty disturbed soil samples were collected at depths of 80 and 120 cm. Three different types of soils are identified: clayey soil with high plasticity, clayey soil with low plasticity, and poorly graded silty to clayey sand soil. Furthermore, active clay minerals were identified with a significant proportion of montmorillonite (14.24 %), illite (24.65 %), kaolinite (28.78 %), and chlorite (32.34 %). The results indicated that a considerable part of the study area has high expansion potential, but most parts of Obhor area have low to none potential of soil expansiveness.

Slope Monitoring Study Using Soil Mechanics Properties and 4-D Electrical Resistivity Tomography Methods

In past applications of electrical resistivity survey, users commonly focussed on the Earth's subsurface to locate water table, bedrock, etc. Nowadays, this electrical resistivity method is used by engineers for shallow subsurface investigations. This paper presents the integration of soil mechanics properties with electrical resistivity tomography methods and recommends the proper selection of resistivity array (survey planning) and appropriate inversion constraint parameters (data processing) that are able to deliver optimum resistivity tomography model results. This study identifies the empirical correlations of the soil's properties such as shear strength parameters, moisture content, void ratio, porosity, saturation degree, and Atterberg's limits with the electrical resistivity values (resistivity tomography models). A total of 11 undisturbed clayey sand soil samples was collected at different distances, depths, and times and were tested under both infield and laboratory conditions during the slope monitoring period. The soil mechanics properties of the soil samples were obtained right after the electrical resistivity survey was made. It is shown that the electrical resistivity values are greatly influenced by the soil mechanics properties. Thus, the electrical resistivity (4-D) survey using the optimized Wenner-Schlumberger array (high data density) is capable of reliably enhancing the conventional outcome of the Earth's subsurface investigation.

Performance Evaluation of Human Hair Fiber Reinforcement on Lime or Cement Stabilized Clayey-Sand

Earthen masonry is generally brittle, weak and susceptible to water ingress damage. Several techniques of earthen masonry stabilization and reinforcement are available for improving their overall engineering properties. However, the inclusion of fiber into soils has a unique advantage. This is because friction between fibers and soil particles increases bonding between the particles. The present study attempts to investigate the resultant strength and ductility behavior when 0.5%, 1% and 2% randomly distributed human hair fibers are used to reinforce both cemented and un-cemented clayey-sand (50% sand and 50% bentonite) soils. The composite soils were tested under laboratory conditions and examined for compaction and unconfined compression strength (UCS) tests. The experimental results show that stress-strain behavior, failure, and strength characteristics of clayey-sand improved with inclusion of fibers. However, the effect of fiber reinforcement is more significant with stabilized clayey sand compared with the control soil mixture. Based on the compressive strength , the cement stabilization had better strength with bentonite clayey sand (50% sand and 50% bentonite) compared to the lime treatment. Addition of cement and lime affected dry density , optimum moisture content, and failure strain of the clayey-sand.

Behavior of a 31 m high excavation supported by anchoring and nailing in residual soil of gneiss

A 31m high excavation located at Niterói/RJ in residual soil from gneissmigmatite with high NSPT (>40) was performed for a building construction. Two levels of anchored walls were considered in order to guarantee stability. When the construction of the upper wall was almost completed, a crack parallel to the direction of the wall formed in the hilltop. It was decided that the construction works would cease in order to perform additional studies. The presence of reasonably continuous soil layers of different colors (gray, white, and red), with thicknesses varying from a few millimeters to some centimeters, clearly observed in the face of the cut slope. The inclination of these soil layers varied along the length of the excavation, and they were in an orientation that was unfavorable for the stability of the cut. Laboratory tests were performed and the presence of high activity clay mineral and low shear resistance parameters was determined for the gray clayey sand soil. Those weakness planes, i.e., the thin layers of gray clayey sand, had not been properly detected in the previous studies that supported the design of those walls. Considering these new findings, the original project was reviewed, and it was decided that soil nailing would be used for accomplish the excavation to the original planned level. The upper wall was also reinforced with additional anchors. Load measurements in the nails indicated that the stress state in the soil mass corresponded to the active condition at the final level of excavation. Significant lateral movements were measured and varied according to the direction and inclination of the cited weakness planes. It was observed that the reliquiae planes were fundamentally important to the behavior of the excavation.

Quantifying the Erodibility of Streambanks and Hillslopes Due to Surface and Subsurface Forces

<abstract> <bold><sc>Abstract.</sc></bold> The erosion rate of cohesive soils due to fluvial forces is usually computed using an excess shear stress model. However, no mechanistic approaches are available for incorporating additional forces, such as seepage, into the excess shear stress model parameters. Recent research incorporated subsurface (seepage) forces into a mechanistic detachment rate model for streambeds. The new detachment model, the Modified Wilson model, was based on two modified dimensional parameters (b₀ and b₁) that included seepage forces. The objective of this study was to modify the parameters (b₀ and b₁) to quantify the influence of seepage on erodibility of cohesive streambanks and to compare the results to those obtained from tests on horizontal beds. A new miniature version of a submerged jet erosion test device (“mini” JET) and a seepage column were utilized to derive the parameters of the Modified Wilson model for a silty sand soil and a clayey sand soil across a range of uniform seepage gradients. The experimental setup was intended to mimic a streambed and a streambank when the “mini” JET and seepage column were placed in vertical and horizontal orientations, respectively. The soils were packed in a standard mold at a uniform bulk density (1.5 to 1.6 Mg m^-3) near the optimum water content. Seepage forces influenced the observed erosion with a non-uniform influence on b₀ and b₁ as functions of the hydraulic gradient and density. Expected theoretical differences between Wilson model parameters for streambanks and streambeds were not consistently observed for these erodible soils, most likely due to variability in streambed and streambank samples in terms of soil preparation, packing, and seepage gradient establishment. The influence of seepage forces can be predicted by the Modified Wilson model parameters in both vertical and horizontal experimental setups using JETs on soils without seepage. Additional research is needed on the behavior of several mechanistic soil parameters under the influence of seepage.

Development of Powered Groundnut Harvester for Small and Medium Holdings in North Kordofan State in Western Sudan

A Powered groundnut harvesting machine was designed and fabricated. The optimum machine diggers were selected using computer simulation method in ANSYS 11 program. The machine was tested in sandy and clayey sand soil. It was found that the effective time and total time recorded by the machine in sandy soil were lower than in clayey sand soil by 0.050 hr. Fuel consumption rate in sandy soil was lower than that in clayey sand soil by 0.7 L / ha. Machine field speed in sandy soil was higher than speed in clayey sand soil by 0.69 km / hr, it was also found that the values of machine theoretical field capacity, effective field capacity in sandy soil were higher than the values in clayey sand soil by 0.061 ha / h and 0.048 ha / h respectively while field efficiency in clayey sand soil was higher than that in sandy soil by 1.2 %. The differences were significant at 0.05 level and it can be concluded that the machine is efficient in harvesting the groundnut crop particularly in clayey sand soil. Hydraulic system and gear box were needed to enhance maneuverability and to control the digging speed of the machine.

Deriving Parameters of a Fundamental Detachment Model for Cohesive Soils from Flume and Jet Erosion Tests

Abstract. The erosion rate of cohesive soils is commonly quantified using the excess shear stress model, which is dependent on two major soil parameters: the critical shear stress (I„ c ) and the erodibility coefficient (k d ). A submerged jet test (jet erosion test, or JET) is one method that has been developed for measuring these parameters. The disadvantage of using the excess shear stress model is that parameters I„ c and k d change according to erosion conditions, such as soil structure, soil orientation, type of clay, presence of roots, and seepage forces. A more mechanistically based detachment model, called the Wilson model, is proposed in this article for modeling the erosion rate of soils using hydraulic analysis of a JET. The general framework of the Wilson model is based on two soil parameters (b 0 and b 1 ). The objectives of this study were to: (1) develop methods of analysis of the JET to determine parameters b 0 and b 1 for the Wilson model in a similar fashion to the previous methodology developed for open-channel flow, and (2) compare the excess stress model parameter (k d ) and the Wilson model parameters (b 0 and b 1 ) determined from the flume tests and JETs for two cohesive soils. Flume tests, treated as the standard test method, and original and “mini” JETs were conducted on two soils to independently measure the excess shear stress model parameter (k d ) and the Wilson model parameters (b 0 and b 1 ). Soil samples of two cohesive soils (silty sand and clayey sand soils) were packed in a soil box for the flume tests and the JETs at water contents ranging from 8.7% to 18.1%. No statistically significant differences were observed for the excess shear stress model parameter (k d ) and for the Wilson model parameters (b 0 and b 1 ) when determined from the flume tests and JET devices, except for b 1 with the original JET. The Wilson model is advantageous in being a more mechanistic, fundamentally based erosion equation as compared to the excess shear stress model. The Wilson model can be used in place of the excess shear stress model with parameters that can be estimated using existing JET techniques.

Evaluation of Cavity Formation and the Use of Cut-off Wall to Reduce the Risk of Washing Subsurface Fine Material

This study shows the results of mapping numerous cavities and distress which appeared and detected in Qassim area, Saudi Arabia. This phenomenon was observed near a school building and residential area and became a serious risk to occupants and residents. The survey was carried out applying geotechnical techniques which included advancing rotary boreholes to depths of 23 m to 30 m with sampling and testing. The evaluation process also included resistivity imaging profiles using 2D electrical resistivity measurements. Results obtained from this research showed a thick top layer of silty clayey sand soil rich of gypsum and carbonate presenting a hazardous and high-risk soil type. The percentage of fines that are likely to be washed out as a result of chemical disintegration and exposure to significant hydraulic gradient was of great concern. Assessment was made using combined geotechnical and geophysical approach in addition to chemical tests. Based on the data collected and analysis of test results a practical solution was suggested to solve this problem. The use of “cut-off wall” in order to reduce the level of subsurface scour and cajuvity formation were found appropriate. The depth of the cut off wall was determined based on the subsurface geological profile. Advantages of this approach and concerns need to be considered in adopting typical solutions that are presented.

Electroosmosis in the Remediation of Tropical Soils Contaminated with Cadmium: Effect of the Incubation Time

This paper addresses the effect of cadmium incubation time on the electroosmotic decontamination of three residual soils from the Zona da Mata Norte, Minas Gerais State, Brazil. The laboratory testing program included the following: (i) materials, encompassing a lateritic clayey silty sand (soil 1), a non-lateritic clayey silty sand (soil 2), and a non-lateritic sandy silty clay (soil 3) impregnated with an aqueous solution of cadmium nitrate in the concentration of 100 mg.L-1; (ii) soils and mixtures specimens compaction at the Standard AASHTO compaction effort; (iii) mixtures contaminant incubation times of 1, 10 and 20 days before performing the electroosmotic decontamination tests; (iv) sequential extraction analysis in order to identify the different contaminant chemical forms in the mixtures specimens. The results support that incubation time and soils chemistry and mineralogy were significant and influential factors in the decontamination process, as well as the 20 days incubation period was the optimum among the tested times.

Clayey Sand Soil's Behaviour Analysis and Imaging Subsurface Structure via Engineering Characterizations and Integrated Geophysicals Tomography Modeling Methods

The geoelectrical resistivity and seismic refraction surveys which were used in this study on the test site, delivered a detailed image of the near-surface conditions in generally very good. Electrical resistivity and seismic refraction analysis proved that a combination of these integrated study of the physical environmental data provided a reasonable compromise between measurement time and image resolution. Quantitative interpretation of the resistivity and seismic models based on soil's parameters determined using laboratory practices and field survey could reproduce the range of resistivity and seismic values found on the site very well. The model explains the ambiguity in between resistivity and clayey sands found on the site and predict the dominant role of water saturation. Geophysical methods are used in this research in purpose to determine the internal structure of a soil mass. Various geophysical methods and their merits for imaging subsurface structures and condition are discussed. Seismic methods are often the most suitable because the measurements depend on the mechanical properties which are also important in the mechanical calculation of soil's behaviour analysis. Other geophysical method, such as geoelectric resistivity, is useful to determine the internal structure, but require a correlation of found boundaries with mechanical properties. This research was conducted to investigate the subsurface structures and conditions through geotechnical engineering properties and its geophysical characteristics. The computation analysis is used in this research in purpose to investigate clayey sand soil's behaviour. Electrical resistivity test and engineering laboratory practices such as soil strength test, liquid limit test, plastic limit test and grain size distribution test was also carried out to investigate clayey sand soil behaviour in Batu Uban, Penang area during monitoring period.

Tropical Clayey Sand Soil's Behaviour Analysis and Its Empirical Correlations via Geophysics Electrical Resistivity Method and Engineering Soil Characterizations

Soil is a heterogeneous medium which consist of liquid, solid, and gaseous phases. The solid and liquid phases play an essential role in soil spontaneous electrical phenomena and in behaviour of electrical fields, artificially created in soil. Soil electrical properties are the parameters of natural and artificially created electrical fields in soils and influenced by distribution of mobile electrical charges, mostly inorganic ions, in soils. Geophysical method of electrical resistivity was used for measuring soil electrical properties and tested in different soil studies. Laboratory tests were performed for the numbers of clayey sandy soil samples taken from Batu Uban area. The empirical correlations between electrical parameter, percentage of liquid limit, plastic limit, plasticity index, moisture content and effective soil cohesion were obtained via curvilinear models. The ranges of the soil samples are changed between 229 Ωm to 927 Ωm for resistivity (ρ), 6.01 kN/m2 to 14.27 kN/m2 for effective soil cohesion (C'), 35.08 kN/m2 to 51.47 kN/m2 for internal fiction angle (?'), 38% to 88% for moisture content (W), 33% to 78% for liquid limit (WL), 21% to 43% for plastic limit (Wp) and 11% to 35% for plasticity index (PI). These empirical correlations model developed in this study provides a very useful tool to relate electrical resistivity with effective cohesion, internal friction angle (strength), void ratio, porosity, degree of saturation, moisture content, liquid limit, plastic limit and plasticity index in context of medium-grained of clayey sandy soil that is, its fluid behaviours.

Changes of hydraulic conductivity of silty clayey sand soil under the effects of municipal solid waste leachate

Effect of municipal solid waste (MSW) on the hydraulic conductivity of soil is an important factor for designing liner systems. Leachate samples were collected within a landfill and a composting factory leachate lagoon. Soil samples from the bottom of the Esfahan, Iran, landfill were collected. Effects of the leachates on permeability of the soil samples were investigated. The results of study showed that at the highest level of leachate concentration (100%), the soil showed 20% increase in permeability (k) value from compost leachate and 10% reduction in landfill leachates. Compost and landfill leachates had shown contrasting impact on the soil permeability. Key words: Hydraulic conductivity, landfills, leachate.

Biodegradation of Polycyclic Aromatic Hydrocarbons (PAHs) in Petroleum Contaminated Soils

Polycyclic aromatic hydrocarbons are a class of potentially hazardous chemicals of environmental and health concern. PAHs are one of the most prevalent groups of contaminants found in soil. Biodegradation of complex hydrocarbon usually requires the cooperation of more than single specie. In this research biotreatment of PAH (phenanthrene) was studied in a solid- phase reactor using indigenous bacteria isolated from two petroleum contaminated sites in Iran, (i.e., Tehran refinery site with clayey-sand soil composition and Bushehr oil zone with silty-sand soil composotion). Phenanthrene (C 14 H10 ) was made in three rates (100, 500, and 1000 mg/kg of soil) synthetically and was conducted with two bacterial mixed cultures for a period of 20 weeks. Highest removal (more than 85 %) of phenanthrene with rates of 100, 500 and 1000 mg/kg in clayey-sand soil with BMTRS (Bacterial Mix of Tehran Refinery Site) consortium was achieved within 3, 5 and 14 weeks, respectively as for silty-sand soil composition with BMBOZ (Bacterial Mix of Bushehr Oil Zone) consortium was achieved within 10, 17, and 19 weeks, respectively. Results for phenanthrene biotreatment in solid phase reactor revealed a significance relationship between concentration and type of microbial consortium with the removal efficiency of phenanthrene over the time (P value<0.001). Furthermore, there was a significant relationship between soil type with removal efficiency of phenanthrene over the time (P value=0.022). That means the bioremediation of the lower concentrations of phenanthrene needs shorter time compared with the higher concentrations. Microbial analysis using confirmative series tests and analytical profile index (API) kit tests showed the Pseudomonas fluorescence, Serratia liquefaciens, Bacillus and Micrococcus strains as dominant bacteria in the mixed cultures.

Evaluation of a Quasi-steady-state Respiration Test in a Full-scale Biopile

A quasi steady state respiration test based on Fick’s law with a correction term for advective flux, for estimating petroleum hydrocarbon degradation rates, was evaluated in a full-scale (3,000 m3) biopile study. A contaminated clayey sand soil with an average TPH content of 1,421 ± 260 mg kg−1 soil was treated in a biopile with a fixed venting and heating system. Temperature in the biopile ranged from 12.1 to 36.6°C and soil water content from 15.2 to 35.8 m3 H2O m−3 soil. Oxygen concentrations in the biopile showed a rapid decrease with depth, before venting and reached constant atmospheric concentration during venting. Measured oxygen consumption in the biopile ranged from −0.04 to −0.68 mol O2 m−3 soil day−1. Average oxygen consumption rates calculated with the quasi-steady-state method were significantly (P < 0.05) lower then the oxygen consumption rates calculated with the transient method. It was suggested that the oxygen diffusion was underestimated by the diffusivity models used and that further research is needed to determine relative effective diffusion coefficients in biopiles. Although both respiration testing and petroleum hydrocarbon concentration showed a decrease of oxygen consumption in time, the estimated degradation rate was low compared to the actual decrease in petroleum hydrocarbon concentration. Additional work will have to be done to acquire a more precise knowledge of the relationship between respirometrically determined degradation rates and the actual change in petroleum hydrocarbon concentration in the soil.

Effect of Long-Term Effluent Recharge on Phosphate Sorption by Soils in a Wastewater Reclamation Plant

The Soreq recharge basins, used for wastewater reclamation employing the Soil-Aquifer Treatment (SAT) system, have been recharged, on average, by about 1,800 m depth of secondary effluent during their operation period of ∼25 years. An estimated amount of ∼6 kg P m−2 was added to the soil/sediment column during this period. The objective of this study was to compare phosphorous sorption characteristics of representative pristine soils in the Soreq recharge site to those of the basin soils sampled after a long period of effluent recharge. Batch isotherm experiments were conducted: samples of one g of soil were equilibrated with 25 mL of 0.02 M NaCl solution containing 0–3.2 mM of phosphate for 7 days at 25± 1∘C and P sorption was measured. Long-term effluent recharge significantly decreased the maximum P sorption capacity of the top sandy soil (0.15–0.3 m) and only very slightly decreased maximum P isotherm capacity of the deep clayey-sand soil (10–10.5 m). The retention of P in the basin sandy soil primarily involved sorption and surface precipitation reactions on soil carbonates. In the basin clayey-sand soil, P was retained by its sorption on surfaces of Fe, Al, Mn oxide/hydroxides and clay minerals. Long-term effluent recharge increased EPC0, (the equilibrium P concentration in solution at which there is no sorption or desorption to or from the soil under the given conditions), of the basin soils compared to the pristine soils. Due to loading of the top horizons with P by prolonged recharge and reduced P concentration in the effluent, EPC0 of the basin sandy soil is now equal to the average P concentration of the recharged effluents. If effluent P concentration will decrease further, the top sandy soil will become a source of P to the reclaimed water, rather than a sink. The clayey-sand layers and lenses in the vadose zone of the SAT system of the Soreq site offer a large capacity for P adsorption. With gradual leaching of carbonate minerals and synthesis of secondary clay minerals, driven by long-term effluent recharge, P retention mechanisms in the basin soil may be changed, but this process would be extremely slow.

Utilization of fly ash for stabilization/solidification of heavy metal contaminated soils

Pozzolanic-based stabilization/solidification (S/S) is an effective, yet economic remediation technology to immobilize heavy metals in contaminated soils and sludges. In the present study, fly ash waste materials were used along with quicklime (CaO) to immobilize lead, trivalent and hexavalent chromium present in artificially contaminated clayey sand soils. The degree of heavy metal immobilization was evaluated using the Toxicity Characteristic Leaching Procedure (TCLP) as well as controlled extraction experiments. These leaching test results along with X-ray diffraction (XRD), scanning electron microscope and energy dispersive X-ray (SEM–EDX) analyses were also implemented to elucidate the mechanisms responsible for immobilization of the heavy metals under study. Finally, the reusability of the stabilized waste forms in construction applications was also investigated by performing unconfined compressive strength and swell tests. The experimental results suggest that the controlling mechanism for both lead and hexavalent chromium immobilization is surface adsorption, whereas for trivalent chromium it is hydroxide precipitation. Addition of quicklime and fly ash to the contaminated soils effectively reduced heavy metal leachability well below the nonhazardous regulatory limits. Overall, fly ash addition increases the immobilization pH region for all heavy metals tested, and significantly improves the stress-strain properties of the treated solids, thus allowing their reuse as readily available construction materials. The only potential problem associated with this quicklime–fly ash treatment is the excessive formation of the pozzolanic product ettringite in the presence of sulfates. Ettringite, when brought in contact with water, may cause significant swelling and subsequent deterioration of the stabilized matrix. Addition of minimum amounts of barium hydroxide was shown to effectively eliminate ettringite formation. Overall, due to the presence of very high levels of heavy metal contamination along with sulfates in the solid matrices under study, the results presented herein can be applied to the management of incinerator and coal fly ash, boiler slag and flue gas desulfurization wastes.

Field Test of Nonfuel Hydrocarbon Bioventing in Clayey-Sand Soil

A pilot-scale bioventing test was conducted at the Greenwood Chemical Superfund Site in Virginia. The characteristics of the site included clayey-sand soils and nonfuel organic contamination such as acetone, toluene, and naphthalene in the vadose zone. Based on the results of an earlier treatability study, an 80-ft by 80-ft (24-m by 24-m) treatment plot was established in a 35-ft (11-m) vadose zone. Air was injected at a low flowrate for 15 months. Performance monitoring included initial and final soil analysis and periodic soil gas analysis and in situ respiration tests. After beginning aeration, soil gas oxygen levels in the plot rose slowly, reaching 10% at virtually all measured locations in approximately 4 months. In situ respiration rates decreased with time indicating that the site was being cleaned. Soil concentrations of the target contaminants decreased significantly during the test, with > 98% confidence for acetone, naphthalene, benzene, chlorobenzene, and toluene, and > 90% confidence for 1,2-dichloroethane. Almost 80% of the initial mass of the target contamination was acetone. The bioventing system removed approximately 60 kg of acetone. The total oxygen consumed, estimated from respiration rates, was more than 80 times that needed to degrade the mass of contaminants removed as measured by soil analysis. Thus, it is likely that most if not all of the removal of the target compounds was attributable to biodegradation.