Clayey Till Research Articles

Water balance during evaporation and drainage in cover soils under different water table conditions

Experimental measurement and analytical methods were performed to study water flow through four different cover soils: clayey till, silt, coarse sand and fine sand. The upward and downward water fluxes (i.e. evaporation and drainage) were measured in the laboratory under different water table conditions. Both experimental and analytical results indicated that downward water flow (drainage) significantly affected upward water flow (evaporation) during changes in the water table elevation. Deepening water table elevation decreased evaporation in the sands and to a lesser extent in the silt, by promoting gravity-driven drainage and hence limiting water supply to the soil surface. Evaporation and drainage in the fine-textured clayey till did not change significantly as the water table dropped from the soil surface to 1 m below the soil column. The results suggest that the clayey till would be an effective oxygen barrier in sulfide-bearing mine waste covers, while the sands would be effective evaporation and drainage barriers for the till. The experimental results showed that the air entry value and hydraulic conductivity of the soil are critical parameters controlling drainage and evaporation in soils under different water table conditions. The study emphasizes the need to have evaporation and drainage barriers above and below a soil layer intended to maintain high water saturation under adverse environmental conditions, such as high evaporation and a fluctuating water table.

Modeling and measurement of evaporation in moisture-retaining soil covers

Soil covers are widely used in mine waste and landfill applications to protect the environment. For an effective soil cover, the infiltration and oxygen barrier must have a low hydraulic conductivity and also maintain a high degree of saturation. Soil water evaporation significantly affects water content, and as a result the degree of saturation of the soil. Therefore, knowledge of the rate of evaporation at the soil–atmosphere interface is required to estimate the water content of candidate cover soils. Clayey soil is commonly used, either separately as a single cover or in various combinations with other soils as a layered cover system. In this study, water flow through a single clayey till cover and a layered soil cover were modeled using a coupled liquid flow, vapor diffusion and heat transfer finite-element model (SoilCover). The layered soil cover studied was a typical mine-waste soil cover intended to control oxygen diffusion and infiltration in a temperate climate and consisted of three layers: coarse sand and fine sand as upper and lower capillary barriers, respectively, and clayey till as an infiltration and oxygen barrier. The water level was located at the bottom of the cover. Evaporation and drainage predicted by the model reasonably agreed with experimental results and showed that the clayey till would be an effective oxygen barrier in sulfide-bearing mine waste covers, for which a very low oxygen flux is desired in order to achieve environmental protection. The results emphasize the need to have evaporation and drainage barriers above and below clay barriers designed to retain moisture. Further analysis using SoilCover showed that coarse sand would perform better than either fine sand or silt as a protective top layer over a clayey till barrier.

Improved Complex Permittivity Measurement and Data Processing Technique for Soil-Water Systems

Abstract The complex permittivity of soil-water systems is a function of a number of soil properties. By measuring the complex permittivity, one could quantitatively evaluate the soil behavior under existing conditions in the field, as well as identify changes in the environment. This paper describes the improvement of a measurement and data processing technique for the analysis of the complex permittivity of soil. A sample holder described in a previously published study is modified to reduce errors from sample handling and to increase the measurement accuracy and capacity. A modular unit is designed to measure the static conductivity and the complex permittivity of the soil simultaneously. The data processing technique is also presented, including the identification of the frequency for data analysis and extraction of the static conductivity from the measured loss factors. The results of the complex permittivity measurements on a compacted natural clayey till are presented to demonstrate the typical relationships of the soil complex permittivity as they are related to the soil volumetric water content, bulk density, and salinity.

Fracture wall cements and coatings from two clayey till aquitards.

Secondary minerals occurring at the faces of fractures, the only reliable visual evidence of the presence of hydraulically conductive fractures in clayey unlithified aquitards, have been characterized for two uncontaminated field sites, Dalmeny, Saskatchewan, and Laidlaw, Ontario. Preliminary identification of secondary minerals and their variations with depth was made using a Munsell Color Chart. Subsequent microscopic analyses (petrography, electron microprobe analysis, scanning electron microscopy, and X-ray diffraction) were used to identify iron-oxide mineralogy. Iron oxides were identified as goethite, ferrihydrite, and hematite at Dalmeny, where they occur to depths of 10 to 15 m, and goethite and ferrihydrite at Laidlaw, observed to depths of 7 m. In both cases, the identification of ferrihydrite was tentative due to the problems of small sample size and peak overlap in X-ray diffraction. The iron oxides do not form coatings on the surfaces of the fractures as had been previously thought; rather they form cements linking the matrix grains. Thus there is potential for decreased permeability and increased surface reactivity parallel to and inward from the fracture faces. The pattern of iron-oxide distribution suggests that the youngest deposits, and those with the greatest surface reactivity and potential for contaminant retardation, are found at greatest depths in the fractures. Manganese oxides form in isolated clusters in larger pores and indentations, although the exact manganese minerals could not be firmly identified.

Measurement and estimation of pore shrinkage and pore distribution in a clayey till during soil-water characteristic curve tests

The soil-water characteristic curve (SWCC) of fine-grained soils is usually determined experimentally. In many applications, such as design of mine waste covers and landfill liners, the unsaturated permeability function, k(h), is often derived theoretically from the measured SWCC. Implicit in these derivations is the transformation of the SWCC to a pore-size distribution (PSD), typically assumed to be constant and mono-modal. PSDs of a clayey till compacted at various water contents were measured after compaction, after flexible-wall permeability testing, and during and after SWCC tests. The measurements show that the PSD changes significantly during permeability and SWCC testing. A method is advanced for predicting the observed changes in PSD during SWCC testing. PSDs are determined for soil samples subjected to the highest and lowest suctions applied during the SWCC test. The measured PSDs are transformed to account for pore trapping; the transform assumes that flow occurs through two sets of randomly distributed pores in series. To model pore shrinkage, the pores are idealized as elastic cylinders. PSDs measured after different suction applications in the SWCC tests are compared with predictions of the shrinkage model. The method can also be used to predict the SWCC. Measured and predicted values are compared.Key words: landfill liners, mine waste covers, soil-water characteristic curve, pore-size distribution.

Arsenic in glacial drift aquifers and the implication for drinking water--lower Illinois River Basin.

The lower Illinois River Basin (LIRB) covers 47,000 km2 of central and western Illinois. In the LIRB, 90% of the ground water supplies are from the deep and shallow glacial drift aquifers. The deep glacial drift aquifer (DGDA) is below 152 m altitude, a sand and gravel deposit that fills the Mahomet Buried Bedrock Valley, and overlain by more than 30.5 m of clayey till. The LIRB is part of the USGS National Water Quality Assessment program, which has an objective to describe the status and trends of surface and ground water quality. In the DGDA, 55% of the wells used for public drinking-water supply and 43% of the wells used for domestic drinking water supply have arsenic concentrations above 10 micrograms/L (a new U.S. EPA drinking water standard). Arsenic concentrations greater than 25 micrograms/L in ground water are mostly in the form of arsenite (AsIII). The proportion of arsenate (AsV) to arsenite does not change along the flowpath of the DGDA. Because of the limited number of arsenic species analyses, no clear relations between species and other trace elements, major ions, or physical parameters could be established. Arsenic and barium concentrations increase from east to west in the DGDA and are positively correlated. Chloride and arsenic are positively correlated and provide evidence that arsenic may be derived locally from underlying bedrock. Solid phase geochemical analysis of the till, sand and gravel, and bedrock show the highest presence of arsenic in the underlying organic-rich carbonate bedrock. The black shale or coal within the organic-rich carbonate bedrock is a potential source of arsenic. Most high arsenic concentrations found in the DGDA are west and downgradient of the bedrock structural features. Geologic structures in the bedrock are potential pathways for recharge to the DGDA from surrounding bedrock.

Measurement and estimation of pore shrinkage and pore distribution in a clayey till during soil-water characteristic curve tests

Measurement and estimation of pore shrinkage and pore distribution in a clayey till during soil-water characteristic curve tests

00/01182 Genesis of coal beds and coal-bearing formations

The sorption and desorption of heterocyclic organic compounds in a complex multisolute system to a natural clayey till was investigated. The composition of the solutes reflect a simplified composition of an aqueous phase in contact with coal tar. Sorption was studied for two ratios (s:l) of clayey till (solid) to aqueous phase (liquid). The effect of the complex mixture of solutes on sorption of the four heterocyclic compounds: benzofuran, dibenzofuran, benzothiophene, and dibenzothiophene is evaluated by comparison with their sorption measured in single-solute systems. Sorption of the four compounds is affected by the complex mixture, with sorption decreases for all four compounds at high s:l ratio indicating competitive sorption. The effect on sorption of the individual compounds is not related to solubility or hydrophobicity of the compounds. Freundlich-type isotherms are observed for all compounds in the high s:l-ratio experiments, but for the most hydrophobic compounds isotherms are close to linear. The sorption of N-compounds and benzofuran is apparently influenced by cation exchange and dipole–dipole attraction to clay minerals. At high concentrations a dramatic increase in the sorption of the most strongly sorbing compounds is observed in the low s:l-ratio experiment. The dramatic increase in sorption appears to be a result of multimolecular layer sorption or condensation on surfaces in the clayey till at high surface density of organic compounds, and the data are fitted by BET (Brunauer, Emmet, and Teller) type 2 isotherms. The increase may or may not be induced by the presence of N-heterocyclic compounds sorbed by cation exchange and dipole–dipole attraction. The desorption of the compounds was studied for the low s:l ratio where multimolecular layer formation apparently had occurred. Partially irreversible sorption, hysteric Langmuir type desorption with isotherms of very high Kl coefficient, or behaviour reflecting dissolution of a condensed phase is observed.

Multiple Linear Regression of Complex Permittivity of a Till at Frequency Range from 200 MHz to 400 MHz

The complex permittivities of 40 compacted natural clayey till specimens at various water content, density and pore fluid salinity as independent variables are analyzed. A multiple linear regression model is constructed and the statistical analyses for the relative permittivity (e') and dielectric loss (e '')-regression equations in the frequency range from 200 to 400MHz are performed. It is concluded that the multiple linear regression model of the complex permittivity is highly significant. The coefficient of determination (R 2) and standard error (SE) of the linear regression equations vary with frequency. At 200MHz for e ' -regression and 300MHz for e '-regression, all three independent soil variables are significant at a probability of 0.95. The complex permittivity regression model with two variables at 250MHz for e 'and 350 for e '-regression equations. Upon further study on the effect of soil mineralogy to the complex permittivity, the results obtained in this study may serves as the principle of the characterization of subsurface and detection of soil and groundwater contamination.

Transport and biodegradation of creosote compounds in clayey till, a field experiment

The transport and biodegradation of 12 organic compounds (toluene, phenol, o-cresol, 2,6-, 3,5-dimethylphenol, naphthalene, 1-methylnaphthalene, benzothiophene, dibenzofuran, indole, acridine, and quinoline) were studied at a field site located on the island of Funen, Denmark, where a clayey till 10–15 m deep overlies a sandy aquifer. The upper 4.8 m of till is highly fractured and the upper 2.5 m contains numerous root and worm holes. A 1.5–2 m thick sand lens is encountered within the till at a depth of 4.8 m. Sampling points were installed at depths of 2.5 m, 4 m, and in the sand lens (5.5 m) to monitor the downward migration of a chloride tracer and the organic compounds. Water containing organic compounds and chloride was infiltrated into a 4 m×4.8 m basin at a rate of 8.8 m 3 day −1 for 7 days. The mass of naphthalene relative to chloride was 0.39–0.98 for the sampling points located at a depth of 2.5 m, 0.11–0.61 for the sampling points located at a depth of 4 m, and 0–0.02 for the sampling points located in the sand lens. A similar pattern was observed for eight organic compounds for which reliable results were obtained (toluene, phenol, o-cresol, 2,6-, 3,5-dimethylphenol, 1-methylnaphthalene, benzothiophene, and quinoline). This shows that the organic compounds were attenuated during the downward migration through the till despite the high infiltration rate. The attenuation process may be attributed to biodegradation.

Sorption of heterocyclic compounds from a complex mixture of coal-tar compounds on natural clayey till

The sorption and desorption of heterocyclic organic compounds in a complex multisolute system to a natural clayey till was investigated. The composition of the solutes reflect a simplified composition of an aqueous phase in contact with coal tar. Sorption was studied for two ratios (s:l) of clayey till (solid) to aqueous phase (liquid). The effect of the complex mixture of solutes on sorption of the four heterocyclic compounds: benzofuran, dibenzofuran, benzothiophene, and dibenzothiophene is evaluated by comparison with their sorption measured in single-solute systems. Sorption of the four compounds is affected by the complex mixture, with sorption decreases for all four compounds at high s:l ratio indicating competitive sorption. The effect on sorption of the individual compounds is not related to solubility or hydrophobicity of the compounds. Freundlich-type isotherms are observed for all compounds in the high s:l-ratio experiments, but for the most hydrophobic compounds isotherms are close to linear. The sorption of N-compounds and benzofuran is apparently influenced by cation exchange and dipole–dipole attraction to clay minerals. At high concentrations a dramatic increase in the sorption of the most strongly sorbing compounds is observed in the low s:l-ratio experiment. The dramatic increase in sorption appears to be a result of multimolecular layer sorption or condensation on surfaces in the clayey till at high surface density of organic compounds, and the data are fitted by BET (Brunauer, Emmet, and Teller) type 2 isotherms. The increase may or may not be induced by the presence of N-heterocyclic compounds sorbed by cation exchange and dipole–dipole attraction. The desorption of the compounds was studied for the low s:l ratio where multimolecular layer formation apparently had occurred. Partially irreversible sorption, hysteric Langmuir type desorption with isotherms of very high K l coefficient, or behaviour reflecting dissolution of a condensed phase is observed.

Transport of creosote compounds in a large, intact, macroporous clayey till column

The transport in macroporous clayey till of bromide and 25 organic compounds typical of creosote was studied using a large intact soil column. The organic compounds represented the following groups: polycyclic aromatic hydrocarbons (PAHs), phenolic compounds, monoaromatic hydrocarbons (BTEXs), and heterocyclic compounds containing oxygen, nitrogen or sulphur in the aromatic ring structure (NSO-compounds). The clayey till column (0.5 m in height and 0.5 m in diameter) was obtained from a depth of 1–1.5 m at an experimental site located on the island of Funen, Denmark. Sodium azide was added to the influent water of the column to prevent biodegradation of the studied organic compounds. For the first 24 days of the experiment, the flow rate was 219 ml day −1 corresponding to an infiltration rate of 0.0011 m day −1. At this flow rate, the effluent concentrations of bromide and the organic compounds increased very slowly. The transport of bromide and the organic compounds were successfully increased by increasing the flow rate to 1353 ml day −1 corresponding to 0.0069 m day −1. The experiment showed that the transport of low-molecular-weight organic compounds was not retarded relative to bromide. The high-molecular-weight organic compounds were retarded significantly. The influence of sorption on the transport of the organic compounds through the column was evaluated based on the observed breakthrough curves. The observed order in the column experiment was, with increasing retardation, the following: benzene=pyrrole=toluene= o-xylene= p-xylene=ethylbenzene=phenol=benzothiophene=benzofuran<naphthalene<1-methylpyrrole<1-methylnaphthalene=indole= o-cresol=quinoline<3,5-dimethylphenol=2,4-dimethylphenol<acridine<carbazole<2-methylquinoline<fluorene<dibenzofuran<phenanthrene=dibenzothiophene. This order could not be predicted from regularly characteristics as octanol/water-distribution coefficients of the organic compounds but only from experimentally determined data. The results indicate that a thin clayey till cover of the type described in this paper does not protect groundwater against contamination by low-molecular-weight organic compounds.

Sorption of heterocyclic compounds on natural clayey till

The sorption of benzofuran, dibenzofuran, benzothiophene, and dibenzothiophene from water on natural clayey till from two locations in Denmark was investigated in single solute systems for a large range of solute concentrations. The variation in sorption of a basic (quinoline) and a neutral (dibenzofuran) heterocyclic compound in samples from different depths in the natural clayey till was also investigated. The sorption isotherms for all four compounds and both clayey tills were highly non-linear, and the overall best fits were obtained with Freundlich isotherms. These observations indicate that sorption is not strictly hydrophobic or that the affinity for organic material is influenced by the sorbed solute, and that multiple components contribute to the sorption. The f oc normalized linear K d-value and Freundlich K f-coefficient were compared with estimated K oc values. Hydrophobic sorption appears to dominate for dibenzofuran, benzothiophene and dibenzothiophene, whereas benzofuran and in particular quinoline sorption is influenced by inorganic sorption sites. This corresponds reasonably well with expectations based on estimates of critical organic carbon content. The sorption of quinoline at the uppermost sample was slightly higher (10%–20%) than for the deeper locations. This is possibly related to a slightly higher clay content at this depth. The sorption of dibenzofuran was higher at the deepest location (approximately 65%) than for the above locations. The only measured geological parameter which was significantly different in the sample from the deep location was the content of fine silt.

Transport and biodegradation of creosote compounds in a large, intact, fractured clayey till column

An experiment was conducted using a large, intact column of fractured clayey till to study the transport and biodegradation of 25 organic compounds typical of creosote. The column (0.5 m in height and 0.5 m in diameter) was collected from a depth of 2.5–3 m at an experimental site on the island of Funen, Denmark. For the first 82 days of the experiment, the column was infiltrated with water containing nitrate, but no organic compounds. During this period, significant nitrate removal and nitrite production were observed indicating that denitrification occurred in the clayey till. After 82 days, a mixture of 25 organic compounds with a total concentration of approximately 70 mg l −1 was added to the influent water together with a conservative tracer (92 mg bromide l −1). Most of the organic compounds were transported as rapidly as bromide, and only carbazole, dibenzofuran, fluorene, dibenzothiophene, and phenanthrene were significantly retarded. No extensive loss of organic compounds was observed during this period, which was attributed to the high concentration of applicated organic compounds. After 40 days, the influent concentration of organic compounds was lowered by a factor of 5; subsequently, significant biodegradation of phenol, ethylbenzene, toluene, quinoline, indole, p-xylene, and o-cresol was observed. Additionally, o-xylene, naphthalene, 1-methylnaphthalene, phenanthrene, fluorene, 2-methylquinoline, carbazole, acridine, benzothiophene, dibenzothiophene, benzofuran, dibenzofuran, pyrrole, 1-methylpyrrole, and benzene were biodegraded to some degree when oxygen was added concomitantly with nitrate (92 days after the addition of organic compounds). Pyrrole, 1-methylpyrrole, and benzene were only slightly biodegraded. The biodegradation of benzene was likely inhibited by the presence of pyrrole and/or 1-methylpyrrole. The study has shown that the transport of low-molecular-weight organic compounds through fractured clayey till may occur as rapid as the transport of bromide. Consequently, there is a high risk of groundwater contamination if aquifers are overlain with fractured clayey till with properties similar to the till used in this column study. The study has also shown that the till provides an environment where biodegradation of some organic compounds may occur when oxygen is provided. However, the concentration of oxygen present in water will often not be sufficient for complete biodegradation of the organic compounds at the concentrations known to be typical for creosote sites.

Diffusion of heterocyclic compounds from a complex mixture of coal–tar compounds in natural clayey till

The diffusion of coal–tar compounds in natural clayey till was studied experimentally. Cores were exposed to a solution with near constant concentration of coal–tar compounds in a multi-component mixture for 5 months, the cores were subsequently sub-sampled and analyzed. Diffusion profile data strongly indicates highly non-linear sorption isotherms. For dibenzofuran, dibenzothiophene, 2-methylquinoline, carbazole, phenanthrene, and fluorene, the profiles indicated significantly stronger sorption at high solute concentrations than at low solute concentrations, and than expected based on linear sorption isotherms. This corresponds with observed dramatic increase in sorption of these compounds at high surface density. For benzofuran, benzothiophene, quinoline, phenols, naphthalenes, and BTEXs the profiles indicated significantly weaker sorption at high solute concentrations than at low solute concentrations and than expected based on linear or Freundlich isotherms. The observed diffusion profiles have important implications with respect to transport of dissolved coal–tar compounds in multi-component mixtures of high concentrations as expected near immiscible phase coal–tar sources. Breakthrough times and concentration levels are affected significantly.

The influence of soil structure and stress history on the soil–water characteristics of a compacted till

The soil–water characteristic defines the relationship between the soil suction and gravimetric water content, w, or the volumetric water content, θ, or the degree of saturation, S. Theoretical and empirical relationships are available to model the unsaturated soil properties such as the coefficient of permeability and shear strength using the soil–water characteristic and the conventional saturated soil properties. These procedures are attractive to engineering practitioners because rigorous laboratory tests on unsaturated soil are difficult and time-consuming and, therefore, costly. Various investigators generally have used the soil–water characteristic over a limited suction range (usually 0 to about 1500 kPa) to model the unsaturated soil behaviour. Soils, however, change from a saturated to a dry condition over a range of suctions from 0 to 1000 000 kPa. A rationale for extending the soil–water characteristic up to 1000 000 kPa is provided in this paper, and a method for estimating the residual state of saturation is presented. Soil–water characteristics were developed for the entire range of suctions from 0 to 1000 000 kPa on statically compacted clayey till specimens prepared at three different initial water contents. The influence of initial water content, soil structure and stress history, as it relates to the soil–water characteristic, has been studied and is reported in this paper. The initial water contents selected for this study represent the dry of optimum, optimum and wet of optimum conditions with corresponding densities determined from the standard AASHTO test. The results indicate that initial moulding water content has considerable influence on the resulting structure (and aggregation), which in turn influences the soil–water characteristics. In the low suction range (i.e. 0–1500 kPa), macrostructure governs the soil–water characteristics for specimens compacted with dry of optimum initial water contents and they exhibit a steeper slope. However, for the specimens compacted at wet of optimum, microstructure governs the soil–water characteristic behaviour. The soil–water characteristic of the specimens compacted at dry of optimum are influenced by the stress history; however, stress history appears to have no significant influence on the soil–water characteristics of the specimens compacted with wet of optimum conditions. The soil–water characteristic behaviour of specimens compacted with optimum initial water contents lies in between those of specimens compacted with water contents that are dry and wet of optimum. It appears that soil–water characteristics are not significantly influenced either by the soil structure (aggregation) or the stress history for the high suction ranges (i.e. 20 000–1 000 000 kPa).

Sediment Characterization and Porewater Isotope Chemistry of Quaternary Deposits from the St. Clair Delta, Ontario, Canada

Walpole Island is part of a large freshwater delta located at the St. Clair River mouth in southwestern Ontario. Quaternary deposits on the island consist of Lake Nipissing to Lake St. Clair stage deltaic sediments underlain by a thick sequence of Late Wisconsin glaciolacustrine rhythmites and massive clayey till. These sediments overlie a thin freshwater aquifer resting on Paleozoic bedrock. Porewater δ18O, δD, and δ13CDICprofiles of cores taken from northern and central Walpole Island show that older (>10,000 yr B.P.), deeper glaciogenic porewaters have mixed with, and have been displaced by, younger modern surficial water. In contrast, isotopic profiles from a core taken from southern Walpole Island indicate that modern St. Clair River waters have penetrated the entire 20-m core via fractures, effectively displacing all glaciogenic porewater. Fracturing and faulting are clearly visible throughout the southern core, and their presence may be attributed to its location on the trend of the Electric Fault. These features are interpreted as the product of renewed Holocene fault movement. Porewater Na+, K+, Ca2+, and Mg2+concentration gradients are consistent with the presence of upwardly diffusing deep brine, for which the Electric Fault may serve as a conduit.

DNAPL Transport Through Macroporous, Clayey Till Columns

AbstractThis paper provides the first experimental determination of the rates and distribution of transport of a dense, nonaqueous phase liquid (DNAPL) through a naturally bioporous and fractured clayey till deposit. Until now, assessment of DNAPL behavior in this type of deposit has relied on theoretical studies. Predictions of DNAPL transport have proven to be uncertain as a result of difficulties in measuring critical parameters such as DNAPL entry pressure and flow behavior in response to natural fracture/biopore apertures and the degree of interconnection of these structures. In the present investigation, the migration of free product trichloroethylene (TCE) was studied by means of two undisturbed clayey till columns (dimensions: 0.5 m diameter by 0.5 m long) under in situ effective soil stress conditions. The experiments revealed that transport of TCE was restricted to biopores in one column and fractures in another column, bypassing the low‐permeability clayey matrix. Effective porosities of the columns, i.e., bio‐pores and fractures, were two to three orders of magnitude lower than total porosities (25% and 32%, respectively, for the two columns), i.e., macropores and matrix. Single phase water flow rates through the columns at water‐saturated conditions followed a linear relationship with hydraulic gradient. TCE flow could not be predicted from the single‐phase calculations because of non‐linearity observed between applied TCE injection heads and resulting TCE flow. TCE flow rates were 24 and 10.3 m/day at TCE gradients of 1.18 and 0.91, respectively. The observed flow rates indicate that in cases where vertical biopores or fractures fully penetrate clayey till aquitards, a low‐viscosity DNAPL (e.g., TCE) may quickly enter underlying aquifers. The experiments further indicate that 100 liters of a low‐viscosity DNAPL are sufficient to contaminate approximately 25 to 100 m3 of till material because of the small effective porosity constituted by the biopores and fractures.

Microbial Reduction of Structural Iron in Clays—A Renewable Source of Reduction Capacity

AbstractThe ability of microorganisms to reduce structural Fe in minerals has become recognized as an important mechanism in the oxidation of pollutants in soils and sediments. To better understand the factors controlling the concentration of Fe(II) in natural deposits, samples of clayey till from sites near Havrebjerg and Sparresholm, Denmark, were incubated for up to 160 d with a combination of Pseudomonas bacteria strains. These strains were previously shown to reduce structural Fe(III) to Fe(II) in smectites. According to Eh, pH, microbial activity, and structural Fe(II) content, the incubation could be separated into four phases of which the second phase (from 4 to 10 d after inoculation) generated the most structural Fe(II). Microbial activity increased the structural Fe(II) content from approximately 10% to 20–34% of total clay Fe, a lower level than that obtained for chemically reduced subsamples (76–79%), but more similar to values typical of unweathered tills of Denmark (about 50%). In addition to microbial activity, the particle size (fine or coarse clay) and the Fe(II) content of the clay minerals prior to incubation were very important factors affecting the extent of Fe reduction. The results indicate that structural Fe in clay minerals of clayey till is very reactive and can undergo extensive in situ redox cycling. Electron transfer to structural Fe(III) of the weathered, oxidized clay minerals may renew natural redox barriers in imperfectly drained soils and sediments and thereby control geochemical processes.

Evaluation of chloride and pesticide transport in a fractured clayey till using large undisturbed columns and numerical modeling

Saturated groundwater flow and tracer experiments using fluorescent dye, chloride, and the herbicides mecoprop and simazine were carried out in the laboratory using three large‐diameter (0.5 m) undisturbed columns of fractured clayey till. Hydraulic conductivity of the columns ranged from 10−5 m/s in the shallowest column (1 m dept)) to 10−7 m/s in the deepest column (4 m depth) and were similar to field‐measured values for these deposits. Results of the tracer experiments are consistent with a conceptual model of advective transport along the fractures combined with diffusion into the fine‐grained matrix between the fractures. Arrival of the chloride tracer in the effluent was highly retarded relative to fracture flow velocities calculated on the basis of the cubic law and measured values of fracture spacing and hydraulic conductivity. The herbicides were more strongly retarded than the chloride at low flow rates, but at higher flow rates the herbicides arrived with the chloride, indicating the influence of nonequilibrium sorption of the herbicides to fracture walls and the matrix solids. The columns were dismantled following the tracer experiments and mapping under UV light showed that nearly all of the visible, weathered fractures (and the few root holes in the case of the shallowest sample) were active transport pathways, with the dye appearing mainly on the fracture surfaces and as a “rim” in the adjacent matrix. Concentration profiles measured perpendicular to the fracture surfaces showed that the herbicides had also moved into the matrix, apparently by diffusion. Simulations of solute transport with a discrete fracture flow/matrix diffusion model showed that the simulations could be “fit” to the data if all of the visible fractures were hydraulically active, but could not be fit if all or most of the flow was channelled through just the primary fractures (defined by prominent oxidation stains). Simulations with an equivalent porous media (EPM) model could not fit the data using the measured total porosity as the effective porosity. The simulations could likely be fit with a smaller value of effective porosity, but this would limit applicability to field situations because fitted effective porosity is expected to change with physical scale and residence time of the solute in the soil.