Soil hydraulic properties are often estimated based on laboratory data or pedotransfer functions dependent on soil physical properties, which often do not consider potential impacts of soil roots or fungal hyphae. Here, we first review current knowledge of how these soil biotic components affect hydraulic properties, then we conducted laboratory experiments to specifically test if the presence of roots and mycorrhizal fungi had a significant effect on the hydraulic properties of two soils with contrasting textures: Flint sand and Hamblen silt loam. Soil cores were seeded with (Panicum virgatum) and grown in a greenhouse over three separate growth periods. The endophytic fungus Serendipita indica was injected as liquid inoculant into designated mycorrhizal cores. Saturated hydraulic conductivity (Ksat) measurements were made with a constant head permeameter, and soil water retention curves were obtained by the evaporation method, supplemented at the dry end for Hamblen silt loam with water activity meter data. Retention curve parameters were obtained by fitting the van Genuchten equation to the resulting measurements. Mean root volume ratios were higher in the mycorrhizal inoculated treatment than in the uninoculated treatment for both soils. For Flint sand, analysis of variance revealed that Ksat was reduced by the presence of roots as compared to bare soil. This was likely due to roots clogging soil pores. Results also indicated the presence of roots changed the shape of the water retention curve for Flint sand by increasing water content at saturation and by reducing the slope of the curve. These changes suggested roots created additional porosity and broadened the pore-size distribution. The presence of mycorrhizal fungi accentuated the root effects. The influence of roots and mycorrhizal fungi on hydraulic properties was less obvious for the Hamblen silt loam, as none of the treatments differed from each other at p < 0.05. The results highlight the necessity to consider the impact of root and fungal structures on models of soil hydraulic properties.

Deep, underground repositories are needed to isolate radioactive waste from the biosphere. Because bentonite is an integral component of many multibarrier repository systems, information on the hydraulic behavior of bentonite is crucial for modeling the long-term viability of such systems. In this paper the hydraulic behavior of bentonite samples was analyzed as a function of aggregate size, and samples were subjected to hydrothermal treatments involving contact with NaCl, KCl, and deionized water. Neutron and X-ray imaging were used to quantify water sorption into packed bentonite samples and bentonite swelling into the water column. The distance between the original clay-water interface and the wetting front was determined as a function of time. Average water uptake exhibited a square-root-of-time dependence in freshly prepared samples, but more variable rates were observed for samples previously in contact with water. The radiography was supported by small-angle neutron scattering analysis and ultra-small-angle neutron scattering analysis of aggregate size distributions and by inelastic neutron scattering to understand the physicochemical environment of the sorbed water. Results showed that hydrothermal treatment with KCl had the greatest effect of increased water transport in the bentonite, possibly as a result of the interaction of K+ with smectite layers in the clay.

ABSTRACTMost fracture data analysis techniques for attributes such as dip and aperture, treat the attributes independently of their respective spatial locations. A power‐law cumulative frequency for fracture apertures, for example, tells us nothing about their spatial distribution. Lacunarity is a technique for analyzing multi‐scale binary and non‐binary data and is ideally suited for analysis that relates an attribute (e.g., aperture) to its spatial distribution. In a previous study, we showed that scale‐dependent heterogeneity of fracture spacing can be analyzed using lacunarity in order to identify whether fractures occur in clusters. To determine if such clusters contain the largest fractures that control fluid flow through a fracture network, it is imperative that size attribute data be integrated with information about fracture spacing. Here we introduce the novel concept of lacunarity ratio (LR), which is the lacunarity of a given non‐binary data set normalized to the lacunarity of its random counterpart. This technique can delineate the relationship between attributes and spatial clustering by determining scale‐dependent changes in persistence and anti‐persistence. LR is implemented to test if large fractures are statistically found within fracture clusters or if they are randomly distributed at a given scale of observation. The technique is then applied to five different data sets with spacing values together with aperture, length and dip values respectively. The LR‐technique thus developed can help in identifying the occurrence of large or steep fractures with respect to fracture clusters, which in turn, can help improve modeling strategies.

Infiltrated stormwater from a regenerative stormwater conveyance (RSC) investigated in Knoxville, TN, was found to substantially interact with the surrounding groundwater system. The degree of interaction resulted from seasonal fluxes in water table elevation, varying weather patterns and event sizes, and most significantly, the presence of two underground sewer lines. Mounding was observed below the RSC during small to medium storm events, which shifted to a sink for areas adjacent to the conveyance during larger events. The most prominent factor influencing trends in groundwater interactions appeared to be the presence of the sewer lines. This study shows the importance of understanding how the urban subsurface infrastructure network will influence groundwater interactions in order to avoid unintended consequences or short-circuiting of treatment mechanisms. These findings inform the scientific community of potential challenges when constructing infiltration-based stormwater controls in urban environments where a high density of buried infrastructure may be present.

Abstract Spontaneous imbibition is a capillary‐driven phenomenon in which a wetting fluid displaces a nonwetting fluid from voids without any increase in external pressure. This study compared forward predictions of early‐time displacement of air by water within Mode I fractures in 14 low‐porosity rock cores using a new fractal model with those based on an established parallel plate model. Spontaneous imbibition was measured using dynamic neutron radiography, along with independent determinations of equilibrium contact angle, fracture aperture width, and fracture surface fractal dimension, Ds. The predicted uptake curves generally agreed with the experimental data. However, both models overpredicted the height of the wetting front at any given time. This overprediction may be due to lateral losses of wetting fluid to the matrix by spontaneous imbibition through fracture surfaces. The predictions of the fractal model were consistently closer to the observed values than those of the parallel plate model, and for the best cases, their upper and lower confidence intervals bounded the data points. In 12 out of 14 cases, the RMSD for the fractal model was less than that for the parallel plate model. A paired t test indicated that, on average, the RMSD for the fractal model was significantly lower than that for the parallel plate model. This statistically improved prediction can be attributed to the retardation of predicted uptake achieved through the introduction of a fracture surface roughness parameter (i.e., Ds) in the fractal model.

Information on menisci dynamics and equilibrium contact angles, theta(e), is needed for modeling multiphase flow of fluids in geologic formations. The wettability of fracture faces is relevant to applications such as waterflooding in enhanced oil recovery and evaluation of caprock integrity for geologic carbon sequestration. We investigated the vertical capillary rise of air-water menisci on exposed fracture faces for a sandstone, a shale, and three granites. The experimental design approximated that employed in the Wilhelmy plate method. Replicate fracture faces were prepared by fracturing cylindrical rock cores using the Brazilian method and splitting the fractured cores apart along their fracture planes. Meniscus onset was imaged from a cross-sectional perspective using dynamic neutron radiography, and quantified with change point analysis. An existing model for meniscus onset fitted the experimental data very well (median R-2 = 0.96). Capillary rise was typified by t(2/3) scaling (where t is time), followed by a constant equilibrium height, z(e). The average time taken for the menisci to reach z(e) was 0.39 s, with no differences between rock types. The menisci achieved a greater vertical extent on the fracture faces of the sedimentary rocks as compared to those of the granites. Apparent theta(e) values were calculated from the best fit estimates of z(e) using the Wilhelmy plate equation for a smooth and planar vertical surface. Despite the fractal roughness of some of the fracture faces, there was a significant linear relationship (R-2 = 0.80) between the apparent theta(e) values and intrinsic theta(e) values measured on polished surfaces of the same rocks using the sessile drop method. The regression equation indicated the apparent theta(e) values were consistently greater than the intrinsic values. This overestimation may be due to averaging of pixels at the air-water-solid interface, a required step in the image analysis. Further exploration of the range of applicability and measurement accuracy of this proposed new technique would be valuable.

The relationship between fracture aperture (maximum opening; [Formula: see text]) and fracture width ([Formula: see text]) has been the subject of debate over the past several decades. An empirical power law has been commonly applied to relate these two parameters. Its exponent ([Formula: see text]) is generally determined by fitting the power-law function to experimental observations measured at various scales. Invoking concepts from fractal geometry we theoretically show, as a first-order approximation, that the fracture aperture should be a linear function of its width, meaning that [Formula: see text]. This finding is in agreement with the result of linear elastic fracture mechanics (LEFM) theory. We compare the model predictions with experimental observations available in the literature. This comparison generally supports a linear relationship between fracture aperture and fracture width, although there exists considerable scatter in the data. We also discuss the limitations of the proposed model, and its potential application to the prediction of flow and transport in fractures. Based on more than 170 experimental observations from the literature, we show that such a linear relationship, in combination with the cubic law, is able to scale flow rate with fracture aperture over [Formula: see text]14 orders of magnitude for variations in flow rate and [Formula: see text]5 orders of magnitude for variations in fracture width.

The effect of solvent extraction on pore space was examined on a suite of samples from the Eagle Ford Shale Formation with varying lithologies and maturities. Several solvents ─toluene, cyclohexane, methanol, dichloromethane, and hydrochloric acid─ were contacted with shale samples, extracting the compatible organic matter. The porosity in these extracted shale samples was compared to unmodified samples. The amount and type of organic matter extracted were determined using Gas Chromatography ─ Mass Spectrometry, and the porosity was determined by (Ultra) Small Angle Neutron Scattering. Mostly alkanes and aromatics were detected in the extracts, but other portions of bitumen may also have been present. Only higher molecular weight alkanes were extracted with hydrochloric acid, suggesting that physical dissolution of carbonate minerals may have liberated this portion of organic matter and the solvent was not able to penetrate the bitumen and kerogen to extract the lower molecular weight alkanes. Additionally, a decrease in porosity with extraction was observed and attributed to a dominant mechanism of kerogen swelling due to kerogen-solvent interaction.

Models of fluid flow are used to improve the efficiency of oil and gas extraction and to estimate the storage and leakage of carbon dioxide in geologic reservoirs. Therefore, a quantitative understanding of key parameters of rock-fluid interactions, such as contact angles, wetting, and the rate of spontaneous imbibition, is necessary if these models are to predict reservoir behavior accurately. In this study, aqueous fluid imbibition rates were measured in fractures in samples of the Eagle Ford Shale using neutron imaging. Several liquids, including pure water and aqueous solutions containing sodium bicarbonate and sodium chloride, were used to determine the impact of solution chemistry on uptake rates. Uptake rate analysis provided dynamic contact angles for the Eagle Ford Shale that ranged from 51 to 90° using the Schwiebert-Leong equation, suggesting moderately hydrophilic mineralogy. When corrected for hydrostatic pressure, the average contact angle was calculated as 76 ± 7°, with higher values at the fracture inlet. Differences in imbibition arising from differing fracture widths, physical liquid properties, and wetting front height were investigated. For example, bicarbonate-contacted samples had average contact angles that varied between 62 ± 10° and ∼84 ± 6° as the fluid rose in the column, likely reflecting a convergence-divergence structure within the fracture. Secondary imbibitions into the same samples showed a much more rapid uptake for water and sodium chloride solutions that suggested alteration of the clay in contact with the solution producing a water-wet environment. The same effect was not observed for sodium bicarbonate, which suggested that the bicarbonate ion prevented shale hydration. This study demonstrates how the imbibition rate measured by neutron imaging can be used to determine contact angles for solutions in contact with shale or other materials and that wetting properties can vary on a relatively fine scale during imbibition, requiring detailed descriptions of wetting for accurate reservoir modeling.

An explicit relationship between seepage properties and microscopic structure of porous media is being sought for the development of oil and gas resources. An effective method to accurately characterize and quantify the microscopic structure of porous media is a key issue. Fractal geometry can take advantage of several physically-based parameters to analyze microstructures of reservoir rocks. In this study, three fractal structural parameters, fractal dimension, lacunarity and succolarity, were employed to characterize scale-invariant complexity, heterogeneity, and anisotropy of rock microstructures, respectively. Twelve three-dimensional digital cores of sandstone reservoir rocks were used to evaluate permeability in terms of fractal dimension, lacunarity and succolarity. The parameters were utilized to quantitatively characterize differences in core micro-structure and predict their effects on permeability. Due to the confounding influence of porosity, the fractal dimension was not an accurate predictor of the variation in permeability on its own. Instead, the results reveal that lacunarity and succolarity were better able to predict differences in structure and permeability. Succolarity, in particular, showed an exponential relationship with permeability, yielding a coefficient of determination of 0.940. Using a combination of fractal structural parameters in the place of pore-size distribution, can provide a better explanation of the relationship between fluid flow, and the heterogeneous structure and anisotropic physical properties of reservoir rocks.

The breakthrough curve obtained from a single-well push-pull test can be adjusted to account for dilution of the injection fluid in the aquifer fluid. The dilution-adjusted breakthrough curve can be analyzed to estimate the reaction rate of a solute. The conventional dilution-adjusted method assumes that the ratios of the concentrations of the nonreactive and reactive solutes in the injection fluid vs. the aquifer fluid are equal. If this assumption is invalid, the conventional method will generate inaccurate breakthrough curves and may lead to erroneous conclusions regarding the reactivity of a solute. In this study, a new method that generates a dilution-adjusted breakthrough curve was theoretically developed to account for any possible combination of nonreactive and reactive solute concentrations in the injection and aquifer fluids. The newly developed method was applied to a field-based data set and was shown to generate more accurate dilution-adjusted breakthrough curves. The improved dilution-adjusted method presented here is simple, makes no assumptions regarding the concentrations of the nonreactive and reactive solutes in the injection and aquifer fluids, and easily allows for estimating reaction rates during push-pull tests.

Core Ideas Spontaneous imbibition was measured in rock fractures using neutron radiography. Early‐time uptake of water displacing air showed a square root of time dependency. Fracture sorptivity was quantified from the slope of the early‐time uptake data. Fracture sorptivity increased with increasing fracture aperture width. Fracture sorptivity decreased with increasing fracture surface roughness. Fractures in low‐porosity rocks can provide conduits for fluid flow. Numerous researchers have investigated fluid flow through fractures under saturated conditions. However, relatively little information exists on spontaneous imbibition in fractures, whereby a wetting fluid displaces a non‐wetting fluid by capillarity. We investigated spontaneous imbibition of water displacing air in a suite of fractured low‐porosity sedimentary and igneous rock cores (5.08‐cm length by 2.54‐cm diameter). Mode I fractures were induced in the cores by compression between opposing parallel flat plates. The following physical properties were measured: bulk density, ρb; solid‐phase density, ρs; porosity, ϕ; contact angle, θe; fracture aperture width, xgeo; and fracture surface roughness, Wr. The wetting front in each fracture was imaged using dynamic neutron radiography. Early‐time uptake exhibited a square root of time dependency, and was quantified by linear regression, with the slope equal to the fracture sorptivity, Sf. Estimates of Sf ranged from 10.1 to 40.5 mm s−0.5, with a median value of 25.0 mm s−0.5. There was a statistically significant effect of rock type on Sf, with igneous rocks generally having lower mean values than sedimentary rocks. Differences in ρb, ρs, ϕ, and θe between the rock types did not contribute significantly to the variation in Sf. However, xgeo and Wr were significantly correlated with Sf. These correlations indicated that Sf increases with increasing xgeo, as predicted by early‐time capillary theory, and decreases with increasing Wr, analogous to the decrease in fracture permeability with increasing surface roughness observed under saturated flow conditions.

Commercial production of algal biofuels is currently limited by high capital costs, particularly costs of synthetic pond liners that minimize seepage of cultivation fluids, salts, and nutrients into the subsurface beneath outdoor algae cultivation facilities. Even some profitable nutraceutical companies bear the high costs of pond liners. However, studies of animal waste holding ponds and aquaculture facilities have shown that underlying soils can effectively "self-seal", potentially eliminating the need for plastic liners. Here, we explored the potential for self-sealing with algae to provide an alternative to synthetic pond liners in unlined algae cultivation ponds. Laboratory-scale soil column experiments were used to investigate soil clogging by algae, a topic rarely discussed in the literature. Columns composed of fine sand, loamy sand, and loam soil displayed reductions in saturated hydraulic conductivity of the entire soil column of up to two orders of magnitude in response to infiltration by the alga Scenedesmus dimorphus suspended in algal culture nutrient solution at a density of 5 x 10(8) cells L-1 Scanning electron microscope imaging showed a dense algal layer on the surface of each soil column, thickly coating mineral grains with algal material and filling pore spaces. The algal coating was absent in samples at > 3 cm depth in the soil columns, and measurements of chlorophyll content also confirmed that algal cells were mostly confined to the soil surface. In this study, the application of an algal suspension contributed significantly to soil sealing. This new application of biological sealing technology should be studied further to determine its utility and durability as an alternative to plastic liners, perhaps in conjunction with soil compaction to achieve hydraulic conductivities beneath algal production ponds that are protective of groundwater quality.

Spontaneous imbibition (SI) is a capillary-driven flow process, in which a wetting fluid moves into a porous medium displacing an existing non-wetting fluid. This process likely contributes to the loss of fracking fluids during hydraulic fracturing operations. It has also been proposed as a method for an enhanced recovery of hydrocarbons from fractured unconventional reservoirs. Numerous analytical and numerical approaches have been employed to model SI. Invariably, these idealize a fracture as the gap formed between parallel flat surfaces. In reality, rock fracture surfaces are rough over multiple scales, and this roughness will influence the contact angle and rate of fluid uptake. We derived an analytical model for the early-time SI behavior within a fracture bounded by parallel impermeable surfaces with fractal roughness assuming laminar flow. The model was tested by fitting it to experimental data for the SI of deionized water into air-filled rock fractures. Twenty cores from two rock types were investigated: a tight sandstone (Crossville) and a gas shale (Mancos). A simple Mode I longitudinal fracture was produced in each core by compressive loading between parallel flat plates using the Brazilian method. Half of the Mancos cores were fractured perpendicular to bedding, while the other half were fractured parallel to bedding. The two main parameters in the SI model are the mean separation distance between the fracture surfaces, [Formula: see text], and the fracture surface fractal dimension [Formula: see text]. The [Formula: see text] was estimated for each core by measuring the geometric mean fracture aperture width through image analysis of the top and bottom faces, while [Formula: see text] was estimated inversely by fitting the SI model to measurements of water uptake obtained using dynamic neutron radiography. The [Formula: see text] values ranged from 45[Formula: see text][Formula: see text]m to 190[Formula: see text][Formula: see text]m, with a median of 93[Formula: see text][Formula: see text]m. The SI model fitted the height of uptake versus time data very well for all of the rock cores investigated; medians of the resulting root mean squared errors and coefficients of determination were 0.99[Formula: see text]mm and 0.963, respectively. Estimates of [Formula: see text] ranged from 2.04 to 2.45, with a median of 2.24. Statistically, all of the [Formula: see text] values were significantly greater than two, confirming the fractal nature of the fracture surfaces. Future research should focus on forward prediction through independent measurements of [Formula: see text] and extension of the existing SI model to late times (through the inclusion of gravity) and fractures with permeable surfaces.

Contact angle measurements for gas-liquid-rock systems are important for modeling multi-phase flow and transport in the subsurface. These data are needed in applications such as the extraction of oil and gas resources, geologic sequestration of carbon dioxide, contaminant fate and transport, and aquifer recharge through the vadose zone. Contact angles are frequently measured with the sessile drop method. Previous research has largely ignored the dynamic behavior of sessile droplets on geologic materials. This study investigates the dynamic behavior of sessile water droplets on prepared rock surfaces in the presence of air. Droplet diameter and advancing contact angle were determined at 0.5 s intervals for similar to 90 s on flat polished disks of Burlington limestone, Crossville sandstone, Mancos shale, Sierra White granite, Vermilion Bay granite, and Westerly granite using a Krtiss DSA 30 Drop Shape Analyzer. The droplet diameter and advancing contact angle data sets were nonlinearly regressed against time using two different two-parameter models. The median coefficients of determination for the fits were 0.85 and 0.96, respectively. The resulting parameter estimates were used to compute the apparent equilibrium contact angle, theta(e), for each disk following droplet diameter stabilization. Estimates of theta(e) ranged from 37.2 degrees for Mancos shale to 75.6 degrees for Burlington limestone. Analysis of variance indicated statistically significant differences in O e between the rock types at the 95% confidence level. The variability of O e on the polished rock surfaces, as quantified by the coefficient of variation (CV) for theta(e), varied between similar to 3 and similar to 9%; there were no significant differences in CV between the rock types. Neutron radiography indicated changes droplet morphology over time were due to the spontaneous imbibition of water into the rock matrix. The transient analysis employed in this study permits a more meaningful estimate of the equilibrium contact angle for rocks than taking the initial value or averaging over time as is frequently done.

This entry focuses on fragmentation procedures and associated indices for assessing soil aggregation on the basis of the size distribution and stability of fragments after mechanical disruption. When choosing between fragment size distribution and structural stability, procedures will depend on the use to be made of the indices. In general, if the procedure is to characterize the field soil condition without consideration of disruptive processes, then measurement of soil fragment size distribution after low-energy input is recommended. Structural stability measurements are most useful for characterizing soil susceptibility to wind and water erosion. In determining the stability of the soil to mechanical disruption, it is important to measure the distribution of soil fragments relative to an initial undisturbed state.

Understanding of fundamental processes and prediction of optimal parameters during the horizontal drilling and hydraulic fracturing process results in economically effective improvement of oil and natural gas extraction. Although modern analytical and computational models can capture fracture growth, there is a lack of experimental data on spontaneous imbibition and wettability in oil and gas reservoirs for the validation of further model development. In this work, we used neutron imaging to measure the spontaneous imbibition of water into fractures of Eagle Ford shale with known geometries and fracture orientations. An analytical solution for a set of nonlinear second-order differential equations was applied to the measured imbibition data to determine effective contact angles. The analytical solution fit the measured imbibition data reasonably well and determined effective contact angles that were slightly higher than static contact angles due to effects of in-situ changes in velocity, surface roughness, and heterogeneity of mineral surfaces on the fracture surface. Additionally, small fracture widths may have retarded imbibition and affected model fits, which suggests that average fracture widths are not satisfactory for modeling imbibition in natural systems.

In the recent article by Paradis et al. (2018), an error was made when developing a conceptual model and an analytical solution to describe the one-dimensional displacement of the center of mass of a non-reactive tracer during a single-well injection, drift, and extraction test (push-pull test).