Formation Of Preferential Flow Paths Research Articles

Transport of explosive chemicals from the landmine burial in granular soils

The transport of explosive-related chemicals (ERCs) in soils was studied during water infiltration and evaporation processes as a function of soil water content and temperature. The experiments were conducted in two 100 cm uniform cylindrical columns packed with homogeneous sand, and instrumented with air and water pressure sensors and sampling ports to monitor hydraulic conditions and ERCs concentration profiles in soil. TNT and DNT crystals were placed in a porous membrane and buried as a point source near the surface of the soil. Spatial and temporal concentration distributions of conservative solutes were used to evaluate transport behavior of TNT and DNT in soils. Velocity variations and comparison with the numericalmodel HYDRUS-2D indicate the presence of preferential flow paths. Water content and movement near TNT and DNT buried source highly influence their transport in soils and near soil-atmospheric surfaces. The formation of preferential flow paths are attributed to disturbances of soil properties by burial of ERC sources and water content heterogeneities. By the analysis of velocity variations, disturbances near the buried source resulted in hydraulic heterogeneities and preferential flow near the source, which influence their transport away from the source. Preferential flow causes faster movement and greater dispersion of the solutes during infiltration periods, and influences the rate of mixing in the system.

Sorption of nalidixic acid onto sediments under batch and dynamic flow conditions

The sorption of nalidixic acid (NA) onto artificial sediments composed of Fe-oxides, Al-oxides, clay and quartz sand was studied under both static batch and dynamic flow conditions. Kinetic and equilibrium sorption experiments showed that the presence of clay increased the sorption capacity of synthetic sediment and that bentonite had the highest sorption coefficient compared to kaolinite. Solute reactive transport experiments showed that the breakthrough point and steepness of the breakthrough curve (BTC) were dependent on both clay type and water velocity. Agreement between batch and column results in terms of sorbed amount and retardation factor was poor regardless of the sediment tested. The presence of even a small amount of clay (3%) can decrease the permeability of the mixed bed and lead to the formation of preferential flow paths in the column system. Different sets of column experiments at various flow rates gave rise to different sorbed amounts at complete breakthrough, leading to chemical nonequilibrium in the flow system. However, this kinetic limitation cannot completely explain the breakthrough behavior of NA in the bentonite column, suggesting that other factors are responsible for this disparity. The presence of immobile water regions was unlikely in the column since more than 90% of the pore water was mobile according to the tracer tests. The inaccessibility of reactive sites and limited physical diffusion in the clay-packed column may play a significant role under flow-through conditions. The reactive transport behavior of NA is therefore strongly dependent on the mineralogy of the clay present in sediments.

Simulation of Infiltration Processes in the Unsaturated Zone Using a Multiscale Approach

We consider the infiltration of a wetting fluid into a homogeneous porous medium and the formation of preferential flow paths with saturation overshoots. These are caused by dynamic capillary pressure effects which can be modeled by the rate-dependent approach of Hassanizadeh and Gray. To track the overshoot wave numerically, we propose an extended Heterogeneous Multiscale Method. In the approach, the rate-dependent model takes the role of a microscale model. The algorithm is applied to several infiltration problems.

Four-year performance evaluation of a pilot-scale evapotranspiration landfill cover in Southcentral Alaska

Alternative landfill covers utilizing evapotranspiration (ET) as the primary mechanism for protecting the waste layer from aerial moisture represent promising tools for cold region solid waste management. However, ET covers have not been evaluated for use in subarctic climates. As the functionality of an ET cover is driven primarily by climactic variables, climate-specific field tests are required prior to widespread implementation. The objective of this study was to evaluate the four-year performance of two competing pilot-scale landfill covers built atop drainage lysimeters near Anchorage, AK. The compacted soil cover (CSC) was designed and constructed according to standards prescribed by Alaska solid waste regulations. The alternative ET cover design was based upon a preliminary modeling study. After four years, the two adjacent lysimeters had each received a total of 1636 mm precipitation. Over that period, 201 mm moisture drained from the ET lysimeter, compared to 292 mm in the CSC lysimeter. The difference in drainage rates between the two covers was most apparent during the autumn season, when the drainage rates for both covers were at their annual maximum. The lower autumn and annual drainage rates observed in the ET lysimeter after the first year were potentially due to higher moisture storage capacity in the ET cover soils and/or formation of preferential flow paths in the CSC soils. Analysis of soil temperature, precipitation, and drainage data indicated that negligible amounts of winter precipitation infiltrated the ET cover during winter, and that the frozen soils promoted runoff over drainage during the spring melt. These results indicate that similar ET cover designs merit consideration for broader use in subarctic conditions.

Changes in Preferential Flow Path Distribution and Its Affecting Factors in Southwest China

Preferential flow is important in solute transport in the soil, and it indirectly affects the quality of the groundwater. The objectives of the present study were to investigate the distribution characteristics of preferential flow paths under different land-use types (forestland, shrubland, and farmland) and to determine the factors that affect the formation of these paths. Dye tracer experiments were conducted at six sites covering the three land-use types. The relationship of the preferential flow paths to the properties of the soil, root length per soil volume, root holes, and root surface area was analyzed using a partial correlation method. The results show dye coverage of greater than 50% at a soil depth of 0 to 10 cm and dye coverage of less than 30% below a depth of 20 cm in all of the land-use types. The vertical distribution differed widely in the various land-use types. Shrubland and farmland had more preferential flow paths in the topsoil compared with forestland, in contrast to the conditions in the deeper soil layers. The soil chemical properties and the saturated hydraulic conductivity in the preferential flow paths were quite different from those in the soil matrix. The soil organic matter content was approximately 5% to 20% higher in the preferential flow paths than in the soil matrix. The average value of the saturated hydraulic conductivity was 41.3 mm min in the preferential flow paths compared with 21.4 mm min in the soil matrix. The percentage of preferential flow paths in the soil was significantly affected by roots with a diameter of less than 5 mm, root holes, and the root surface area. These findings suggest that land use is significant in the distribution of preferential flow paths, and root characteristics are important in the formation of preferential flow paths.

The effects of reduction of the deposited waste on short-term landfill leachate composition of a landfill: a case study in Norway

Municipal solid waste landfill leachate was sampled over from a landfill receiving varying amounts of municipal solid waste. The investigation aimed to provide information on expected leachate changes in the short term, either after closure of an active landfill, or after a strong decline in the amount of waste deposited at smaller landfills. It was found that during a two year period following a sudden decline in the amount of waste deposited, the levels of various chemical and physical parameters all dropped sharply. The reasons for the decline in discharge levels are thought to be aerobic decomposition taking place in the municipal solid waste just after landfilling, and thus the decline in the impact of this process when there were less fresh waste masses available, and formation of preferential flow paths for the leachate as the municipal solid waste stabilized in the landfill.

A joint velocity‐concentration PDF method for tracer flow in heterogeneous porous media

The probability density function (PDF) of the local concentration of a contaminant, or tracer, is an important component of risk assessment in applications that involve flow in heterogeneous subsurface formations. In this paper, a novel joint velocity‐concentration PDF method for tracer flow in highly heterogeneous porous media is introduced. The PDF formalism accounts for advective transport, pore‐scale dispersion (PSD), and molecular diffusion. Low‐order approximations (LOAs), which are usually obtained using a perturbation expansion, typically lead to Gaussian one‐point velocity PDFs. Moreover, LOAs provide reasonable approximations for small log conductivity variances (i.e., σY2 < 1). For large σY2, however, the one‐point velocity PDFs deviate significantly from the Gaussian distribution as demonstrated convincingly by several Monte Carlo (MC) simulation studies. Furthermore, the Lagrangian velocity statistics exhibit complex correlations that span a wide range of scales, including long‐range correlations due to the formation of preferential flow paths. Both non‐Gaussian PDFs and complex long‐range correlations are accurately represented using Markovian velocity processes (MVPs) in the proposed joint PDF method. LOA methods can be generalized to some extent by presuming a certain shape for the concentration PDF (e.g., a β PDF fully characterized by the concentration mean and variance). The joint velocity‐concentration PDF method proposed here does not require any closure assumptions on the shape of the marginal concentration PDF. The Eulerian joint PDF transport equation is solved numerically using a computationally efficient particle‐based approach. The PDF method is validated with high‐resolution MC reference data from Caroni and Fiorotto (2005) for saturated transport in velocity fields, which are stationary in space and time, for domains with σY2 = 0.05, 1, and 2 and Péclet numbers ranging from 100 to 10,000. PSD is modeled using constant anisotropic dispersion coefficients in both the reference MC computations and our PDF method.

Particle‐based transport model with Markovian velocity processes for tracer dispersion in highly heterogeneous porous media

Monte Carlo (MC) studies of flow in heterogeneous porous formations, in which the log‐conductivity field is multi‐Gaussian, have shown that as the log‐conductivity variance σY2 increases beyond about 0.5, the one‐point velocity probability density functions (PDFs) deviate significantly from Gaussian behavior. The velocity statistics become more complex due to the formation of preferential flow paths, or channels, as σY2 increases. Methods that employ low‐order approximations (e.g., truncated perturbation expansions) are limited to small σY2 and are unable to represent the complex velocity statistics associated with σY2 > 1. Here a stochastic transport model for highly heterogeneous domains (i.e., σY2 > 1) is proposed. In the model, the Lagrangian velocity components of tracer particles are represented using continuous Markovian stochastic processes in time. The Markovian velocity process (MVP) model is described using a set of first‐order ordinary and stochastic differential equations, which are easy to solve using a particle‐based method. Once the MVP model is calibrated based on velocity statistics from MC simulations of the flow (hydraulic head and velocity), the MVP‐based model can be used to describe the evolution of the tracer concentration field accurately and efficiently. Specifically, the MVP model is validated using MC simulations of longitudinal and transverse tracer spreading due to a point‐like injection in the domain. We demonstrate that for σY2 > 1, the ensemble‐averaged tracer cloud remains markedly non‐Gaussian for relatively large travel distances from the point source. The MVP transport model captures this behavior and reproduces the velocity statistics quite accurately.

Deformation and Permeability Evolution of Petroleum Cement Paste Subjected to Chemical Degradation Under Temperature

The variation of permeability of typical petroleum cement paste is investigated as functions of mechanical loading and chemical degradation under the temperature of 90°C. In sound material, the permeability classically increases with deviatoric stress due to microcracks and volumetric dilatancy but decreases with confining pressure. Chemical leaching leads to significant increase of porosity of cement paste. However, the permeability of degraded material is lower than that of sound material during triaxial compression tests; this is due to compaction of pores under confining pressure. Further, the permeability variation in degraded cement is much more sensitive to confining pressure than that of sound material. During triaxial creep test, the permeability of degraded material decreases with time while that of sound material increases; this shows that the chemically leached material has a higher potential of volumetric compaction which is a key mechanism of plastic deformation. Coupled chemical degradation and triaxial compression tests are also performed. Under low confining pressure (3 MPa), the permeability increases with propagation of leaching front, and there is formation of preferential flow paths in the axial direction. However, with high confining pressure (10 MPa), there is no increase of permeability during chemical leaching and creation of successive degraded layers in the flow direction.

Are preferential flow paths perpetuated by microbial activity in the soil matrix? A review

Recently, the interactions between soil structure and microbes have been associated with water transport, retention and preferential or column flow development. Of particular significance is the potential impact of microbial extracellular polymeric substances (EPS) on soil porosity (i.e., hydraulic conductivity reduction or bioclogging) and of exudates from biota, including bacteria, fungi, roots and earthworms on the degree of soil water repellency. These structural and surface property changes create points of wetting instability, which under certain infiltrating conditions can often result in the formation of persistent preferential flow paths. Moreover, distinct differences in physical and chemical properties between regions of water flow (preferential flow paths) and no-flow (soil matrix) provide a unique set of environmental living conditions for adaptable microorganisms to exist. In this review, special consideration is given to: (1) the functional significance of microbial activity in the host porous medium in terms of feedback mechanisms instigated by irregular water availability and (2) the related physical and chemical conditions that force the organization and formation of unique microbial habitats in unsaturated soils that prompt and potentially perpetuate the formation of preferential flow paths in the vadose zone.

Stability analysis of a phase-field model of gravity-driven unsaturated flow through porous media

The formation of preferential flow paths during infiltration of water into homogeneous, dry soil is an important phenomenon whose explanation and prediction have remained elusive under the standard theories of multiphase flow in porous media. We have recently proposed a macroscopic phase-field model of unsaturated flow in porous media, which explains why such fingering occurs [L. Cueto-Felgueroso and R. Juanes, Phys. Rev. Lett. 101, 244504 (2008)]. Here we present a linear stability analysis of the proposed model for constant-flux infiltration, which allows a quantitative description of the wetting front instability. The present analysis stresses the critical role of the initial water saturation and applied flux ratio in the asymptotic stability of the system, as well as in the transient growth of perturbations, which is consistent with the experimental evidence. The trends in the frequency and growth factor of the most unstable modes predicted by our analysis are also in quantitative agreement with experimental measurements.

Phosphorus retention in forest soils and the functioning of buffer zones used in forestry

Vaananen, R. 2008. Phosphorus retention in forest soils and the functioning of buffer zones used in forestry. Dissertationes Forestales 60. 42 pp. Available at http://www.metla.fi/ dissertationes/df60.htm Phosphorus (P) retention properties of soils typical for boreal forest, i.e. podzolic soil and peat soils, vary significantly, but the range of this variation has not been sufficiently documented. To assess the usefulness of buffer zones used in forestry in removing P from the discharge by chemical sorption in soil, and to estimate the risk of P leaching after forestry operations, more data is needed on soil P retention properties. P retention properties of soils were studied at clear-cut areas, unharvested buffer zones adjoining the clear-cut and at peatland buffer zone areas. Desorption-sorption isotherms were determined for the humus layer, the mineral soil horizons E, B and C of the Podzol profile and for the surface layer peat (0-15 cm) and the subsurface layer peat (15-30 cm). The efficiency of buffer zones in retaining P was studied at six peatland buffer zone areas by adding Pcontaining solute in the inflow. A tracer study was conducted at one of the buffer zone areas to determine the allocation of the added P in soil and vegetation. Measured sorption or desorption rather than parameter values of fitted sorption equations described P desorption and sorption behaviour in soil. The highest P retention efficiency was in the B horizon and consequently, if contact occurred or was established between the soluble P in the water and the soil B horizon, the risk of P leaching was low. Humus layer was completely incapable of retaining P after clear-cutting. In the buffer zones, the decrease in P retention properties in the humus layer and the low amount of P sorbed by it indicated that the importance of the layer in the functioning of buffer zones is low. The peatland buffer zone areas were efficient in retaining soluble P from inflow. P sorption properties of the peat soil at the buffer zone areas varied largely but the contribution of P sorption in the peat was particularly important during high flow in spring, when the vegetation was not fully developed. Factors contributing to efficient P retention were large buffer size and low hydrological load whereas high hydrological load combined with the formation of preferential flow paths, especially during early spring or late autumn was disadvantageous. However, small buffer zone areas, too, may be efficient in reducing P load.

Retention of phosphorus in peatland buffer zones at six forested catchments in southern Finland

Our current knowledge of the P retention efficiency of peatland buffer zone areas used to reduce sediment and nutrient leaching from forestry areas is insufficient. Especially the role of P sorption by soil in buffer zones needs closer examination as there is considerable variation in the efficiency of P retention. Six sites in southern Finland were chosen for the study. The buffer zone areas varied between 0.1–4.9% of the catchment area. A total of 10 kg of solute PO–P was added to the inflow of the buffer zone areas and the concentrations of PO–P in inflow and outflow were measured for 2–4 years. P retention characteristics of the surface peat were determined with sorption-desorption isotherms before and after PO–P addition and the effective buffer zone area over which the added P was spread was determined from soil water samples. P retention in the two largest buffer zone areas was complete (100% retention), and the third largest buffer retained 94%. Retention in the three smallest buffer zones was 24%, 95% and 95% of the added P. As a result of P addition reduction in peat P retention capacity was detected in three out of four cases. The effective buffer zone area varied from 67% to 100% of the total buffer zone area. Factors contributing to efficient P retention were large buffer size and low hydrological load whereas high hydrological load combined with the formation of preferential flow paths, especially during early spring or late autumn was disadvantageous. High P retention capacity in peat contributed to the sustainability of P retention. The study showed that even relatively small buffer zone areas are able to efficiently reduce P load.444

Field Performance of Three Compacted Clay Landfill Covers

A study was conducted at sites in subtropical Georgia, seasonal and humid Iowa, and arid southeastern California to evaluate the field hydrology of compacted clay covers for final closure of landfills. Water balance of the covers was monitored with large (10 by 20 m), instrumented drainage lysimeters for 2 to 4 yr. Initial drainage at the Iowa and California sites was <32 mm yr−1 (i.e., unit gradient flow for a hydraulic conductivity of 10−7 cm s−1, the regulatory standard for the clay barriers in this study); initial drainage rate at the Georgia site was about 80 mm yr−1 The drainage rate at all sites increased by factors ranging from 100 to 750 during the monitoring periods and in each case the drainage rate exceeded 32 mm yr−1 by the end of the monitoring period. The drainage rates developed a rapid response to precipitation events, suggesting that increases in drainage rate were the result of preferential flow. Although no direct observations of preferential flow paths were made, field measurements of water content and temperature at all three sites suggested that desiccation or freeze–thaw cycling probably resulted in formation of preferential flow paths through the barrier layers. Data from all three sites showed the effectiveness of all three covers as hydraulic barriers diminished during the 2 to 4 yr monitoring period, which was short compared with the required design life (often 30 yr) of most waste containment facilities.

Experimental assessment of preferential flow paths in a field soil

Preferential flow paths in the subsoil can cause an accelerated movement of solutes in unsaturated-saturated transport systems. To visualize these zones of preferred flow, an iodide-coloring technique was applied to ten plots in a field. Results indicate the existence of a nonuniform solute front in the subsoil. However, this nonuniformity could be adequately accounted for by using the convection-dispersion equation for describing the relative amount of colored area in the system. Pore water velocities agreed well with measured volumetric water contents, while dispersivities were roughly of the same order of magnitude as those reported in the literature for field-scale experiments. Results did not agree with the accelerated movement found in a previous unsaturated-saturated flow experiment on the same field. The study indicates that the antecedent moisture regime and rainfall intensity have an important effect on the formation of preferential flow paths in a subsoil, thus showing the dynamic nature of this physical phenomenon.

A New Technique for Evaluating the Presence of Preferential Flow Paths in Nonstructured Soils

AbstractA new technique for determining the presence of preferential flow paths in a nonstructured soil profile is described. The presented method is easily applicable in the field, and is based on the formation of an intensely colored complex of I2 with starch. Moreover, the method can be applied to all kinds of nonstructured soils, irrespective of their color. An illustration of the technique is given in case of water and solute movement in a water‐repellent soil, where formation of preferential flow paths was known to occur.