Showing papers on "Hydraulic conductivity published in 1990"

Water Content-Density Criteria for Compacted Soil Liners

Abstract: Soil liners have traditionally been compacted in the field to a minimum dry unit weight over a specified range in water content. This approach evolved from the practice for structural fills for which strength and compressibility are of primary concern. With soil liners, hydraulic conductivity is usually of paramount importance. Hence, the approach used to ensure adequate strength and permissible compressibility is not necessarily applicable to the construction of soil liners. Data are presented to show that the water content-density criteria for compacted soil liners can be formulated in a manner that is different from the approach currently used by many engineers. The recommended approach is based on defining water content-density requirements for a broad, but representative, range of compactive energy, and relating those requirements to hydraulic conductivity and other relevant factors. A case history illustrates the recommended procedure and its implementation. Hydraulic conductivity measurements were performed on three soils to develop an “acceptable zone” on the compaction curve that was used for quality control during construction. As a result of implementing the recommended procedure, construction operations were significantly accelerated.

An experimental investigation of variable density flow and mixing in homogeneous and heterogeneous media

Abstract: This study is an experimental investigation of variable density groundwater flow in homogeneous, layered and lenticular porous media. At the scale of the experiments the flow of dissolved mass in water depends upon both forced and free convection. In addition, density differences as low as 0.0008 g/cm{sup 3} (1,000 mg/L NaCl) between a plume of dense water and ambient groundwater in a homogeneous medium produces gravitational instabilities at realistic groundwater velocities. These instabilities are manifest by lobe-shaped protuberances that formed first along the bottom edge of the plume and later within the plume. As the density difference increases to 0.0015 g/cm{sup 3} (2,000 mg/L NaCl), 0.037 g/cm{sup 3} (5,000 mg/L NaCl), or higher, this unstable mixing due to convective dispersion significantly alters the spreading process. In a layered medium, reductions in hydraulic conductivity of the order of half an order of magnitude or less can influence the flow of the dense plume. Dense water may accumulate along bedding interfaces, which when dipping can result in plume migration velocities larger than ambient groundwater velocities. In a lenticular medium the combination of convective dispersion and nonuniform flow due to heterogeneities result in relatively large dispersion. Scale considerations, further, indicate that convective dispersionmore » may provide an important component of mixing at the field scale.« less

Estimating unsaturated hydraulic conductivity from easily measured soil properties

Abstract: We measured hydraulic conductivity (saturated and unsaturated) on 127 soil cores, which were taken in different horizons of a wide variety of Belgian soil series. The hot air method (Arya et al. 1975) and the crust method (Bouma et al. 1983) were combined to obtain the complete range of hydraulic conductivity from saturation to air-dry. The textural composition in nine fractions, the organic carbon content, and the dry bulk density were determined for each of the sample horizons as well. Four different empirical models were evaluated on their performance in describing the measured hydraulic conductivity curves. The model parameters were estimated by linear and nonlinear regression techniques. It is concluded that the Gardner equation (1958) with three parameters best described the hydraulic conductivity for the given soils. Regression equations for estimating the Gardner parameters were established from simple soil properties, such as soil texture, carbon content, bulk density, and saturated hydraulic conductivity. We found that the three parameters can reasonably well be estimated from the textural composition and the saturated hydraulic conductivity. A one-dimensional sensitivity analysis indicates that the n parameter, representing the slope of the hydraulic conductivity pressure head relation in log-log scale, is most sensitive.

The hydrological response of soil surfaces to rainfall as affected by cover and position of rock fragments in the top layer

Abstract: Rainfall experiments have been conducted in the laboratory in order to assess the hydrological response of top soils very susceptible to surface sealing and containing rock fragments in different positions with respect to the soil surface. For a given cover level, rock fragment position in the top soil has an ambivalent effect on water intake and runoff generation. Compared to a bare soil surface rock fragments increase water intake rates as well as time of runoff concentration and decrease runoff volume if they rest on the soil surface. For the same cover level, rock fragments reduce infiltration rate and enhance runoff generation if they are well embedded in the top layer. The effects of rock fragment position on infiltration rate and runoff generation are proportional to cover percentage. Micromorphological analysis and measurements of the saturated hydraulic conductivity of bare top soils and of the top layer underneath rock fragments resting on the soil surface reveal significant differences supporting the mechanism proposed by Poesen (1986): i.e. runoff generated as rock flow or as Horton overland flow can (partly) infiltrate into the unsealed soil surface under the rock fragments, provided that they are not completely embedded in the top layer. Hence, rock fragment position, beside other rock fragment properties, should be taken into account when assessing the hydrological response of soils susceptible to surface sealing and containing rock fragments in their surface layers. A simple model, based on the proportions of bare soil surface, soil surface occupied by embedded rock fragments, and soil surface covered with rock fragments resting on the soil surface, describes the runoff coefficient data relatively well.

Influence of Clods on the Hydraulic Conductivity of Compacted Clay

Abstract: This investigation of the influence of clod on the hydraulic conductivity of compacted clay showed that clod size during soil processing and compaction significantly influenced the compaction curve and the hydraulic conductivity of a highly plastic, compacted clay soil. The compaction curves for soils with initially large and small clods compacted with standard Proctor effort were significantly different. For smaller clods, the compaction curve was much flatter, suggesting less sensitivity to molding water content.

Mercury intrusion and permeability of Louiseville clay

Abstract: Several theories have been proposed to correlate the permeability (hydraulic conductivity) with the pore-size distribution of soils, and it seemed interesting to determine if these theories could b...

The use of air‐filled porosity and intrinsic permeability to air to characterize structure of macropore space and saturated hydraulic conductivity of clay soils

Abstract: SUMMARY Intrinsic permeability to air of macropore space (ka) is related to macroporosity (ɛ) and organization of macropore space (O). Organization is defined as ka/ɛ. The use of ka for estimating saturated hydraulic conductivity (Ka) is also considered. The relationship between Log (O) and ɛ (Oɛ characteristic) can be used to describe changes to the macropore space of clay soils by amelioration and compaction. The effects of dominant macropore shape can also be identified and calculated as an empirical index of the efficiency of the pore organization E (E=log (O)/ɛ). Intrinsic permeability can then be related to E in a E:ka characteristic. Intrinsic permeability is the parameter most sensitive to structural change and E is mainly influenced by the dominant shapes of the macropores. Thus, the E:ka characteristic is suggested as a basis for studying differences in macropore space as may occur in response to external and internal stresses upon the soil and different systems of soil management, for example increases of packing pores by cultivation or of fissures by gypsum application and loss of packing pores by compaction. Empirical data indicate that Ks of the B horizons of Australian red-brown earths can be estimated from ka of macropore space at a standard potential.

Primary Events Regulating Stem Growth at Low Water Potentials

Abstract: Cell enlargement is inhibited by inadequate water. As a first step toward understanding the mechanism, all the physical parameters affecting enlargement were monitored to identify those that changed first, particularly in coincidence with the inhibition. The osmotic potential, turgor, yield threshold turgor, growth-induced water potential, wall extensibility, and conductance to water were measured in the elongating region, and the water potential was measured in the xylem of stems of dark-grown soybean (Glycine max [L.] Merr.) seedlings. A stepdown in water potential was achieved around the roots by transplanting the seedlings to vermiculite of low water content, and each of the parameters was measured simultaneously in the same plants while intact or within a few minutes of being intact using a newly developed guillotine psychrometer. The gradient of decreasing water potential from the xylem to the enlarging cells (growth-induced water potential) was the first of the parameters to decrease to a growth-limiting level. The kinetics were the same as for the inhibition of growth. The decreased gradient was caused mostly by a decreased water potential of the xylem. This was followed after 5 to 10 hours by a similar decrease in cell wall extensibility and tissue conductance for water. Later, the growth-induced water potential recovered as a result of osmotic adjustment and a rise in the water potential of the xylem. Still later, moderate growth resumed at a rate apparently determined by the low wall extensibility and tissue conductance for water. The turgor did not change significantly during the experiment. These results indicate that the primary event during the growth inhibition was the change in the growth-induced water potential. Because the growth limitation subsequently shifted to the low wall extensibility and tissue conductance for water, the initial change in potential may have set in motion subsequent metabolic changes that altered the characteristics of the wall and cell membranes.

Determination of Fracture Inflow Parameters With a Borehole Fluid Conductivity Logging Method

Abstract: There is much current interest in determining the flow characteristics of fractures intersecting a well bore in order to provide data for use in estimating the hydrologic behavior of fractured rocks. Inflow rates from these fractures into the well bore are usually very low. Moreover, in most cases only a few percent of the fractures identified by core inspection and geophysical logging actually conduct water, the rest being closed, clogged, or isolated from the water flow system. A new procedure is proposed and a corresponding method of analysis developed to locate water-conducting fractures and obtain fracture inflow rates by means of a time sequence of electric conductivity logs of the borehole fluid. The physical basis of the analysis method is discussed, and the procedure is applied to an existing set of data, which shows initiation and growth of nine conductivity peaks in a 900-m section of a 1690-m borehole, corresponding to nine water-conducting fractures intersecting the borehole. By applying our analysis to these nine peaks, the flow rates and the salinity of the water from these fractures are determined. These results are used with other information to obtain transmissivities of the nine fractures, which are validated against independent hydraulic measurements by packer tests. The salinities measured in fluids from the fractures are also validated against salinity values obtained by chemical sampling of fluids from different depths of the borehole. The applicability of this technique is discussed in the context of a borehole-testing program.

Soil physical properties after 100 years of continuous cultivation

Abstract: A study was conducted to evaluate the effects of 100 years of continuous soil and crop management on soil physical properties at Sanborn Meld, University of Missouri. Undisturbed soil cores were used to evaluate bulk density, saturated hydraulic conductivity, water retention, and pore-size distributions. Results indicate that annual additions of manure decreased bulk density by an average of 0.12 g cm−3 compared to unfertilized treatments. Saturated hydraulic conductivity was increased by about nine times with annual additions of 13.5 t ha−1 (6 tons/acre) of manure. Implications are that soil and crop management have only subtle effects on bulk density and pore-size distribution for this soil. However, annual additions of manure increased hydraulic conductivity, which may reduce runoff and the potential for soil erosion.

Macrodispersion in sand-shale sequences

Abstract: Macrodispersion in sand-shale sequences is investigated by a series of numerical tracer tests. Hydraulic conductivity is modeled as a binary, spatially correlated random function. Realizations of the random conductivity field are simulated on a nodal grid discretizing the heterogeneous formation. Corresponding realizations of the random velocity field are obtained by solving the equation for saturated steady state flow. Particle tracking, with flux-weighted tracer injection and detection, is used to generate experimental residence time distributions (RTDs). Moments of the RTD are used to characterize longitudinal tracer spreading. Results show that macrodispersive transport in sand-shale sequences cannot be represented by a Fickian model. RTDs display a bimodal structure caused by the fast arrival of particles traveling along preferential sandstone channels and by the much slower arrival of particles following tortuous routes through sandstone and shale. The relative importance of channeling and tortuous flow transport mechanisms is determined by sand-shale conductivity contrast, shale volume fraction, and conductivity spatial correlation structure. Channeling is promoted by high conductivity contrasts, low shale fractions, and flow parallel to bedding in anisotropic media. Low contrasts, high shale fractions, and flow perpendicular to bedding act to break up channels and to enhance tracer spreading.

Hydraulic characteristics of municipal refuse

Abstract: A review is made of the hydraulic conductivity and other hydraulic parameters of municipal waste. The hydraulic conductivity can be assessed indirectly from measured field parameters and water balance. A test well penetrating about 100 ft of refuse was installed and pumped for about one day at 20 gpm and 2½\N days at 12 gpm. Drawdowns were measured at three observation wells and the pumped well. Difficulties were encountered during drilling and subsequent monitoring of leachate levels. The hydrogeologic parameters were computed using conventional hydrogeologic analysis. The results of a pumping test of leachate from a municipal landfill are presented. Based on the results of the pumping test, it is concluded that hydraulic conductivity of municipal refuse is about 10\u-³ cm/s. It is also concluded that pumping of leachate from a municipal landfill is feasible for control of leachate release to ground water.

Hydraulic Conductivity Determinations in Unlithified Glacial and Fluvial Materials

Abstract: Experiences with many measurements of the hydraulic conductivity of unlithified glacial and fluvial materials in Wisconsin suggest that hydraulic conductivity must be viewed in terms of the operational scale of measurement, based on the scale of the problem at hand and the volume of the materials of interest Frequently, the hydraulic conductivity of a given lithostratigraphic unit appears to increase as the operational scale of measurement increases In particular, laboratory methods can yield hydraulic conductivities one to two orders of magnitude lower than conductivities determined in field tests on the same materials The operational scale of most laboratory methods is much smaller than the operational scale of most field problems, and laboratory tests, although often logistically and financially attractive, may be of little value in characterizing the hydraulic conductivity of Pleistocene and recent deposits at working field scales

Unsaturated hydraulic conductivity

Abstract: The unsaturated zone plays an important role in the hydrological cycle. It forms the link between surface water and ground water and has a dominant influence on the partitioning of water between them. The hydraulic properties of the unsaturated zone determine how much of the water that arrives at the soil surface will infiltrate into the soil, and how much will run off and may cause floods and erosion. In many areas of the world, most of the water that infiltrates into the ground is transpired by plants or evaporated directly into the atmosphere, leaving only a small proportion to percolate deeper and join the ground water. Surface runoff and deep percolation may carry pollutants with them. Then it is important to know how long it will take for this water to reach surface or ground water resources. Besides providing water for plants to transpire, the unsaturated zone also provides oxygen and nutrients to plant roots, thus having a dominant influence on food and fiber production. Water content also determines soil strength, which affects anchoring of plants, root penetration, compaction by cattle and machinery, and tillage operations. To mention just one other role of the unsaturated zone, its water content has a great influence on the heat balance at the soil surface. This is well illustrated by the large diurnal temperature variations in deserts. To understand and describe these and other processes, the hydraulic properties that govern water transport in the soil must be quantified. Of these, the unsaturated hydraulic conductivity is, if not the most important, certainly the most difficult to measure accurately. It varies over many orders of magnitude not only between different soils but also for the same soil as a function of water content. Much has been published on the determination and/or measurement of the

Response of the water level in a well to Earth tides and atmospheric loading under unconfined conditions

Abstract: The response of the water level in a well to Earth tides and atmospheric loading under unconfined conditions can be explained if the water level is controlled by the aquifer response averaged over the saturated depth of the well. Because vertical averaging tends to diminish the influence of the water table, the response is qualitatively similar to the response of a well under partially confined conditions. When the influence of well bore storage can be ignored, the response to Earth tides is strongly governed by a dimensionless aquifer frequency Q′u. The response to atmospheric loading is strongly governed by two dimensionless vertical fluid flow parameters: a dimensionless unsaturated zone frequency, R, and a dimensionless aquifer frequency Qu. The differences between Q′u and Qu are generally small for aquifers which are highly sensitive to Earth tides. When Q′u and Qu are large, the response of the well to Earth tides and atmospheric loading approaches the static response of the aquifer under confined conditions. At small values of Q′u and Qu, well response to Earth tides and atmospheric loading is strongly influenced by water table drainage. When R is large relative to Qu, the response to atmospheric loading is strongly influenced by attenuation and phase shift of the pneumatic pressure signal in the unsaturated zone. The presence of partial penetration retards phase advance in well response to Earth tides and atmospheric loading. When the theoretical response of a phreatic well to Earth tides and atmospheric loading is fit to the well response inferred from cross-spectral estimation, it is possible to obtain estimates of the pneumatic diffusivity of the unsaturated zone and the vertical hydraulic conductivity of the aquifer.

Combining laboratory and field measurements to define the hydraulic properties of soil

Abstract: In situ field measurements and laboratory determinations are presented of the saturated and unsaturated flow properties of two contrasting soils. Field measurements were obtained with ponded rings or disc permeameters of different radii. A pressure-transient outflow technique was used in the laboratory on undisturbed cores. The soil water diffusivity function from this, when integrated, provides a good rendition of the growth of the field-measured sorptivity as the surface potential, ψo, approaches zero. The undisturbed wetting hydraulic conductivity K(ψ) from the cores of both soils merged neatly with the near-saturated field results. One soil from within the herbicide strip of an apple orchard had a smoothly continuous K(ψ) befitting its texture. However, the K(ψ) of the other soil, from a dairy pasture, displayed a matrix-macropore dichotomy due to its high level of soil floral and faunal activity. Here, a mean pore size weighted for unsaturated flow, when ψo < −100 mm, was 17 ± 4 µm. For ponded infiltration, this changed abruptly to 2.2 ± 0.6 mm. Large and connected macropores caused K to change three orders of magnitude as ψo went from just −100 mm to zero.

Surface measurements of the hydraulic properties of a tilled and untilled soil

Abstract: The surface characteristics of soil can have a profound effect on the hydrology of tilled land. Apposite measurements of the surface hydraulic properties of Plainfield sand (Wisconsin, U.S.A.), a Typic Udipsamment, were used to assess the hydrologic impact of 5 years tillage by either moldboard plow or no-till. The crop was always corn (Zea mays L.). The “mean” pore size (λm), weighted in a way relevant to the flow of water through the soil surface, was computed here from saturated and unsaturated measurements of sorptivity (S0) and hydraulic conductivity (K0). Disc permeameters of dissimilar radii were used at two unsaturated supply-potential heads of ψ0=−100 mm and −20 mm to find S0 (ψ0) and K0 (ψ0). At saturation (ψ0=0), infiltration rings of contrasting radii were employed. The saturated and unsaturated values for S0 and K0 of the plowed soil were either the same as, or greater than the corresponding values for the no-till soil. Combination of the values for the saturated S0 and K0 showed that the no-till soil had a λm=1.34 (±0.67) mm, while in the plowed soil the “mean” pore size during saturated flow was only 0.19 (±0.18) mm. The large λm, and the high coefficient of variation, for the no-till soil was presumed to be related to the macropore network associated with the decay of crop residue in the less-sorptive matrix. The small homogeneous λm of both the saturated and unsaturated plowed soil reflects the annual pulverization of the soil surface by tillage.

Seasonal variation in field-saturated hydraulic conductivity in two swelling clay soils in Sweden

Abstract: SUMMARY A simple method (the inversed auger hole method) for measuring field-saturated hydraulic conductivity (Kfs) was investigated. Measurements were carried out in the spring, summer and autumn at three depths in two Swedish clay soils (Ultuna and Limsta, with clay contents of 45–60%0 and 65–80%, under barley and grass/clover ley respectively). Seasonal fluctuations in Kfs at Limsta were more pronounced, and were observed deeper in the profile. This was attributed primarily to larger structural changes due to a higher capacity for swell/shrink (normal shrinkage over the available water range) and an earlier drying up of the soil under grass/clover ley. It was shown that the measured Kfs values were strongly correlated with the total inter aggregate (macro-) porosity (et), estimated from a simple model of soil shrinkage. Combining the data from both soils, a single power-law relation was adequate (r= 0.73) to describe the variations in Kfs with et

Effective unsaturated hydraulic conductivity of layered sands

Abstract: Accurate estimates of field-scale hydraulic conductivities of unsaturated heterogeneous soils are very difficult to obtain. In the present study, various approaches to determining effective conductivity values for heterogeneous sands are compared with laboratory measurements. The unsaturated hydraulic conductivity, K(ψ), of two homogeneous sands and one layered sand composed of the two homogeneous sands was measured using the steady-state flux control method. The averaged K(ψ) curves of the two homogeneous sands using a direct averaging approach were compared with the measured layered sand K(ψ) data. The result shows that the geometric mean of hydraulic conductivity-suction curves of the coarse and medium sands approximates the measured hydraulic conductivity-suction curve of the layered sand. The observed suction variance and effective hydraulic conductivity were compared with expressions developed from a stochastic theory. The results support the stochastic approach.

Specific Storage as a Poroelastic Coefficient

Abstract: A definition for the specific storage coefficient Ss is given which is unambiguous for general isotropic three-dimensional aquifer elasticity. In every representative elementary volume, Ss is the fluid volume released from storage per unit decline in hydraulic head, per unit bulk volume, under conditions such that there is no strain in two orthogonal directions, and the total normal stress in the third orthogonal direction is constant. The specific storage coefficient is a point property of the aquifer and is defined independently of problem domain stress and head boundary conditions. The expression for Ss in terms of aquifer and fluid compressibilities is identical to the familiar forms obtained assuming zero horizontal strain and constant overburden in an aquifer, although it is not restricted to these conditions. As a point property of the fluid-saturated material, the specific storage coefficient is one of four constants in the general constitutive poroelastic equations relating three-dimensional aquifer stress and strain to fluid pressure and dilatation. Written in terms of Ss, these equations show that pore fluid mass diffusion is governed by a diffusivity equal to the ratio of hydraulic conductivity to specific storage under arbitrary boundary conditions. It is shown that Ss controls slow compressional body wave velocity in the low frequency limit and that the uniaxial aquifer compressibility α is not necessarily related to the vertical direction.

Effective hydraulic conductivity for gradually varying flow

Abstract: Consider the problem of flow in a porous medium with hydraulic conductivity which fluctuates locally about a mean value. The flow is unsteady but gradually or slowly varying, i.e., the correlation length of head fluctuations is considerably larger than the correlation length of hydraulic-conductivity fluctuations. The equations which must be satisfied by the effective conductivity tensor are derived under general conditions using a method of volume averaging and spatial moments. The generality of the derived equations is shown by replicating some known results.

Effect of Soil Properties on Unsaturated Hydraulic Conductivity Pore-Interaction Factors

Abstract: The variability of the pore-interaction factor, , for a microscopic model proposed for predicting unsaturated hydraulic conductivity, K( ), from soil water-retention, , data was examined in relation to soil particle-size data and indices, bulk density, organic C, parameters for the van Genuchten function, an index (W) of total energy of drainage and soil-series, toposequential, and geographic groupings. The exhibited no trend relationship to any of the soil properties tested. However, a change in the distribution of was observed in relation to the geometric-mean particle diameter (Gd) and other soil textural variables. For Gd < 0.08 mm, variability was much larger than for soils with Gd 0.08 mm. The latter consisted entirely of soils in the sand and loamy sand textural groups. A similar distributional relationship also occurred for vs. W. Examination of on three sites of the Hecla soil series and on related toposequential soils indicated that classification of on the basis of soil series or of soil-association groupings is a feasible strategy for parameter estimation on some soils. The exponential factor for a macroscopic model was also investigated. The was found to be related to W as an exponential function over the full data range. However, vs. W was nearly identical to a linear function for W data extending from 0 to 400 cm(where is the density of water). The was also strongly related to Gd as a power function, and to other textural variables as exponential functions. The increasing slope of the power function for Gd < 0.08 mm indicated a large potential error of prediction for K on fine-textured soils. View complete article To view this complete article, insert Disc 5 then click button8

Saturated hydraulic conductivity of clayey tills and the role of fractures.

Abstract: The background for the present knowledge about hydraulic conductivity of clayey till in Denmark is summarized. The data show a difference of 1-2 orders of magnitude in the vertical hydraulic conductivity between values from laboratory measurements and field measurements. This difference is discussed and based on new data, field observations and comparison with North American studies, it is concluded to be primarily due to fractures in the till.