Hydraulic conductivity, of soil

It is sometimes claimed that mucilage and similar gels may help to maintain hydraulic conductivity between root and. soil (52). However, the hydraulic conductivity of soils is often substantially decreased when soils are irrigated with waste water. Apart from the inducement of sodicity, which is real in many cases, the decreases in hydraulic conductivity are attributed largely to the production of microbial biomass, particularly extracellular polysaccharides (e.g.. Ref. 53). These extracellular polysaccharides form gels that may store large quantities of water and allow water and ions to diffu.se through them at rates not much less than those of free water, but they could be expected to restrict mass flow of water and thus nutrients, to roots (54). 

McNeal, B. L., D. A. Layfield, W. A. Norvell, and J. D. Rhoades. 1968. Factors influencing hydraulic conductivity of soils in the presence of mixed salt solutions. Soil Sci Soc. Am. Proc. 32 187-290. 

Crawford, J. W. 1994. The relationship between structure and the hydraulic conductivity of soil. Eur. J. Soil Sci. 45 493-502... 

Faybishenko, B. Con arison of Laboratory and Field Methods for Determination of Unsaturated Hydraulic Conductivity of Soils. Proceedings of the International Workshop on Characterization and Measurement of the Hydraulic Properties of Unsaturated Porous Media, van Genuchten, M. Th., Leij, F., Wu, L., Eds. 1998 pp 279-292. 

The effect of grafted and ungrafted guargum on turbulent flow of water and on hydraulic conductivity of soil. 

The Effect of Grafted and Ungrafted Guargum on Turbulent Row of Water and on Hydraulic Conductivity of Soil... 

Other factors affecting performance include the presence of toxic material, the redox potential, salinity of the groundwater, light intensity, hydraulic conductivity of the soil, and osmotic potential. The rate of biological treatment is higher for more permeable soils or aquifers. Bioremediation is not applicable to soils with very low permeability, because it would take a long time for the cleanup process unless many more wells were installed, thus raising the cost. 

Other models successfully employ a simple water routing system. Each layer of soil is assumed to hold all water entering the layer up to the field capacity. When the water content of a soil layer exceeds the field capacity, water drains downward to the next layer at the rate specified by the hydraulic conductivity of the saturated soil in the layer. 

U.S. EPA s rationale for the requirement of composite bottom liner option in the final doubleliner rule is based on the relative permeability of the two liner systems.13 The results of numerical simulations performed by U.S. EPA,10 which compared the performance of a composite bottom liner to that of a compacted soil bottom liner under various top liner leakage scenarios, showed that liquids passing through defects in the top FML enter the secondary LCRS above the bottom liners. The hydraulic conductivities of bottom liner systems greatly affect the amount of liquids detected, collected, and removed by the secondary LCRS. 

U.S. EPA bases its 1 gallon/acre/day leak detection sensitivity on the results of calculations that show that, theoretically, an LDS overlying a composite bottom liner with an intact FML component can detect, collect, and remove liquids from a top liner leak rate <1 gallon/acre/day. This performance standard, therefore, can be met with designs that include a composite bottom liner. Based on numerical studies, one cannot meet the leak detection sensitivity with a compacted soil bottom liner, even one with a hydraulic conductivity of 10-7 cm/s. Therefore, the emphasis of this standard is on selecting an appropriate bottom liner system. 

Figure 26.8 shows the influence of sodium bentonite on the hydraulic conductivity of the silt/sand soil. The addition of only 4% or 5% sodium bentonite to this particular soil drops the hydraulic conductivity from 10 4 to 10 7 cm/s, a rather dramatic reduction. 

Darcy s law applied to a soil liner shows the rate of flow of liquid q directly proportional to the hydraulic conductivity of the soil and the hydraulic gradient, a measure of the driving power of the fluid forcing itself through the soil and the cross-sectional area A of the liner (see Figure 26.9). 

The investigators divided the collection units into a number of subunits, each subunit measuring 3 ft by 3 ft. A total of 250 different collection units underneath the soil liner were monitored independently to determine the rate of flow. The objective was to correlate the variability of the hydraulic conductivity of the liner with the molding water content of the soil and with the dry density of the compacted soil. 

At the Keele Valley landfill, just outside Toronto, however, some excellent field data have been obtained. At this particular site, a 3-ft clay liner spanning 10 acres is monitored by a series of underdrains. Each underdrain measures 15 m2 and is made of HDPE. The underdrains track the liquid as it moves down through the soil liner. The underdrains have been monitored for more than 2 years and have consistently measured hydraulic conductivities of about 1 x 10-8 cm/s. Those field values are essentially identical to the laboratory values. 

The water and ions in the double layer are attracted so strongly electrochemically to the clay particles that they do not conduct fluids. Fluids moving through the soil go around the soil particles and also around the double layer. The hydraulic conductivity of the soil, then, is controlled very strongly by the thickness of these double layers. When the double layers shrink, they open up flow paths resulting in high hydraulic conductivity. When the layers swell, they constrict flow paths, resulting in low hydraulic conductivity.20-24... 

Consequently, if the free ions in the soil water are leached out, the double layers swell tremendously, pinching off flow paths and resulting in very low hydraulic conductivity. Data have shown hydraulic conductivity to be as much as two to three orders of magnitude lower when measured with distilled water than with other kinds of water. For this reason, distilled water should not be used in testing the hydraulic conductivity of a clay liner. 

An experiment was conducted using a soil called SI, an illitic clay containing chlorite from Michigan. Two sets of data showd the results of permeation of the regular soil, first with water and then with pure reagent grade heptane. The heptane caused the hydraulic conductivity of the regular compacted soil to skyrocket. About 8% cement was then added to the soil. 

Francis GS, Fraser PM (1998) The effects of three earthworm species on soil macroporosity and hydraulic conductivity. Appl Soil Ecol 10 11-19... 

Tufenkji N, Elimelech M (2004) Correlation equation for predicting single-collector efficiency in physicochemical filtration in saturated porous media. Environ Sci Technol 38 529-536 Turner BL, Kay MA, Westermann DT (2004) Colloid phosphorus in surface runoff and water extracts from semiarid soils of the western United States. J Environ Qual 33 1464-1472 van Genuchten MT (1980) A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Sci Soc Am J 44 892-898... 

Laboratory tests have achieved hydraulic conductivities of less than 4 x 10 ° cm/sec in soils. 

Research is currently focusing on ways to lower the hydraulic conductivity of engineered biofihns to make them an effective barrier to contaminant transport this includes the introduction of ultramicrobacteria (UMB) to penetrate the soil matrix. 

This technology is designed to treat clays soils, nonstratified soils, soils containing more than 50% silt or clay, and soils with hydraulic conductivities of less than 0.001 cm/sec. 

The typical density of rock and soil materials is approximately 2.65 g cm-3—the density of quartz. The high hydraulic conductivity of sandy sediments is ideal for producing groundwater. Groundwater flow velocities have been found to be in accordance with Darcy s law ... 

The phenomenon of soil dispersion with respect to Na+ loads (magnitude of ESP or SAR) appears to be unique to all soils on at least one particular point. As the total salt or Cl- concentration in the water increases, the dispersion index decreases and the saturated hydraulic conductivity increases (Fig. 11.6). When this occurs, the soil-water system becomes toxic to plants and organisms owing to high osmotic pressures. When chloride concentration in solution increases beyond 6000 mg L 1, Na ions near clay surfaces begin to dehydrate because of high osmotic pressure in the surrounding solution. This causes clay particles to flocculate (flocculation is the reverse of dispersion) and, consequently, the saturated hydraulic conductivity of the soil increases. 

Contantz, J. 1982. Temperature dependence of unsaturated hydraulic conductivity of two soils. Soil Sci. Soc. Am. J. 46 466-470. 

Generally, 70 to 75% of the water vaporized on land is transpired by plants. This water comes from the soil (soil also affects the C02 fluxes for vegetation). Therefore, after we consider gas fluxes within a plant community, we will examine some of the hydraulic properties of soil. For instance, water in the soil is removed from larger pores before from smaller ones. This removal decreases the soil conductivity for subsequent water movement, and a greater drop in water potential from the bulk soil up to a root is therefore necessary for a particular water flux density. 

Figure 9-20. Changes in the hydraulic conductances of the root (Z, 00 ), the root-soil air gap (Lgap), the soil ), and tlie overall pathway (Love,all see Eq. 9.14) as the soil dries over a 30-day period. Note that Lovera11 is dominated by Z, over the first 6 days of drought, by Lgap from 6 to 20 days, and later by L 1. [Data for young roots of desert succulents are adapted from Nobel and Cui (1992a) used by permission of Oxford University Press.]...

Nobel, P.S., and Cui, M. 1992a. Hydraulic conductances of the soil, the root-soil air gap, and the root Changes for desert succulents in drying soil. J. Exp. Bot. 43 319-326. 

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