Moisture potential, soil

The modern procedure to minimise corrosion losses on underground structures is to use protective coatings between the metal and soil and to apply cathodic protection to the metal structure (see Chapter 11). In this situation, soils influence the operation in a somewhat different manner than is the case with unprotected bare metal. A soil with moderately high salts content (low resistivity) is desirable for the location of the anodes. If the impressed potential is from a sacrificial metal, the effective potential and current available will depend upon soil properties such as pH, soluble salts and moisture present. When rectifiers are used as the source of the cathodic potential, soils of low electrical resistance are desirable for the location of the anode beds. A protective coating free from holidays and of uniformly high insulation value causes the electrical conducting properties of the soil to become of less significance in relation to corrosion rates (Section 15.8). 

Soil moisture can have a significant effect on decomposition (Swift et al. 1979). This is due, in part, to the fact that soil moisture can affect the metabolism of decomposer microorganisms. This effect can be modified by soil texture because bioavailable moisture is determined, in part, by the suction with which water is held between soil particles (matric potential). Thus, the calibration of soils to a known matric potential can lead to the assessment of the effect of bioavailability of moisture in soil (Hillel 1982) and allow for the comparison of process rates between soils at the same matric potential (Orchard and Cook 1983). 

It is also a measure of the moisture stress in plants or soil, measured in megapascals. A more negative value indicates greater moisture stress. Soils with no moisture stress have a water potential of 0 to -1 mPa. Two methods of measuring soil water potential are the heat-dissipation method or the electrical-resistance method. 

Soil Moisture Potential. Work must be performed to remove water from any saturated or partially saturated material. This work is, in effect, a negative potential energy, and provides a useful measure of the total force holding water in the material. 

Fig. 4.1. (A) Germination of Oryzopsis holciformis as a function of soil moisture potential in loess soil, o-—o in soil for 2 days, o—o in soil for 8 days. (B) Final germination percentage of Oryzopsis holciformis as a function of soil moisture potential in sand of different particle sizes (shown in mm). After Dasberg and Mendel, 1971 [14]...

Several important soil biological, chemical, and physical processes have major effects on the environment and are important for understanding the basis for several important environmental issues including air, soil, and water pollution, climate change, and the fate of pollutants and other materials added to soil. The magnitude and rate of many of these processes are affected by abiotic factors, such as temperature, aeration, soil water content, and soil moisture potential. 

Soil moisture potential Tensiometer or pressure plates... 

Low Level Waste. The NRC 10CFR61 specifies the nature of the protection required for waste containers (20). Class A wastes must meet minimum standards, including no use of cardboard, wastes must be solidified, have less than 1% Hquid, and not be combustible, corrosive, or explosive. Class B wastes must meet the minimum standards but also have stabiHty, ie, these must retain size and shape under soil weight, and not be influenced by moisture or radiation. Class C wastes must be isolated from a potential inadvertent intmder, ie, one who uses unrestricted land for a home or farm. Institutional control of a disposal faciHty for 100 years after closure is requited. 

Many factors affect the mechanisms and kinetics of sorption and transport processes. For instance, differences in the chemical stmcture and properties, ie, ionizahility, solubiUty in water, vapor pressure, and polarity, between pesticides affect their behavior in the environment through effects on sorption and transport processes. Differences in soil properties, ie, pH and percentage of organic carbon and clay contents, and soil conditions, ie, moisture content and landscape position climatic conditions, ie, temperature, precipitation, and radiation and cultural practices, ie, crop and tillage, can all modify the behavior of the pesticide in soils. Persistence of a pesticide in soil is a consequence of a complex interaction of processes. Because the persistence of a pesticide can govern its availabiUty and efficacy for pest control, as weU as its potential for adverse environmental impacts, knowledge of the basic processes is necessary if the benefits of the pesticide ate to be maximized. 

If water losses from the surrounding area are a major component of the total evaporative losses of the pond, then soil moisture conditions will be expected to be high. Under non-limiting soil moisture conditions vegetative moisture losses are often defined as "potential" losses. Evaporative losses in this case would not be expected to differ greatly from free water evaporation. The literature recommends... 

The existence of an electrical potential causes not only cation and anion movement but also migration of moisture toward the cathode. This movement of water (electroendosmosis) is due to the asymmetrical nature of the polar groups of the water molecule. In arid regions water leaving the anode area may cause the soil surrounding the anodes to become so dry that proper current densities cannot be maintained along the line. To alleviate this, some pipe-line companies have had to transport water into desert areas to re-moisten anode beds. 

The effect of the backfill is to lower the circuit resistance and thus reduce potential loss due to the environment. The additive resistances of the anode/backfill and backfill/soil are lower than the single anode/soil resistance. Backfills attract soil moisture and reduce the resistivity in the area immediately round the anode. Dry backfill expands on wetting, and the package expands to fill the hole in the soil and eliminate voids. 

A typical profile of the pressure potential of soil moisture tested by a tensometer across the free-water surface shows a negative pressure (lower than atmosphere pressure) in the capillary zone (Figure 18.2). The negative pressure in the capillary zone indicates that the capillary zone belongs to the unsaturated zone. 

Tensiometer Measures the matric potential of a given soil, which is converted to soil moisture content Commonly consists of a porous ceramic cup connected to a pressure-measuring device through a rigid plastic tube... 

Performance data Percolation is being measured with a lysimeter connected to flow monitoring systems, soil moisture is being measured with water content reflectometers, and soil matric potential and soil temperature are being monitored with heat dissipation units. From November 1999 to July 2002, the capillary barrier cover system had a cumulative percolation of 0.5 mm. Total precipitation was 837 mm over the 32-month period. Additional field data were collected through 2005. 

The French drain (also called a channel drain or floating slab) is a construction feature that appears to provoke strong reaction from its defenders and detractors alike. French drains are only a concern in basement foundations. This slab detail is a standard feature in new houses in parts of the country as varied as New York and Colorado, but in other places it is virtually unknown. French drains are used in areas with expansive soils, such as parts of Colorado, to protect the slab from damage if the wall moves. In central New York State, the main function of the French drain is to drain away water that may seep down the walls. One national builder has discontinued and now prohibits the use of French drains in houses because of the potential for radon problems. This builder states that French drains also have been found to significantly increase indoor moisture levels. 

The TDE moisture module (of the model) is formulated from three equations (1) the water mass balance equation, (2) the water momentum, (3) the Darcy equation, and (4) other equations such as the surface tension of potential energy equation. The resulting differential equation system describes moisture movement in the soil and is written in a one dimensional, vertical, unsteady, isotropic formulation as ... 

Figure 6.6. Long-term changes of Uts of metals in a loessial soil from Israel. The soil received metal nitrates under field the capacity moisture regime (after Han and Banin, 1999. Reprinted from Water Air Soil Pollut, 114, Han F.X and Banin A., Long-term transformations and redistribution of potentially toxic heavy metals in arid-zone soils. II Incubation under field capacity conditions, p 245, Copyright (1999), with permission from Springer Science and Business Media)...

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