Soils layers

Sorbed pesticides are not available for transport, but if water having lower pesticide concentration moves through the soil layer, pesticide is desorbed from the soil surface until a new equiUbrium is reached. Thus, the kinetics of sorption and desorption relative to the water conductivity rates determine the actual rate of pesticide transport. At high rates of water flow, chances are greater that sorption and desorption reactions may not reach equihbrium (64). NonequiUbrium models may describe sorption and desorption better under these circumstances. The prediction of herbicide concentration in the soil solution is further compHcated by hysteresis in the sorption—desorption isotherms. Both sorption and dispersion contribute to the substantial retention of herbicide found behind the initial front in typical breakthrough curves and to the depth distribution of residues. 

Soil does not provide as solid a base as rock. The strength of a foundation built on soil and its ability to withstand an earthquake will therefore depend upon the quality and depth of soils which may be formed of a number of soil layers of different stratifications and depths. Sandy soil or soil with sedimentary deposits, for instance, will have less strength and will provide a weaker base, as such soils may settle more during a ground movement. 

The aluminum smelter solid wastes, in the form of spent pot lining, are disposed of in engineered landfills that feature clay or synthetic lining of disposal pits, provision of soil layers for covering and sealing, and control and treatment of any leachate. Treatment processes are available to reduce hazards associated with spent pot lining prior to disposal of the lining in a landfill. Other solid wastes... 

Another group of effects consists in blocking the channels of losing water from the soil layer, i.e., the hydraulic conductivity responsible for the gravitational flow, and of physical evaporation. All these effects provide an increase of the water content of the soil and, consequently, improve the water supply of plants, which is reflected in the three last columns in Table 8. According to the data of various authors, an increase in the soil water content (AW) in sandy soils lies in the range of 10-35% at doses up to 0.2% in a number of cases [10, 11, 58, 131-133] the dependencies of AW on the doses of the hydrogels added have been studied. 

Finally, one more phenomenon is connected with the dependence of swelling on pressure. It is quite clear that the soil layer exerts certain pressure on the SAH particles and this pressure depends on the depth. This factor additively reduces SAH efficiency as a water absorbent. We possess some data, yet unpublished, which directly indicate some decrease of the SAH swelling in soils with the depth of their residence. 

Figure 6-7 illustrates the runoff paths for HOF and SOF, as well as for subsurface stormflow and groundwater flow. Subsurface stormflow is a moderately rapid runoff process in which water flows to a stream through highly permeable surface soil layers (without reaching the water table). Note in Fig. 6-7 that while HOF and subsurface stormflow may occur over a large fraction of an infiltration-limited hillslope, SOF occurs over a smaller portion adjacent to the stream. 

Fig. 11. Bulk soil water content and root ABA content in consecutive 10 cm soil layers on days 3, 9,12 and 18 after withholding water (A) from maize plants growing in 1 m deep soil columns. Well-watered plants (A) received water daily throughout the experimental period. Points are means of five measurements. Modified from Zhang Davies (1989).

H. M. Helal and D. Sauerbeck, A method to study turnover processes in soil layers of different proximity to roots. Soil Biol. Biochem. 15 223 (1983). 

Abdul et al. (1990) reported that upper layers of soil were saturated with automatic transmission fluid (ATF) after a storage tank leaked 208,000 gallons onto the surrounding soil. Lower soil layers contained less ATF volumetric ATF concentrations ranged from <10% to 40% at 80-0 cm above the ATF table. 

Monolithic covers, also referred to as monofill covers, use a single vegetated soil layer to retain water until it is either transpired through vegetation or evaporated from the soil surface. A conceptual design of a monolithic cover system is shown in Figure 25.2. 

Capillary barrier cover systems consist of a finer-grained soil layer (like that of a monolithic cover system) overlying a coarser-grained material layer, usually sand or gravel, as shown conceptually in Figure 25.3. The differences in the unsaturated hydraulic properties between the two layers minimize percolation into the coarser-grained (lower) layer under unsaturated conditions. 

The finer-grained layer of a capillary barrier cover system has the same function as the monolithic soil layer that is, it stores water until it is removed from the soil by evaporation or transpiration mechanisms. The coarser-grained layer forms a capillary break at the interface of the two layers, which allows the finer-grained layer to retain more water than a monolithic cover system of equal thickness. Capillary forces hold the water in the finer-grained layer until the soil near the interface approaches saturation. If saturation of the finer-grained layer occurs, the water will move relatively quickly into and through the coarser-grained layer and to the waste below. 

Lateral flow (L) within the soil layer containing plant roots is small for most landfill cover situations and is zero for lysimeters with sidewalls. During the course of a hydrologic year, ASW is usually small in comparison to the other terms, but it may be large on a daily basis. A primary focus for the design is PRK below the ET landfill cover as represented by the rearranged equation ... 

Numerous factors control AET and thus control the hydrologic performance of an ET cover. Soil-water content, rate of root growth, and total root mass strongly affect the rate of AET. AET is also affected by whether wet soil is available in surface soil layers, deeper in the profile, or in... 

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. 

Agricultural interests have amended existing soil properties to improve productivity their experience demonstrates the power of knowledge of soil properties and the ability to control them.14 A primary benefit of these amendment efforts was improvement in soil-water-holding capacity and increased rate of water removal from all soil layers by plants. The benefits of soil modification remain effective for decades. There is opportunity for similar improvements in soil during ET landfill cover design and construction. Control of ET cover soil properties has potential to enhance cover performance and should add little to construction cost. 

Control layers, such as those used to minimize animal intrusion, promote drainage, and control and collect landfill gas, are often included for conventional cover systems and may also be incorporated into ET cover system designs. For example, a proposed monolithic ET cover at Sandia National Laboratories in New Mexico will have a biointrusion fence with 1/4-in. squares between the topsoil layer and the native soil layer to prevent animals from creating preferential pathways, potentially resulting in percolation. The biointrusion layer, however, will not inhibit root growth to allow for transpiration. At another site, Monticello Uranium Mill Tailings Site in Utah, a capillary barrier ET design has a 12-in. soil/rock admixture as an animal intrusion layer located 44 in. below the surface, directly above the capillary barrier layer. 

In more recent applications, several types of ET cover designs also have incorporated synthetic materials, such as geomembranes, which are used to enhance the function of minimizing water into the waste. For example, the Operating Industries Inc. Landfill in California has incorporated a soil layer with a geosynthetic clay liner in the design. The cover system for this site will reduce surface gas emissions, prevent oxygen intrusion and percolation, and provide for erosion control.68... 

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