High-rainfall areas

Movement of carbonates and salts can also occur in a similar fashion. As these minerals are weathered in the upper soil profile, their component ions go into solution and are moved down through the soil by rainfall entering the soil. As the water moves down the soil there may not be enough water to move the ions out of the soil, so they precipitate in a lower horizon where they accumulate. Such accumulations are common in arid environments with limited rainfall. In high rainfall areas, carbonates and salts are usually completely removed from the soil through leaching. 

The alkali and alkaline earth metals are examples of relatively simple cations that occur in only one oxidation state and are surrounded by water (see Figure 6.1). The most common of these ions in soil, in order of decreasing abundance, are calcium (Ca2+), magnesium (Mg2+), potassium (K+), and sodium (Na+). Sodium is typically present in very small amounts in high-rainfall areas, whereas it may have a relatively high concentration in low-rainfall areas. 

High rates of water influx remove SiO at low solution concentrations. Normal ground water and streams carry about 17 ppm SiO and less in high rainfall areas (Davis, 1964). In some weathering profiles silicification or deposition of silica has been observed. Most often the form of the phase deposited is crypto-crystalline, either opal or chalcedony. In these cases quartz grains do not show overgrowths (Elouard and Millot,... 

Figure 4.8 A modification of the Gibbs (1970) plot shows that evaporation-controlled rivers are in arid regions, rock-controlled rivers are in intermediate rainfall areas, and atmospheric-controlled rivers are in high rainfall areas. (Modified from Gibbs, 1970.)...

COVERING THE SOIL. Covering the soil with mulch films or organic material is an alternative to leaving the soil bare. In high-rainfall areas with heavy soil, however, this system tends to lead to waterlogging. 

The leaching and precipitation of copper at Rio Tinto in Spain this century is described by Taylor and Whelan (Tl) and an extensive report on the present-day technology of copper leaching practices in the western United States is discussed by Sheffer and Evans (SIO). Modified practices have been successfully used throughout the world both in low- (SlO) and high-rainfall areas (B7). 

The percentage of the CEC occupied by specific cations influences soil pH and associated characteristics relevant to plant growth and soil biological activity. Only the most strongly held cations remain in soils in high rainfall areas. Al dominates the CEC, hydrolyzing when released from the solid phase to the soil solution to form acidic soils with pH values often below 5. 

Drainage. Avoid drainage of any kind near the outside of the pond walls. Rain water, particularly, must drain away from the pond perimeter. If the pond is constructed in a high rainfall area, footer drains should be installed to keep runoff from infiltrating down around the walls of the pond. 

Iron - Although iron is the most abundant element in soils, levels of available iron often are limiting in calcareous soils, and to a much lesser extent in sandy soils in high-rainfall areas. Soil applications of most iron sources are not effective for crops, so foliar sprays are the recommended control method [3]. Plant species vary considerably in their susceptibility to iron chlorosis. Growing iron-tolerant species or varieties within a species is one of the main methods of controlling iron chlorosis on low-iron soils. 

Clay soils acid soils especially in high rainfall areas, chalk and limestone soils and peats poor grassland... 

High rainfall areas over 700mm annual rainfall, or over 250mm excess winter rainfall Deep clayey soils Deep sUty soils Organic soils Peat soils... 

Soluble salts of the soil Water in the soil should most properly be considered as the solvent for salts of the soil the result being the soil solution. In temperate climates and moderate rainfall areas, the soil solution is relatively dilute, with total dissolved salts ranging from 80 to 1 500 p.p.m. Regions of extensive rainfall show lower concentrations of soluble salts as the result of leaching action. Conversely, soils in arid regions are usually quite high in salts as these salts are carried to the surface layers of the soil by water movement due to surface evaporation. 

If we assume that the TCDD is contained in the surface 6 inches of the soil profile since it is relatively immobile (5), then the 2,4,5-T at the 947 lbs of active ingredient/acre treatment would have had to contain 2.1 ppm TCDD to be observed. At the lower application rates of 584 and 160 lbs/acre, the 2,4,5-T would have had to contain 3.5 and 12.5 ppm TCDD in the technical materials to have 1 ppb in the top 6 inches of soil. Since the soil is sandy and high rainfall occurred in the area, maximum movement of materials in soil may occur causing TCDD to be present deeper in the profile. If the TCDD moved uniformly throughout the 36 inch soil profile, then six times more TCDD would have had to be present in the original 2,4,5-T for detection. This would have required the presence of 12.6, 21.0, and 75.0 ppm TCDD in the 2,4,5-T applied in the three treatments. These calculations are based on the assumption that no degradation occurred in or on the soil. 

The applications and limitations of passive gas control systems must also be understood. They can be used at virtually any site where there is the capability to trench or drill and excavate to at least the same depth as the landfill. Limiting factors could include the presence of a perched water table or rock strata. Passive vents should generally be expected to be less effective in areas of high rainfall or prolonged freezing temperatures. 

The pH of Soils. The pH of soils (see Textbox 48) varies widely with location and climate the local climate of a region usually provides a clue as to the pH of the soil. In high-rainfall regions, for example, soils are generally acid. Alkaline soils, in contrast, typically occur in low-rainfall areas the actual value of the pH varies over a wide range, from below 5 in very acid... 

Arid areas (0.05 < P/PET < 0.20) mean annual precipitation values up to about 200 mm in winter rainfall areas and 300 mm in summer rainfall areas but more importantly inter-annual variability in the 50-100% range. Pastoralism is possible but without mobility or the use of groundwater resources is highly susceptible to climatic variability. 

Semi-arid areas (0.20 < P/PET < 0.50) highly seasonal rainfall regimes and mean annual values up to about 800 mm in summer rainfall areas and 500 mm in winter regimes. Inter-annual variability is nonetheless high (25-50%) so despite the apparent suitability for grazing of semi-arid grasslands, this and other sedentary agricultural activities are susceptible to seasonal and inter-annual moisture deficiency. 

The most productive wetlands are on mineral soils, often developed on alluvial deposits in fiuxial wetlands. Nutrients and fertile sediments seasonally flow into these areas under high rainfall and surface water flow. 

All Groundwater from fractured bedrock and surface drainage Humid I - very low concentrations occur in areas of very high rainfall or very high relief ... 

Leaching, the passage of water through the soil profile, is a process that occurs in soils in areas of moderate to high rainfall, and which results in the loss of exchangeable cations (such as Ca, Mg, K, and Na - see Section 5.5). Unless the rate of weathering of soil minerals is sufficiently rapid to replace these losses, the exchange sites will become occupied by H" and which are acidic cations. 

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