More than 30% of the surface of the earth consists of arid and semi-arid soils, which are generally too dry to produce good yields. If enough fresh water is available and the soil condition is suitable, these soils can be irrigated and used for arable land. These regions are therefore thought to be new promising lands for food production in the future. For this reason in past dacades, much attention has been paid to arid and semi-arid soils and more agricultural development projects are being planned and carried out. However, many projects have failed in the past, and others fail even now, because some years after irrigation the salinity or alkali hazard increases. The soil profile investigations of salts accumulation related to the depth of ground water level, some field experiments using lysimeter and model experiments in the laboratory to clarify the mechanisms of salt accumulation are illustrated in this paper. From field investigations and lysimeter experiments of irrigated lands in Khuzistan State, Itan, the author presents the following cases to clarify the essential processes of soil salinization. Case 1 was caused by the formation of shallow groundwater due to seepage from earth canals and intensive irrigation such as basin irrigation. This case was the most dominant to soil salinization development process in arid irrigated lands. Case 2 was caused by the logging of irrigation water at the extremely hard pan formation under the plough layer caused by the compactness of tractor loading. This case was usually found in desalinized fields with clay-like texture. The hard pan is apt to become an impermeable layer logged with irrigation water,and provides a secondary site which functions to supply sodium salts to surface horizon. Case 3 was caused by the addition of salts to the soil irrigated by water with high electric conductivity. Chemical analyses of the irrigation water applied to arid land in Iran, for example, showed that about 1.8kg of the total dissolved salts were contained in 1 m5 of irrigation water. Case 4 was caused by cultivation under the low leaching fraction resulting from the extremely high evapotranspiration of the dry summer. The lysimeter experiment revealed that cultivation during periods of high temperature and low humidity reguires much irrigation water, as calculated by the leaching fraction, to keep soil salinity below a specified level. The mechanism of salt accumulation in arid lands was also examined through the experimental pedology view. Experiments were carried out by using soil columns saturated with salt solution which capillarily provides from the constant groundwater level.Most calcium and magnesium ions in the salt solution are adsorbed rapidly at the cation exchange sites of the soil when the solution contacts with the soil. Large amouuts of sodium ions, on the other hand, still remain in the from of free ions which moves easily with the soil solution. When the cation exchange sites adsorbing calcium and magnesium ions are dried out, there ions are detached from the sites and deposited on the surface of the soil column as salts. Furthermore sodium salt precipitation occurs directly from soil solution when the evaporation at the surface portion of the column proceeds.
This paper regards the behaviour of iron and manganese in an aquatic environmnd from the viewpoint of thermodynamics. The important oxidation states of metals are bi —and tri—valency for iron,and tri—and quadri — valency for manganese in water. The metal concentration is determined by certain environmental conditions, such as solubility of metals, themodynamic states of pH,Eh and electron activity,other metal concentration in water,organic and inorganic chelate agents,organics,clay,carbonates,oxidizing agents and so on. The behaviour of iron and manganese in lakes depends on the seasonal cycle of water mass,which causes changes in the dissolved oxygen concentration in water. Iron and manganese precipitate under the oxidation state and dissolve under the reduction state from sediments. Heavy metals in sediments (Cd, Zn, Pb, Ni, Cu) were determined by atomic absorption spectrometry. Correlation analysis among the metal concentrations in sediments was done. High correlation coefficientss were obtained between two metals. Probability distributions of metal concentration in sediments were disscussed. Most metal concentrations showed logarithmic normal distribution. Different chemical species of metals in sediments were partitioned into selective chemical fractions.
Heavy metal ions in the soil solution are considered to be in equilibrium with the complexes of soluble ligands, exchangeable ions, what adsorbed on hydroxides of Al and Fe, and aluminol group of allophane, and the complex of humic substances and insoluble metal conpounds. Equilibrium equations derived from the law of mass action and donnan* s equilibrium theory fitted with Ca—Zn and Ca—Cd exchange on montmorillonite, while Gapon’s equation did not. The solubility decrease, due to the fall of the redox potential of soils which causes sulfides to form, was most prominent in Cd. Fractionation of heavy metals in the soil showed that the amount of Cu was relatively high in the humus complexes and that of Cd was so in the exchangeable form, while that of Zn was relatively low in the fractions which react directly with soil solution. Soil contamination by heavy metals raised relative amounts of these fractions. The concentrations of metals in the percolated water of uncontaminated paddy soil in lysimeter experiments were about the same as those in clean streams. The income and outgo of Cd in the soils used in the experiments were most severely influenced by the redox state of the soils through the uptake by rice plants, while the incomes of Cu and Pb balanced their outgoes approximately.
There is no doubt that some elements in all waste materials disposed of on land sooner or later react with soil constituents, and reach the food chain via surface and underground water, resulting in a potential threat to the health of human beings. Recently, because of the increasing quantity and variety of wastes,this threat has been enhanced. This paper presents information on the movement of trace elements involving radionuclides, based on data obtained from the ongoing research program, the author* s experience, and reviews of the literature. A number of abiotic (physical, chemical) and biotic reactions take place, which affects the rate of movement of contaminants from sources to the surrounding soil. Although sharp distinctions of these processes cannot be made, relatively simple examples are described. Physical processes in the subsurface environment, composed of advection, dispersion, preferential flow, and so on, affect the movement of contaminants, though for low ground water flow less than ca.10 3cm/s molecular diffusion may dominate the contaminant transport. It is noted that the dispersion coefficient itself is not a parameter representing the dispersion phenomena,but the ratio of the coefficient divided by the interstitial velocity is related to the extent of the diffusivity. Among biological processes involving microorganisms, the three important mechanisms influencing migration of trace metals are oxidation—reduction, mineralization—immobilization, and production of organic constituents. Chemical processes affect contaminant transport by causing interactions between ionic compounds and geologic materials, such as chemical adsorption and ion exchange processes, or by affecting the form of the contaminant,i.e., hydrolysis and precipitation. For nonionic compounds, the adsorption is commonly hydrophobic with organic carbon on the soil surface. Based on the information on the environmental processes mentioned above, it is pointed out that an improved understanding about transport/interaction mechanisms of individual/mixed contaminants is needed to limit any risks from subsurface contamination in the future.
It is essential to make clear a system analysing material transport in soils, in order to increase food production and to conserve the natural environment. This report describes a quantitative concept of physical,chemical and biological phenomena making up a system of material transport in soils, such as pore structure in soil, absorption of ions to soil particles, capture of solid particles in pores,decomposition of organic matter, and dissolution and precipitation of chemical materials in soil water. In conclusion,it is stated in detail that prediction of material transport in soils will be successful after such actions are quantitatively described by using mathermatical equations and after they are combined with material transport equations.