Soil, leaching into the

Several facts have emerged from our studies with 2,7-DCDD and 2,3,7,8-TCDD. They are not biosynthesized by condensation of chloro-phenols in soils, and they are not photoproducts of 2,4-dichlorophenol. They do not leach into the soil profile and consequently pose no threat to groundwater, and they are not taken up by plants from minute residues likely to occur in soils. Photodecomposition is insignificant on dry soil surfaces but is probably important in water. Dichlorodibenzo-p-dioxin is lost by volatilization, but TCDD is probably involatile. These compounds are not translocated within the plant from foliar application, and they are degraded in the soil. 

The majority of trichloroethylene present on soil surfaces will volatilize to the atmosphere or leach into the subsurface. Once trichloroethylene leaches into the soil, it appears not to become chemically transformed or undergo covalent bonding with soil components. When trichloroethylene was absorbed onto kaolinite and bentonite, the nuclear magnetic resonance (NMR) spectra showed no evidence of chemical reactions (Jurkiewicz and Maciel 1995). Because trichloroethylene is a dense nonaqueous phase liquid, it can move through the imsaturated zone into the saturated zone where it can displace soil pore water (Wershaw et al. 1994). 

Table III illustrates the impact of adsorption on the leaching of organic chemicals in the soil. A water input of 305 cm was used, which is equivalent to a full year of precipitation in the eastern United States. In a soil with a field capacity of 30%, the water would penetrate 1017 cm. Mirex with a very large Kqc is practically immobile after a full year of precipitation, it is still on the surface. It is likely that any compound adsorbed this strongly would be carried off the land surface by soil erosion instead of being leached into the soil. In contrast, DBCP, which is very weakly adsorbed, penetrates the soil profile almost as far as the water does.

DBCP. The predictions suggest that DBCP is volatile and diffuses rapidly into the atmosphere and that it is also readily leached into the soil profile. In the model soil, its volatilization half-life was only 1.2 days when it was assumed to be evenly distributed into the top 10 cm of soil. However, DBCP could be leached as much as 50 cm deep by only 25 cm of water, and at this depth diffusion to the surface would be slow. From the literature study of transformation processes, we found no clear evidence for rapid oxidation or hydrolysis. Photolysis would not occur below the soil surface. No useable data for estimating biodegradation rates were found although Castro and Belser (28) showed that biodegradation did occur. The rate was assumed to be slow because all halogenated hydrocarbons degrade slowly. DBCP was therefore assumed to be persistent. 

Mlrex. Mirex does not leach into the soil profile and is predicted to volatilize only slowly. There Is no evidence for any rapid transformation so it should be considered persistent. Because It is so strongly adsorbed to the soil and stays on the surface, a major loss from terrestrial systems would probably be erosion and transport Into surface waters. 

Atmospheric PAH depositions are usually from very dispersed sources but they cover significant amounts of land surface. PAH concentrations from these sources are typically quite low in soil and they are adsorbed strongly to soil particles. Consequently, there is minimal leaching into the soil below and the adsorbed PAHs tend to resist biodegradation, volatilization, and/or photolysis. If low concentrations of HMW PAHs, such as benzoMpyrene, need to be reduced below some established risk threshold (often determined in a site-specific manner by the regulatory officials), bioremediation of these low concentrations will likely be a desired alternative because of the scale and magnitude of the problem. 

Chemical residues and waste generated in the manufacturing of methamphetamine pose a serious danger to the environment. This waste is often poured down the drain, into storm sewers, or into crudely dug pits in the ground. These chemicals can leach into the soil and groundwater and cause contamination for many years. 

With the carbon supply in forest litter equal to 421.lt/km2, from which 12.95 t/km2 are annually leached into the soil, with 82.5% of this amount remaining in the upper soil layer (which can be up to 8 cm thick), we get C18 = 0.31. 

Dichloral urea (DCU) is typical of the substituted ureas. These compounds vary in water solubility from about 5 to about 3000 p.p.m. Consequently, some of them are readily leached into the soil. They are stable to oxidation and hydrolysis and are relatively persistent. They are slowly broken down by soil microorganisms and are likely to remain active in the soil for several seasons. 

The second class of pesticide transport processes is that of movement of dissolved or particulate-sorbed pesticides in water. Leaching of pesticides hais been recognized as a critical process, if not in the sense of the absolute percentage of applied pesticide leached into the soil profile, then with respect to the contamination of groundwater by trace quantities of pesticide. Several pathways of dissolved pesticide movement with leaching water are recognized (18). 

From the cinnamic acids or phenyl propanoids described above, / -oxidation and truncation of side chains yields a variety of benzoic or simple phenolic acids [28], Rao et al., [22] identified gallic acid (18), gentisic acid (19), protocatechuic acid (20), />-hydroxybenzoic acid (21), oc-resorcyclic acid (22), vanillic acid (23) and salicylic acid (24) in C. arietinum and showed that overall, leaf content of all phenolic compounds was much greater than in roots and stem. They postulated that the production of these compounds may enhance the activity of indole acetic acid oxidase or may express antimicrobial properties when leached into the soil. However, Singh et al. [24] showed that the production of both 18 and 24 by C. arietinum was induced when treated by the culture filtrate of Sclerotium rolfsii along with the phenyl propanoids 14, 15 and 17 mentioned above. 

Inhibitors are released from roots, chiefly, but the leaves of some plants, such as Encelia farinosa, a desert shrub, contain inhibitors. If in the leaves, the inhibitor may be either leached into the soil or the leaves may fall to the ground and undergo decay. The berries of some plants are also the source of growth-inhibiting substances, such as parasorbic acid found in the mountain ash (Sorbus aucuparius). 

Another interesting case of phytotoxicity, mentioned by Bonner (1950), is that of Encelia farinosa, a desert shrub. This plant, unlike most desert shrubs, does not harbor a growth of annuals under its branches. Investigations gave no indication of excretion of toxins from the roots but when leaves were added to soil, pepper, tomato and com plants were severly injured or killed barley, oats and sunflower were little affected. The toxic substance was shown to have the structure 3-acetyl-6-methoxybenzaldehyde. When leaves of the plant fall on the ground they retain their toxicity for a year or more even though much of the phytotoxin is leached into the soil. 

Animal manures are sometimes used as trash mulches and merely left on the surface. In such cases decomposition is retarded because of lack of moisture, and some of the ammonia formed may be volatilized. Most of the nutrients are, however, leached into the soil and any losses sustained are balanced against the benefits derived from soil protection against erosion. The more bedding material in the manure, the more effective this type of use is likely to be. 

Landfills contain a tremendous amount of plastic waste. As the plastics degrade, pollutants leach into the soil and gases escape into... 

Beside selection, waste site design created additional problems. There was little design for dumps and landfills. Someone dumped waste or piled it up. See Case 16-3. Sometimes waste got covered. Little attention was given to preventing dump contents from leaching into the soil below or to nearby areas. 

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