Soil-applied pesticides properties

In order to properly understand pesticides in the context of the soil microbial ecosystem, it is important to consider the relevant properties of the soil environment, the metabolic capabilities of the soil microbial community, and the uses and dissipation routes of soil-applied pesticides. 

The documented occurrence of pesticides in surface water is indicative that runoff is an important pathway for transport of pesticide away from the site of application. An estimated 160 t of atrazine, 71 t of simazine, 56 t of metolachlor, and 181 of alachlor enter the Gulf of Mexico from the Mississippi River annually as the result of runoff (47). Field application of pesticides inevitably leads to pesticide contamination of surface runoff water unless runoff does not occur while pesticide residues remain on the surface of the soil. The amount of pesticides transported in a field in runoff varies from site to site. It is controlled by the timing of runoff events, pesticide formulation, physical—chemical properties of the pesticide, and properties of the soil surface (48). Under worst-case conditions, 10% or more of the applied pesticide can leave the edge of the field where it was applied. 

Uses Soil and turf wetting agent for hydrophobic soils apply alone or with pesticides, iiq. fertilizers nonphytotoxIc aids management and conservation of water by altering surf. tens, props, of surf, water from rainfall or inigation Properties Emerald gm. cl. liq. solv. odor completely sol. in water sp.gr. 0.99 b.p. 

Endosulfan enters air, water, and soil when it is manufactured or used as a pesticide. Endosulfan is often applied to crops using sprayers. Some endosulfan in the air may travel long distances before it lands on crops, soil, or water. Endosulfan on crops usually breaks down within a few weeks. Endosulfan released to soil attaches to soil particles. Endosulfan found near hazardous waste sites is usually found in soil. Some endosulfan in soil evaporates into air, and some endosulfan in soil breaks down. However, it may stay in soil for several years before it all breaks down. Rainwater can wash endosulfan that is attached to soil particles into surface water. Endosulfan does not dissolve easily in water. Most endosulfan in surface water is attached to soil particles floating in the water or attached to soil at the bottom. The small amounts of endosulfan that dissolve in water break down over time. Depending on the conditions in the water, endosulfan may break down within 1 day or it may take several months. Some endosulfan in surface water evaporates into air and breaks down. Because it does not dissolve easily in water, only very small amounts of endosulfan are found in groundwater (water below the soil surface for example, well water). Animals that live in endosulfan-contaminated waters can build up endosulfan in their bodies. The amount of endosulfan in their bodies may be several times greater than in the surrounding water. More information on the chemical and physical properties of endosulfan can be found in Chapter 3. More information on its occurrence and fate in the environment can be found in Chapter 5. 

Oxime carbamates are generally applied either directly to the tilled soil or sprayed on crops. One of the advantages of oxime carbamates is their short persistence on plants. They are readily degraded into their metabolites shortly after application. However, some of these metabolites have insecticidal properties even more potent than those of the parent compound. For example, the oxidative product of aldicarb is aldicarb sulfoxide, which is observed to be 10-20 times more active as a cholinesterase inhibitor than aldicarb. Other oxime carbamates (e.g., methomyl) have degradates which show no insecticidal activity, have low to negligible ecotoxicity and mammalian toxicity relative to the parent, and are normally nondetectable in crops. Therefore, the residue definition may include the parent oxime carbamate (e.g., methomyl) or parent and metabolites (e.g., aldicarb and its sulfoxide and sulfone metabolites). The tolerance or maximum residue limit (MRL) of pesticides on any food commodity is based on the highest residue concentration detected on mature crops at harvest or the LOQ of the method submitted for enforcement purposes if no detectable residues are found. For example, the tolerances of methomyl in US food commodities range from 0.1 to 6 mg kg for food items and up to 40 mg kg for feed items. ... 

The key properties of pesticides that control their accumulation in sediment and biota are hydrophobicity and persistence. Generally, pesticides were found to have the potential to accumulate in sediments and in biota if they had a water solubility of < 1 mg/L or an octanol-water partition coefficient of (fC0w) > 1000 and a soil half-life of > 30 d. Pesticides that are rarely detected in sediment or in biota have high water solubilities and short soil half-lives. A few pesticides that are moderate in hydrophobicity and persistence can also be detected in the environment to some extent. Most of the current-use pesticides have relatively high water solubilities and short soil half-lives, and are not likely to accumulate in biota. However, some currently used pesticides that are intermediate in hydrophobicity and persistence are likely to be detected (Table 4). It is also important to take into account where, and in what amounts, these pesticides are applied. 

Typically, only 0.01-10% of the mass of pesticide compounds applied to fields is detected in streams [91]. The remaining 90-99% of pesticides adsorb to soil, percolate into groundwater, or volatilize [79]. The major degradates of the most heavily used herbicides found in surface water have not been studied widely. Many chemical properties of pesticides affect the amounts transported to streams. In general, acetanilide herbicides are more soluble in water, and thus more mobile than are the triazines [92], The solubilities of sulfonated degradates of acetanilides (ethane sulfonic acid, or ESA), can be 10 times the solubility of the parent compound [93]. The greater mobilities of the degradates of the acetanilides (amide family) can explain these com-... 

Adsorption is very important for the biological properties of the chemicals. Many soil pesticides may be applied in higher quantities when the soil has strong adsorption properties. Adsorption inactivates and makes toxicants less harmful and reduces leakage, but on the other hand, it can make the pesticides more recalcitrant to microbial degradation. The adsorption process is quite fast, and often less than an hour is needed to produce equilibrium. The opposite process, desorption, takes longer and sometimes a low residue is bound irreversibly. 

Pesticide and soil properties determine the mobility and degradation of applied chemicals. The interaction of the organic molecules with soil solids varies according to chemical structure, organic matter and clay content, soil pH, and in some cases concentration. Degradation rates are influenced by pH, substrate concentration,... 

Water leaves the field either as surface runoff, carrying pesticides dissolved in the water or sorbed to soil particles suspended in water, or as water draining through the soil profile, carrying dissolved pesticides to deeper depths. The distribution of water between drainage and runoff is dependent on the amount of water applied to the field, the physical and chemical properties of the soil, and the cultural practices imposed on the field. These factors also impact the retention and transformation processes affecting the pesticide. 

The analysis techniques used were FTIR to study this effect and the optional use of theoretical calculations to justify the obtained results by means of computational chemistry tools. Using QSAR properties, we can obtain an estimate of the activity of a chemical from its molecular structure only. The QSARs have been successfully applied to predict soil sorption coefficients of non-polar and nonionizable organic compounds, including many pesticides. Sorption of organic chemicals in soils or sediments is usually described by sorption coefficients. The molecular electrostatic potential (MESP) was calculated using the AMBER/AM 1 method. These methods give information about the proper region by which compounds have intermolecular interactions between their units. 

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