Soil degradation half-life

Soil degradation half-lives in soil 10-100 yr (estimated, Suzuki et al. 2000). 

Further examples of these steric halogen effects are the hydrolyses in soil degradation half-life (DT50) of the herbicidal nitriles bromoxynil (8 1962, Brominal , May Baker) [26], ioxynil (9 1962, Actril , May Baker) [27], and diclobenil (10 1960, Casoron , Philips Duphar) [28] (Fig. 35.5a). 

Soil. An estimated degradation half-life of 270 d was reported for pentachlorobenzene in soil (Beck and Hansen, 1974). 

Soil. The half-life in soil is <1 wk (Hartley and Kidd, 1987). Diflubenzuron degrades more rapidly in neutral or basic conditions but more slowly under acidic conditions (pH <6) (Ivie et al., 1980). 

CASRN 2164-17-2 molecular formula CioHnF3N20 FW 232.21 Soil. In soils, fluometuron rapidly degrades (half-life approximately 30 d) to carbon dioxide, polar, and nonextractable compounds (Hartley and Kidd, 1987 Humburg et al., 1989). 

Soil. Ambrosi et al. (1977a) studied the persistence and metabolism of phosalone in both moist and flooded Matapeake loam and Monmouth fine sandy loam. Phosalone rapidly degraded (half-life 3-7 d) but mineralization to carbon dioxide accounted for only 10% of the loss. The primary degradative pathway proceeded by oxidation of phosalone to phosalone oxon. Subsequent cleavage of the 0,0-diethyl methyl phosphorodithioate linkage gave 6-chloro-2-benzoxazolinone. Although 2-amino-5-chlorophenol was not detected in this study, they postulated that the condensation of this compound yielded phenoxazinone. 

The enzyme parathion hydrolase is active enough to be used effectively to degrade high concentrations of diazinon in soil. The half-life of diazinon in soil treated at 2000 ppm was 5.6 hours. 

Amitraz degrades in the environment to N-(2,4-dimethylphenyl)-N -methyl formamidine. Amitraz has low potential of leaching in soils. Its half-life in soil has been reported to be less than 1 day. 

Persistence in the soil is generally low (half-life under aerobic and anaerobic conditions is 21 and 68 days, respectively). In sterile soil, the half-life is almost 1 year. Azinphos-methyl adsorbs strongly to soil particles and has low solubility in water. Biodegradation and evaporation are the primary routes of elimination from soil but azinphos-methyl is also degraded by ultraviolet light. Degradation is more rapid at higher temperatures. Azinphos-methyl has a short half-life in surface waters (2 days). Hydrolysis is more prominent under alkaline conditions but the compound is relatively stable in water below pH 10. The half-life on crops is 3-5 days under normal conditions. 

In an aerobic soil, the half-life of brodifacoum is 14 days. If released into water, brodifacoum is expected to adsorb to suspended solids and sediment. Volatilization from water surfaces is not expected to be an important fate. The potential for bioconcentration in aquatic organisms is high. Brodifacoum is stable to hydrolysis in the environment. Brodifacoum is degraded by UV light when in solution. 

If released to soil, chlorobenzilate is expected to have low mobility, and therefore unlikely to leach into groundwater. Volatilization from soils is not expected to be a significant fate process. The half-life of chlorobenzilate in fine sandy soils was estimated to be 10-35 days, and degradation was primarily microbial. In silty clay loam and clay soils, the half-life of chlorobenzilate was estimated to be 10.8-15.1 and 29.5-169.1 days, respectively. If released into water, chlorobenzilate is expected to adsorb to particulate matter and sediment. Bioconcentration factors in carp were 224-709, indicating the potential for moderate to high accumulation in aquatic organisms. If released into air, chlorobenzilate will exist in both vapor and particulate phases. The half-life of... 

Methoxychlor is very persistent in soil. Its half-life is 120 days. Methoxychlor degrades much more rapidly in aerobic conditions than in anaerobic conditions. Methoxychlor is tightly bound to soil and is insoluble in water. The risk to groundwater should be low. Movement of the pesticide is likely via adsorption to suspended soil particles. 

As a soil fumigant methyl bromide leaves no toxic residue in soils. The volatile gas rises into the atmosphere. Methyl bromide is an ozone-depleting substance. Although methyl bromide is very soluble in water, its high vapor pressure in various soil types indicates a low tendency to adsorb to soils and rapid evaporation. Methyl bromide has a half-life in air estimated from 0.3 to 1.6 years. Degradation is primarily due to photolysis. In soils, the half-life is 0.2-0.5 days. In water, a half-life of 3 h was calculated. 

Naled is practically nonpersistent in soil, with half-life of less than 1 day. It degrades in sunlight to di-chlorvos (DDVP). Naled does not bind strongly to soils, but is not highly soluble in water. It is moderately volatile. Soil microorganisms break down most of the naled in the soil. Naled is rapidly broken down in water, with a half-life of 2 days. Plants reductively eliminate bromine from naled to form DDVP, which may evaporate or be further modified. 

In the ambient atmosphere, NDMA should be rapidly degraded upon exposure to sunlight. The half-life for direct photolysis of NDMA vapor is on the order of 5 to 30 minutes. In surface water exposed to sunlight, NDMA would also be subject to photolysis. On soil surfaces, NDMA would be subject to removal by photolysis and volatilization. The volatilization half-life of NDMA from soil surfaces under field conditions has been found hours. In subsurface soil and in water beyond the penetration NDMA would be susceptible to slow microbial decomposition under both aerobic and anaerobic conditions. In aerobic subsurface soil, the half-life of NDMA has been found to be about 50 to 55 days. Degradation has been found to proceed slightly faster under aerobic conditions than under anaerobic conditions. 

Fomesafen is weakly adsorbed by soils. Its adsorption coefficients (kj are in the range of 0.5 to 3.0 depending on the organic matter content of the soils. Under aerobic conditions fomesafen degrades slowly in soil, its half-life is generally greater than 6 months. 

Analyses of residues in the soils after incubation showed that the persistence of diazinon was considerably shorter in the previously treated soil than in the untreated soil. The half-life value for diazinon in previously treated soil was 1.7 days while in the untreated soil it was 9.9 days. Most of the insecticide added to the previously treated soil was lost within 10 days. Paddy water from the same fields were tested also for diazinon-degrading activity (17). Again water from a rice field treated previously with diazinon inactivated the insecticide more rapidly than did the water from an untreated field. In the water from the previously treated field the insecticide dissipated completely within 3-5 days of incubation after an initial lag of 1-2 days (17, 18). Table II summarizes the results of the study on the stability of diazinon in soil and paddy water. The data indicated clearly that a factor capable of degrading diazinon developed in rice fields of the Institute farm after insecticide applications. The diazinon-degrading factor, found in the diazinon-treated rice fields in the Institute farm, was noticed also in three other locations in the Philippines (19). 

Moderate and Low Persistence. The Green Screen threshold values for moderate persistence are half-life 7-40 days in water and 30-60 days in soil sediment. The water value is in the range of the IJCVirtual Elimination Task Force s moderate persistence value for water. The Green Screen threshold values for low persistence are set to reflect the ability of a chemical to rapidly degrade half-life <30 days in soil or sediment or <7 days in water or ready biodegradability (defined in Appendix 1). 

Soil The half-life in soil was reported to be 2-8 weeks (Hartley and Kidd, 1987). When maleic hydrazide was applied to muck, sand and clay at concentrations of 0.7 and 2.7,1.0 and3.75 and 0.85 and3.4 ppm, 86 and 100, 87 and 100 and47 and67% degradation yields were obtained, respectively (Hoffman et ah, 1962). 

In basic soils mepiquat chloride is nonpersistent and is rapidly degraded. There is a half-life of two years in other soil types, but 86—93% is metabolized within 30 days (10). Breakdown does not appear to be a function of microbial activity. 

A study investigating the breakdown of clopytaUd [1702-17-6] reported half-Hves on different soils of approximately 2—7 weeks in a laboratory incubation (400) it was indicated that carryover was likely to occur in field soil. Picloram degrades and does not accumulate in field soil although low residue levels do persist for several years (401). The half-life for triclopyr [55335-06-3] is reported to be two weeks in two Canadian soils (402), and it has been shown to be rapidly degraded by aqueous photolysis (403). 

Linear alkylbenzenesulfonate showed no deleterious effect on agricultural crops exposed to this material (54,55). Kinetics of biodegradation have been studied in both wastewater treatment systems and natural degradation systems (48,57,58). Studies have concluded that linear alkylbenzenesulfonate does not pose a risk to the environment (50). Linear alkylbenzenesulfonate has a half-life of approximately one day in sewage sludge and natural water sources and a half-life of one to three weeks in soils. Aquatic environmental safety assessment has also shown that the material does not pose a hazard to the aquatic environment (56). 

Studies have found that methyl parathion degrades more rapidly in anaerobic soil than in aerobic soils (Adhya et al. 1981, 1987 Brahmaprakash et al. 1987). An average half-life of 64 days was determined for nonflooded (aerobic) soils compared to an average half-life of 7 days in flooded (anaerobic) soils (Adhya et al. 1987). In experiments with " C-labeled methyl parathion, 35% of the labeled compound was recovered from nonflooded soil after 28 days, compared with 9% recovered from flooded soil (Brahmaprakash et al. 1987). 

Most of the trichloroethylene used in the United States is released into the atmosphere by evaporation primarily from degreasing operations. Once in the atmosphere, the dominant trichloroethylene degradation process is reaction with hydroxyl radicals the estimated half-life for this process is approximately 7 days. This relatively short half-life indicates that trichloroethylene is not a persistent atmospheric compound. Most trichloroethylene deposited in surface waters or on soil surfaces volatilizes into the atmosphere, although its high mobility in soil may result in substantial percolation to subsurface regions before volatilization can occur. In these subsurface environments, trichloroethylene is only slowly degraded and may be relatively persistent. 

---