Half-lives, degradation

Soil. Loehr and Matthews (1992) studied the degradation of phenol in different soils under aerobic conditions. In a slightly basic sandy loam (3.25% organic matter) and in acidic clay soil (<1.0% organic matter), the resultant degradation half-lives were 4.1 and 23 d, respectively. 

Sediment degradation half-lives in sediment 20-200 yr for all homologues (estimated, Suzuki et al. 2000). 

Ground water t,/2 = 96-5040 h, based on estimated unacclimated aqueous aerobic and anaerobic degradation half-lives (Howard et al. 1991) reported t,/2 < 180 d (Bottoni Funari 1992)... 

Solutions of nimodipine are incompatible with some plastics, including polyvinyl chloride. The solutions are also light-sensitive, and should be protected from light [3]. The degradation half-lives of nimodipine are 16 and 56 h following exposure of aqueous solution of the drug to UV light... 

Hydrolytically unstable Maintaining exposure concentrations. Toxicity of breakdown products. Comparison of degradation half-lives to the exposure regimen used in testing. Classification requires expert judgement, should be based on measured concentrations, and needs to address the toxicity of significant breakdown products. 

In contrast to measuring 0C or values, the measurement of pesticide degradation half-lives (t. / ) in soils is a much more difficult and time-consuming task. Hence, the spatial variations in t. trs could not be assessed in as much detail as we did with OC and K. . Metolachlor and aldicarb TTR half-lives were measured in a selected number of soil samples collected from the Georgia field site. Soil samples were selected to represent the three major soil series present at the site (Clarendon, Ardilla, and Tifton) and the four major morphologic soil horizons (0-20, 25-46, 48-63, and 94-107 cm) within the crop root zone. Soil samples collected from 4 depths at 10 sites were used to characterize aldicarb TTR degradation rates. Metolachlor half-lives were measured in soils taken from four depths at one site for each soil series and also in samples collected at two depths (0-20 cm and 94-107 cm) at 6 sites. 

Table I. Variations in Metolachlor Degradation Half-Lives (days) Measured in Soils from the Georgia Site...

The measured variability in aldicarb TTR degradation half-lives are summarized in Table II. Data shown for Clarendon soil were averaged over 4 sites, and the values for Ardilla and Tifton soils are averages for 3 sites. Among the sampling sites, J-4 and J-18 were the farthest apart (about 233 m), whereas the sites 1-15 and K-15 were the closest (about 30 m). The greatest... 

Table III is a summary of mass balances performed for each chemical and degradation half-lives at 85°F, in open system, and using sandy loam soil of 14 percent and 22 percent moisture.

Table III. Mass Balance and Degradation Half-lives for the...

Pesticides Pesticide Properties in the Environment The efficacy and environmental impact of these compounds depends on their distribution and persistence. This monograph contains data on > 340 active ingredients including, aqueous solubility, vapor pressure, and pK. Soil behavior is defined by sorption coefficients and degradation half-lives. In most cases a range of referenced values is sited along with selected values. 

In the calculations presented, heptachlor is degraded into heptachlorepoxide in all environmental media (with a fraction of formation ff = 0.9), aldrin is degraded into dieldrin in all environmental media (jf = 0.9), too, whereas DDT degrades into DDE in the atmosphere (jf = 0.9), and in equal parts (hothff= 0.5) into DDE and DDD in all the other media. Degradation half-lives were extracted as experimental values from the literature [37,38] where possible, or calculated with QSAR software (especially for OH reactions) [39]. 

Figure 17.1.2. The ranges in degradation half-lives for the chlorinated aliphatic hydrocarbons, F-11 and F-113, and ketones in surface water, groundwater, and soil (data from Howard et al. " ).

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