half-life time DT50
soil-water partition coefficient KOC
retardation factor R
octanol-water partition coefficient KOW
The organic compounds in soil undergo a lot of change. Most pesticides break down or degrade over time as a result of several chemical and microbiological reactions in soils. Some are broken down by sunlight. These processes result in the final degradation of the compound into the mineral compounds CO2, H2O, HCl, SO2, etc. As they degrade, some pesticides produce intermediate substances (metabolites) whose biological activity may also have environmental significance.
Compounds with an extremely long degradation time are considered persistent. Persistent compounds disperse into the environment without undergoing any changes.
The value of the degradation is given by half-life DT50. Half-life DT50 is a measure of the amount of time it takes for 50 percent of the parent compound to disappear from soil or water by transformation. The important degradation processes are biological processes (biodegradation) and physicochemical processes (hydrolysis, photolysis, etc.).
The major degradation products are listed in Appendix 3. Additional remarks are made for the degradation products that are expected to have a significant influence on the environment.
Biodegradation is the transformation of a substance by micro-organisms. Under environmental conditions, biodegradation can be affected by a number of factors, including the presence of oxygen (aerobic/anaerobic conditions) and nutrients, the population size of the required microorganisms, and the micro-organisms' adaptation.
During hydrolysis a compound is split by contact with water, undergoing a chemical reaction in which a part of the molecule of the reacting substance is replaced by an OH group. This process is strongly affected by the acidity of the environment (pH).
Photolysis is the breaking down of a compound as a direct result of irradiation.
The mobility of a pesticide in water indicates the water's solubility and pesticide sorption (KOC), or retardation factor. When a pesticide enters soil, some of it adheres to soil particles (particularly organic matter) through a process called sorption, and some dissolves and mixes with the soil water.
Classification of degradability in soil (biological and physicochemical transformations)
|>180||Very slightly degradable|
The organic compounds that dissolve in groundwater move more slowly than groundwater because of sorption to the soil particles. The solubility of a pesticide and its sorption in soil are inversely related: i.e. increased solubility results in less sorption.
Pesticide solubility is an important factor in waste disposal. Solubility can indicate the maximum amount of pesticide in solution in any accidentally contaminated water. The solubility of a compound in water is given in mg/L at 20°C. Compounds with high degrees of solubility are expected to leach into groundwater.
Soil-water partition coefficient KOC
The partition coefficient KOC is defined as the ratio of pesticide concentration in a state of sorption (i.e. adhered to soil particles) and the solution phase (i.e. dissolved in the soil water). Thus, for a given amount of pesticide, the smaller the Koc value, the greater the concentration of the pesticide in solution. Pesticides with a small Koc value are more likely to leach into groundwater than those with a large Koc value.
Sorption for a given pesticide is greater in soils with a higher organic matter content. Thus, pesticide leaching is thought to be slower in those soils than in soils lower in organic matter.
Because of the large range of Koc values, use the logarithm of Koc. A compound's mobility in soil is classified according to Table C.
The Koc value is used to determine the parameter known as retardation factor R. This factor indicates the delay of the pesticide transport rate in comparison with the water rate.
Classification of solubility in water
Classification of mobility
Bioaccumulation indicates the tendency of a compound to accumulate in organisms. The most useful index for quantifying pesticide bioaccumulation is the partition coefficient KOW.
Octanol-water partition coefficient KOW
The octanol-water partition coefficient KOW is defined as the ratio of the pesticide concentration in two unmixed liquids, octanol and water. The compounds with a high KOW value (i.e. readily soluble in octanol and less soluble in water) accumulate in organisms.
|Impact mainly because of high mobility in groundwater (risks owing to spreading)|
|Impact mainly because of bioaccumulation, toxicity, high persistency and immobile character (risks owing to direct contact with pesticide or consumption of contaminated fruits or vegetables)|
|Polyaromatic Hydrocarbons (PAHs)|
|Polybrominated biphenyls (PBB)|
|Impact because of degradation products|
|No dominant impact|
|Dinoseb and dinoseb salts|
|Polychlorinated biphenyls (PCB)|
|Polychlorinated terphenyls (PCT)|