**2. Pesticide properties**

The physical and chemical properties that make pesticides effective for pest control also create a potential for surface and ground-water contamination. The fate of a pesticide applied to soil depends largely on two of its properties: persistence and adsorption (adsorption is inversely related to solubility). Persistence is the "lasting power" of a pesticide. Most pesticides in the soil break down or "degrade" over time as a result of several chemical and microbiological reactions.

Generally, chemical reactions result in only partial deactivation of pesticides whereas soil microorganisms can completely break down many pesticides to carbon dioxide, water and other inorganic constituents. Some pesticides produce intermediate substances called metabolites as they degrade. The biological activity of these substances may or may not have environmental significance. Microbes decrease rapidly below the root zone so pesticides leached below this depth are less likely to be microbially degraded. However, some pesticides will continue to degrade by chemical reactions after they have left the root zone.

Degradation time is measured in half-life. Half-life refers to the amount of time it takes for a pesticide in soil to reach half the activity level it had at the time of application (i. e., for a pesticide with a half-life of 30 days, 50 percent of the pesticide will have degraded after 30 day). Pesticides having short half-lives often do not persist in the soil long enough to leach into groundwater. Chemicals with long half-lives are highly persistent and have a greater change of leaching into groundwater. To describe potential persistence, scientists classify pesticides as follows:


Pesticides are divided into many classes. The pesticide classes are shown in Table 1(Squibb, 2002). The adsorption process binds pesticides to soil particles, like iron fillings or paper clips stick to a magnet. Adsorption occurs because of the attraction between chemicals and soil particles. Pesticide molecules are positively charged. For example, are attracted to and can bind to negatively charged clay particles. Strongly adsorbed pesticides are less subject to through soil than weakly adsorbed pesticides. On the other hand, strongly adsorbed pesticides are more subject to loss via surface runoff. Factors controlling pesticides

through childhood. For example, differences in the chemical biotransformation capacity of the human fetus and developing child can be both protective and potentially detrimental to normal development Regarding this point, there is little direct information regarding the specific metabolism of xenobiotics, much less pesticides, in children or the fetus. Overriding differences in biotransformation in the fetus is the probable role of maternal metabolism of xenobiotics affecting the level of fetal toxicant exposure. Polymorphisms of maternal phase 1

Deterioration of surface and ground water quality represent the most significant adverse environmental impact associated with agricultural production. Degradation of surface and ground water quality has been identified as the primary concern with respect to the impact of agriculture on the environment. The degradation may occur as a result of the leaching of agricultural chemicals soil or biological organisms to surface waters. In this study, it is

The physical and chemical properties that make pesticides effective for pest control also create a potential for surface and ground-water contamination. The fate of a pesticide applied to soil depends largely on two of its properties: persistence and adsorption (adsorption is inversely related to solubility). Persistence is the "lasting power" of a pesticide. Most pesticides in the soil break down or "degrade" over time as a result of

Generally, chemical reactions result in only partial deactivation of pesticides whereas soil microorganisms can completely break down many pesticides to carbon dioxide, water and other inorganic constituents. Some pesticides produce intermediate substances called metabolites as they degrade. The biological activity of these substances may or may not have environmental significance. Microbes decrease rapidly below the root zone so pesticides leached below this depth are less likely to be microbially degraded. However, some pesticides

Degradation time is measured in half-life. Half-life refers to the amount of time it takes for a pesticide in soil to reach half the activity level it had at the time of application (i. e., for a pesticide with a half-life of 30 days, 50 percent of the pesticide will have degraded after 30 day). Pesticides having short half-lives often do not persist in the soil long enough to leach into groundwater. Chemicals with long half-lives are highly persistent and have a greater change of leaching into groundwater. To describe potential persistence, scientists classify

3. Persistent chemicals Half-life greater than 100 days (Mahler et al.,

Pesticides are divided into many classes. The pesticide classes are shown in Table 1(Squibb, 2002). The adsorption process binds pesticides to soil particles, like iron fillings or paper clips stick to a magnet. Adsorption occurs because of the attraction between chemicals and soil particles. Pesticide molecules are positively charged. For example, are attracted to and can bind to negatively charged clay particles. Strongly adsorbed pesticides are less subject to through soil than weakly adsorbed pesticides. On the other hand, strongly adsorbed pesticides are more subject to loss via surface runoff. Factors controlling pesticides

will continue to degrade by chemical reactions after they have left the root zone.

1. Non-persistent chemicals Half-life less than 30 days 2. Moderately persistent chemicals Half-life of 30 to 100 days

and phase 2 enzymes may play a key role in these exposure events (Garry, 2004).

evaluated the surface and ground water contamination by pesticides.

**2. Pesticide properties** 

pesticides as follows:

1997).

several chemical and microbiological reactions.


Table 1. The Main classes of Pesticides

adsorption include pesticide charge; soil pH, temperature and water content; the presence of previously adsorbed chemicals that have a stronger bond to soil particles; and the amount and type of organic matter present. In general, pesticide adsorption relates inversely to pesticide solubility in water. Highly soluble pesticides are weakly adsorbed and pose a greater threat of groundwater contamination.

Four chemical properties that affect pesticide movement are solubility, adsorption, volatility and degradation.

**Solubility:** The tendency of a pesticide to dissolve in water affects its leaching potential. As water seeps downward through soil, it carries with it water- soluble chemicals. This process is called leaching. Water solubility greater than 30 mg/L has been identified as the flag for a

Pesticides and Their Movement Surface Water and Ground Water 415

pesticides into groundwater. In fine textured soils, macropores, which are principally root

The advantages and disadvantages of pesticide pollution controlling technique are determined by many factors, which require a comprehensive evaluation method adopted in the evaluation of pesticide pollution controlling techniques. But in the average comparison experiment of pesticide pollution controlling techniques, an intuitive analysis and simple nature description of the ecologic, economic factors under the technique effects are made and the analysis results are independent to each other, a systematic and comprehensive evaluation of advantages and disadvantages of candidate techniques compared is difficult to be made. The change and development of these factors themselves is a grey change process. The Grey System Theory put forward by Deng Julong in 1980s is a new method of solving problems of few data, poor information and uncertainty, which takes the systems of "small sample", "poor information" and "uncertainty" with part information known and part unknown as the subject, mainly by finding valuable information through creation and development from the "part" information known, so as to achieve correct description and effective monitoring of rules of the system operation and evolution. At present, the Grey System Theory is widely applied in many scientific fields, but no literatures of pesticide pollution controlling evaluation can be found. During the pesticide pollution controlling evaluation, the information provided by limit system investigation and spatial–temporal detection data is not complete and certain and the vegetable field pesticide controlling system is a grey system. Based on this point, this paper has made a comprehensive comparison of the pesticide pollution controlling techniques in the vegetable production by adopting a relational analysis method of the Grey System Theory. The chemical pesticide provides a necessary guarantee for the output increase, but the pesticide abuse has led to

channels and wormholes, may contribute to the leaching of pesticides.

daily worsen of the ecosystem of agricultural fields (Jiang et al., 2009).

greater potential for groundwater contamination exists.

Pesticides in groundwater are an extremely serious problem. The turnover rate for groundwater may be as a few months, but more commonly years and decades are needed to replace the water in an oxygen-free environment are much less effective in breaking down pesticide chemicals. Extremely slow dilution and breakdown means that the contaminant will be present for a long time. The most critical hazard of contaminated groundwater is the potential for toxic effects in man and domestic animals that drink the water. Contamination of an underground aquifer cannot be easily corrected. Doing so requires drilling purge wells and pumping the water to the surface. Pumping may have to be continued for a long time to remove all the contaminated water. The process is extremely expensive. Preventing groundwater contamination is the best solution to what could be a hazardous situation. Numerous instances of groundwater contaminated with pesticides have been identified. In some cases, small communities have had to use bottled water until other sources of drinking water were developed. At this time, the full extent of groundwater contamination is not known. Pesticides have been found in groundwater in numerous instances, however, and it seems apparent that more instances will be discovered as more and more underground aquifers are sampled and tested for the presence of pesticides. The time it takes for pesticides to travel to groundwater decreases as the depth to groundwater decreases. Generally, the depth to groundwater is least in spring and greatest in late summer. If spring rains come shortly after pesticide application and water table is close to the surface, a

**3.1 Pesticides in groundwater** 

potential leached. Highly soluble pesticides have a tendency to be carried in surface runoff and to be leached from the soil to groundwater. Poorly soluble pesticides applied to soil but not incorporated have a high potential for loss through runoff or erosion.

**Adsorption:** Adsorption refers to the attraction between a chemical and soil particles. Many pesticides do not leach because they are adsorbed, or tightly held by soil particles. Pesticides which are weakly adsorbed will leach in varying degrees depending on their solubility. Adsorption depends not only on the chemical properties of the pesticide but also on the soil type and amount of soil organic matter present. Even strongly adsorbed pesticides can be carried with eroded soil particles in surface runoff. The potential for a pesticide to be adsorbed is called the adsorption partition coefficient (Kd). The lower the partition coefficient is the greater the pesticide leaching potential.

**Volatility:** The tendency of a pesticide to become a gas, similar to the evaporation of water will affect its loss to the atmosphere by volatilization. If a pesticide is highly volatile (has a high vapour pressure) and is not very water soluble, it is likely to be lost to the atmosphere and less will be available for leaching to groundwater. Highly volatile compounds may be come groundwater contaminants, however if they are highly soluble in water. For most pesticides, loss through volatilization is insignificant compared with leaching or surface losses. Volatile pesticides may cause water contamination or other problems from aerial drift. Environmental conditions such as temperature, humidity and wind speed affect volatilization losses. Special surfactants or carriers can be used to reduce volatilization losses.

**Degradation:** A pesticides rate of degradation (persistence) in soil also affects leaching potential. Pesticides are degraded or broken down into other chemical forms by sunlight (photodecomposition) by microorganisms in the soil and by a variety of chemical and physical reactions. The longer the compound lasts before it is broken down that is the longer it persist the longer it is subject to the forces of leaching and runoff (Hairston, 1995).
