**5.2 Herbicides**

Herbicides are the major class of pesticides to control weeds. Little attention is paid to herbicides as a source of pollutants; mainly because with a few exception; most herbicides have not appreciable mammalian toxicity. Among toxic herbicides are paraquat (LD50=125 mg/kg) and dinoseb (LD50=58 mg/kg); however widely used herbicides including 2,4-D and glyphosate are not highly toxic to mammals. On the other hand groups of herbicides that have potential to persist in soil and enter surface water include triazines, sulfonylureas, phenylureas and uracils. Laboratory experiments have shown that among four triazines; prometryn and terbutylazine half-lives were 263 and 366 days in ground water respectively. The half lives of simazine and atrazine were shorter than prometryn and terbutylazine (Navarro et al. 2004).

Sulfonylureas are high potent herbicides group effective at very low dose (10-15 g/ha), for that reason persistent herbicides from previously sprayed farms may damage the next crop. These herbicides are able to penetrate into deeper layers of the soil profile, where they have a relatively high persistence. A number of sulfonylureas were detected in wetland sediments. Etametsulforun methyl, sulfosulforun and metsulforun-methyl were determined in wetland sediments with mean concentration ranging from 1.2 to 10.0 µg kg-1 (Degenhardt et al. 2010). According to Cessna et al (2006) a half-life of 84 days was observed for metsulforun-mathyl in farm dugouts. Residues of 10 herbicides were detected in prairie farm dugouts. 2,4-D was the most frequent with median concentration 0.05 µg L-1(Cessna & Elliot, 2004).

Based on these studies, herbicides have different tendency for binding to soil colloids and so have different movement ability.

#### **5.3 Fungicides**

Fungicides are substances that destroy or inhibit the growth of fungi. Fungicides are used in agriculture and industry. Early fungicides were organic derivatives of metals such as mercury. Organomercury fungicides were widely used as seed dressing to control diseases of cereals. Although mercury content of these fungicides formulation were less than 5%, the main concern is the side effects of residues remaining in the environment long enough to enter soil and water. Both inorganic and organic compounds of mercury are toxic, however organic compounds are more lipophilic than inorganic and are liable to adsorption by soil colloids and storage in fat depot. Bioconcentration factor up to 100000 times is reported for the methyl mercury content in fish (USEPA, 1980). Dithiocarbamates (e.g. mancozeb, thiram, zineb and maneb) are the first synthetic organic fungicides. Some fungicides are toxic to aquatic organisms. Maneb is highly toxic to fish and triadimefon is highly toxic to crustaceans. Dithiocarbamate fungicides have low persistence. Among high persistent group of fungicides are triazoles (penconazole, myclobutanil and flusilazole), carboximides (boscalid) and pirimidines (fenarimol) (Wightwick et al., 2010).
