Here is what one source of info says: 

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Breakdown of the Chemical in Soil and Groundwater

Simazine is persistent and does not adsorb strongly to soil particles (Koc = 138 g/ml). In combination with a lengthy soil half-life, these factors suggest that simazine is likely to contaminate groundwater. Its tendency to leach is limited by its low solubility in water (6.2 ug/ml). Soil half-lives of 36 to 234 days on sandy loam, 16.3 to 25.5 weeks on loamy sand and silt loam, and 75 days on an unspecified soil type have been reported. Residual activity remains for 2-7 months (2-4 kg/ha) after application. Simazine does adsorb to clays and mucks (26, 27, 29).

Simazine is subject to decomposition by ultraviolet radiation, but this effect is small under normal field conditions. Loss from volatilization is also insignificant. In soils, microbial activity probably accounts for decomposition of a significant amount of simazine. Simazine has little if any lateral movement in soil, but can be washed along with soil particles (29).