From the National Pesticide Information Center, run by Oregon State University under a cooperative agreement with the US EPA:
"If permethrin gets into the soil, it is broken down by microorganisms. Sunlight may also break down permethrin on the soil surface and on the surface of water. Permethrin does not mix well with water. When permethrin gets into surface water like lakes or streams, it sticks very strongly to sediment and can stay there for more than a year. Since permethrin sticks to sediment and does not mix well with water, it won't usually contaminate groundwater. Permethrin does not evaporate very easily when it is applied to surfaces. Permethrin was applied indoors near a window in an experiment where it was exposed to daylight. After 20 days, 60% of the permethrin that was applied was still on the surface.
If permethrin is applied to plants, it may stay on the leaves for between 1 and 3 weeks. Scientists applied permethrin to soil and then planted sugar beets, wheat, lettuce and cotton in the soil. Scientists found trace amounts of the permethrin residue in the edible parts of the plants at 30 and 120 days after planting. Trace amounts of permethrin have been found in foods including bananas, collard greens, squash and watermelon. However, less than 1% of the more than 1,700 food samples tested had detectable levels of permethrin."
Bottom line: permethrin is relatively persistent in the environment. A Good Thing if you're needing prolonged pest control. A Bad Thing if you're worried about human exposure. So, be careful about where you apply it.
In contrast, the half-life of the herbicide glyphosate (Round-Up) in the soil is about 24 hours. So, within 10 days of application, less than 0.1% of the active ingredient remains. It's one of the safest herbicides there is.
-science guy Paul