A wide variety of toxic organic chemicals are applied to many different types of soils in farm fields and in urban settings. The chemicals are readily involved in transfer processes, whereby the chemicals move from place to place, and in transformation processes, whereby the chemicals are degraded into other products. The chemicals must be effective in performing the task they were designed for, such as killing weeds, but they must not be too mobile to leach into groundwater or too long lasting to carryover and injure following crops. Field and laboratory studies of each of these processes are carried out to evaluate the environmental movement and dissipation of these toxic organic chemicals.

Mass-balance investigations of radiolabeled or formulated products are carried out in soil column field lysimeters under natural conditions. Soil, water, air, and plant samples are analyzed for active parent chemicals and their by-products to produce mass-balance profiles of distribution of the compounds over the growing season. The measured values are fed into computer models to provide snapshots of the fate of specific chemicals over time periods of from 1 day to 1 year after application. The information is used to aid in the registration of newly developed chemicals and in supporting the discontinued use of unsafe ones, or in developing application equations to improve the safety of selected chemicals.

Sorption studies using aqueous solutions of organic chemicals, such as herbicides, and selected soil constituents, such as soil organic matter and/or clay minerals, reveal the capacity of selected soil colloids to bind specific organic chemicals. Cationic herbicides and weakly basic chemicals bind in the greatest amounts, followed by low water soluble nonionizable compounds. Highly soluble and weakly acidic chemicals are bound to soil in the lowest amounts. Bonding mechanisms are examined using X-ray diffraction, supercritical fluid extraction (SFE) techniques, and other instrumental methods to confirm the bonding mechanisms involved.

Application to soils of selected sorbents, such as activated carbon, can reduce the biological activity of organic chemicals as was done when toxic polychlorinated biphenyls (PCBs) were illegally dribbled along North Carolina highways by liquid "junkman". Soil incorporation of the chemicals with the carbon reduced bioactivity of the chemicals to plants and other organisms by 85 to 100 percent. Such remediation methods are used world wide to reduce the toxicity of organic contaminants.

Organic chemicals like the herbicide metolachlor can move downward through the soil, but factors such as surface cover have a pronounced effect as shown in this 90 day snapshot profile of chemical distribution. More of the chemical is retained in the soil surface in turf or sod cover than is retained in fallow or mulched areas. Other factors that regulate movement of chemicals include the type of soil, longevity of the chemical, rate of application, amount of water input, and soil temperature. If less than 0.1% of the chemical applied to the surface reaches the 90 cm (3 foot) depth it can contaminate the ground water.