Fate Of Soil Contaminants
Contaminants in soil may be present in solid, liquid, or gaseous phases. When liquids are released, they move downward through the soil and may fill pore spaces, absorb onto mineral or organic surfaces, dissolve into soil water, or volatilize into the soil atmosphere. Most hydrocarbons exist in more than one phase in the soil horizon. Insoluble soil contaminants travel downward through the unsaturated, or vadose zone to reach the saturated zone, or water table, where voids between soil particles are filled with fluid. Their behavior in the saturated zone depends on their density. Light compounds float on the water table, while denser compounds may sink. Although many hydrocarbon compounds are not very soluble in water, even low levels of dissolved contaminants may produce unsafe or unacceptable groundwater quality. Other contaminants such as inorganic salts, nitrate fertilizers for example, are highly soluble and move rapidly through the soil environment. Metals like lead, mercury, and arsenic demonstrate a range of behaviors; some chemically bind to soil particles and are thus, immobile, while others dissolve in water and are transported widely.
Pore water containing dissolved contaminants, called leachate, is transported by groundwater flow, which moves both horizontally and vertically away from the contaminant source. Point source groundwater pollution often forms a three-dimensional plume that decreases in concentration with distance from the source and time since introduction of the contaminant. A portion of the contaminant, called the residual, is left behind as ground-water flow passes, resulting in a longer-term contamination of the soil after the contaminant plume has receded. If the groundwater velocity is fast, hundreds of feet per year, the zone of contamination may spread quickly, potentially affecting wells, surface water and plants that extract the contaminated water in a wide area.
Over years or decades, especially in sandy and other porous soils, groundwater contaminants and leachate may be transported over distances of miles, resulting in a situation that is extremely difficult and expensive to remedy. In such cases, immediate action is needed to contain and cleanup the contamination. However, if the soil is largely comprised of fine-grained silts and clays, contaminants will spread slowly. Comprehensive understanding of site-specific factors is important in evaluating the extent of soil contamination, and in selection of an effective cleanup strategy.