Groundwater transport is the movement of substances, particularly contaminants, in underground water. The zone of groundwater in which contaminants are concentrated is called a groundwater plume. Plumes have various shapes, sizes, and transport rates. The different factors that affect groundwater transport include the aquifer geology and hydrology as well as the physical, chemical, biological, and radiological properties of the contaminants in the groundwater.
Aquifers are underground rock and soil formations that are saturated with water. Groundwater moves through the pores between soil and rock particles and through cracks, crevices, and fractures in hard bedrock. Contaminant movement is greatly affected by the types of rocks and soils that are present. Important geological factors include the texture and size of the particles and the physical characteristics of the pores and other spaces through which groundwater travels.
Permeability, or hydraulic conductivity, is the ease with which groundwater moves through underground formations. Large, loosely-packed particles, such as gravels and heavily fractured bedrock are more permeable than clays, tightly packed silt particles, and solid rock. The size, distribution, and interconnectedness of the open spaces largely determine how easily groundwater and any contaminants it contains can migrate from place to place.
Chemical properties are also important factors in groundwater transport. Some contaminants dissolve in water, while others are insoluble. Dissolved substances actually become part of the groundwater and are not easily separated from it. Insoluble substances behave much more independently. They may be very dense and not easily carried along. They also may become lodged within pores or snagged on jagged particle edges. The chemical nature of the contaminants and the temperature and pH of the groundwater largely determine which substances are soluble or insoluble in the water.
Dissolved contaminants, or solutes, are carried along with the groundwater as it flows. This transport process is called advection. The contaminants basically move at the same flow rate and in the same direction as the groundwater. Advectional groundwater transport is most common in highly permeable aquifers with large networks of interconnected pores or spaces.
Solutes become dispersed throughout groundwater due to mechanical mixing and molecular diffusion. Mechanical mixing occurs as a natural consequence of the movement through and around soil and rock particles. Molecular diffusion is mixing that happens at the molecular level between some compounds and water. Dispersion gradually dilutes contaminant concentrations and creates an elliptically shaped plume in which contaminants are highly concentrated near the trailing edge of the plume and diluted toward the leading edge.
Some contaminants, such as petroleum products, do not readily dissolve in groundwater. Instead, they float atop the water table or sink to the bottom of the aquifer, depending on their density. They may undergo some dispersion, but their plumes move much more slowly than the plumes of solutes and contaminants that easily disperse through the groundwater.
Effects that slow down groundwater transport are called retardation factors. A major retardation factor is adsorption. This occurs when contaminants become attached or stuck to soil or rock particles due to electrostatic attractive forces. Compounds that are not easily dissolved or dispersed in groundwater readily undergo adsorption. Other retardation factors include pore friction and the filtering effect that occurs when solid, insoluble contaminants quit moving because they become lodged within pores or snagged by jagged soil or rock particles.
Finally, contaminants can undergo physical, chemical, biological, or radiological transformations that change their groundwater transport rates. A contaminant may change phases, for example, from a liquid to a gas. Chemical reactions between contaminants and natural biological and radiological processes can also convert one compound to another. The newly created compounds may move faster or slower than the previous compounds.
