Groundwater transport involves the movement of contaminants in groundwater, forming plumes of various shapes and sizes. Factors affecting transport include geology, hydrology, and properties of contaminants. Aquifers are saturated with water and permeability affects movement. Chemical properties also play a role, with some contaminants dissolving while others do not. Advection is the transport of solutes with groundwater, while retardation factors slow down movement. Contaminants can also undergo transformations that affect transport speed.
Groundwater transport is the movement of substances, especially contaminants, in groundwater. The area of groundwater where contaminants concentrate is called the groundwater plume. Plumes come in various shapes, sizes and speeds of transport. The different factors affecting groundwater transport include the geology and hydrology of the aquifer, as well as the physical, chemical, biological and radiological properties of contaminants in groundwater.
Aquifers are underground rocks and soil formations saturated with water. Groundwater moves through pores between soil and rock particles and through cracks, fissures and fractures in hard rock. The movement of contaminants is strongly influenced by the types of rocks and soils 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 compacted particles, such as gravels and heavily fractured bedrock, are more permeable than clays, compacted silt particles, and solid rock. The size, distribution and interconnectedness of open spaces largely determine how easily groundwater and any contaminants it contains can migrate from one location to another.
Chemical properties are also important factors in groundwater transport. Some contaminants dissolve in water, while others are insoluble. In fact, the dissolved substances become part of the aquifer and are not easily separated from it. Insoluble substances behave much more independently. They can be very dense and not easily transported. They can also lodge inside the pores or get caught in the jagged edges of the particles. 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 groundwater as it flows. This transport process is called advection. Contaminants move at essentially the same rate and in the same direction as groundwater. Advetional transport of groundwater is most common in highly permeable aquifers with large networks of interconnected pores or spaces.
Solutes are lost in groundwater due to mechanical mixing and molecular diffusion. Mechanical mixing occurs as a natural consequence of movement through and around soil and rock particles. Molecular diffusion is the mixing that occurs at the molecular level between some compounds and water. Dispersion gradually dilutes contaminant concentrations and creates an elliptically shaped plume where 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 dissolve easily in groundwater. Instead, they either float atop the water table or sink to the bottom of the aquifer, depending on their density. They may experience some dispersal, but their plumes move much more slowly than plumes of solutes and contaminants which are easily dispersed into groundwater.
Effects that slow down groundwater transport are called retardation factors. An important delay factor is adsorption. This occurs when contaminants attach or stick to soil or rock particles due to electrostatic attractive forces. Compounds that are not easily dissolved or dispersed in groundwater are easily adsorbed. Other retarding factors include pore attrition and the filtering effect that occurs when solid and insoluble contaminants stop moving because they lodge within the pores or become entangled in jagged soil or rock particles.
Finally, contaminants can undergo physical, chemical, biological or radiological transformations that modify the transport speed of groundwater. A contaminant can change phase from, for example, a liquid to a gas. Chemical reactions between contaminants and natural biological and radiological processes can also convert one compound into another. Newly created compounds can move faster or slower than previous compounds.
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