Groundwater Contamination
Groundwater contamination can originate on the surface of the ground, in the ground above the water table, or also in the ground below the water table. There are several types of activities that can lead to groundwater contamination at each level. Where a contaminant originates is a factor that affects its actual impact on ground-water quality. For example, if a contaminant leaks on the surface of the ground or is injected into the ground above the water table, it may then have to move through numerous layers of soil and other underlying materials before it is able to reach the groundwater. There are many sources of groundwater contamination.
Often the biggest groundwater contamination threats are related to the way a well is constructed. If the annulus of a well is not sealed properly it can provide a path for contaminants to move from the ground surface to the aquifer. Older wells can become contaminated or can cause good wells to become contaminated. This occurs through the seepage of contaminated surface water or poor quality, shallower groundwater into a deeper good quality aquifer.
As the contaminant moves through these layers, a number of processes are taking place (e.g., filtration, dilution, oxidation, biological decay) that can lessen the eventual impact of the substance once it eventually reaches the groundwater. The effectiveness of these processes is also affected by both the distance between the groundwater and where the contaminant is introduced as well as the amount of time it takes the substance to reach the groundwater.
If a contaminant is introduced directly into the area below the water table, the primary process that affects the impact of the contaminant is dilution by the surrounding groundwater. In comparison with streams or rivers, groundwater tends to move very slowly and with extremely little turbulence. Therefore, once the contaminant reaches the groundwater, little dilution or dispersion generally occurs. Instead, the contaminant forms a concentrated plume that flows along the same path as the groundwater. Among the factors that determine the form, size, and rate of movement of the contaminant plume are the amount and type of contaminant as well as the speed of ground-water movement. Because groundwater contamination is hidden from view, it can go undetected for years until the supply is tapped for use.
Substances that can contaminate groundwater can basically be divided into two categories: substances that occur naturally and substances introduced or produced by man’s activities. Substances that occur naturally include minerals such as iron, selenium, and calcium. Substances resulting from man’s activities include synthetic organic chemicals and hydrocarbons (e.g., pesticides, solvents, petroleum products); landfill leachates (liquids that have dripped through the landfill and then carry dissolved substances from the waste materials), containing such substances as heavy metals and organic decomposition products; bacteria; salt; and viruses. A significant number of today’s groundwater contamination problems begin with man’s activities and can be introduced into groundwater from a variety of sources.