Contaminated groundwater can be a significant environmental challenge. How contaminants move in the groundwater and how to address or remediate groundwater is a common topic of seminars, webinars, and articles (you can view our series of webinars on assessment and remediation of groundwater).
While not an uncommon issue, few environmental professionals are fluent in their knowledge of groundwater. Personally, my environmental undergraduate degree barely touched on groundwater, let alone groundwater contamination. Many of my colleagues, on the other hand, are groundwater experts.
Ask the Groundwater Experts
Recently, I asked two of our experts in groundwater, Dr. Michael Sklash, P.Eng,. and Dr. Fatemeh Vakili (see their bios) to provide some basic groundwater awareness.
Basic Groundwater Understanding
Is groundwater a “stream” of water beneath the surface?
Only in rare cases does groundwater flow like a stream. Karst areas, areas of limestone with caves, can have streams. In most cases, groundwater flows though pores between soil particles, between minerals in some rock types, and through fractures in rocks and clayey soils (see Geology and Groundwater Basics).
How deep is groundwater in Ontario, and does it vary?
The depth to groundwater in Ontario varies mostly due to geology and topography (world-wide, climate also is a factor). Groundwater, generally, is deepest at the top of hills and shallowest in stream valleys (where the groundwater level may be above ground level).
How quickly does a spill on the surface reach groundwater?
The time required for a release to reach the groundwater depends on the depth of the water table (top of the groundwater), the geology (like sandy or clayey soil), the release rate, the duration of the release, and the type of release. For example, a small, brief release of Bunker C oil in an area where the water table is deep and the soil is clayey may never reach the water table. A large release of a less-viscous liquid like trichloroethylene (TCE) in an area where the water table is shallow and the soil is sandy can reach the water table very quickly (see Chemical Behaviour in Groundwater).
If groundwater is contaminated, do contaminants move at the same rate as the groundwater?
Some contaminants, like chloride from road salt, move at the same velocity as the groundwater. Some contaminants, like gasoline, move more slowly than groundwater as they tend to be adsorbed into soil and organic matter. For example, the four common contaminants associated with gasoline releases (benzene, toluene, ethylbenzene, and xylenes) all move slower than the groundwater. They also move at different rates because of their specific chemistries.
Do contaminants sink, float, or mix in the groundwater?
Some chemicals, like chloride, are miscible (readily dissolve) in groundwater. Other chemicals are immiscible in groundwater to varying degrees. Some of these are less dense than groundwater (like gasoline) and, hence, float on groundwater. These are commonly known as light non-aqueous liquids (LNAPLs). Some chemicals are denser than, and sink in, groundwater (like TCE). These are known as dense non-aqueous liquids (DNAPLS). However, these chemicals do dissolve in groundwater to some extent and, once dissolved in groundwater, these chemicals will not float or sink but rather move with the groundwater.
Finally, what do I do if groundwater on my site is contaminated?
The most important thing is to “put your arms around the problem;” that is, determine the nature and extent of the impact before trying to remediate the groundwater (see High Resolution Site Investigation Tools).
Understanding groundwater and contaminant movement in groundwater is an important part of an environmental professional’s career. Hopefully, the information above, including the links to the webinars, will provide you with some helpful groundwater insights.
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