GROUNDWATER
examine THe fate and transport of groundwater contaminants before settling legally By Fatemeh Vakili and Michael Sklash
W
hen it comes to the assessment and remediation of impacted groundwater, common questions are “who is responsible?” and “who is going to pay?”. In some cases, the responsible party is known, and the focus can turn to questions of risk. How long will it take for the contamination to reach a water supply well or a surface water body? Will the groundwater contamination adversely affect indoor air quality or impact utilities? In other cases, determining the responsible party is difficult. For example, when you have multiple owners of a business over time, multiple poten-
tial sources in close proximity, or complex subsurface conditions, there may be some uncertainty about when and where the release occurred. In these cases, it is important to know the history of the site, groundwater flow conditions, and how contaminants move and what processes affect their concentrations. Groundwater contamination frequently originates from surface or near-surface releases of chemicals or waste fluids, such as from underground storage tanks or pipes, spills, and landfills. When a contaminant release occurs, the contaminant moves downward through the subsurface soil or rock under gravity. Spread of the contaminant into the subsurface from a release depends on many factors, including: the nature of the release (amount, duration, nature
of the chemical, the subsurface geology, preferred pathways, and the depth to the water table). The water table is the top of the saturated zone where the pores in the soil or rock are filled with water. Other factors to consider include: • Is the release a miscible or immiscible chemical (i.e., does it readily mix with water)? Chemicals such as chloride from landfills and road salt, and nitrate from agriculture, are miscible in water. • If immiscible, is the release a chemical that is less dense than water (a “light nonaqueous phase liquid” or “LNAPL” such as gasoline or oil) or denser than water (a “dense nonaqueous phase liquid” or “DNAPL” such as trichloroethene [TCE])? • Was the release large enough to continued overleaf…
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December 2021 | 17
