Dissolution of organic liquids in groundwater

Abstract

Among the most pervasive and persistent sources of groundwater contamination are organic liquids that enter the subsurface from leaks, spills, and improper disposal. Organic liquids with specific gravity greater than unity are known as DNAPLs (dense non-aqueous phase liquids) and these are especially troublesome because they tend to sink well below the watertable. Remediation technologies for subsurface source zones containing DNAPL are not yet able to achieve levels of mass removal sufficient to restore the groundwater or to meet regulatory requirements. Therefore, it is natural to inquire about the benefit of partial mass removal. This subject has been and is being addressed by many researchers for single-component DNAPLs. This dissertation considers the dissolution of multi-component DNAPLs; a process that is inherently unsteady. A mathematical technique called TMASST (transient multiple analytical source superposition technique) was developed to calculate the unsteady rate of dissolution of both single and multiple component DNAPLs. Source zones are thought to be comprised of many subzones that contain DNAPL. The number, location, and geometry of the subzones are not known. This problem is circumvented by synthesizing a new mass transfer rate coefficient that applies to the source zone as a whole. This field-scale parameter is much smaller in magnitude than the values of mass transfer rate coefficient measured in laboratory experiments. Calculations show that multi-component DNAPLs dissolve more slowly than single-component DNAPLs. Individual component solubilities and the type of mixture are the important DNAPL characteristics affecting the source longevity. Sources are estimated to persist for many decades and the dissolved concentrations of DNAPL components in waters passing through the source zone are calculated to remain well above the regulatory limits until virtually all the DNAPL has been dissolved away.