Many sites have groundwater impacts associated with soils of varying permeability.  When a source of groundwater contamination is isolated in a lower permeability zone, diffusion from low permeability soils can be the dominant long-term transport process into higher permeability soils.  As many in situ remedies are difficult to properly implement in lower permeability soils, diffusive transport should be accounted for when considering in situ remedial options relative to the site goals.

What drives diffusive processes, is the magnitude of the impact within the lower permeability soils and the distance between that source and the soil permeability interface (in essence: the concentration gradient). The further something has to travel via diffusion (assuming it is transported or diluted within the high permeability zone), the lower the mass flux will be and more importantly, the lower the resultant aqueous concentration will be.

A simple analogy would be like standing next to a person with strong cologne on a windy day.

The closer the person is to you, the stronger the smell will be. If you were in a sealed room with the person, the distance wouldn’t matter because eventually the odor in the room would be the same throughout. The groundwater flow in the higher permeability soils is like the wind and the person’s perfume is the source of contamination. The greater the distance between the source (cologne) and the flow in the permeable unit(wind), the lower the concentration will be in the flowing groundwater.

As the amount of mass lost via diffusion is usually very small when compared to the total mass of the source contaminant, this process can adversely impact groundwater concentrations in the more permeable unit for an unacceptable amount of time relative to state and federal regulations.

Our next blog post will discuss on of the in situ remedial strategies that could be applicable for reducing the extent of diffusive transport and how the magnitude of impacts can be of significant importance.