Darcy’s Law Groundwater Velocity and Plume Arrival Time

In his 1999 article titled “On Misuse of the Simplest Transport Model”¹ Ernesto Baca explained why it is not a good idea to calculate the rate of travel of a contaminant plume front, or its time of arrival at a receptor location, using the Darcy’s Law groundwater velocity. The missing factor is dispersion.

Baca noted that the Darcy’s Law velocity equation: 

V = Ki/n

only accounts for advection of the dissolved contaminant. He stated that, by measuring the plume from the source location to the far extent of the tip of the plume, one is implicitly considering both advection and dispersion. Therefore, for a relatively mobile dissolved contaminant being transported by seeping groundwater, the larger the dispersive effects, the more your estimate of arrival time will be off.

The toe of the contaminant plume will arrive earlier (i.e. in less time) that you would predict based on a Darcy’s Law groundwater velocity estimate.

The process is depicted in this video:

The video file is available for download. The video presents an analysis of a relatively mobile (slightly adsorbed and retarded) chemical, is for illustrative purposes only, and does not represent McLane Environmental’s analysis of, nor conclusions regarding, any particular real site or plume.

¹ Baca, E. 1999. On the Misuse of the Simplest Transport Model. Groundwater v 37, no 4, Jul-Aug 1999.