Unit: Contaminant Transport in Groundwater Some Quiz Questions Unit: Contaminant Transport in Groundwater

Question for understanding (of the phenomenon of diffusion) For the same boundary conditions, the solution of the equation for transport due to diffusion describes the spreading of the contaminant (in 1D): in the air* of the unsaturated zone in saturated soil In which medium to you expect faster contaminant spreading? Why? *exact solution in dry unsaturated zone – what is different when Sw  0?

Question for understanding & memory When we use the equation for contaminant transport what do we assume about sorption? * the asterisk denotes that the coefficient of hydrodynamic dispersion and advection velocity are divided by the retardation factor, R

Question for understanding With inspiration from the figure on the next slide, draw the diagram of concentration as a function of time at some distance from the contaminant source x  0, without sorption ( = without retardation), due to: (a) advection (b) advection + hydrodynamic dispersion

Validity limits for using the one-term solution of the advection-dispersion equation Comparison of exact (two terms)-approximate solution (one term) of the advection-hydrodynamic dispersion equation: dimensionless time (X axis) – concentration ration C/Co (Y axis), original figure from Sauty (1980), modified

Question for understanding: Compare (4) – (2) (i. e Question for understanding: Compare (4) – (2) (i.e. show with a drawing the expected concentration profile with and without contaminant degradation at rate λ) C/Co 1 t

Question for understanding An aqueous solution of two contaminants has been released in the subsurface and reached the water table. You use the same transport equation to study their spreading in groundwater. From the parameters listed below, mark which ones you expect to be the same and which different in the equation used to describe the spreading of each contaminant: Duration of contaminant release Same  Different  Contaminant concentration at the source Same  Different  Advection velocity Same  Different  Retardation factor Same  Different  Coefficient of diffusion Same  Different  Coefficient of mechanical dispersion Same  Different  Half life Same  Different 

Reference Sauty, J.-P. (1980). An Analysis of Hydrodispersive Transfer in Aquifers, Water Resources Research, 16:1:145-158. Sauty’s figure was reproduced for better quality from Fetter, C.W. (1999). Contaminant Hydrogeology, 2nd edition, MacMillan (Figure 2.9).