Ritardation Factor on mass transport

???
Hi! Can someone help me, please?

I've to simulate a mass transport of a contaminant (perchloroethylene) in a phreatic acquifer.
I need to simulate the effect of "NATURAL ATTENUATION" which, considering long period, tends to reduce the concentration of PCE in water.
My question is: what is the mass transport coefficient t6hat considers the effect of natural attenuation? Is it the Retardation factor? I sow that the ritardation factor is calculated by Henry, Langmuir, Freundlich isotherms but, to calculate the R factor I need coefficients like"K" or "b1,b2" or "k1,k2". Where can I find in lecterature these coefficients? What are their meants?

Thanks
Manuela Girotto

I hope this is not a too late answer

in Hydrogeology an groundwater Modelling by Neven Kresic second edition 2006 page 485 (CRC Press), you could read :
"As defined in the USEPA directive, on applying monitored natural attenuation (MNA) at contaminated sites (...),natural attenuation processes in general include a variety of physical, chemical, or biological processes that, under favorable conditions, act without human intervention to reduce the mass, toxicity, mobility, volume, or concentration of contaminants in soil or groundwater.  These [i]in-situ [/i] processes include biodegradation; dispersion; dilution;sorption;volatization; radioactive decay; and chemical or biological stabilization , transformation, or destruction of contaminants."

by the way in the same "good" book page 476 "(...) Because of sorption, the contaminant movement in groundwater is slowed down relative to the average groundwater velocity. This effect is callaed retardation (...)

For Modelling natural attenuation of chlorinated solvent you can use your  search Engine on BIOCHLOR

My colleague Daniel find this two interresting links

[color=blue]doctorflood.rice.edu/envi518/Handouts/Lisbon/06_Biochlor.ppt
www.afcee.brooks.af.mil/products/techtrans/download/biochlor_database.pdf[/color]

we hope this answer could help you