Hello Yohannes, this depends on your particular aquifer system configuration. In this case the hydrocarbons may not be adsorbing simply because there isn't enough particular matter. Also, different hydrocarbons have different decay rates so this also needs to be taken into account. Also, there could be different sources of hydrocarbons and, possibly, geogenic sources (depending on the setting). I suggest you study your aquifer system as well as the surface environment to determine: distribution coefficients (Kd), decay processes (depending on the contaminant), and of course travel time (e.g. transmissivities of aquifers). You may also want to use a tool such as BIOSCREEN (US EPA) as a prelminary way of exploring the possible reasons for this contamination persistance. Hope this helps.

Hello Mauricio

Thanks for the explanation !