Thanks Brian. Your description of the "activity Kd model" does help.
I have tried using both the Kd approach and a surface complexation model (SCM) to account for uranium attenuation along the flow path. In reference to a SCM, you said the magic words: "if you can parameterize it." I have the age-old problem of desiring more site-specific data to inform my SCM. When I try to incorporate a SCM with the FeOH+.sdat database and our estimates of HFO concentrations in the aquifer, the predicted attenuation of uranium is much lower than we observe in the field. A Kd model gets me closer to what we observe, but as you imply, these models are not ideal, especially when the pH of the system is changing. I will continue to think about this.
Thank you.