Abstract
Stagnant zones occur in saturated and unsaturated flow systems, a product of physical and chemical processes, such as heterogeneous and unconnected flow paths (including dead-end pores and pathways), water held by capillary forces at particle surfaces, secondary mineral formation, cementation processes, or trapped gasses. These zones can serve as a sink or source of solutes and reaction products. Several modeling programs have the capability to simulate these stagnant zones in reactive transport, such as PHREEQC, which uses twined mobile and immobile columns. The distribution and transfer of solutes between the mobile and immobile zones are controlled by parameters that may be scaled by the user to match field-observed conditions. Scaling factors are commonly used in geochemical modeling to resolve differences between laboratory and field-observed behavior. However, applying scaling in the mobile-immobile system is more dynamic because there is diffusion-controlled movement of solutes between the mobile and immobile columns. There are advantages to this method for geochemical modeling. Foremost, no mass is discounted or lost in the system, it is maintained and tracked in stagnant system. As such, measured reaction rates and physical parameters can be more easily honored in the model. Additionally, reaction products removed by the stagnant system can diffuse into the mobile system, which more closely simulates what is observed in the field. Disadvantages of this scaling method are similar to those with all scaling factors in that relating the scaling factor to real-world physical, hydraulic, and geochemical properties and processes often relies on observed field behavior.