Groundwater flow and age in topography-driven groundwater flow systems with geological barriers

Faults in hydrogeological systems can act as conduits or barriers for groundwater flow. However, the effect of faults on groundwater flow and age has not been widely studied, particularly in topography-driven flow systems (i.e., Tóthian flow systems). This study established Tóthian models through HydroGeoSphere and compared age distributions between models with and without fault zones. Hydraulic conductivity of the aquifer was set at 1 m/d, whereas that of the fault zones (Kf) was varied at 0.001–0.75 m/d to simulate barrier effect and at 5–20 m/d to mimic conduit effect. Simple (aquifer thickness 100 m) and complex Tóthian models (aquifer thickness $\ge$ 1500 m) were both considered. Our results show that, when the fault zones act as conduits, the groundwater is slightly younger than it would be without the fault zones, regardless of simple or complex Tóthian models. When the fault zones act as barriers, in most simple Tóthian models, groundwater cannot flow across the fault zones, with new local flow systems forming on both sides. Groundwater age thus increases upstream but decreases downstream of the fault zones. In the other simple Tóthian models (Kf at 0.25–0.75 m/d), groundwater can flow across the fault zones at some depths. Age changes are more pronounced in parts with flow parallel to the fault zones than those in other parts. In all complex Tóthian models with fault zones as barriers, new local and intermediate flow systems are formed upstream and downstream of the fault zones. Age changes mainly occur in deep parts of the aquifer.