The North Tabriz Fault, a barrier to groundwater flow in an alluvial aquifer northwest of Tabriz, Iran

Abstract

Geological structures, specifically faults, may influence the permeability of geological units and so can be an important factor in the movement of groundwater and hydrogeological processes. This study is an investigation of the local influence of the North Tabriz Fault on the hydrogeology of the adjacent unconfined alluvial aquifer. The study area is located northwest of the city of Tabriz, Iran, on the North Tabriz fault zone. The North Tabriz Fault is a prominent tectonic structure with strike-slip right-lateral kinematics and is considered as one of the most active faults in Iran. This study shows that the fault has a major influence on the hydrogeological characteristics of the local aquifers. Sharp changes in the position of aquifer bedrock and aquifer thickness over relatively short distances (<200 m) suggest a near-vertical dip for the fault evidencing its reverse movement. The fault displacement has reduced the saturated aquifer thickness in the northern sector because the northern block (hanging wall) is thrust over the southern block (foot wall). As a result of barrier behavior prohibiting flow across it, the fault has channelized groundwater roughly parallel to its strike. Sharp changes of EC and chloride values over short distances across the fault show that there is no significant leakage or groundwater mixing across the barrier. A Chadha diagram indicates a Ca–Mg–Cl water type for the majority of northern water samples and Ca–Mg–HCO3 type for all southern samples, without any transitional modes. From Gibbs plots, rock type determines the groundwater hydrochemistry. Abundant Ca + Mg in all of the southern water samples are interpreted as related to carbonate weathering and dissolution, increasing SO4 more rapidly than Cl there. Gypsiferous marl and sandstone are prominent throughout the study area, especially in the southern part, and thus can be the sources of excess SO4. As a result of this study, subsurface structural evidences such as hydraulic head patterns, flow path variations and changes in groundwater hydrochemical properties across the fault are useful tools to understand the influence of fault zones on aquifer characteristics and the hydrogeological behavior of these zones.