Injana Formation is the most extended geological formation in Salahaddin Governorate/ Iraq. About 10% of the studied area is covered by the outcrops of the formation as a recharge area. The formation is a subsurface within the unsaturated zone in 5% of the total studied area, while it exists within the saturated zone in about 85%; it is a major confined groundwater aquifer. Therefore, the hydrogeological system of the layers needs to be re-evaluated to describe the successions of aquifers and confining layers and their relation with each other.
The lithology, depths, water table, saturated thickness, hydraulic characteristics of the aquifers, and the lateral and vertical variations of these characteristics were adopted to classify the hydrogeological system. The lithological composition is mainly composed of alternating successions of claystone, siltstone and sandstone with some differentiation within the studied area.
The Quaternary and, occasionally, the Mukdadiya Formations are dry or of secondary aquifer, except in limited areas of the governorate. Injana Formation represents the major upper aquifer in the area, especially in the western bank of Tigris River. The outcrops of the formation are adjacent to Makhul and Hamrin anticlines; while Al-Tharthar valley represents a recharge area for the groundwater. In the remaining parts of the studied area, the formation represents the main deeper of a confined to semi-confined groundwater aquifer.
The general direction of the groundwater movement in this hydrogeological system is towards the discharge area represented by Tigris River and Tharthar Lake, which is compatible with the topographic slope. The formation is classified as a multi-layer aquifer hydrogeological system.
