Calculating the Hydraulic Characteristics of Bai Hassan Aquifer in Erbil City/Northern Iraq

Iraq lacks surface water resources, so groundwater extraction is prioritized to meet the demands of the industrial, agricultural, and domestic sectors. The chosen location is the Bai Hassan Formation’s major aquifer for the city of Erbil. The hydrogeological system in the study area within the central basin of the Erbil Plain, is considered the largest part of the basin and water reservoir, which is a confined aquifer. The study aims to describe the hydraulic characteristics of the limited characteristics of the aquifer in Erbil, northern Iraq. The wells that were studied are 300 m deep and penetrate the Bai Hassan Formation and Quaternary sediments. The general direction of the flow of Groundwater in the region is generally toward the western regions. Using methods of Jacob straight line and theis recovery methods, the results of the pumping test conducted in 5 wells without observation wells indicated a transmissivity values range between 80.295-299.6 m2 / day and hydraulic conductivity values range between 0.65-3.565 m/day and this reflects the wide horizontal and vertical variation in the layers of the reservoirs. The measured specific capacity for these wells ranges from 40.4 to 79.6 m2/day, and there is an inverse relationship between the specific capacity and well drawdown.


Introduction.
In Erbil city, the Bai Hassan Formation is a major aquifer, and it can be considered as a single aquifer that is not confined when taken on a regional scale, Due to the scarcity of surface water, and due to its importance for agriculture and industry.Most of the basin's settlements rely on groundwater as their primary source of water for their water delivery systems, making groundwater a key water resource in the basin.
Groundwater consumption has expanded significantly recently because of population and economic growth, and too much groundwater extraction is becoming a significant issue in the basin.Systems for managing water resources must be operated to adapt to climate change variability.
The age of the Ba ‫ﻩ‬ Hassan Formation dates to the Pliocene.Formation discoveries appear in the eastern, northwestern, and southwestern parts of the central basin of the Erbil Plain.This formation is the newest in the stratigraphic sequence in Iraq.The thickness of the formation is unknown, as its thickness in the central basin of the Erbil Plain reaches (2000-3000) m [1] The formation consists of alternating concretions and gravel with layers of silt, clay, and sand.The size of the gravel increases in the upper parts of the formation.The rocks of the formation are characterized by their high porosity and permeability, and it is considered one of the best geological units storing water.Lateral and vertical changes in it are common, and the formation is overlain by recent deposits.Numerous studies have been done on the hydraulic properties of the groundwater in the western desert in general [2], [3], The Rock discoveries in the central basin of the Erbil Plain cover a period extending from Upper Miocene-Recent, Figure 1 shows the geological map of the central basin of the Erbil Plain, and Figure 2 shows the topographic map of the central Erbil basin, on which the studied wells were, installed [4].
The movement of groundwater depends on hydraulic gradient and hydraulic conductivity, and the movement of groundwater is affected by other factors, the most important of which are: the nature of the aquifer, porosity, areas of feeding and filtration, as well as the nature of the fluids moving within the aquifer [5].The general direction of groundwater movement is from east to west, Figure 3.
The central basin of Erbil has a gradual slope from east to west, reaching about 3 degrees [6].The area is topographically uneven, and it is covered with sandy pebble layers or a succession of sandstone, silt, and clays, so it has high permeability and porosity, which helps the water to filter down.
The rocks of the Bai Hassan Formation contain clastic rocks (claystone, slatestone, and conglomerate), represent the groundwater aquifer in the research area [7] [8].
The current study's goals are to Description the hydraulic characteristics of the constrained features of the groundwater aquifer in Erbil, Northern Iraq.  the groundwater in the central basin of the Erbil plain, which is about 1400 km2 in size, and the Erbil area of about 70 km2 for Erbil city [8], [9], [10].The chosen area is situated between latitude 36° 08′ 01′′ -36° 13′ 58′′ N and longitude 43° 58′ 00′′ -44° 03′ 06′′ E, figures (4), and figure (5) shows the hydrogeological basin of the Erbil Plain.The researched region is situated at the base of the foothills of the Chumchamal-Butma Subzone.The predominant climate in the area is dry to semi-dry, according to climatic data obtained in Erbil station during the period (2021-1990), which reveals that the total falling rain was 418.25 mm [8].Dry to semi-dry weather predominates in the area [10].The rocks exposed at the central Erbil plain are from the Upper Miocene-Recent, and they comprise deposits from the Quaternary as well as the Ingana, Muqdadiya, and Bai Hassan formations [11].The exposed rocks are thick claystone alternating with coarse conglomerate [12].
In general, the main water aquifer in the study area can be counted as an unconfined aquifer, depending on the type of sediments that make up it and the level of stable water as well as its location in the stratigraphic column.Some of them implicitly contain several semi-confined aquifers due to the presence of Odaysat of clay rocks and solid clay between the sandy and pebble layers that carry water.On this basis, some aquifers in the study area are type of multi-layered aquifers, which are hydraulically connected, i.e. mutually related.

Theis recovery method [15]
In this method, the recovery of the water level with time is calculated directly after stopping pumping, and the residual drawdown (s´) is calculated.This method is accurate because the recovery of the water level occurs naturally, and thus we overcome the problem of fluctuation of the groundwater level during pumping In this method, a semi-logarithmic relationship is drawn between (t/t`) on the logarithmic xaxis and (s`) on the normal y-axis, and Δs` is measured for one logarithmic cycle, Whereas t Time since pumping started (total pumping time plus residual drop measurement time) t` Time since pumping stopped (residual drop measurement time) The following equations are applied:

Jacob Straight Line Method [16]
In this method, the water level decrease (s) is measured with time (t) until we reach the steady state.A curve of drawdown with time is drawn on a semi-logarithmic paper, where the drawdown (s) is on the normal vertical axis and time is on the logarithmic horizontal axis, then Δs is measured for one logarithmic cycle, from which the values (T, K) can be calculated as follows [17], [18]:

……………………..(5)
Whereas T : Transmissivitym²/day, Q : static discharge m³/day 's : The amount of water drawdown in the well for one log cycle, m s : The amount of drawdown measured in the well, m. sc : Corrected drawdown, m.D : saturated thickness, m K : Hydraulic conductivity m/day The values of (T, K) can be calculated using the corrected drawdown values

Hydrogeology
The main water reservoir in the city of Erbil is located in the Bai Hassan Formation, the oldest alluvial deposits, and the newer sediments and they are linked together hydraulically.On a small scale, due to the presence of mud lenses in some areas, which were observed through lithological sections of some wells studied in the region, this indicates the existence of some areas in which the reservoir is almost confined.In general, the main water aquifer in the study area can be counted as an unconfined aquifer, depending on the type of sediments that make up it and the level of stable water as well as its location in the stratigraphic column.Some of them implicitly contain several semi-confined aquifers due to the presence of Odaysat of clay rocks and solid clay between the sandy and pebble layers that carry water.On this basis, some aquifers in the study area are of the type of multi-layered aquifers, which are hydraulically connected, i.e. mutually related.

composition and geomorphology of the region
The central basin of Erbil has a gradual slope from east to west, reaching about 3 degrees The area is topographically uneven, and it is covered with sandy pebble layers or a succession of sandstone, silt and clays, so it has high permeability and porosity, which helps the water to filter down.

Estimating the Aquifer's Hydraulic Characteristics
A crucial component of groundwater investigations is estimating the layers' physical characteristics.The examination of pumping test results from the wells reveals the hydraulic characteristics of the aquifer.In this study, five wells were chosen for pumping and recovery trials in the north, middle, and south of the specified area in the Erbil basin.Next, it was decided on the specific capacity, hydraulic conductivity, and transmissivity.

Table 1 :
Table 1 displays the test outcomes.The results show various amounts and ranges for hydraulic conductivities between 0.65 and 3.565 m/day and for transmissivity between the ranges of 80.295 to 299.599 m 2 /day.The precise daily capacity ranges from 40.4 to 79.6 m 2 /day.Because lithological variances suggest variations in the permeability and porosity of the rock constituting an aquifer, it is expected that different hydraulic characteristics will be seen.Additionally, the Erbil aquifer's classification as a Nauruz Q/17 confined aquifer is to blame for these differences in values.Well W4 had the highest transmissivity value, and the well W5 had the lowest value.The majority of the weak areas (fractures, joints, and bedding planes) are reflective to the Erbil carbonate rocks' high water solubility.The hydraulic properties from pumping test analysis for Bai Hassan aquifer.