Adjustment of Original DRASTIC model by means of Lineament Density to Map Groundwater Vulnerability: Case Study in Rania Basin, Kurdistan Region, Iraq

Groundwater has never been heavily relied on as a water source in Northern Iraq as it has been in the last two decades due to the rapid and often unplanned urbanization, industrial and agricultural projects. This paper attempts to present a concise groundwater vulnerability assessment of Rania basin to the local and regional planning authorities to ensure a more sustainable development in the area. The focus of the study is the Rania basin, which is a part of Dokan sub-basin in North East Iraq. The initial groundwater vulnerability assessment is mapped with standard DRASTIC model. It is then modified by adding “Lineament Density Index” to the original seven DRASTIC parameters due to the previously established close relationship between flow and yield of groundwater with lineament. The area is categorized into five vulnerability index zones of; very low (26%), low (32%), medium (31%), high (11%) and very high (0.012%). The modified model offeres a slightly different vulnerability classification of; very low (16.61%), low (35.45%), medium (30.32), high (17.57) and very high (0.05%). Measured Nitrate concentration is used to validate the assessment results. A progressive increase in nitrate concentration somehow reflects the different vulnerability zones identified by the DRASTIC models in the area. Samples of wet season show 15.96 mg/l, 17.68 mg/l and 20.1 mg/l for very low vulnerability, low vulnerability and medium vulnerability zones when classified by modified DRASTIC model.


Study area
Rania basin is located in the northeastern part of Iraq. It

Geology of the study area
According to Jassim and Goff (2006), Rania basin, is located in the Sulaimaniyahya-Zakho subzone of high-folded zone, in the unstable platform in terms of tecto-struactural condition.
The structure of the area has strong orogeny-induced uplifted units that are very complex, Though relatively uniform, ones (Jassim and Goff, 2006

Hydrogeology
According to Jawad (2008), Rania is located in the sub basin of Dokan that is a part of the lower Zab basin. The basin has a high potential of groundwater resource, which can be used

2 Standard DRASTIC model
To assess the groundwater vulnerability of the basin, the first model to consider, was standard DRASTIC. The model is a combination of seven parameters with specific weights (1 to 5) and rates (1 to 10) assigned to each parameter as explained in (Table 1). Abbreviations from each parameter are taken to make the short form of "DRASTIC". Geological and hydrogeological characters are the fundamental criteria used to assign the label unit of the map.
The equation below is a linear representation of all factors involved to find the vulnerability index by standard DRASTIC model:

DI DwDr RwRr AwAr SwSr TwTr IwIr CwCr
In a way that, DI is the DRASTIC Index, w = weighting coefficient and r= rating coefficient.
D is the depth to water table, R is recharge, A is the aquifer media, S is the soil media, T is the basin topography, I is the impact of unsaturated zone and C represents the basin hydraulic conductivity.
The basin was mapped and classified into several categories in terms of groundwater vulnerability with Geographic Information System (GIS Arc Map 10.5). All the rates and weights were reached at through Delphi technique, i.e. employing the professional's academic and field-based insight to evaluate the risk in the area under certain circumstances (Rahman, 2008).
As all the process of degradation and natural attenuations of the contaminants are time consuming, the greater depth of the water table, often plays a positive role to delay the contaminant to reach the aquifer, hence providing more chance for attenuation. All the measurements of depth to water table used to construct (D_Map) for this research have been collected in wet season, since water table is at its shallowest depth which is crucial in groundwater vulnerability to contamination. The data was partly collected by the researcher using electrical sounder and mainly from the archives of the Sulaimani Groundwater Directorate (SGD). The data set ranges from 2.5m, the shallowest, to 104m the deepest. The Depth layer in raster format was created using an ArcMap tool of Inverse Distance Weighted (IDW) and divided the basin into nine zones according to the recommended rates given in (Table 1).
The nine classes were (less than 4.5m, 4.5 -7.5, 7.5 -10, 10 -12.5, 12.5 -15, 15 -19, 19 -23, 23 -30 and more than 30m). Class 1 or the lowest vulnerability class is designated to the deepest water table, more than 30m mostly in the north, east and center of the area. Over exploitation in the villages scattered over the mountainous area having a higher altitude may explain the deeper water table. In contrast, shallower water table of less than 4.5 having a higher vulnerability index rating of 9 is seen in the southern and western area closer to the Dokan Lake.
The rate value of 1 (very low) was assigned to aquifers of lower hydraulic conductivity or less than 4m/day and rate of 2 (low) was given to aquifers with hydraulic conductivity higher than 4m/day. Greater density values of lineament may indicate more potential groundwater contamination.

Maps of lineament and density of lineaments
The Rania basin map of lineament distribution was prepared using images from The Landsat 8 Thematic Mapper Plus of 5 th Nov. 2020 with cell size of (30 x 30) meters and the grey scale Operational Land Imager (OLI) spectral band. The lineament distribution over the basin was extracted with the help of PCI Geomatica technique. Figures. 4A and 4B show the thematic and the lineament distribution maps of the study area respectively. Spatial analysis tool from ArcMap 10.5 was used to build the lineament density map. The chances of moving a pollutant towards the groundwater increases with higher intensity of lineament features in the studied area.

Map of lineament rating and lineament index
The alluvial deposits in the Rania basin shows many linear features that represent higher potential of increased porosity and permeability. Furthermore, the movement of groundwater in the Rania basin is mostly through the fractures due to their abundance in greater portion of the aquifers in the area. Therefore, the most effective parameter to be added to the DRASTIC model in assessing the basin's groundwater vulnerability more precisely is lineament density.
The lineament density maps with ranges and rating (Fig. 6A and 6B) made in GIS according to values taken from Table 2. The value of (5) was given to the lineament density as a weight coefficient based on its valuable importance (Al-Rawabdeh, 2014) and finally the index map was constructed by multiplying the rated lineament map to the mentioned weight, using the map algebra tool in ArcMap 10.5.

Modification of standard DRASTIC model with lineament index map
To make the DRASTIC model represent a more realistic vulnerability assessment of groundwater, the lineament index is added to the generic model. This is because a strong relationship of lineament with groundwater occurrence, flow and production has been established in previous studies (Lattman and Parizek, 1964). When the lineament index is

Index validation with nitrate
The obtained vulnerability maps from both standard as well as modified DRASTIC models

Standard vulnerability mapping assessment
As shown in In general terms, 58% which is greater than the half of the studied area are assessed to have very low and low vulnerability to pollution, while only 10.64% of the area is assessed as highly and very highly vulnerable (Table 3).

Lineament assessment
Landsat 8 images (ETM+, 2020) was used to extract lineament distribution map and hence the lineament density map. Five classes of lineament density were identified in Rania basin (Fig. 6C). The lineament density classes and their covered area percentages are demonstrated in Table 2 occupy an area of 66.28km 2 , 23.75 km 2 and 13.98km 2 or 5.22%, 1.87%, and 1.1% of the total area correspondingly. The map of lineament ratings (Fig. 6B) shows the basin divided according to density ratings from 1 -5. A small portion of about 1.09% in the basin having the highest lineament intensity (Class -1) was given a high rating value of 5 whereas rating value of 1 was given to areas having the lowest lineament density (Class-V). Ratings of 2, 3 and 4 were given to areas with lineament density classes of (IV, III and II) correspondingly.
The final lineament index map was prepared by multiplying the lineament rating map in raster format designated weight of lineament parameter (5) through map algebra in GIS (Fig. 6C).
The produced index map was then divided to five zones of (5, 10, 15, 20 and 25), occupying area already demonstrated with map of lineament ratings.

Modified DRASTIC model assessment
The produced map from Modified DRASTIC assessment (Fig. 6D) (Table 3). There is about 10% decrease in the area covered by the very low vulnerability class and 6% increase with regard to the class of high vulnerability compared to the results offered by the standard DRASTIC evaluation.
However, the variation in the areas covered by other classes of high, medium and low vulnerability is less than 5%. The low variation between the two models suggests the marginal influence of lineament density index on the assessment of groundwater vulnerability in the area. This is because 91.72% of the basin has the lowest lineament distribution ranging (0 -1.05) as represented on (Table 2) and (Fig. 6A).

Validation of the modified DRASTIC method with concentration of Nitrate (NO 3 -)
To validate the vulnerability models, Nitrate (NO 3 -) is considered as a reality check for the DRASTIC model in both generic and modified phases. Nitrate does not enter the water supply through dissolved minerals but comes from nitrogen cycle. Therefore, it can be used as an indicator for pollution of water resource (Secunda et al, 1998).
Collected samples of water from 39 wells were analysed for this study. They were taken in October 2018 and May 2019 to represent the variations in Nitrate concentration that may exist between dry and wet seasons ( Table 3)

Conclusions
The main purpose of carrying out this research is to evaluate the groundwater in the Rania basin in terms of its vulnerability to pollution. Drastic model as the most widely used and its easy-to-use approach was considered for the assessment. However, it was soon realized that despite its good performance for intrinsic vulnerability on a larger scale, the standard DRASTIC model comes short to offer, an accurate and a closer to reality, assessment of the groundwater pollution on a smaller and detailed scale. Therefore, the generic model was modified through adding an extra parameter of Lineament Density to its original seven.
The vulnerability index values that produced through the final modified DRASTIC model (DI+DL) ranged between 56.19 and 226.2.
According to the index values and the maps produced by both models, the Rania basin is divided into five vulnerability ranks of (very high, high, medium, low and very low). Although no major and decisive discrepancies appear between the intensity and the classifications of the intrinsic vulnerability classes produced by each model, never the less, slight deviations can be noticed between the areas covered by each vulnerability classes. Areas ranked as very highly vulnerable accounts for the smallest section of the studied basin while the low vulnerability class covers the largest portion of the basin. To

Declarations: 1
• Ethics approval and consent to participate: Not applicable 2 • Consent for publication: Not applicable 3 • Availability of data and materials: The datasets used and/or analysed during the 4 current study are available from the corresponding author on reasonable request. 5 • Competing interest: The authors have no relevant financial or non-financial interests 6 to disclose. 7 • Funding: No funds, grants, or other support was received. 8