eparation of the seven DRASTIC layers was conducted as prescribed by the authors with modification of the net recharge component. Geographical information system (GIS) was employed in the assessment.
2.3.1. Depth to Water Level
Most parts of the study area have groundwater level within 5 - 30 feet, it reaches up to about 50 - 75 feet in some locations such as Sabon Gayan. In some locations around Badiko and Tudun Wada, groundwater level is found within 5 feet above mean sea level. A thematic layer of this parameter for the study area was created using the spatial analyst function of Arc GIS 10.0 according to the model rating (Figure 2).
2.3.2. Net Recharge
In the study area, there was no readily available data on recharge from documentary sources, as such a simple formula proposed by  was used to determine the net recharge:
Slope was generated by reclassifying the thematic layer of topography which was secured from the Digital Elevation Model of the area (section 2.3.6). Soil
Table 1. Data Type and Sources.
Figure 2. Thematic Layer of Depth to Water Level (ft) in Kaduna Metropolis. Source: Data Analysis (2015).
permeability as used in the formula, is a qualitative value given to the three soil types in the area (section 2.3.4). Sandy soils was rated 5, sandy loam 3 and loamy sand 4. Rainfall is expectedly uniform in the whole study area and exceeded 1000 mm according to literature, a uniform rating of 4 which signifies rainfall greater than 850 mm was adopted. Thematic layer of net recharge was produced using Equation 3. Recharge pattern is somewhat uniform in the whole study area and follows the pattern of topography. Using the model builder of Arc GIS 10.0, the qualitative classification of recharge was achieved (Figure 3).
2.3.3. Aquifer Media
According to literature and VES report, the study area is completely overlaid by uniform aquifer media, the weathered metamorphic/igneous rock which has the DRASTIC rating between 3 - 5. Typical rating of 4, as suggested by  was adopted in most parts of the study area. The value was adjusted in some areas to reflect the degree of consolidation of the aquifer materials. In some areas around Malali and Unguwan Rimi, where the materials are highly consolidated according to the VES reports, a rating of 3 was adopted. A rating of 5 was adopted in some areas such as Unguwan Pama, Romi and Barnawa due to the extreme weathering of the aquifer media. The result is depicted on Figure 4.
2.3.4. Soil Media
A scanned copy of Soil map of Nigeria produced by  was georefrenced, digi-
Figure 3. Qualitative Classification of Net Recharge in Kaduna Metropolis. Source: Data Analysis (2015).
Figure 4. Thematic Layer of Aquifer Media in Kaduna Metropolis. Source: Data Analysis (2015).
tized and the study area was extracted and vectorized. From the map and its accompanying report, two major soil textural classes were present in the area, the sandy loam and loamy sand. The third soil group which is the recent alluvium soil found within the course and the flood plain of River Kaduna, Rivers Tubo, Chidawaki, Rumana among others was described as sandy soil in the report, as such a rating of 9 was adopted for the category. Sandy loam which can be seen in the north, northeast, and eastern tips of the study area, was rated 6 according to the model. Loamy sand which occupies most parts of the study area, but not considered by the model was adjusted to 6.5. A thematic soil map of the area (Figure 5) was produced according to this rating using Arc GIS 10.0.
Topography of the area was generated using Digital Elevation Model (DEM) of Arc GIS 10.0. Slope percent was then calculated using the slope function of the same software. It was classified according to the DRASTIC rating as can be seen on Figure 6.
2.3.6. Impact of Vadose Zone
The vadose zone media of the study area is metamorphic/igneous formation which has the theoretical rating of between 2 - 8. A typical rating of 4 was adopted for most parts of the area according to the DRASTIC rating. The rating
Figure 5. Thematic Layer of Soil Media in Kaduna Metropolis. Source: Data Analysis (2015).
was also adjusted to 2 and 3 for massive igneous around Malali and partially weathered formation around Malali, Unguwan Rimi and the like. Using the same software, the vadose zone map of the area was produced (Figure 7).
2.3.7. Hydraulic Conductivity
Information on this parameter appeared scanty in the study area. However, almost all the available information documented through pumping test of some boreholes in the area, indicated that the conductivity falls within the range of 1 - 100 gallon per day per square feet, thus, a rating of 1 was adopted for the parameter throughout the study area (Figure 8).
3. Result and Discussion
3.1. Drastic Index Vulnerability Mapping
Using the model builder of Arc GIS 10.0, Equation (1) was inputted and the model runs to produce the final DRASTIC vulnerability map of Kaduna metropolis (Figure 9).
The highest DRASTIC value obtained in the whole study area was 131 while the lowest was 77. Theoretically, the highest value of normal DRASTIC is 223 while the lowest is 65. The values can thus be graded into five qualitative classes (Table 2).
Figure 6. Thematic Layer of Slope in Kaduna Metropolis. Source: Data Analysis (2015).
Figure 7. Thematic Layer of Vadose Zone Media in Kaduna Metropolis. Source: Data Analysis (2015).
Figure 8. Thematic Layer of Hydraulic Conductivity in Kaduna Metropolis. Source: Data Analysis (2015).
Figure 9. Unclassified DRASTIC Vulnerability Map of Kaduna Metropolis. Source: Data Analysis (2015).
Table 2. Classification and Description of DRASTIC Index Values.
Source: Discerned from  based on the theoretical values.
Putting this in mind, it is noticeable that, the groundwater vulnerability to contamination of the entire study area falls within very low, low and slightly moderate vulnerability (Figure 10). However, the essence of this study is to determine which area is more vulnerable to groundwater contamination than others, as such, the obtained values were retained and reclassified (via “reclass” function of ArcGIS) into five classes of very low, low, moderate, high and very high vulnerability to contamination (Figure 11) as suggested by  .
From the classified DRASTIC vulnerability map, one can rightly see that, majority of the study area falls within very low, low and moderate vulnerability to groundwater contamination. Very low vulnerability areas can be seen in the northern part around Turunku and Sabon Gida, in the northeast around Rinagi,
Figure 10. Classified DRASTIC Vulnerability Map of Kaduna Metropolis Based on Total DRASTIC Scores. Source: Data Analysis (2016).
Figure 11. Classified DRASTIC Vulnerability Map of Kaduna Metropolis. Source: Data Analysis (2015).
and in the southwest around Kasewa Hill. The same vulnerability class occurred in the southern part of the study area around Bakin Kasuwan Gwari and south- eastern tip.
Low vulnerability areas can be seen in the northwest around Rumana and Zato, and southwest around Jimmu, Nima and Kasewa Hill. In the south and southeast, low vulnerability class is predominant as can be seen around Kukau, Kakau and Kankomi respectively, the same class can be found in the north around Afaka and Rigachikun, in the northeast around Butonu and in the center at Babban Saura and Kamazo.
Moderate vulnerability happens to be the most dominant class in the study area. Encircling the high vulnerability areas, moderate vulnerability occurs to the north around Rigachikun Forest Reserve, Afaka and Kawo. It can also be seen around Gwogote and Tagwaye in the west, as well as most eastern and southern parts of the study area.
High groundwater vulnerability areas occupied most of the central parts of Kaduna metropolis especially within the township, Tudun Nupawa and Unguwan Shanu. It also occupies the northern part of the metropolis stretching from NDC, Rigachikun, Maraban Jos and Birnin Yero. The same class occurs in Kaduna south around Kakuri, Makera and Sabon Tasha, and in the southeast around Chidunu and Anguwan Tanko. Other areas belonging to this category are Kwane and Kadi in the west as well as Gwarso in the north-western part of the study area.
Very high vulnerability class occupies very small portion of the study area. It can be seen as an encircled area in the east, and in scattered form in the west, south-west, as well as southern part of the study area. Fewer areas also occurred at the center around River Mashi and north-western part of the area.
3.2. Map Production, Data Quality and Reliability
It is generally believed that, the quality of any map is determined by the quality of the input data used in map production. In this research, several types and sources of data were consulted, extracted, gathered and synthesized for the production of the thematic layers used in the production of the final DRASTIC vulnerability map. Its believed that, the data used for the task and at this scale, represent the best quality information which is available at the moment. However, with improved data quality, the map quality will also be greatly improved. Mapping and models in general, are simplifications of the complex reality. Groundwater vulnerability assessments and mapping as put forward by  are “a means to synthesize complex hydrogeological information into a form useable by planners, decision and policy makers, geoscientists and the public”.
From the previous section, it can be concluded that groundwater system in most parts of Kaduna metropolis is low to moderately vulnerable to contamination. However, this does not preclude the occurrence of real pollution incidence in the area which may be tight to the potential or actual contaminants sources (hazards) present. In view of this, a low vulnerability area with very high hazards may be exposed to higher risk of groundwater contamination than a highly vulnerable area with low or very low hazards. Conversely, contamination incidence may be low in a highly vulnerable area with high hazards, but with adequate control measures to safeguard the groundwater system. The vulnerability map will serve as screening tool and guide the administrators where to direct resources (more vulnerable areas) when there is limited resources. It may be used for preventive purposes through prioritization of areas where groundwater protection is critical. This will help in reducing the cost of groundwater monitoring in the area.
That notwithstanding, it is recommended that land use(s) with high groundwater potential hazards be located in the low vulnerability areas. Where an existing land use with higher contamination hazard is already located on a highly vulnerable area, more sophisticated control measures should be put in place. Proper documentation of potential and existing groundwater contaminants sources in the area, will therefore, be vital for sustainable groundwater quality management in Kaduna metropolis.
Rating of Drastic Index Model (Aller et al. 1987).
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