The purpose of this work is to study the vulnerability of the Quaternary aquifer that lies beneath the N’Djamena city Chad. The subsoil of N’Djamena city Chad is made up of a multilayered aquifer in which there are two main aquifers located respectively at a depth of about 10 and 60 m, between the two there is an intermediate aquifer at about 30 m depth. It is this latter water table, generally captured by human-powered pumps, that is the subject of this study. Because of anarchic garbage dumping, wastewater discharge, latrines scattered throughout the city, chemical fertilizers and herbicides used on the banks of Chari River and its tributary the Logone for market gardening, the quality of the water in this aquifer is highly threatened. Moreover, it has been noting that the sources of pollution are constantly increasing in conjunction with the growth of the population, so the knowledge and protection of groundwater are necessary. We have therefore carried out a study of intrinsic vulnerability using two mapping methods (GOD and SI), as mapping is recognized as an effective tool for decision support in the case of safeguarding water resources. The results obtained by the GOD method show that 38% of the study area is covered by high vulnerability, 29% by moderate vulnerability, 21% by low vulnerability and 21% by the very low vulnerability. With the SI method, 54% of the study area is covering by low vulnerability and 46% by the low and moderate vulnerability. The coincidence rate of low nitrate values in groundwater with areas of very low and low vulnerability is 91% and 76% for the GOD and SI methods, respectively. Although these observations validated the different maps obtained, the SI approach seems to be the most adequate for vulnerability tracing in our study area.
Water is necessary for all life on planet and is a factor in promoting the health of individuals and the socio-economic development of human communities [
Groundwater is not immune to surface pollution since it is largely renewed by rainwater that falls on the surface before infiltrating through the soil to the water table, carrying with it certain undesirable products. However, natural groundwater is generally free of contamination, especially when it is deep, because of the purifying power of soil; but in recent decades, there has been an increased degradation of this resource.
The process of water degradation, although very slow, can have serious effects [
In N’Djaména city Chad, groundwater contamination is essential of anthropogenic origin [
The decontamination of water is not always easy, because it requires large financial means, which is not always within the reach of developing countries and especially Chad. In view of this, it is necessary that measures be taken to know and protect the groundwater of the N’Djamena city Chad.
In this context, measures to help protect and prevent groundwater pollution become an important step towards which many efforts must be provided. Among these measures, the mapping of areas vulnerable to pollution [
There are many methods for estimating groundwater vulnerability, and these are broadly dividing into three groups [
In the case of this study, we chose the parametric index mapping methods GOD and SI. The GOD method was developing by [
The aim of this work is to study the vulnerability of the Quaternary aquifer under N’Djamena city Chad, using the GOD and SI parametric methods. The highlighting of nitrate mapping, which is an indicator of anthropogenic pollution, is part of the validation process of vulnerability maps. The parameter “land use” of the SI method allows taking into account the risks linked to anthropic actions likely to generate groundwater pollution and contaminant aquifers of Ndjamena Chad.
N’Djamena, capital of Chad, is the largest city in country. It was creating on April 22, 1900 and established as a commune in 1919. Geographically, N’Djamena city Chad (study area) is located in western Chad on the border with Cameroon between 12˚06'59'' North and 15˚04'20'' East. N’Djamena city Chad has a population of about 1,500,000 (INSEED, 2017) with an annual growth rate of about 7% and a density of about 83 inhabitants/ha according to the 2013 National Symposium report. The city is located on a relatively flat area with an average elevation ranging from 280 to 320 meters (
According to the literature [
Vulnerability class GOD | vulnerability index |
---|---|
Very low vulnerability | 0 - 0.1 |
Low vulnerability | 0.1 - 0.3 |
Moderate vulnerability | 0.3 - 0.5 |
High vulnerability | 0.5 - 0.7 |
Extreme vulnerability | 0.7 - 1 |
there throughout the wet season before dissipating either by evaporation or infiltration, or by the combined effect of the two phenomena, thus giving rise to more or less permanent pools depending on their size. There is also the presence of a large canal that crosses the city from north to south and whose purpose is to drain wastewater.
The subsoil of N’Djamena city Chad contains two superimposed aquifer levels [
The data and information collected for this study came from several sources. The Water Chadian Society (STE), the Centre of Geographic Documentation (CDG) of the Ministry in charge of Environment, Water and Fisheries and various drilling companies, provided them to us. The different data collected concern the drilling depths, lithological sections and pumping tests. The piezometric data comes from fieldwork conducted in March 2018 as part of this study and supplemented by those of the town hall conducted during the same year (N’djamena Town Hall, 2018).
The results of nitrate analyses were obtaining from the database of the National laboratory of water (LNE). From the Lansat 8 image of N’Djaména city Chad obtained from the National center for the reseach and development (CNRD) in 2017. We were able to generate with ArcGIS version 10.4 software the drainage flow map of the hydrographic network, the depth map of the aquifers, the recharge map, the type of aquifer, or the piezometric level as well as the topographic map.
The information on the land use of N’Djamena Chad plain comes from the land use database of the Agriculture Ministry and more precisely from the service in charge of the Information System for Rural Development and Land Use (SIDRAT) collected in 2016. Various software programs were using to process all these data and Surfer was using for the processing and spatial analysis of the data. The land use map (OS) was obtaining from the processing and classification of the Lansat 8 image of SIDRAT following the SI weights [
The GOD method was developing by [
IGOD = Gi ∗ Op ∗ Da (1)
Gi, Op, and Da represent the index values of aquifer type (G), water table depth (O), and aquifer unsaturated zone layer lithology or geologic features (D), respectively. The value of the GOD index varies between 0 and 1 (
The different ranges of GOD index obtained are relating to the vulnerability classes (
The SI (Susceptibility Index) method was developing in Portugal by [
SI is a derived version of the DRASTIC model developed by [
In the vulnerability assessment process, the DRATOS model considers five parameters. The first four parameters are similar to the four parameters used in the DRASTIC method (D: depth to water table, R: effective aquifer recharge, A: aquifer lithology, and T: topography). Therefore, the ratings corresponding to the different classes of parameters used in the DRASTIC method are the same as those used by the SI method. In addition to the common parameters, the land use (OS) parameter is added, which takes into account the impact of anthropogenic activities [
The land cover (OS) parameter was obtaining by processing the Landsat image SIDRAT Database, 2016.
For this parameter, we used the [
The maximum value of 100 is assigned to industrial landfills, garbage dumps and mines.
The weights assigned to the SI parameters vary from 0 to 1 depending on the importance of the parameter in the vulnerability (
Parameter: Land Use and corresponding Land Use (LU) values | |
---|---|
Land use class (OS) | LU factor value |
Industrial landfill, garbage dump, mines | 100 |
Irrigated perimeter, rice fields, irrigated and non-irrigated annual crops | 90 |
Quarry, shipyard | 80 |
Covered artificial areas, green areas, continuous urban areas | 75 |
Permanent crops (vines, yards, olive trees, etc.) | 70 |
Discontinuous urban areas | 70 |
Pastures and agro-forestry areas | 50 |
Aquatic environments (tides, salt flats, etc.) | 50 |
Forest and semi-natural areas | 0 |
Index (VI) is calculating by summing the products of the scores by the weights of the corresponding parameters, as summarized by the formula below:
ISIouIDRATROS = 0.186 ∗ D + 0. 212 ∗ R + 0. 259 ∗ A + 0. 121 ∗ T + 0. 222 ∗ OS (2)
This method presents four degrees of vulnerability according to the index values obtained (
Apart from the two models, we used the ArcGIS software developed by ESRI. This Geographic Information System (GIS) software, equipped with powerful mathematical functions and tools, allowed us to work both in vector, raster and raster systems. The most used tools on ArcGIS are the “Interpolate IDW” module for the analysis and spatialization of point data, the “Reclassify” module for the attribution of coasts and the “Mapcalculator” module for the cross-referencing operations between the different thematic maps. The comparative flowchart of the realization of the vulnerability map from the GOD and SI methods is shown in
Parameter | D | R | A | T | OS |
---|---|---|---|---|---|
Weight | 0.186 | 0.212 | 0.259 | 0.121 | 0.222 |
Degree of vulnerability | Vulnerability index |
---|---|
Low | <45 |
Moderate | 45 - 64 |
High | 65 - 84 |
Very high | 85 - 100 |
The GOD vulnerability map (
Several hydrogeological parameters such as aquifer type, effective recharge, depth and lithology of the aquifer were mapped by the process of interpolation (spatial analysis) of the technical data of the boreholes on ArcGIS. As for the vulnerability maps, they are produced by raster calculation applied to the vulnerability index formula GOD [
Five vulnerability index ranges were identified by the GOD method, with vulnerability index values ranging from 0.126 to 0.657. The analysis of these index ranges gives four classes of vulnerability to pollution (
➢ The class of very low vulnerability, which is located in the northern and eastern part of the study area, it occupies the smallest proportion because it represents only 13% of the area. This class is explaining by the presence of layers with clay dominance. The static levels in this area are generally between 15 and 17 m.
➢ The class of low vulnerability is located in the northern and northeastern part of the study area where it represents 20% of the area of the study area. This is the intermediate vulnerability between the very low vulnerability class and the moderate vulnerability class. The static levels in this zone are generally between 13 and 14 m.
➢ The moderate vulnerability class occurs in the center and west along a strip and in the southern part of the study area. It represents 27% of the total area. In this area the static level is approximately between 11 and 12.5 m.
➢ This moderate degree of vulnerability can be explaining by the nature of the unsaturated zone made up of silty sand that is not very permeable to infiltration, to which is added the occupation of the land by housing. These conditions, favor the infiltration of contaminants and make these sectors sensitive to pollution, they must therefore be monitored.
➢ The high vulnerability class occurs in a band along the southern part of the study area. This class occupies relatively all of the areas along the Chari and Logone rivers (Sabangali, Farcha, Melezi, Ngueli, Walia, Ndigangali, Ngomba) and represents about 40%, or a little more than a third of the study area. The high degree of vulnerability can be explaining by the litho-stratigraphic context of the unsaturated zone dominated by sands. Static levels in this zone are generally between 8 and 10 m.
The SI method allowed us to obtain 5 classes of vulnerability indexes for our area study varying between 4.01 (minimum value) and 45.72 (maximum value). These indices were dividing into two classes of vulnerability pollution (
➢ Low vulnerability class represents 54% and occupies for most part the northern end and center of the plain of area study. This less severe vulnerability class may be relating to the nature of the unsaturated zone made up of clay, which is not very permeable and which could probably act as a purifier for pollutants.
➢ Moderate vulnerability class occurs in a strip along the southern part, is distributing almost all along the banks of Chari and Logone rivers, and extends towards the center of total area study; it is also found in blocks in northern part of area study. This class represents 46% of area. The areas affected by moderate and specific vulnerability contain mainly crop perimeters, swamp gardens or forest areas. The moderate degree of vulnerability of these areas can be explained by the fact that these areas are possibly marked by the use pollutants (fertilizer, herbicide), and they are constituted of sands and silts which are very permeable because of their porosity. In these areas of moderate vulnerability, there are also areas where the sewage system is defective, areas previously filled with garbage before the construction of houses.
In order to test and validate pollution vulnerability map, many authors [
These different levels overlap indifferently with the very low, low, medium and high vulnerability class (
On the validity map of vulnerability pollution by GOD method (
Component heads identify the different components of your paper and are not topically subordinate to each other. On the SI pollution vulnerability validity map (
Degrees of Vulnerability According to GOD | Nitrate content | Degrees of Vulnerability According to GOD | Nitrate content | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
0 - 15 mg/L | 15 - 50 mg/L | >50 mg/L | 0 - 15 mg/L | 15 - 50 mg/L | >50 mg/L | ||||||||
Number of points | Total % | Number of points | Total % | Number of points | Total % | Number of points | Total % | Number of points | Total % | Number of points | Total % | ||
Very low | 17 | 51% | Very low | ||||||||||
Low | 14 | 41% | 6 | 100% | 6 | 100% | Low | 26 | 76% | 6 | 100% | ||
Moderate | Moderate | 8 | 23.52 | ||||||||||
High | 3 | 8.82% | High | 6 | 100% |
to the moderate vulnerability zone. Six (6) values between 15 and 50 mg/L are also in the Low Vulnerability Zone and the six (6) values greater than or equal to 50 mg/L nitrate are in the Moderate Vulnerability Zone.
Comparing the percentages of surfaces occupied by the different classes of vulnerability according to SI and GOD (
This observed difference in the number of classes may be relating to the fact that the class boundaries and dimensions that are assigned to different parameters are not absolute. This implies that the standard class boundaries may not reflect the reality on the ground where one class may surround different hydrogeological units [
Based on the results obtained from the two (2) methods (GOD and SI), it appears that the GOD method overestimated the vulnerability compared to the SI method (presence of the high vulnerability class). This finding is in agreement with some work done in some sub-Saharan regions such as Burkina [
It should be noting that in the realization of vulnerability maps by both SI and GOD methods, we are faced with a number of difficulties. Indeed, one of the difficulties related to the elaboration of parameter maps is the process of interpolation whose reliability depends on the data used for their realization. This interpolation can lead to errors in the realization of parameter maps, because it is only reliable within the intervals delimited by the point data [
Vulnerability Class | Percentage of Area by Class and Method (%) | |
---|---|---|
SI or DRATOS (occupied surface) | GOD (occupied surface) | |
Very low | 0% | 12% i.e. 4740 ha or 47.4 Km2 |
Low | 54% or 21,330 ha or 213.3 Km2 | 21% or 8295 ha or 82.95 Km2 |
Moderate | 46% or 18,170 ha or 181.7 km2 | 29% or 11,455 ha or 114.55 km2 |
High | 0% | 38% or 15,010 ha or 150.1 Km2 |
➢ Comparison of Maps and Methods (GOD and SI)
Knowing that our study area is an urban area located in a semi-arid climate, in this context we can consider that the classes of moderate and high vulnerability represent areas threatened by pollution. They cover about 67% and 46% for GOD and SI respectively.
The moderate and high vulnerability zones in the central part of the study area correspond to areas where the influence of human activities responsible for increasing anthropogenic pollution has been noted in numerous studies ( [
The vulnerable areas are also found all along the Chari River, this finding was also made by [
It should also be noting that in these vulnerable areas that border the Chari River on both sides, apart from the influence of the above-mentioned sources of pollution, the use of pesticides and fertilizers contributes to the deterioration of groundwater quality. In these areas, the population also cultivates vegetables; therefore, these are areas to be closely monitored.
The very low and low vulnerability class covers about 33% and 54% of the area study for GOD and SI respectively. They are generally, found in the northern and northeastern part of area study and are probably relating to the low permeability of the clay-rich unsaturated zone formations and the almost zero recharge rate. Moreover, when moving away from the river towards the north, the water table becomes deeper and deeper and is therefore relatively protected. To validate vulnerability maps, we compared the distribution of nitrates in the groundwater of N’Djamena city Chad with the distribution of vulnerability class.
The nitrate measurement campaign carried out in our area study shows that there are relatively high values of nitrate in both low vulnerability and medium and high vulnerability areas according to the methods (
The rate of coincidence between the nitrate levels in the water of the city of N’Djamena Chad and the different class of vulnerability degrees allows us to observe the following.
For GOD method, 91% of nitrate concentrations below 15 mg/L are measured in a very low-to-low vulnerability zone and about 9% in a high vulnerability zone; 100% of values between 15 and 50 mg/L are measured in a low vulnerability zone; 100% of values above 50 mg/L are also measured in a low vulnerability zone. For SI method, 76% of the concentrations below 15 mg/L are measured in a low vulnerability area and about 24% in a moderate vulnerability area, 100% of the values between 15 and 50 mg/L in a low vulnerability area; 100% of the values above 50 mg/L in a high vulnerability area.
In both cases, we note on the one hand a good correspondence between the zones of low concentrations of Nitrates and the zones of low vulnerability considered as “well protected”. Thus, we can consider that this map of spatial distribution of nitrate rate allows validating the maps of vulnerability pollution. However, we note the presence of high nitrate levels in areas of very low and low vulnerability, which confirms that the water tables of N’Djamena city Chad are likely to be threatened locally by the infiltration of pollutants. This situation is quite possible because the notion of vulnerability is not synonymous with current pollution, but rather with a predisposition of these areas to possible contamination, if nothing is undertaken to protect them [
At the SI level, we have the best coincidence (100%) between Nitrate concentrations above 50 mg/L and the high vulnerability zone. Considering that high nitrate levels are related to anthropogenic pollution not intrinsic pollution (hydrogeological context of the area), we can say that this high coincidence rate shows that the specific SI method better assesses nitrate vulnerability in the case of our study.
In this study, we chose two methods (GOD and SI) to evaluate the vulnerability to chemical pollution of N’Djamena city Chad. The results show that there is a difference in the number of class’s degrees of vulnerability. We obtained four (04) classes of vulnerability for GOD method: the high vulnerability class (38%), the moderate vulnerability class (29%), the low vulnerability class (21%) and the very low vulnerability class (12%), against two (02) classes of vulnerability for SI method, namely the classes of low vulnerability (54%) and moderate (46%).
The vulnerability mapping of our area study shows that the moderate and high vulnerability areas cover about 67% and 46% respectively for GOD and SI. These areas include certain neighborhoods in the center (Am Riguebé, Ridina, Paris-Congo, Moursale), in the east (Gassi) and in the north (Achawayil, Lamadji), as well as all the areas along the Chari and Logone rivers (Sabangali, Farcha, Melezi, Ngueli, Walia, Ndigangali, Ngomba).
The analysis of vulnerability maps (GOD and SI) to groundwater pollution in N’Djamena Chad shows that the vulnerability degree is a function of lithology and permeability of soil; in fact, the risk of pollution of water table of N’Djamena city Chad is greater in the sandy facies than in the clay facies of the unsaturated zone.
Comparing the distribution of nitrates in the groundwater of N’Djamena city Chad and the distribution of vulnerability classes, we note that 91% of samples with very low nitrate levels (0 to 15 mg/L) coincide with the very low-to-low vulnerability zones for GOD method. Whereas with the SI method the coincidence rate is 76% with the low vulnerability zones that, we consider being “well protected” zones. This overlay allows us to say that the maps elaborated reflect the reality on the ground. Apart from that, we also found that the coincidence rate between high nitrate concentrations (>50 mg/L) and the high vulnerability class (100%). This shows that the specific SI method assesses nitrate vulnerability better than the GOD method. This finding is quite logical because the SI method integrates land use in its formula.
In any case, it seems undeniable that the groundwater of Ndjamena Chad is exposed to the risks of pollution linked to both the hydrogeological context of the aquifer system and to human activities. The risk of nitrate pollution of the city’s water is real but variable in places. It is therefore desirable to monitor areas with high nitrate content (an indicator of anthropogenic pollution). Water quality and health are inseparable couple. Such a risk of pollution can pose a long-term problem for public health.
In view of the problem and the real issues related to the risk of vulnerability groundwater in the Ndjamena city Chad, in terms of operational perspectives, it is relevant to develop (N’djamena Town Hall) an interministerial strategic plan for integrated management of pollution risks on natural resources.
This will not only help to raise awareness among the population on behavioral change in terms of integrated management groundwater resources (IWRM) but will also help to preserve natural resources. It is important not to lose sight of the promotion of rational use of phytosanitary products (fertilizers, pesticides, herbicides, etc.) and of the environment (promotion of hygiene and basic sanitation services) in the areas concerned. Finally, a new research base can be launched through studies of the sanitary and environmental impact of nitrate and other pollutants in anomalous or vulnerable areas.
The authors declare no conflicts of interest regarding the publication of this paper.
Deubalbe, D., Kadjangaba, E., Bongo, D., Djimouko, S., Mbaigane, J.C.D. and Mahmout, Y. (2021) Vulnerability Evaluation of Groundwater of N’Djamena City: Contribution of the Parametric Methods GOD and SI. Journal of Environmental Protection, 12, 472-489. https://doi.org/10.4236/jep.2021.127030