Nitrate Pollution of Groundwater Based on GIS in the City of Daloa, West-central Cote d’Ivoire

SAMPAH GEORGES EBLIN1, KOUAKOU SERAPHIN KONAN1, OI MANGOUA JULES MANGOUA1, VALENTIN NEDEFF2*, ANDREI VICTOR SANDU3,4, NARCIS BARSAN2*, ION SANDU3,5 1Jean Lorougnon Guede University, Department of Earth Sciences, Laboratory of Environmental Science and Technology, BP 150 Daloa, Cote d’Ivoire 2Vasile Alecsandri University of Bacau, Faculty of Engineering, Department of Environment Engineering and Mechanical Engineering, 156 Calea Marasesti, 600115, Bacau, Romania 3Gheorghe Asachi Technical University of Iasi, Materials Science and Engineering Faculty, 53A D. Mangeron Blvd., 700050, Iasi, Romania 4 Romanian Inventors Forum, 3 Sf. Petru Movila St., Bloc L11, III/3, 700089 Iasi, Romania 5 Alexandru Ioan Cuza University of Iasi, Arheoinvest Interdisciplinary Platform, Scientific Investigation Laboratory, 11 Carol I Blvd., 700506 Iasi, Romania

Large African cities are experiencing uncontrolled growth in area and demography leading to anarchic occupation of urban space. The settlement of populations in this urban space is often not followed by a basic sanitation system so that human activities threaten the quality of water resources. The mechanism of pollution of groundwater is an evolutionary process in space and time, difficult to master [1][2][3][4][5][6][7][8][9][10][11][12]. The impact of these pollutions appears in the deterioration of the quality of the water resources that become unsafe for the population in the majority of cases and even unusable for the other uses in some cases [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. Thus the alteration of the aquifer has gradually become a global concern [15,16].
The water of the Lobo River treated for the consumption of populations of the municipality of Daloa is burdened in organic matter, pollutants and other toxic substances [17]. The inefficiency of the treatment gives the tap water a colour, odours and unpleasant taste. Doubt about the quality of water, coupled with the weak distribution network [18], forces a large part of the city's population to turn to traditional well water and springs. These resources are superficial aquifers naturally exposed to all forms of pollution [19], some of which come from anthropogenic activities such as agricultural practices, uncontrolled landfills and wild sanitation [20]. On the other hand, traditional wells and domestic wastewater collection systems coexist in households, often at distances of less than 10 m.
According to [21], globally, wastewater is the main source of pollution of groundwater resources. In addition, like cities in developing countries, wild garbage dumps abound in places [22,23] and especially upstream of the lowlands, areas par excellence of emergence of spring waters of the city. In Daloa, studies of the quality of these resources are very sketchy and even non-existent [24]. Thus, there is a risk of health related to the consumption of traditional wells and springs water. This study aims to enquire about the state of this aquifer through the assessment of nitrate from 29 water points. Geographical Information System (GIS) were used to perform geographic analyses to highlight the spatial distribution map of nitrates in groundwater. Indeed, the map is necessary and unavoidable because it makes it easy to identify areas with potential health risk.

Experimental part Study area
The city of Daloa is located in west-central Côte d'Ivoire between longitudes 6°24' and 6°29' West and latitudes 6°50' and 6°55' North ( fig. 1). This an area par excellence of coffee and cocoa production. In the decades 1960-1980, vast movements of land colonization will lead to a rapid demographic growth in the region [25]. The 2014 General Population and Housing Census estimates the population of Daloa City at around 320,000.

Materials and method
This study required a digital camera for shooting and a GPS for positioning the 29 sampling points as provided by the protocol (table 1) In the ArcGIS 10.2 environment, Spatial Analyst Tools IDW interpolated the resulting nitrate concentrations from sampling points (known x, y coordinates), to elaborate the nitrate map.

Results and discussions
Nitrate concentrations obtained are sometimes very high, with a maximum value of 301.1 mg/L. In spring waters, concentrations are low to moderate; except spring water near the general hospital (49.3 mg/L), which is close to the WHO drinking water guidelines (50 mg/L). On the other hand, in well waters, nitrate concentrations are sometimes very high, well above the WHO standard (table  2).    This map shows that the shallow aquifer of the city of Daloa is mostly covered by nitrate concentrations beyond the WHO drinking water standard (40% of the total surface area); then come the high concentrations (33% of the study area) and moderate concentrations (25%) (fig. 4). Low and very low nitrate concentrations cover insignificant surfaces and are generally obtained in the peripheries that represent the extension areas of the city ( fig. 3).
concentrations were observed in some wells. On the other hand, the high standard deviation (77.27 mg/L) between the extreme values shows that this parameter evolves significantly from one well to another. It can thus be said that the impact of agriculture is not considerable on the superficial aquifers of the city. According to [27], maximum nitrate concentrations are not necessarily located under agricultural parcels. In some cases, the contribution of septic tanks can cause nitrate pollution greater than agriculture [28]. Indeed, rapid poorly controlled urbanization contributes to nitrate pollution of groundwater [29].
In Daloa, wells are generally dug in households that lack sanitation. Toilet waters and greywater (sinks, kitchen, washing machine, shower, etc.) are respectively discharged into septic tanks and wells lost. The study shows that contaminated areas are concerned with the town centre and are mostly represented by the old districts where domestic wastewater collection systems would fail. The condition of the old facilities raises some concerns about groundwater, which is a vital resource. These conclusions were drawn following studies carried out in the municipality of Abidjan (Cote d'Ivoire) where the nitrate concentrations obtained are very high in the old districts strongly urbanized [30,31]. The impact of pit latrines on the quality of groundwater has been subject of several studies [32,33]. This study reveals sometimes ver y high nitrate concentrations in well waters (21%) which confirm shallow aquifers pollution in the city of Daloa by domestic wastewater. Highly variable concentrations depend on a   Agriculture is considered to be the main cause of the increase in the concentrations of phytosanitary products in the receiving environment, the first emitter of nitrogen pollution and the second emitter of phosphorus [26]. Therefore the high nitrate concentrations in well water could be justified by intense agricultural activity in the region of Daloa. However, very (17%) to low (17%) nitrate number of factors including the water level in the well and the location of wells in relation to endogenous sources of contamination [19,34].
For drinking water, the limit value of nitrate concentration is 50 mg/L; beyond that, its poses health risks. In fact, consumption of water that is highly contaminated with nitrates can lead to various health problems [35,36], such as methemoglobinemia, gastric cancer, goiter, congenital malformations, hypertension, etc. An article published in 2008 by [37] mentions that between 1945 and 1970, nearly 2000 cases of methemoglobinemia were reported in the world literature; most of these cases being associated with the consumption of private well water with a high concentration of nitrates. It is safe to say that the users of these wells are exposed to health risks. A study conducted by [18] at the Orly district of Daloa, to establish a relationship between water supply patterns and population health, showed that 64.03% of waterborne diseases would be associated with the consumption of well water.

Conclusions
Nitrate concentrations in shallow aquifers in the city of Daloa range from 2.5 to 301.1 mg/L with an average of 44.1±66.57 mg/L. In spring waters, they range from 5 to 49.3 mg/L with an average of 17.71±14.00 mg/L. Extreme concentrations are obtained in well water with an average of 55.97±77.27 mg/L. These results show that the high (10%) and ver y high (21%) nitrate concentrations obtained in the sampled wells are attributable to pollution of the water table by discharges of domestic effluents, especially septic tanks. These wells pose a public health risk because excess nitrates in drinking water can be at the origin of many diseases mentioned in the literature. It is therefore imperative to proceed with their immediate closure.
The resulting map is a decision support tool that can allow authorities to anticipate a health disaster.