VULNERABILITE DES AQUIFERES DE LA PLAINE VOLCANIQUE DU NOUN AUX ACTIVITES AGRICOLES (CAMEROUN). VULNERABILITE OF THE NOUN VOLCANIC PLAIN AQUIFERS TO AGRICULTURAL ACTIVITIES (CAMEROUN).

法语. The Noun volcanic plain is a southern west entity of the Bamoun Plateau which was the site of various volcanic episodes. Aquifers distribution in this environment constitutes the principal water source of the area, intended to human consumption and agro-pastoral uses. Current observations indicate the prevalence of certain diseases and the disappearance of certain species not targeted by the farmers. Thus, an exploration of the agricultural system was undertaken in order to establish the link with water resources quality in this large agricultural basin of Cameroun.  This brings us to explore the handling and nature of the agricultural entrants used and to analyze the hydrogeochemical data of the soil and water resources concern. Such knowledge will make it possible to establish the link between agriculture practice and water quality, which is a key point of ongoing reflections on agricultural systems durability in the large production basins. Beyond the Noun plain, the whole volcanic and wet zones for which the intensification of agricultural activities is fore seen like an adapted alternative to the territorial and environmental stakes, which would be concerned. This study proposes to contribute to the scientific and operational challenges evoked.   Located between longitudes 10°30'-10°45'E and latitudes 5°26'-5°39'N the Noun plain covers a surface of approximately 900 km2 on which pyroclastic projections and volcanic ash settled partly. With an average altitude of 1200 meters, it is roughcast of three aligned volcano-plutonic massif from north to south (Mbam, 2335m, Nko-Gam, 2268m; Mbatpit, 1988m) (Fig. 1) and about forty more recent Strombolian cones. The South is particularly characterized by volcanic rocks of various origins. The thickness of these lava flows varies 10 cm (at Ngouodam) to 50m (at Nkouondja and Nkouotlouom)  The Noun plain enjoys an equatorial climate of moderated monsoon by altitude. The seasonal variations are very weak. During the rainy season, maximum precipitations are observed from August to September and one can count approximately 185 of rainy days per year. There are two distinct seasons, a dry season going from November to March and a rainy season from April to October. The data from the weather Center at Koutaba and the Hydroelectric dam of Bamendjin, obtained over 10 years, show that the Noun plain has an annual average pluviometry of 2500 mm. 65% of annual relative moisture and 25°C of average annual temperature. These climatic conditions are contributing to the extension of agro-pastorals activities. These last years, climate has been characterized by strong temperature variation which accelerate evaporation and evapotranspiration. For this study, six localities were targeted with three types of sampling corresponding to surface water, wells and boreholes.  The multi-disciplinary approach adopted in this study includes field observations and chemical analyses which constitute the traditional mode of investigation in hydrology and in environmental science. Sampling points selected on topographic maps and aerial photographs were readjusted on the field. Water samples retained come from 9 sources; 5 crater lakes, 18 wells (6 in Foumbot, 6 in Baïgom and 6 in Bamkouop) and 12 boreholes (5 in Foumbot, and 7 in Mbamkouop), with water depths going from 6 to 25 m. A rainy event was also sampled in Foumbot. The samples treated are from strongly enthropized wetlands.  The localization and the altitude of the selected sampling sites were given by the use of Garmin GPS. Water of wells were drawn using plastic buckets attached with ropes, while water of boreholes was collected by pumping during 5-15 min before sampling.Water of collector was filled in plastic bottles of 500 ml after being rinsed with sample and was preserved from air in order to avoid evaporation.Physical parameters such as pH, electrical conductivity (EC) and temperature of water samples from stream were measured in situ by using pH/EC/Temperature DKK TOA-meter and photometer HACH.  Nitrates and similar components that are nitrites and ammoniac nitrogen were determined by molecular spectrophotometry absorption to respective wave lengths of 585 Nm, 500 Nm, and 655 Nm. For pesticides, water samples stabilized on the field were analysed at the Hydrological Research center (CRH) of Cameroun and at the Agricultural research Institute for Development by the GC-ECD method (Gas Chromatography with Electron Captures Detector) with the precision that the distinction of the various residues of pesticides was done by retention time. The measured parameters were treated by statistical methods and were compared with the World Health Organization (WHO).  The Noun plain is covered with hard rocks (10%) and pyroclastic projections (90%). Volcanic ash and graded lapilli reaches locally 10 cm to 50 m thickness the peneplain (Martin and Sieffermann, 1966; Mouncherou et al., 2011); lapilli, blocks and bombs at strombolians cones level and ponds. These formations lie on a Precambrian granito-migmatitic base carrying diaclases and other lineaments oriented according to SW-NE and N10-20E direction (Njonfang, 1998). The hydrographic network fit observed features in this basin (Fig. 2).The exploration of the agricultural production system has made it possible to identify fertilizers, 21 commercial names which gather mineral fertilizer as ammonium sulphate (9%), calcic Cyanamide (9%), Urea (26%), various (14%). manures of animal origin (17%) and vegetable (15%) (Fig. 4).  With regard to pesticides, 91 commercial products were identified with 67 active matters. Organophosphorus pesticides (37 %) are used. Organochlorite pesticides (11 %) are the less used among the most significant chemical classes (organophosphorus, organochlorites, pyrethrinoid, carbanates) (Fig. 6). These represent about 112 of fertilizer products used for about 10 gardening products (tomato, salad, water melon, spotted sweet pepper…). Pesticides cause serious damages on environment because of the combined effects of their anarchic and abusive method of application, as well as their persistence and toxicity.   The Results of the analyses carried out in the study area holds that 80% of the water resources is contaminated. The contamination level exceeds the standards prescribed by WHO in 1986. This standard stipulates that water quality intended for household use should not contain more than 0.1ug/l of a distinct active matter and 0.5µg/l of total active matters. Whereas in analyzed wells, active matters like lindane, difocol and endosulfan has concentrations higher than 0.1µg/l. This water contamination generates serious public health dangers. Several discomforts, namely nauseas, giddiness, vomiting and intoxications are related to water use. The observed signs predict of a strong disturbance of the nervous system and especially of endocrine according to studies of Berryman & Giroux (1994).  In the study area, transfers of suspended material are limited in spite of the visible signs of erosion on the field. We noted a more significant nitrate concentration in early rains in April, whereas in February the concentrations are weak. This indicates infiltration and transfer starting from surrounding sources. As a matter of facts, nitrogen flux arises at 95 % in nitrates forms; organics and ammoniac forms being negligible. The reduced shape of nitrogen comes from natural protein decomposition (animal and vegetable), as well as domestic or agricultural industrial wastes. In natural water, its concentration can vary from 0.1 to 10 mg/L (Chevery, 2001). Presence of natural Nitrates in water comes from organic nitrogen degradation. Meanwhile, agricultural activities contribute to raise this rate. Thus, nitrates content of the various sites (Fig. 8) during heavy rainfall period shows contamination with acceptable limits. Similar results were obtained by Nikolaidis & al. in 2008 at Evros in Greece on ground water of an intensive agricultural zone where Nitrate contents are in the background range and allowable values (Lital et al., 2008).   The geological characteristics of Noun plain structures, as well as the tectonic deformations, joints, fractures, shearing and other discontinuities offer a potential asset for the agricultural production and fluids movement in this large basin. This is what is important and attractive towards the zone and the vulnerability of water resources.  This system put forward the use of very varied fertilizers ranging from organic waste and less valorized plants. Results show disproportion of parasites eradication measures and the use of prohibited product. The using mode of these products in the locality of Kouparé, Mbamkouop, Koufen and Foumbot is not convenient. Applicators and residents are thus subject to various troubles (nauseas, vomiting, dizzy spell…). Species like butterflies, bees and certain edible insects are disappearing. The remark done here is that damage goes beyond the agricultural and concerned zones.Certain water bodies are invaded by vegetable species (algae) probably linked to nitrate excesses. As for pesticides, the situation is hardly more brilliant; residues of phytosanitary molecules are observed in half of the rivers and in one third of the ground water's (wells).  Analyses results show that nitrates rate varies between 10 and 38.25mg/L on average (Fig. 8) and is close to the threshold of portability as indicated by WHO. Whereas pesticides active matters (the Malathion, the chlorpyrifos ethyl and methyl, lindane, the endosulfan, the cypermétrine and the deltaméthrine…) (Fig.8), largely exceed the allowed concentration standard defined by WHO (2004).  On sites, pollution is more marked at Mféchiéya with a total average concentration of 1.80µg/l which is 6 times higher than the value guides; followed by Mfossett where the total average concentration is 1.58µg/l being 5 times the standard. Sites of Koufen and Koupare total average concentrations are respectively 1.24µg/l and 1.20µg/l which are 4 times the standard. There are thus the least contaminated after Mbaïgom and Mbamkouop which presents total average concentration of 0.91µg/l and 0.75µg/l respectively (Fig. 9). The bad agricultural practices on a porous soil and the low aquifer depth contribute to increase the contamination risks of cultures and water resources.  In prospect, the resolution of the problem cannot only be lawful; farmers should be voluntarily engaged in more environmental friendly practices. These approaches can be diverse, namely the networks of sustainable agriculture and reasoned agriculture. Among these practices, we can note those which fall within competence of agronomic choices. They are techniques which aim to limit pollution and withdrawing of water:·         Avoid the abusive utilization of fertilizers and pesticides in the wetland or the recharge area; ·         Limit washing by making recourse to nitrate traps cultures which store nitrates in their tissues before restoring them to cultures sow the following year; ¶·         Limit pesticides treatments according to real dangers;·         Limit and split fertilizer contributions during the year;·         Limit irrigation excesses which wash nitrates and pesticides;·         Support meadow which protects the soil and reduces phosphorus and nitrate scrubbings and does not require phyto-sanitarian treatment;·         Valorise organic fertilizers on the whole cultures and meadows (they are often concentrated on corn);·         Develop the direct sowing, a non-ploughing technic which allow to maintain even to increase outputs, while reducing production costs, fertilizers and pesticides washed quantities;·         Choose vegetable species adapted to climatic conditions of our areas;·         Preserve soil moisture by covering it with bark or branches of crushed trees, etc.  Other solutions consist in sparing natural vegetation zones, namely forests in edge of rivers, hedges and slopes which retain water and polluting elements washed in parcels. Farmers will have to assign a small part of their surface in cultures to "pasture bands" which are grass covered surfaces along the rivers. These pasture bands allow re-infiltration of water and fixing then degradation of many pollutants.  According to the differences between the data sampling campaigns, spreading of fertilizers and pesticides in rainy season on agricultural spaces of the Noun volcanic plain results immediately in a pollution of water bodies and inversely a reduction of their use involves an instantaneous and progressive improvement of water quality. Indeed, before being able to reach water resources, these elements have to percolate through grounds or slopes; which can take few days to several years. This result supports the thesis according to which water resources of the area are renewable.At the end of this work, it is important to note that water resources at the Noun plain (constitute essentially of volcanic formations) are renewable and peculation is rapid through the soil or slopes. On the other hand, water remains a little longer in the aquifer.  However, the systematic use of fertilizer and phytosanitary product, the output of farming activities has greatly increased. This intensive agriculture is without consequences on the environment.  Nowadays, agriculture have to evolve, remain productive but must also preserve actors (farmers are first to handle dangerous product), the health of the persons consuming the agricultural product and environment. These changes cannot be implemented without an evolution of the exploitation system.  So, it will be unrealistic to come back in the old agricultural system. Today stakes is to innovate and support innovation in new practices which permit at the same time to produce sufficient output to ensure a decent income to farmers while respecting ecosystem balance. It is therefore important to manage catchment and their water resources in an integrated way. This management should be collectively done by associating all users of water in order to build together a good solidarity around the vital resource.