Pollution nitrique des eaux souterraines au Tadla (Maroc) Ground water nitrate pollution in Tadla (Morocco)

Résumé

Dans les périmètres irrigués du Maroc, la pollution nitrique diffuse des eaux souterraines augmente le risque de détérioration de la qualité des ressources hydriques et engendre un risque sanitaire pour la population rurale s'approvisionnant le plus souvent directement de l'aquifère. La présente étude, réalisée dans le périmètre du Tadla (3600 km²), située au centre du Maroc, vise à faire l'état des lieux de la contamination des eaux souterraines par les nitrates et à identifier les causes potentielles de cette pollution.

Le suivi de la concentration des eaux souterraines en nitrates a été réalisé durant la période août 1996 - avril 1998 dans un réseau de cent puits. Les paramètres du milieu physique et des pratiques culturales ont été déterminés soit par des mesures au champs, des analyses au laboratoire ou par enquête auprès des agriculteurs dans un réseau de 40 sites. Les résultats sont présentés sous forme de cartes thématiques par le SIG.

Le niveau actuel de la pollution nitrique des eaux souterraines dans le périmètre irrigué du Tadla commence à devenir critique. De part la pollution ponctuelle mise en évidence aux alentours de quelques agglomérations urbaines et à l'aval des sucreries, les zones dont les teneurs en nitrates des eaux souterraines dépassent 50 mg NO_3- /l sont localisées en aval des Béni Amir et en aval et à l'Est des Béni Moussa Est. Béni Moussa Ouest reste en général moins polluée que Béni Moussa Est. Les apports excessifs en eau et en engrais azotés pour les cultures pratiquées (blé, betterave, cultures maraîchères, arboriculture et luzerne), sont les causes directes de la pollution diffuse. Par ailleurs, il est certain que la texture du sol (argileuse, argilo-limoneuse et limono-argileuse), la teneur en argile (29% à 55%) et tous les paramètres qui leurs sont corrélés notamment la réserve utile du sol, réduisent le risque de lixiviation des nitrates en profondeur et assurent en conséquence un contrôle naturel de la pollution nitrique des eaux souterraines. Néanmoins, tous les autres facteurs étudiés notamment le niveau des nappes faible par rapport au sol (<4m-12m), la vitesse d'infiltration du sol en milieu saturé généralement élevée (20-450 mm/h), les teneurs assez importantes du sol en matière organique (1.5%-3.5%) et en azote total (0.10%-0.15%), joueraient positivement sur la vulnérabilité de la nappe à cette pollution. De même, le pouvoir de minéralisation élevé des sols couplé aux conditions écologiques favorables à une minéralisation intense de la matière organique native du sol, assez caractéristique des régions irriguées à climat méditerranéen, seraient en partie responsable de cette pollution. Néanmoins, Aucune corrélation n'a été mise en évidence entre la teneur en nitrate des eaux souterraines et les combinaisons linéaires des variables du milieu physique. Ceci serait dû à l'homogénéité du milieu physique et à l'accumulation de la pollution suivant le sens d'écoulement des nappes de l'amont vers l'aval hydraulique. Vu le niveau d'intensification agricole que connaît cette région, la situation risquerait de s'aggraver rapidement induisant, en dehors de toute mesure de prévention, la détérioration de la qualité des eaux souterraines dans cette région.

Mots-clés:

• Pollution,
• eaux souterraines,
• nitrates,
• caractéristiques physico-chimiques du sol,
• rejets urbains,
• pratiques agricoles,
• Maroc

Abstract

In Morocco, diffuse ground water pollution by nitrates in irrigated areas has caused an increase in the risk of water quality deterioration. This has generated a health risk in rural areas, since most of the rural population get their drinking water supply from the aquifer. The present study was carried out in the Tadla region. The plain of Tadla is situated in the centre of Morocco, in the Oum Erbia river basin. It covers a surface area of 3600 km². The Oum Erbia river (160 km long) divides this plain into two large irrigated areas (Beni Amir in the east (3300 ha) and Beni Moussa in the south (6900 ha)), with different hydrological characteristics. The irrigation water comes from the Oum Erbia river, the Bin El Ouidane dam and from two important waters tables: Beni Amir (190 Mm³) and Beni Moussa (250 Mm³). The objectives of this study were to quantify the degree of ground water contamination by nitrates and to identify the potential causes of this pollution.

The monitoring of nitrate concentrations in ground water was carried out during the period of August 1996 to April 1998, in 100 wells in the region of study. The physical environment and agricultural practices were determined either by field measurements, laboratory analyses or by investigations with farmers at 40 selected sites. The results are presented by thematic maps using a geographical information system.

The level of ground water nitrate pollution is becoming a serious problem. The point pollution is localized around or downstream of urban built up areas such as Fquih Ben Saleh, Beni Mellal, Souk Sebt and Oulad Ayad. This pollution, caused by household wastes, is likely aggravated by waste dumping from the sugar refineries (SUBM, SUTA and SUNAT) as well as by waste water reuse in agriculture. Other zones with nitrate concentrations more than 50 mg NO₃ -/L were observed in wells situated downstream from Beni Amir and east of Beni Moussa. West Beni Moussa was generally not as polluted as east Beni Moussa, but an increase in ground water nitrate levels was observed in some downstream wells in west Beni Moussa. The diffuse nitrate pollution has been attributed to land use, irrigation and fertilization practices. These practices may be responsible for nitrate leaching beyond the root zone during the cropping season and may generate large quantities of residual inorganic nitrogen at harvest time. This represents a potential risk of nitrate ground water pollution in the next draining period. The physical characteristics of soil texture (clay, clay-silt and silty clay), percentage clay (29 % - 55 %) and their correlated parameters (especially the useful storage capacity) act to reduce the risk of nitrate leaching and ensures a control on ground water nitrate pollution. However, other observations including: a low water level table compared to the soil surface, particularly in the middle of the irrigated area and around the edge of the Oum Erbia river (<4 - 12m); a high soil infiltration rate in unsaturated soil, which varied from 20 -100 mm/h (permeable soils) to 200 - 450 mm/h (extremely permeable soils); a high level of organic matter in the soil (1.5% - 3.5%) and; total nitrogen contents (0.10% - 0.15%) would increase the aquifer vulnerability to ground water nitrate pollution. Similarly, the high soil mineralization capacity in the 0-60 cm soil layer (100 - 600 kg N /ha), combined with the ecological conditions, favoured an intense mineralization of the native organic matter of soil. This high mineralization capacity, which is characteristic of irrigated Mediterranean areas, could be partly responsible for this pollution. However, no correlation was found between ground water nitrate contents and linear combinations of physical environmental variables. This could be due to the homogeneity of the physical environment and to the movement of water in the saturated soil, in the direction of ground water flow, inducing the accumulation of nitrate pollution in the hydraulic downstream area of the water table.

Considering the degree of agriculture in this area, the situation could worsen rapidly, thus inducing significant ground water deterioration. In attempts to avoid this situation, many important measures at the regional scale are necessary, in order to rationalize agricultural practices and to increase the farmers' awareness of ground water nitrate pollution. These urgent preventative actions should be adopted in order to combine agricultural productivity and durability with water economy and water quality preservation. Some programs are in progress within this region. The objectives are essentially to reduce nitrate excess and to introduce more efficient irrigation techniques such as the pivot (frontal ramp) recommended for cereals and sugar beet, and localized irrigation system proposed for vegetable crops and arboriculture. Also, since the Oum Erbia river basin has also been subjected to a clean-up of urban and industrial pollution, it would be interesting to carry out a more precise study of nitrogen dynamics and nitrate leaching at the plot scale, especially for the most common practiced crops in the region (wheat, sugar beet and vegetable crops). This would advance the understanding and management of nitrogen cycles in the crop-water-soil system.

Finally, the database included as part of this study may be integrated in a water and nitrate movement simulation model. This will allow the prediction of nitrate leaching losses and identify, through scenario analysis, the links that should be acted on, in order to reduce the risk of ground water nitrate pollution in this region.

Keywords:

• Pollution,
• ground water,
• nitrate,
• soil physical and chemical characteristics,
• agricultural practices,
• urban pollution