Ground water nitrate pollution in Tadla (Morocco)
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.