447
Nitrate contamination of alluvial groundwater in the Ziz basin, southeastern Morocco
Z. Lgourna (a)*, N. Warner(b), L. Bouchaou(a), S. Boutaleb(a), T. Tagma(c), M. Hssaisoune(a), N. Ettayfi(a), and A. Vengosh(b)
(a) Laboratory of Applied Geology and Geo-Environment, Ibn Zohr University, Agadir, Morocco
(b) Division of Earth & Ocean Sciences, Duke University, Durham, NC, USA
(c) Polydisciplinary Faculty of Khouribga, Hassan 1st University of Settat, Morocco
*Corresponding author. E-mail : zainablg@gmail.com
Received 13 Sept 2014, Revised 10 Oct 2014, Accepted 24 Oct 2014
Abstract
Nitrate pollution usually originates from diffused sources, like intensive agriculture and unsewered sanitation or point sources, such as septic tanks and human and animal wastes. This study aims to investigate the degree of the influence of nitrate contaminants sources on groundwater. The groundwater in the Ziz basin has been polluted by municipal wastewater, and agricultural activities. The nitrate concentrations at fifty seven sampling points within the basin, ranged from 0 mg/L to 315.6 mg/L. In comparison to the Moroccan drinking water standards based on World Health Organization (WHO) standards of 50mg/L, 25%
of water samples showed higher nitrate concentrations above 50 mg/L and 36 % samples exceed the recommended limit of 25 mg/L, while, nitrates concentrations of 64% water samples were low than 25 mg/L. The analysis of nitrates variations suggests a contribution of the irrigation return flow containing nitrogenous compounds which can migrate downward from the unsaturated zone to groundwater, and transferring contaminants, and the high level of nitrates around the population agglomerations, which are not served by sewage systems, constitutes an additional source of nitrate pollution due to widespread and common use of septic tanks for domestic waste water. The nitrate pollution of groundwater in the Ziz Basin has negative effects on water quality, mainly, for the population that depends extensively on groundwater for drinking and other domestic needs. Thus, all agricultural and groundwater management measures should be applied to secure a sufficient water quality.
Keywords: Groundwater quality, Nitrates Pollution, Salinity, Arid area, Ziz basin.
1.
Introduction
In arid areas of the southeastern of Morocco, the competition for scarce water resources is intense and the combination of population growth, economic and agriculture development, results in overexploitation of the limited water resources. Water pollution is an additional problem in water resources, which becomes increasingly evident, and has led to serious ecological and environmental problems. As part of these arid areas, in the Ziz basin, water supplies for population are derived from both surface water and groundwater
448 and usually boreholes in rural areas are the main source of the extracted groundwater. The increasing demands of agricultural practices require high exploitation of water resources, and resulted in depletion of the groundwater resources and degradation of their quality. However, the aquifers are vulnerable to pollution by infiltration of surface contaminants, including nitrate derived from irrigation back flow and sewage, especially in many of the villages that are not served by sewer systems.
The objective of this study is to evaluate the sources that control the water chemistry and the nitrate pollution.
2.
Study area
At the Southeastern Morocco, the Ziz basin is located in an arid area and extends from an altitude higher than 1500m in the High Atlas Mountains to the plain of Tafilalet where the altitude is below than 1000 m.
This basin comprises a series of oases and one of the oldest dams in Morocco (Hassan Eddakhil). The stratigraphy is composed of (a) Precambrian Schist and limestones outcropping in Erfoud (b), Paleozoic schist, quartzites and sandstones; (c) Triassic red marls, doleritic basalts and anhydritic red marls; (d) Upper Jurassic green marls and dolometic limestones; (e) Infracenomanian sandstones, clays and Anhydrite (f) Cretaceous marls with gypsum, sandstones and limestone; (7) Eocene and Pliocene silts and alluvial deposits [1-2-3]. The climate is classified as an arid area, and the annual average of rainfall ranging from 250mm/y in the High Atlas to 130 mm/y in the Errachidia area and below to 75mm in the Tafilalt plain [4].
3.
Sampling and analysis
The 57 water samples were collected from groundwater and surface water throughout the different parts of the studied area in order to display a total representation of the basin.
Figure 1. Location map of the study area, showing the spatial distribution of samples collected in the Ziz basin.
449 The water samples are plotted with them location and numbers in (figure 1) to display their spatial distribution in the Ziz basin. The parameters such as, pH, temperature and conductivity (µS/cm) were measured in-situ. The analyses were carried out in Duke University in USA using different techniques for their contents on major and trace elements and also for the content of water stables isotopes
4.
Results and Discussions
Results of the study reveals that total dissolved solids (TDS) for investigated water ranges between 136.6 mg/L and 15011.1 mg /L and Electrical Conductivity ( E.C) values range from 484 µS/cm to 28300 µS/cm.
The salinity shows a gradual increase from the north to the south of the basin along the water flow path.
The linear correlations between TDS mg/L and other major dissolved constituents (figure 2) Cl- (R² = 0, 93), SO42- (R² = 0,781), and Na+ (R² = 0,764) suggest that water composition is controlled by an influence of evaporates dissolution. Nitrate content values range from 0 mg/L to 315.6 mg/L. In comparison to the Moroccan drinking water standards based on World Health Organization (WHO) standards of 50 mg/L, the figure 3 and table1 illustrate that a total of 14 wells (25%) showed higher concentrations, and 20 wells (36
%) exceed the recommended limit of 25 mg/L, while, nitrates concentrations of 35 samples (64%) were low than 25 mg/L.
Figure 2. Variations of major elements concentrations (mg/L) vs. TDS (mg/L) of groundwater and surface water in the Ziz Basin. Samples are sorted according by geographical distributions.
450 Figure 3. Frequency distribution of NO3 contents.
Table 1. Elementary statistics of nitrate contents (mg/L) in sampled water in Ziz Basin.
Upstream Downstream
Minimum 7.3 0
Maximum 315.6 250.9
Mean 48.7 47.2
Standard deviation 70.6 66.1
NO3- > 50 mg/L 6 8
Figure 4. Plot of nitrate concentration (mg/L) vs. total dissolved solids (TDS mg/L) of groundwater and surface water in the Ziz Basin. The NO3 – TDS correlation presents two main trends. Samples represented
by spatial distribution in the basin.
451 The relationship of nitrates versus Total Dissolved Solids (TDS) doesn’t show a linear correlation (figure 4).
However, it’s noticed: (i) a trend 1 of increasing chloride and TDS with a low range of nitrate values (<
25mg/L); (ii) a trend 2 with a relative increase between TDS and Nitrates, especially in the wells in agricultural and animal farm areas or near to human agglomerations where groundwater is contaminated with high nitrate concentrations more than 50mg/L
The analysis of nitrates variations suggests a contribution of the irrigation return flow containing nitrogenous compounds accumulated in the unsaturated zone formed by sediments with variable permeability. The high nitrate concentrations occurred within the agricultural activities areas, and mainly, in flood irrigated fields with poorly managed irrigation system; groundwater is easily altered by pollutants such as mineral fertilizers and products of organic compounds from agriculture. Moreover, the widespread and common use of septic tanks for domestic waste water constitutes an additional source of nitrate pollution and explains the high level of nitrates around the population agglomerations that are not served by sewage systems [5-6].
The anthropogenic effect on the groundwater quality in the Ziz Basin is mainly due to agricultural activities.
Since, the farmers have a tendency of flood irrigation use, and within the entire study area, most of the population depends extensively on groundwater for drinking and other domestic needs, overexploitation of aquifers induced mixing of groundwater with water resulting from irrigation return flow or sewage effluent.
5.
Conclusion
The results indicate that water quality in Ziz basin is controlled by chemical weathering of lithologic formations and anthropogenic activities. High nitrate content reflect contamination by agriculture activities along the basin. The anthropogenic effect is clearly visible in high nitrate content, and may result from agricultural backflow, wastewater or pollution from septic tanks.
Thus, the vulnerability of water resources to nitrate contamination in Ziz basin can be controlled by improvement of water use and groundwater management measures that should be applied to secure a sufficient water quality.
Acknowledgements
This study was supported from the Science for Peace program in NATO (Project ESP.MD.SFPP 983134).
We thank the Hydraulic Agency of Guir-Ghris-Ziz basin for their help throughout the project.
References
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