European Journal of Scientific Research ISSN 1450-216X / 1450-202X Vol. 147 No 4 November, 2017, pp. 412-425 http://www. europeanjournalofscientificresearch.com Salinization Risk Assessment of Soil and Groundwater: A Case Study in Sidi El Hani Basin (Central-Eastern Tunisia) M’nassri Soumaia Département du Génie des Systèmes Horticoles et du Milieu Naturel Université de Sousse, Institut Supérieur Agronomique de Chott Mariem BP 47, 4042 Chott Mariem, Tunisie E-mail: [email protected] Tel: +21696714748 El Amri Asma Département du Génie des Systèmes Horticoles et du Milieu Naturel Université de Sousse, Institut Supérieur Agronomique de Chott Mariem BP 47, 4042 Chott Mariem, Tunisie E-mail: [email protected] Dridi Lotfi Département du Génie des Systèmes Horticoles et du Milieu Naturel Université de Sousse, Institut Supérieur Agronomique de Chott Mariem BP 47, 4042 Chott Mariem, Tunisie E-mail: [email protected] Tagorti Mohamed Département des Sciences Biologiques Institut Supérieur de Biotechnologie de Monastir, BP 56, 5000 Monsatir, Tunisie E-mail: [email protected] Hachicha Mohamed Laboratoire de Valorisation des Eaux Non Conventionnelles Institut National de Recherches en Génie Rural, Eaux et Forêts, BP 10, 2080 Ariana, Tunisie E-mail: [email protected] Majdoub Rajouene Département du Génie des Systèmes Horticoles et du Milieu Naturel Université de Sousse, Institut Supérieur Agronomique de Chott Mariem BP 47, 4042 Chott Mariem, Tunisie E-mail: [email protected] Abstract The aquifer of Sidi El Hani, located in the central-eastern part of Tunisia, is the main source for irrigation activities. However, in the last decades, it is largely threatened by salinization. Therefore, a campaign of groundwater samples was carried out in the Ouled Chamekh plain. A campaign of soil samplings was also performed in a parcel located in the study area. A 27 soil samples were collected with a total depth reached 30 m. In addition to the experimental approach, we used statistical and hydrochemical analysis to assess the Salinization Risk Assessment of Soil and Groundwater: A Case Study in Sidi El Hani Basin (Central-Eastern Tunisia) 413 major processes of salinization of soil and groundwater. The results show that the degradation of the groundwater quality mainly results from natural factors such as rock weathering and anthropogenic factors associated with the irrigation water return. Moreover, based on the generalized Residual Alkalinity (RA) concept, calcite and gypsum have a tendency to precipitate when the soil solution gets concentrated under evaporation. This is in agreement with the saturation index. Furthermore, the RA reveals that the soil solution follows a neutral saline pathway. Keywords: soil degradation, groundwater salinization, residual alkalinity, concentration factor, Sidi El Hani.. Introduction Nowadays, groundwater salinization is considered as a common chanllange faced by many countries all over the world. This problem is most severe in arid and semi-arid regions where freshwater resources tend to be scarce. The groundwater salinization is largely caused by natural processes that can be accelerated by the anthropogenic activities. Indeed, the salinization of groundwater can be due to evaporation, water-rock interaction (Risacher and Fritz, 2009; Lucas et al ., 2010), seawater intrusion (Fadili et al., 2016), upconing of saline waters from deep layers (Zammouri et al., 2007) and irrigation return flow (Jiang et al., 2009; Najib et al., 2017). Saline groundwaters used in irrigation, particularity in arid and semi-arid regions (Bouarfa et al., 2009), affect the soil as well as the crop yields (Louati et al., 2015). The most harmful effects associated to its use are sodification, salinization and alkalinization, which may alter soil structure (Hachicha et al., 2010). The unconfined aquifer of Sidi El Hani, located in the central-eastern part of Tunisia, constitutes the only water table resource available for agricultural use. This is, especially attributed to the scarcity of surface water, which is limited to the sabkhas and wadi Cherita. In these last decades, this aquifer suffers from an increasing vulnerability to salinization due to an ever increasing population and irrigation needs. As a consequence, the water salinity varies between 4 and 8 g/l (Majdoub et al., 2012). Therefore, the irrigated lands of this region are characterized by a progressive salinization (Louati et al., 2015), which may lead, in the medium term, to a reduction in agricultural land and to a future economic instability of the area. Furthermore, it is more important to understand the relation between the groundwater quality and soil processes in order to help farmers to adopt the appropriate soil and water management strategies and integrate the sustainable management of groundwater resources. The current study aims to assess the major processes of soil and groundwater salinization using a combination of physical, chemical and mineralogical analysis, in sequence to obtain a general understanding on the soluble salts availability in the system soil-groundwater. 2. Material and Methods 2.1 Study Area The study area is the Ouled Chamekh plain located in center-east of Tunisia. It is surrounded by the mountain chains of El Guessat and Ktitir that barely reach more than 100 m of altitude in the western part. The eastern and southern parts are occupied by the endorheic depression of the sabkhas of Sidi El Hani and Cherita (Fig. 1). It is characterized also by an undeveloped hydrographic network (Essefi et al., 2013). The study area is classified as a semi-arid area with an annual average precipitation of 270 mm. January and February are considered as the coldest months of the year, where the average maximum temperature is 16°C and the minimum is 8°C. August is considered as the hottest month 414 M’nassri Soumaia, El Amri Asma, Dridi Lotfi, Tagorti Mohamed, Hachicha Mohamed and Majdoub Rajouene with an average temperature of 30°C. The average annual relative humidity is around 71 and 50%, respectively, in November and July. Figure 1: Location map of the Ouled Chamekh plain According to the geological setting, the study area is extended from the lower Pleistocene to Holocene Fig. 2. The Pleistocene is mainly composed of detrital formations, limestone, marls and a lenticular of gypsum that are outcropped in the El Guessat and Ktitir mountains (Tagorti et al., 2013; Essefi et al., 2014). The Holocene is the dominant formation in the study area. It is characterized by sands, silts, clays, and gypsum. The structural framework is characterized by the presence of several synclines and anticlines. In this plain, two anticlines were detected such as the El Guessat and Ktitir anticlines. These structures are separated by a vast plain which take place within a collapsed zone with a synclinal form. They were occupied by the sabkhas of Sidi El Hani and Cherita, which are composed of salt with evaporate deposits of halite and gypsum. According to the tectonic network, the structure of the study area is strongly controlled by the outcrops of the Mio-Plio-Quaternary such as the faults of Ktitir, Hajeb Layoun and Sidi El Hani. The fault line of Sidi El Hani causes the subsidence of the sabkha of Sidi El Hani in the eastern boundary of the study area. The direction of groundwater flow of Sidi El Hani aquifer is mainly from the west to the sabka of Sidi El Hani in the north-east and to the sabkha of Cherita in the south with a velocity ranging between 29 10 -6 and 49 10 -6 m/s. Referring to a previous piezometric surface map (Dridi et al., 2014), the thickness of Sidi El Hani aquifer varies from 3 m, at the eastern part of the basin, up to 50 m, in the center. The hydraulic head varies between 35 and 80 m NGT (Tunisian General leveling System). Irregularities of the water table contours indicate high variation of the hydraulic conductivity of the alluvial aquifer from the west to the south and east due to the heterogeneity of sediments. The transmissivity varies between 2×10 -4 and 5×10 -3 m2/s and the hydraulic gradients are in the order of 0.3 to 0.45%. Salinization Risk Assessment of Soil and Groundwater: A Case Study in Sidi El Hani Basin (Central-Eastern Tunisia) 415 Figure 2: Geological setting of Ouled Chamekh plain (ONM, 1985: Echelle 1:50000) 2.2 Sampling and Analytical Techniques For this study, 49 water samples were collected in March and April 2015 from different wells that are used for irrigation. The static levels of the wells vary between 5 and 26 m (Fig. 3a). All sampling water was taken after pumping water for around 15 to 20 min. In addition to the groundwater, a campaign of soil samplings was also performed in a parcel located in the study area (Fig. 3b). The 27 soil sampled depth reached 30 m. various techniques, depending on the lithology, were used for extracting soil samples. Core drilling was suitable for clayey layers, whereas a rotary drilling using a tri-cone roller bit was performed in sandy layers. Figure 3: Location of the water samples (a) and the parcel of soil samples (b). 416 M’nassri Soumaia, El Amri Asma, Dridi Lotfi, Tagorti Mohamed, Hachicha Mohamed and Majdoub Rajouene 2.2.1 Ground Water and Soil Physio-Chemical Analysis Water temperature, pH and electrical conductivity (EC) were measured in situ. The collected groundwater samples were analyzed in the laboratory. For total dissolved salts (TDS), cations (Ca 2+ , 2+ + + - - 2- Mg , K and Na ) and anions (Cl , HCO 3 and SO 4 ), the standard methods suggested by the American Public Health Association were used (APHA, 2005). TDS were measured by evaporating a pre-filtered sample to dryness, K + and Na + were determined by flame atomic absorption 2+ 2+ - - spectrophotometry. In the case of Ca , Mg , Cl and HCO 3 , they were analyzed by volumetric 2- methods.