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Quantitative status, vulnerability and pollution of groundwater resources in different environmental and climatic contexts in Sardinia and in Ethiopia

Bushra, Abdelwassie Hussien (2012) Quantitative status, vulnerability and pollution of groundwater resources in different environmental and climatic contexts in Sardinia and in Ethiopia. [Doctoral Thesis]

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Abstract

The Raya Valley is located in the northern part of Ethiopia within the Regional State of Tigray. The valley is part of the series of grabens at the western margins of the Afar Depression. Rainfall in this area is erratic both in time and space; hence it has suffered from a number of severe droughts and associated famines, and is chronically food-insecure. The surface water resources are characterized by streams, which flow strongly during the short rainy season and no perennial flow of surface water there is during the dry period, except the minor seepages that die out within a short distance in the valley. Therefore, groundwater is the only source for both domestic and irrigational development. Previous studies were confined to developing these resources. Hence, the objective of this research, in the Raya Valley, is to understand and to characterize the groundwater system of Raya Valley, using hydrogeology, groundwater chemistry and isotope hydrology tools and to come up with different options that may help to monitor for efficient management of the groundwater. In the meantime, this work incorporated water balance evaluation and identification of the degree of vulnerability to contaminants of the Rio Mannu di Pabillonis Basin in the central part of Sardinia, Italy. Finally corresponding results were compared in relation to their climatic differences. Field traverses have been undertaken to collect relevant information regarding geology, hydrology, and hydrogeology in the Raya Valley (in 2009, 2010 and 2011) and in the Rio Mannu di Pabillonis Basin (in 2010 and 2011). An inventory of groundwater levels of boreholes and springs was made. Land-use was also investigated during field visits to be used as ground truth for the verification of existing maps. The isopiezometric/static water level contour maps show that the groundwater flow toward the centre and then all flow to the south in a similar fashion to the surface water, in the Raya Valley. In the Rio Mannu Basin, the groundwater flow direction is also to the centre of the basin and finally toward the north, following the topographic gradient. Water samples, from Raya Valley and Rio Mannu Basin, were collected and analyzed and results were compared with Ethiopian and Italian drinking water standards, respectively. For most of the chemical constituents, water samples are within the safety standards with regard to the Raya Valley, whereas some elevated NO3- values have been noticed in areas where the dominant activity is animal farming in Rio Mannu basin. In situ water temperature and electric conductivity measurements in the Raya Valley have shown distinct zonation along west to east, with the highest values recorded in the east. In the meantime, Ca-HCO3 water type dominates the western boreholes and springs, while Na-Cl-SO4 water type dominates the eastern boreholes and springs. The dominant water type is Na-Cl in Rio Mannu Basin. Rain water samples of two consecutive rainy seasons (2009 and 2010) from different elevations were analyzed for stable isotope (oxygen and hydrogen). The results were used to construct a Local Meteoric Water Line (LMWL) of the area. Additional 17 borehole and 3 spring samples were collected and analyzed for the same isotopes in the same laboratory. Correlation between δ 18O and δ2H (LMWL) is δ2H = 7.4756 δ18O + 11.827, (r2= 0.9959) and hence the later data were plotted alongside the meteoric water to define the possible sources of the groundwater recharge in the area of interest. Most of the borehole and spring samples plotted nearly along the local meteoric water line. But, the eastern boreholes and the hot spring, in the east, plotted away from the LMWL in the direction of higher water-rock interaction, with regression equation δ18O and δ2H (LMWL) δ2H = 6.7244 δ18O – 0.0795, (r2= 0.998). This indicates the existence of another source of water recharge to the Raya Valley. The Thornthwaite and WetSpass models were applied to simulate the hydrological water balance of both areas. In the Thornthwaite method the water balance was made using average, less than and greater than average precipitations in order to observe which of the water balance components are more affected positively or negatively. Thus the results signify that Cumulative Available Water is more sensitive to variation in precipitation and follows the same direction of precipitation, while Actual Evapotranspiration follows the reverse way, i.e. it decreases when precipitation increases and vice-versa. Thus, 86.5% of the average precipitation is lost as evapotranspiration, 4.3% becomes surface runoff; and 9.2% is groundwater recharge in the Raya Valley. Meanwhile, it is 73.5% for evapotranspiration, 11.8% for surface runoff, and 14.7% for groundwater recharge in the Rio Mannu Basin. Seasonal and annual evapotranspiration, surface runoff, and groundwater recharge are the main outputs of the WetSpass model. Accordingly, 81.1% of the precipitation in the basin is lost through evapotranspiration, 5.9% becomes surface runoff, and 12.9% is groundwater recharge in the Raya Valley. And 54.4% of the precipitation in the basin is lost through evapotranspiration, 6.8% becomes surface runoff, and 39.8% is groundwater recharge in the Rio Mannu Basin. The intrinsic vulnerability map of the Raya Valley was prepared using SINTACS program with the help of ArcGIS. After running the model, the areas inherently vulnerable where rated as very low to high based on the results of the model. The alluvial deposit, including the Gerjalle swampy area, was rated as high vulnerable, demonstrating the higher risk in the main target area of the valley. The Raya Valley is a graben running parallel to the western escarpment of the Afar Depression, in a north- south direction. Description of the local geology and cross-sections along different orientations elucidate that the valley floor is mainly composed of basin fill deposits underlain by moderately to highly fractured basalt. The thickness of the alluvial deposit is variable and ranges from about 18 to more than 311 meters. The minimum thickness is in the western and eastern flanks of the valley, whereas the maximum thickness is obtained towards the eastern part of the valley center. No significant impermeable layer that can act as a confining bed is observed. The gradient of the alluvial sediment is low (0.006 in a total length of over 60 kilometers along the flow direction) and the alluvial aquifer is inter-granular containing gravel, sand and with some intercalation of silt and clay. The groundwater recharge, as explained above, has two sources, precipitation and groundwater inflow. Therefore, the surface water divide does not coincide with the groundwater divide. The hydraulic conductivity is variable both in lithology and space, thus it varies between 0.03-0.37 m/day in the volcanic rocks, and 0.11-26 m/day with mean value of 2.8 m/day, in the alluvial sediments. Evidences from boreholes pierced in the volcanic rocks, beneath the alluvial sediments, show significant quantities of water. Therefore, an attempt to mathematically model the area should verify the groundwater inflow and consider the weathered and fractured part of the volcanic rocks as a second aquifer.

Item Type:Doctoral Thesis
Date:09 March 2012
Tutor:Barbieri, Giulio
PhD classes:Ciclo 24 > Scienze della terra
Coordinator:Cidu, Rosa
Institution:Universita' degli Studi di Cagliari
Divisions:Dipartimenti (fino a dicembre 2011) > Dipartimento di Ingegneria del territorio
Subjects:Area 04 - Scienze della terra > GEO/05 Geologia applicata
Uncontrolled Keywords:Recharge, groundwater temperature, stable isotopes, vulnerability to pollution
ID Code:673
Deposited On:14 Mar 2012 10:53

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