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Colorado State University (2013)

Isotope and noble gas study of three aquifers in central and southeast Libya

Al Faitouri, Mohamed S. E

Titre : Isotope and noble gas study of three aquifers in central and southeast Libya

Auteur : Al Faitouri, Mohamed S. E

Université de soutenance : Colorado State University

Grade : Doctor of Philosophy (Ph.D.) 2013

Résumé
Libya suffers from a shortage in water resources due to its arid climate. The annual precipitation in Libya is less than 200 mm in the narrow coastal plain, while the southern part of the country receives less than 1mm. On the other hand, Libya has large resources of good quality groundwater distributed in six basin systems beneath the Sahara. In 1983, the Libyan government established the Great Man-Made River Authority (GMRA) in order to transport 6.5 million cubic meters a day of this groundwater to the coastal cities, where over 90% of the population lives. This large water extraction of one million cubic meters per day (or greater) from each wellfield has the potential to greatly stress the water resources in these areas. This study focuses on three GMRA wellfields in two sedimentary basins (Sirt and Al Kufra) in central and southeast Libya. The Sarir wellfield is located within the Sirt basin and consists of 126 production wells ; the Tazerbo wellfield in the Al Kufra basin has 108 wells ; and the proposed Al Kufra wellfield is also in the Al Kufra Basin and will have 300 production wells. With the large amount of water to be extracted from these aquifers, it is necessary perform hydrogeological studies to better characterize the physical and chemical properties of the aquifers in order to provide the information needed for planning over the proposed 50-year life span of the projects. For this study, water samples were collected for analyses of the stable isotopes of water ( 2 H (D) and 18O), 14C and noble gases. These data are used to determine the age of the water in the aquifers and when recharge occurred and to estimate the climate at the time of recharge (paleotemperatures). In addition, the helium content of the water will allow for the determination of the fluxes of helium in the crust at the locations of the aquifers. These fluxes include the helium produced in situ in the aquifer and fluxes from outside the aquifer (crust and mantle helium). Three extraction wells were sampled from Sarir for 14C analysis and the ages of these samples range from 9800 to 15200 ybp. At Tazerbo, five extraction wells were sampled and the age of the water averages 24000 ybp over a depth range of 150 m. In the Al Kufra area there were two wells sampled located approximately 100 km apart and at two different depths. The apparent age at the shallow well is 12000 ybp and at the deep well it is 21000 ybp. These ages indicate recharge during wet periods during the late Pleistocene and early Holocene. Recharge possibly occurred along ancient lakes and paleoriver channels The stable isotope compositions of water in all three aquifers are greatly depleted relative to that of modern precipitation. The results from the northern most aquifer (Sarir) is the most enriched and plot below the Global Meteoric Water Line (GMWL) and show evidence of potential effects of evaporation. The waters from the Al Kufra Basin aquifers are more depleted and plot on the GMWL. These results suggest that the groundwater recharge during the late Pleistocene and early Holocene was recharged at a time in which the regional monsoonal patterns were different than today. Other workers have concluded that the paleomonsoons most likely came from the southeast out of the Atlantic causing the waters to be depleted relative to modern precipitation. Noble gas recharge temperatures (NGRTs) allow for the determination of average annual temperature during the time of recharge and to compare these values with the current annual temperatures. Noble gas recharge temperatures were determined for one sample from Sarir, three samples from Tazerbo and one from Al Kufra (Table 4). For Sarir, the NGRT was 18°C which is about 5 degrees cooler than the average annual temperature today. For Tazerbo, one sample yielded a temperature of 20°C and the other two yielded temperatures of 25°C, compared to the present average annual temperature of 23°C. For the sample from Al Kufra, the recharge temperature is 19°C compared to the present average of 24°C. These results are consistent with findings from around the world that indicate average temperatures being several degrees cooler in the late Pleistocene than today. The analyses of the noble gas data show that there are significant amounts of 4 He in the waters in excess of what is to be expected by water equilibration with the atmosphere. This indicates that for each aquifer, there is a strong component of terrigenic helium (helium produced within the earth due to radiogenic decay of U and Th decay products). Groundwater age in the aquifer can be estimated by determining the in situ production rate of 4 He using the U and Th content (assuming no external sources of helium). This method yields average water ages of 1.3 x 105 years for Al Kufra, 5.1 x 105 years for Sarir and 4.5 x 106 years for Tazerbo. The 4 He ages for Sarir and Al Kufra are about one order of magnitude greater than the 14C ages and the Tazerbo 4 He age is about two orders of magnitude greater. Based on looking at helium isotope ratios, it can be seen that there are external fluxes to the aquifers which would add more helium to the system than could be produced in situ. The fluxes of helium for each aquifer system are determined by dividing the 4 He concentrations by the 14C age. The resulting fluxes are consistent with fluxes determined elsewhere in sandstone aquifers. Helium isotope ratios (3 He/4 He) provided information on the sources of helium in the aquifers. The main sources that can be identified are the ratios of helium produced in situ, the ratio of helium from crustal rocks outside the aquifer and helium that comes from the mantle. It was determined that for the total amount of helium at Sarir, crustal helium accounts for 85%, mantle helium accounts for 14.6% and in situ produced helium accounts for 0.4%. For Al Kufra it is 95% crustal, 3.3% mantle and 1.7% in situ. For Tazerbo, values are 90% crustal, 9.7% mantle and 0.3% in situ.

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