APPLICATION OF 15N ISOTOPES TO MANAGEMENT OF GROUNDWATER QUALITY, JABAL HASOUNA WELLFIELDS, GREAT MAN-MADE RIVER PROJECT, LIBYAN ARAB JAMAHIRIYA

摘要

The Jabal Hasouna wellfi elds are located some 700 kilometres south of Tripoli, the capital of Libya, and are part of the Great Man-Made River Project for water supply to coastal areas. The wellfi elds include 484 deep wells with a total design production rate of 2.5 million cubic metres per day from the Cambro-Ordovician sandstone aquifer. The aquifer is unconfi ned in the southwestern portion of the wellfi elds and the remainder is confi ned, but most of the wells are located in the unconfi ned part. Water quality is good, but concentrations of nitrate up to 133 mg/L occur in the wellfi elds, with slightly higher concentrations in the unconfi ned aquifer. Previous 14C dating of the groundwater gave ages of 10.4 ka ± 0.4 ka to 17.2 ka ± 0.1 ka BP and stable isotope data indicate a meteoric origin from recharge during a humid climate phase of the Sahara. Analysis of δ15N and δ18O from groundwater nitrate showed a range of 7.11 to 12.64‰ for the δ15N and 12.51 to 22.35‰ for the δ18O. At Jabal Hasouna these ranges suggest that soil nitrate may be the source. There appears to be a low rate of denitrifi cation so high nitrate levels will be a long-term concern for water quality.