Water quality in the Hasouna Wellfields, Western Jamahirya System, Great Man Made River Project (GMRP)

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The Hasouna Wellfields, comprising 484 wells designed to produce 2.5 million m3/day of water, are part of the Libyan Great Man Made River Project (GMRP). They are located about 700 km south of Tripoli, in an area of stony desert and sand dunes, forming a rolling topography of low to moderate relief. The water will be conveyed by pipeline for water supply and irrigation to the coastal regions west of Tripoli. Geologically, the study area forms the northern edge of the Murzuq Basin and comprises rock formations ranging in age from Pre-Cambrian to Upper Cretaceous. There are two aquifers: the Cambro-Ordovician (CO) Hasouna Formation (the main aquifer), and the overlying Zimam Formation. The Zimam Formation, which contains saline water, is absent over the part of the field area where the freshwater-bearing, Hasouna aquifer is unconfined. The CO aquifer is major regional aquifer with a transmissivity range between 1500 to 2000 m2/day and a storativity range from 2.2x 105 to 6.4x 10-2 between the confirmed and unconfined areas of the Wellfields. Pumping tests have shown that the CO aquifer is leaky-confined type over part of the study area. Water quality is good with average total dissolved solids (TDS) of 1039 mg/l, average chloride content of 600 mg/l and a positive redox-potential, indicating an oxidizing environment. However, the concentration of dissolved C02 ranges from 3.7 to 97 mg/l and nitrate ranges from 0.0 mg/l to 133 mg/l. These two constituents can result in wellfield operational and consumer health problems respectively. The main objective of this study was to make a complete investigation of the water quality, hydrochemical processes and all the physical and chemical properties of these Wellfields. Special attention was focussed on the problem of corrosion due to the C02 and the nitrate. This research used various methods, including the classification of water type, analysis of field data, study of major and minor cations, anions and trace elements, and the determinationof water aggressivity supported by field study corrosion coupons, hydrochemical processes, water quality for potable uses and isotopic analysis. The calculated saturation and corrosion indices indicated that the Hasouna water is corrosive. The field study using corrosion coupons demonstrated that the corrosive properties of the water can change under short term non-pumping conditions to be scaling in character. Although the GMRP has addressed the problem of C02 by stripping it in de gassing towers before it enters the pipeline, the results of this research will assist the process by identifying areas of high and low C02 concentration. A similar approach of blending low and high nitrate water can be used for the nitrate problem. The origin of high nitrate levels in the Hasouna water is problematic. Similar high levels occur in Sirt and Kufra basins in Libya, (Edmunds and Wright, (1979)). The isotopic analysis of 815N (31 samples) and 8180 (6 samples) from nitrate for the Hasouna water suggested the source could be precipitation and soil nitrate (815N and 8180). The 815N signature is similar to NH4, N03, N2 and effluent/manure. These sources imply a more humid climatic environment than the present. The stable isotope data indicate a meteoric origin for the groundwater, and the radiocarbon age is > 8 ka, which suggests that the Hasouna water is likely to have a late-Pleistocene or very early Holocene origin. At that time the climate was more humid than the present day arid conditions. The water in Hasouna is classified to be class C3, for salinity hazard (High) and class S 1/S2, for Sodium Hazard. This indicates that the blended water is suitable for irrigation with special management to control salinity. The analysis of the speciation properties of the ionic composition of the water indicates undersaturation with all of the plausible mineral species. The hydrochemical processes, which control the composition of the Hasouna water are mixing, ion exchange and dissolution of gypsum and dolomite.
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