Investigation of Chemical and Radiochemical Fingerprints of Water Resources in Siwa Oasis, Western Desert, Egypt

Water resources in Siwa oasis, Egypt; included the Nubian sandstone aquifer system (NSSAS), Tertiary carbonate aquifer system (TCAS) as well as springs and lakes. Existing chemical and radiochemical fingerprints of these resources in the central and western parts of the oasis were examined based on the analyses of 30 water samples collected in 2015. In each sample, the analyzed chemical and radioisotopic constituents were Cl^-, SO₄^2- , CO₃^2-, HCO^{3 -}, Na⁺, K⁺, Ca²⁺ and Mg²⁺, Al, B, Ba, Cd, Co, Fe, Cu, Mn, Mo, Ni, Pb, Sr, V, Zn, Si and Cr, ²²⁶ Ra, ²³²Th and ⁴⁰K. Other physical properties included ph and TDS were measured in the field. The chemical analysis was carried out following the ASTM methods, while the radiochemical measurements were done using a hyper pure germanium (HPGe) gamma ray spectrometry. Results of this work may be advantage ous to promote the hydrologic database in the oasis, where levels of the natural radioactivity in groundwater resources were established for the first time. Water resources were classified based on their contents of TDS into fresh water for the NSSAS, brackish to salty water for TCAS, brackish water for springs and brine water for lakes. The artesian water of the NSSAS and TCAS and water of the springs were alkaline, while the water of the lakes was acidic. Chemical characters of the ground water of the NSSAS reflected a little response to the processes of water-rock interactions, and the vice versa was observed for the groundwater of the TCAS. The ground water of the NSSAS was dominated by HCO^3-Na genetic type characterizing the meteoric origin of water. The ground water of the TCAS was characterized by the presence of three genetic types of water (SO^4-Na, Cl-Mg, and Cl-CA water), indicating the modification of the recharged water from the underlain NSSAS due to the effects of water sediment interactions and mixing with trapped ancient sea water. The chemical properties of spring water were, greatly similar to those of groundwater of the TCAS, which indicated that the springs, under study, discharged their water through faults cutting in the TCAS. Lake water was the more mineralized water in the oasis. It was characterized by the Cl-Ca and SO^4-Na genetic types. The abundance of heavy metals in the TCAS, springs and lakes was, generally, higher than it in the NSSAS. Almost, B, Ba, Sr and Si were detected with different concentrations in all water samples from the oasis, while the concentrations of the rest of metals were found to be below or near the detection limits of the applied analytical procedures. The radiochemical constituents increased in the central part of the oasis, like the TDS, major ions and heavy metals ones. The maximum activity values recorded were 5.6±0.7 Bq/l (NSSAS), 6.7±1.6 Bq/l (TCAS) and 25. 6 ±5.9 Bq/l (TCAS) for ²²⁶Ra, ²³²Th and ⁴⁰K , respectively