Application of Vertical Electrical Sounding for Delineation of Sea Water Intrusion into the Freshwater Aquifer of Southern Governorates of Gaza Strip, Palestine

Abstract Vertical Electrical Sounding (VES) proved to be an important tool for mapping the freshwater-saltwater interface. Seawater intrusion is serious problem especially in coastal areas. 14 Vertical electrical soundings with Schlumberger array were carried out (perpendicular to the coast) in southern part of the coastal aquifer in Gaza Strip (Khan Younis and Rafah Governorates) to study the subsurface geologic formation and delineation of seawater intrusion into freshwater aquifer. Lithology and hydrochemistry data from 12 bore holes were used to integrate the results. The results reveals that 50% of the total sounding are interpreted of 3 layer modes, while the other 50% are of 4 layers model. Interpreted 2D geoelectrical models (in two cross sections parallel to the coast line) showed that seawater intrusion with electrical resistivity less than 2Ωm is dominant along the section parallel to coast line at 400m east of it and for shallow depths (3-10m depth), while in the other section (1000m) east of the coast line seawater intrusion is detected in some places while the other parts still have brackish/fresh groundwater with resistivity more than 10Ωm at different depths (26-50m). ملخص: اثبتت طريقة المقاومة الكهربائية العمودية (VES)  فعاليتها فى تحديد الحدود الفاصلة بين المياه العذبة و المالحة . تعتبر مشكلة تداخل المياه المالحة فى مياه الخزان الجوفى العذب مشكلة ذات اهمية و خاصة فى المناطق الشاطئية. لقد تم تنفيذ 14 مجسة كهربائية عمودية بأستخدام تشكيل شلمبرجير بشكل قطاعات عمودية على على الساحل الجنوبى لقطاع غزة (محافظات خانيونس و رفح) و ذلك بهدف دراسة التركيب الجيولوجى للخزان الجوفى و تحديد مدى تداخل مياه البحر فيه . كما تم الاستعانة بالمعلومات الليثولوجية و الهيدروكيميائية من 12 بئرمياه فى مناطق الدراسة. اظهرت النتائج ان 50% من منحنيات المقاومة الكهربائية المقاسة تمثل نموذج لثلاث طبقات جيولوجية بينما 50% تمثل اربع طبقات. تم اختيار قطاعين متوازيين و موازين لشاطىْ البحر, يبعد الاول 400 م عن الشاطىْ بينما الاخر يبعد 1000م عنه لدراسة تغيرات المقاومة فى بعدين. أظهرت النتائج ان مياه الحر ذات المقاومة الكهربائية أقل من 2 أوم.متر تسود على طول القطاع القريب من الشاطىْ و لاعماق ضحلة (3-10 م), بينما فى القطاع الثانى و الذى يبعد 1000م شرقا تم تحديد أمكن تداخل مياه البحر المالحة فى بعض الاماكن و لم يتم تحديدها فى أماكن أخرى مما يفسر على انه لا زال تلك المناطق تحتوى على مياه عذبة/ مالحة ذات مقاومة كهربائية اكبر من 10 أوم.متر و على أعماق تتراوح بين 26-50 متر

Estimation of Aquifer Parameters from Vertical Electrical Sounding Measurements South Gaza Strip, Palestine

In this study quantitative interpretation of some 14 Vertical Electrical Sounding (VES) points using the Schlumberger array and data from 10 boreholes were applied to study the aquifer geoelectrical/hydrogeological parameters in the southern governorates of Gaza Strip. The quantitative interpretation of (VES) results analyzed in order to find empirical relations between different aquifer parameters. The results represent the predicted hydraulic conductivity (K), transmissivity (Tr), TDS, earth resistivity, water resistivity, porosity and Dar-Zarrouk parameters (Longitudinal conductance and Transverse resistance). The results show the earth resistivity is strongly affected by the chemistry and salinity of the groundwater. The relation between longitudinal resistivity (ρL) and groundwater salinity (TDS) was achieved for groundwater aquifer. The geoelectric (Dar-Zarrouk) parameters determined and interpreted by using the integration of the results, which indicates that longitudinal conductance has a high values at the western part of the study area represents an increase in groundwater movement horizontally, that is a strong evidence of seawater intrusion. Porosity was estimated to be in the range of 38–52% with an average value of 46%. Transmissivity values for the study area were calculated. This result helps to delineate the contact between the saline and fresh groundwater aquifer in the study area

Electrical Resistivity Tomography for Coastal Sea Water Intrusion Characterization along Rafah Area, South of Gaza Strip, Palestine

Resistivity methods mainly electrical resistivity tomography 2D and resistivity sounding 1D represent the main geophysical tools to delineate seawater intrusion into freshwater aquifers. Thirty 2D imaging sections were applied with Wenner-Schlumberger array (NW-SE direction), distributed on six profiles (perpendicular to the coast) in southern part of the coastal aquifer in Gaza Strip (Rafah Governorates), supported by 4 Vertical electrical soundings points with Schlumberger array, to map shallow-depth seawater intrusion into freshwater aquifer and study the subsurface geologic formation. Analysis and interpretation of the electrical resistivity tomography 2D and 1D sounding results reveal that seawater intrusion zone appears at the western part of the study area and extends along Rafah coast with a difference distance and depths. The depths are ranging between (few meters to 20 m) in the western sections, while they are betwee 40 to more than 65 m in the eastern section. The resistivity data results were correlates well with the 3 boreholes data and 3 chemical analyses water samples