INTEGRATED INVESTIGATION TO LOCATE A WASTE DISPOSAL AREA

INTEGRATED INVESTIGATION TO LOCATE A WASTE DISPOSAL AREA 1 FOR OIL DRILLING PRODUCTS G. APOSTOLOPOULOS 2, K. PAVLOPOULOS 3, C. ANTONIADES 4, I. LOUIS 5 and C. VLACHOU 4 Nestoros 1, 17564 P.Faliron, Athens, Greece 2 National Documentation Centre 3 Harokopio University 4 Greek Ombudsman 5 University of Athens, Geophysics & Geothermics Dpt., Panepistimiopolis, Ilisia 15784, Athens, Greece, jlouis@geol.uoa.gr INTRODUCTION Aim of this project was to find an area proper for waste disposal produced by deep drilling for oil in NE Peloponnese, Greece (Fig. 1a). The project deals with legal requirements and geological - geomorphological issues. Geophysics plays an important role in such problems since it gives information for the near surface features of the earth responsible for the migration of pollutants. The project tread the following phases: (a) consideration of all requirements set by legislation so that selected locations can be further investigated (b) analysis of geological and tectonic characteristics of the area, (c) analysis of the geotechnical characteristics of the formations and proposal for further geotechnical investigation, (d) analysis of the hydrogeological and geomorphological characteristics of the area, (e) evaluation of the ecology and ecosystems of the area, (f) selection of the areas fulfilling all legal requirements and have the best characteristics for waste disposal sites, (g) conduct geophysical investigation in the most appropriate areas selected, (h) design geochemical survey soil groundwater monitoring and establishment of relative standards, (i) final selection of the most appropriate area for disposal of drilling products, and (j) proposal for monitoring of the drilling and disposal area. OIL DRILLING PRODUCTS Oil drilling products include (a) drilling water (98%) constituted from salts and heavy metals diluted, Η 2 S, CO 2, formation solids, chemical additives, some radioactive material, water with lack of O 2, organic material etc., (b) drilling fluids constituted from water and oil based heavy metals (Ba, Cr, Cd, Hg, Pb), carrots (2%) and (c) other wastes (<1%) connected to the whole installation of the oil drilling. LEGAL ISSUES According to the legislation, waste disposal areas should be located at least 5 meters away from villages, in areas with the most appropriate geological, hydrogeological and geomorphological conditions to prevent leakage of pollutants. They have to occur away from nearby residential or archeological areas, woods, wetlands or other sensitive areas and they must lie in small altitudes with gentle ground surface slopes. Design for monitoring of the environmental impact after disposal has to be taken into account as well. METHODOLOGY From geological and environmental point of view the area selected for waste disposal should possibly be a natural isolator preventing in that way the leakage of toxic substances or pollutants to the surrounding soil, subsurface, surface and underground water. That means a very good knowledge of the geologic conditions prevailing in the investigated area, ensuring impermeable geological formations (barriers), absence of landslides, etc. Selecting the areas that fulfill legal requirements, the next step is to find which of them fulfill further conditions such as the existence and location of aquifers and their recharge areas, drainage basins, direction of underground water flow, presence of wells or water drillings, quality of underground water, permeability of lithological formations, presence of karsts, quarries and mines locations, seismic risk evaluation etc. Areas fulfilling the above requirements are further examined. Thus, detailed geological and hydrogeological surveys are performed to evaluate specific characteristics such as the presence and extent of shallow underground water, tectonic zones, discontinuities, erosion of geological formations and geotechnical conditions. Finally, areas passing the above test are further investigated with an extended and detailed survey incorporating geophysical methods and boreholes. Finally the area with the best characteristics is selected and the monitoring system is designed. 1 Submitted for presentation at Stavanger 23, 65 th EAGE Conference & Exhibition 1

GEOLOGY-HYDROGEOLOGY The Lower Oligocene Gavrovo flysch formation dominates in the wider region of interest. Green and brown silty marls with intercalations of sandstones and a total thickness of about 25 m characterize the flysch sequence. Thin sandstone intercalations (5 to 1 cm thick) dominate in the southeastern side, while more thick sandstone layers (5 to 1 cm) are observed in the sequence of the north side. Surface faults with E-W and NW-SE direction are observed in the greater area of interest. The small thickness of weathered materials and sandstone beds within the southern and western section of the study area do not create favorable conditions for the presence of significant underground water. Contrary, at the northern section the thick sandstone beds provide favorable conditions for underground water. However, the existence of thick sandstone beds facilitates the migration of pollutants due to the movement of underground water via primary and secondary porosities of sandstones. Drillings or irrigation wells are not observed in the wider region of interest. GEOMORPHOLOGY Gentle ground surface slopes (2-5 %) dominate the areas selected for further investigation. Backward and down cutting erosion problems are not observed. Thin soil covers (1-3 cm) and weathered mantle layers (.5-2 m) were also observed in the areas of interest. V C1 C2' P1 P2 C1' C2 I Figure 1. Location map of the investigated area (a) and CSDD electrode configuration (b). FIELD EVALUATION The oil drillings locations were planned in two sites, east and west of Skolis Mountain. As mentioned, the waste disposal area had to be far away from settlements, archeological sites, wells and drillings, nearby to secondary roads and not far away from the oil drilling locations. The area must have low slopes to avoid landslides and easily excavated material to be used in the future to cover the landfill. The disposal procedure of oil drilling products should not disturb human activities and national transportation. The areas with the best geological, hydrogeological and geomorphological conditions are on the flysch deposits of Gavrovo Zone. Some of the areas were located in the silt-clay face of flysch and since they couldn t cover the above-mentioned conditions they were excluded. For example they were located near a surface water layer, or nearby a tectonic zone or a thrust, adjacent to a torrent (overflow problems), within areas of surface limestone presence or sandstone intercalations (tectonic movements or failure of protective measures may cause water to flow in deeper horizons polluting them). Four areas were selected at the third step for detailed investigation. They were all in the silt-clay zone of flysch with gentle slopes and far from human activities. GEOPHYSICAL EXPLORATION Geophysical methods were used to determine the sequence of the subsurface lithological formations (silt-clay, gravel, sandstone, limestone) in the areas selected for detailed investigations. Especially, the electrical resistivity method was used, since silty and clayey formations exhibit high resistivity contrasts (low values) in respect to the surrounding formations (high values). This discrimination is important since the controlling environmental identity of formations is permeability. Since the faces of deposition are of greater importance than the lateral inhomogeneities we used the Schlumberger configuration of electrodes (Ward, 199). Moreover, to obtain a more detailed image of the subsurface associated with the dips of layers we used the Combined Schlumberger Dipole Dipole (CSDD) array (Fig.1b) of 2

electrodes, (Apostolopoulos, 1996). CSDD array ensures swiftness in data acquisition compared with other elaborate and expensive 2-D resistivity techniques and resolves well for layers dips rather than isolating confined bodies. The dipole-dipole data are transformed to Schlumberger values and a first semi-logarithmic apparent resistivity section can be constructed. The apparent resistivity data are in turn transformed to a specific grid file, the vertical columns of which are the input files for processing and interpretation by using the filter of Johansen (1975) and the conventional 1-D code of Zohdy (1989). DETAILED INVESTIGATION Mastena A Gentle ground surface slopes (2-5 %) with U type sills to prevent soil movements prevail in the area. Limited erosion in depth and thin soil covers are also observed. The area belongs to the Gavrovo flysch formation with marls manifestations on the surface. Surface manifestations of sandstones with siltstone intercalations are observed far to the north. There exist indications for a possible fault tracing in the E-W direction. The interpretation results of a CSDD resistivity profile, shown in Figure 2, suggest the presence of a clay layer on the top, followed by a sandstone layer with an abrupt change in its thickness about in the middle part of the section, indicating in that way the possible existence of a fault. The presence of sandstone can cause leakage to deeper horizons and contamination of groundwater. Since the oil drilling was planned near Mastena A, this area needs special protective measures, such as placing many monitoring wells and geomembranes geotextiles. S 315 31 35 3 295 MASTENA A MAS-A MAS-A-2 MAS-A-3 N 315 31 35 3 295-5 5 1 15 2 25 3 35 4 45 5 55 6 65 Distance (m) 2 4 8 15 2 5 115 Figure 2. Mastena A true-resistivity section. St.Nikolaos The area has the same geological status as Mastena A, with an exception on top of the flysch where unconformable Pleistocene Holocene clay-sand deposits with intercalations of conglomerate are present. The area has gentle slopes (2-5%) with open U type sills. A valley extends to the north with intense erosion in depth, which reveals the top of flysch under the mentioned deposits. Faults with N-S direction have caused an uplift of the area while other faults with an E-W direction have caused regressive erosion processing. The presence of stream created by intensive erosion raises ecological issues since there is a special biota in the area of interest. The processing and interpretation of the CSDD apparent resistivity section (Fig. 3a) resulted to a resistivity depth section (Fig. 3c) in the southern part of which we can observe the presence of a conglomerate formation under the clay sediments with great permeability. Figure 3b is the theoretical response of the model (Fig. 3c) and as we can see is in a very good agreement with the experimental response (Fig. 3a) indicating in that way a reliable solution. The subsurface image indicates that polluted water can reach through the nearby torrent ecologically protected areas. 3

i iii Logarithmic AB/2 Logarithmic AB/2.5.5.5.5-4 -3-2 1 2 3 (a) -4-3 -2 1 2 3 (b) ii NW 2 195 ST-NIK-3 ST-NIK-2 St NIKOLAOS ST-NIK ST-NIK-4 SE 2 195 19 185 18 19 185 18 175 175-45 -4-35 -3-25 -2 5-5 5 1 15 2 25 3 (c) 2 4 8 15 2 5 115 Figure 3. St. Nikolaos: Observed apparent resistivity section (a), true-resistivity section (b), computed apparent resistivity section (c). Ali Aloni The area belongs to the same geologic environment as Mastena A. Thin sandstone intercalations are observed here while the existing faults have caused the uplift of the area. The CSDD resistivity section (Fig. 4) has detected the presence of a clay layer on the top, followed by a sandstone layer with an abrupt change in its thickness about in the middle part of the section, indicating in that way the possible existence of a fault causing a small uplift of the surface. A pseudo 3-D picture of the underground (slices in various altitudes, Fig. 5a) with all CSDD results shows in a more efficient way the presence of thin sandstones and the displacement caused to the fault. With the proper excavation this area can be used as a waste disposal site as it has the advantage that is well hidden, far from human activities. 35 SE B-2 B ALONI A A-2 NW 36 355 35 345 34 335 345 34 335 33-7 -65-6 -55-5 -45-4 -35-3 -25-2 5-5 5 1 15 2 Distance (m) 33 2 4 8 15 2 5 115 Figure 4. Ali Aloni true-resistivity section. 4

ALONI MASTENA C C A B N B 355m -----> 325m -----> 35m -----> 32m -----> 345m -----> 315m -----> 34m -----> 31m -----> 335m -----> 35m -----> 33m -----> 325m -----> 2 4 8 15 2 5 115 Figure 5. Pseudo 3-D distribution of subsurface true resistivity for Ali Aloni and Mastena C investigated areas. Mastena C The fact that the oil drilling site is within Mastena area, an alternative area with good initial indications should be also selected for detailed investigation. Mastena C has the same overall geology as Mastena A with very thin insignificant sandstone intercalations. Old land slides with regressive erosion processing have been observed at the east part of the area. However, they appear as being currently stabilized. A pseudo 3-D picture of the underground (slices in various altitudes, Fig. 5b) with all CSDD results shows in a more efficient way the presence of fine clay material with very low transmissivity. CONCLUSIONS The integrated geological, geomorphological, hydrogeological and geophysical investigations indicated Mastena C with Ali Aloni as the most appropriate alternative sites for waste disposal of oil drilling products. Electrical resistivity method gave additional valuable information to distinguish between equally evaluated areas. AKNOWLEDGEMENTS The research leading to this article was co-sponsored by the General Secretariat for Research and Technology (GSRT) of Greece and the Hellenic Petroleum SA. We wish to thank Mr. A. Parasis and Dr. E. Kamberis, both from Hellenic Petroleum, for their constructive contribution to the environmental and geologic issues of the project respectively. We also thank Mrs R. Koutsi who helped acquire the geophysical data. REFERENCES Apostolopoulos G., 1996. Combined Schlumberger and Dipole-Dipole Array - 2D Approach in Interpretation. Extended Abstracts of 58th EAEG Meeting, Amsterdam, Holland. Johansen H.K. 1975. An interactive computer/ graphic-display-terminal system for interpretation of resistivity soundings. Geophys. Prosp. 23, 449-458. Ward S.H., 199. and induced polarization methods. Geotechnical and Environmental Geophysics. Investigations in Geophysics No 5, Vol. 1, SEG. Zohdy A.A.R. 1989. A new method for the interpretation of Schlumberger and Wenner sounding curves. Geophysics 54, 245-253. 5