Application of self-potential method for monitoring the contamination from Olive oil mills' waste (OOMW)

The self-potential (SP) method is one of the oldest geophysical method which measures the distribution of natural electric potential at the ground surface. Integrated electrical resistivity tomography (ERT) and self-potential techniques are often used to identify groundwater flow direction and possible soil and groundwater contamination. In the present study, the results of ERT and SP measurements are analyzed to characterize a site in western Crete contaminated from olive oil mill waste (OOMW). The production of olive oil is one of the major economic importance in Mediterranean region, particularly in Greece which is the third country (worldwide) in olive-oil production with Island of Crete contributing more than 30% of the total olive oil production in Greece. The production procedure of olive oil generates large volumes of OOMW, which pose serious environmental problems due to their high organic load and rich in inorganic constituents. The OOMWs are usually disposed in poorly constructed shallow evaporation ponds, which often results into overflow or leaking of wastewaters, hence, polluting soil and groundwater. The high electrical conductivity and the high concentration of phenolic compounds are the main features that make OOMWs detectable by means of geoelectric prospecting methods. In the study area, five self-potential surveys from May to July 2014 were carried out along an ERT profile by using the fixed base configuration. The SP field data have been analyzed by using the 2D tomographic approach bas ed on the charge occurrence probability (COP) function and spectral methods. Taking into account the electrical resistivity distribution from the ERT tomography, COP maps are interpreted to provide temporal information about self-potential sources and their possible correlations with contamination from OOMW.