GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 3.2 617 The obtained results showed the complexity related to reading LNAPL migration phenomenon via electrical resistivity behavior. Through the use of the resistivity ratio values between uncontaminated and contaminated condition the relative magnitude of electrical resistivity change was assessed. Fig. 3 highlights three time resistivity section after 81 days from the contamination. The results show, in terms of resistivity ratio, a strong increase of electrical conductivity values as consequence of the biodegradation effects occurring at this stage of the test. The measurements continued for 250 days and the “mean ratio resistivity” (MRR) value for each resistivity sections representing the bulk electrical properties of the subsoil analysed by two boreholes was analyzed. The variability of the MMR was interpreted in four main steps from the contamination phase to the biodegradation one. The laboratory experiments provided results in time-lapse, giving a highly efficient tool to monitor variability of the subsurface contaminant processes, when they are associated to changes in the type and concentration of pore fluids, water resources quality, bacterial activity, and hydrogeological properties (i.e. porosity and hydraulic conductivity) (Sauck et al ., 1998; Atekwana et al., 2004, 2005; Werkema et al., 2003). The test showed the difficulties and uncertainties of interpretation of geophysical techniques. In any case, the geo-electrical measures appear to be a very high resolution support for analysis of environmental type specially if related to dynamic issues. Acknowledgements The present activity has been performed in the frame of the TICaMoSC project “Tecnologie Idrogeofisiche ��� �� ����������������� �� �� ������������ �� ���� ������������� ��� ���������� ��������� ������ per la caratterizzazione ed il monitoraggio di siti contaminati”, FSE Basilicata 2007-2013 funds. References Atekwana, E. A.,. Atekwana E. A, Legall F. D. and Krishnamurthy R. V. (2004) - Field evidence for geophysical detection of subsurface zones of enhanced microbial activity: Geophysical Research Letters, 31, L23603. Atekwana, E. A.,. Atekwana E. A, Legall F. D. and Krishnamurthy R. V. (2005) - Biodegradation and mineral weathering controls on bulk electrical conductivity in a shallow hydrocarbon contaminated aquifer: Journal of Contaminant Hydrology, 80, 149–167. Binley A., Cassiani G., Middleton, R. and Winship P. (2002) - Vadose zone model parameterisation using cross- borehole radar and resistivity imaging. Journal of Hydrology, 267, 147-159. Cassiani G., Bruno V., Villa A., Fusi N. and Binley A.M. (2006) - A saline tracer test monitored via time-lapse surface electrical resistivity tomography. Journal of Applied Geophysics, 59, 244-259. de Groot-Hedlin C.D. and Constable S.C. (1990), Occam’s inversion to generate smooth, two-dimensional models from magnetotelluric data, Geophysics, 55, 1613-1624. Deiana R., Cassiani G., Kemna A., Villa A., Bruno V. and Bagliani A. (2007) - An experiment of non- invasive characterization of the vadose zone via water injection and cross-hole time-lapse geophysical monitoring, Near Surface Geophysics, 5, 183-194. Fig. 3 - Time variation of mean ratio resistivity values recorded for each couple of boreholes with identification of the identified main phases due to the degradation and diffusion of LNAPL in the subsoil.