Geophysical Induced Polarisation (IP) laboratory measurements on E. coli-sand mixtures

The aim of the MIRACHL project is the characterisation and monitoring of in-situ
remediation of chlorinated hydrocarbon contamination using an interdisciplinary
approach and geophysical methods, such as DCIP (direct current induced
polarisation) to investigate the remediation process. To interpret these geophysical field IP data, lab investigations with different kinds of bacteria are necessary to assess the sensitivity of the methods for these specific
applications. A first experiment was conducted with E. coli bacteria. Bacteria were grown together with a rich source of nutrients (Luria-Bertani broth - LB) and mixed in different flasks with sterilised Ottawa sand. These bacteria-sand-mixtures were continuously shaken (30°C, 80 RPM) until defined endpoints (within 21 days) when the mixtures were harvested and packed in a 4-point sample holder to measure SIP (spectral induced polarisation), TDIP (time-domain induced polarisation) and SP (self-potential) under laboratory conditions. The same procedure was repeated with only the media-sand mixture to exclude any influences from just the nutrient and with water-sand mixtures.
The results show a slightly increase in phase and a decrease in resistivity after several days with a decrease in phase that appears to coincide with die-off of the bacteria. Resistivity in general was very low (between 3-10 Ωm) due to the highly conductive LB-media containing 5 g/L NaCl. As a result, the phase effects are very small too. The positive phase which could be observed for the E. coli measurements was surprising and is not yet understood. As expected, the water-sand mixtures showed almost no phase shift and slightly higher resistivity values. The influence of the LB-media (nutrients) is very small and results only in a slightly lower resistivity than the E. coli-sand mixtures but in a higher resistivity than the water-sand samples.
The self-potential measurements show no clear tendency, but this is most likely due to limitations in the sample holder. The TDIP data needs to be further processed but the resistivity values are in accordance with the SIP results.
Scanning electron microscope (SEM) images showed E. coli bacteria attached to the sand grains and this could modify the grain surface (e.g. increasing the grain surface area and/or form a biofilm) and impact geophysical measurements. In the future, to support these observations with quantitative comparisons, the number of bacteria present in the sand will be determined using quantitative polymerase chain reaction (qPCR) to detect bacterial DNA (deoxyribonucleic acid).