Cross-borehole resistivity tomography: Can it be used to plan and monitor in situ remediation and assist risk assessment?

<p><strong>Background</strong></p><p>Evolving <em>In situ</em> methods are showing results for sustainable and efficient plume remediation of groundwater contaminations. By injecting reactive components such as oxidation agents, zero valent iron, substrate and/or bacteria, a treatment zone (TZ) is established. In the TZ, the contamination degrades into harmless components by chemical and/or biological processes. Successful <em>in situ</em> remediation depends on contact between injectants and contamination. Yet, monitoring the spreading of the injectant is difficult by point sampling. The cross-borehole geophysical method DCIP (Direct Current, Induced Polarisation) allows for detailed spatial information on subsurface electrical resistivity and induced polarisation properties. The information can be used to assess the success of the injection and the development over time. Furthermore, the IP properties can be used to infer spatial information on hydraulic conductivity, which can be used in planning of the <em>in situ </em>remediation and in quantification of contaminant mass discharge (CMD) at the site.<strong> </strong>The objective of this study is to develop a cost-efficient method for detailed spatial and temporal monitoring of <em>in situ</em> remediation and to develop better tools to retrieve spatial subsurface information, able to assist and improve CMD based monitoring.</p><p><strong>&#160;</strong></p><p><strong>Approach</strong></p><p>A TZ in a plume of chlorinated ethenes was established by injecting the micro zerovalent iron product and a bacterial culture into the groundwater. A network of 9 geophysical and 16 monitoring wells was established. Cross-borehole DCIP measurements and water samples were taken before and shortly after injection and during the following year. Soil cores were sampled for chemical analysis of iron shortly after injection, and slug tests and grain size analysis. Data from water samples, soil cores and hydraulic tests were compared to the geophysical measurements to assess correlation between water chemistry and electrical resistivity from cross-borehole DCIP. The hydraulic properties inferred from hydraulic tests and cross-borehole DCIP were compared. The hydraulic properties with uncertainties and the contamination data were used to estimate the CMD through the TZ.</p><p><strong>&#160;</strong></p><p><strong>Results </strong></p><p>The changes in electrical conductivity and specific water quality parameters caused by the injection, showed a strong correlation with the geophysical model. The observed correlation enabled a coherent, detailed understanding of both spatial and temporal spreading of the injected components, resulting in a re-injection. Hydraulic tests and hydraulic properties inferred from cross borehole DCIP showed a very good correlation, and applying the hydraulic properties inferred from cross borehole DCIP reduced the uncertainty of the CMD estimate before and after injection. In conclusion, cross borehole DCIP has the potential to improve planning and monitoring of <em>in situ</em> groundwater remediation and to reduce uncertainty of CMD estimation and thereby strengthen CMD as a metric in risk assessment.</p>