Electrical Resistivity Imaging (ERI) was carried out continuously for 10 days to map the subsurface resistivity distribution along a potentially hazardous hillslope at the Jieshou Junior High School in Taoyuan, Taiwan. The reliability of the inverted resistivity structures down to about 25 m depth was examined with synthetic modeling using the same electrode arrangements installed on land surface as in field surveys, together with a DOI (depth-ofinvestigation) index calculated from the ERI data. The subsurface resistivity distribution is consistent with results from well logging. These ERI recordings were taken daily and provided highly resolved imagery of the resistivity distribution underground and illustrated the dynamical fluid-flow behavior due to heavy rainfall infiltration. Using Archie's law, the resistivity distribution was transformed into a map of relative water saturation (RWS), which is strongly correlated with the rainfall infiltration process. We then found that the averaged RWS is significantly correlated with daily precipitation. Our observations indicate that time-lapse ERI is effective in monitoring subterraneous rainfall infiltration; moreover, the preferential flow paths can be delineated according to the changes in averaged RWS derived from the ERI data.
Time-lapse methodology was applied to cross-hole electrical resistivity tomography (CHERT) to investigate two groundwater contamination sites. In the first case study, resistivity profiles were used to delineate the transport direction and spatial distribution of the contaminant, which can serve as a basis for adjusting the remediation treatment by the remediation team. In the second case study, changes in electrical conductivity were used to evaluate the remediation reagent's transport direction and area of effect, and this was used to indirectly verify the effectiveness of the remediation efforts. CHERT equipment was installed simultaneously at the monitoring wells, which enhanced the benefits of the boreholes, enabling them to be even more economical. In large-scale groundwater contamination sites or sites with complex hydrogeological environments, application of CHERT techniques can result in greater amounts of data, particularly in analyzing localized preferential flow paths. This data would be greatly beneficial to the remediation of groundwater contamination sites and long-term groundwater management.
In this study, the self-potential (SP) method was used for the continuous monitoring of naturally occurring electric potential differences at a soil and groundwater contamination site in Southern Taiwan from August to October 2015. At this field site, we set two perpendicular survey lines with 25 potential electrodes for daily measurements. The hourly SP medians of each day were averaged for the inversion of the 85-m long north-south (line I) and 35-m-long west-east (line II) SP tomographies (SPTs). The results are as follows: First, the regional groundwater flow direction was found to be in the north-northeast direction according to the distribution of electric potentials in the SPTs. The rainfall effects and consequent SP response were analyzed, which revealed enhanced regional positive and negative potentials in the SPTs, with a strength of up to-100 mV. In addition, effects of remediation reagent injection were monitored from 13 to 18 October; the results showed increased local potential in the two SPTs. Finally, we assessed the movement of the isopotential lines within a specific distance and determined the apparent velocity of groundwater to be 3.24 ± 0.38 and 0.57 ± 0.08 m day-1 through the dissipation of rainfall effects and local potential increases because of the injection.
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