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 October 13 to 18; 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 m/day and 0.57 ± 0.08 m/day through the dissipation of rainfall effects and local potential increases because of the injection.