In this work, a lab-scale microcosm was designed and used to simulate a porous groundwater aquifer. A minimal media acted as the non-selective pressure for plasmid transfer, while a minimal media with gentamicin acted as the selective pressure. PCR, plate counts and confocal laser scanning microscopy were used to monitor transconjugant and donor persistence in the microcosm. The donor was identified through gfp on the chromosome and red fluorescence was used to identify transconjugants through dsRed on pJP4. The donor persisted for 73 h in a minimal media environment when 1x109 cells were inoculated into the microcosm. Biofilm thickness increased in response to gentamicin addition, potentially increasing interactions between donor and recipient for increased rate of plasmid transfer. A higher number of transconjugants was identified when selective pressure was used due to increased transfer of pJP4. In conclusion, an inhibitory concentration of gentamicin provided sufficient selective pressure for pJP4 transfer.