Stormwater in␣ltration systems (SIS) have been built in urban areas to reduce the environmental impacts of stormwater runoff. In␣ltration basins allow the transfer of stormwater runoff to aquifers but their abilities to retain contaminants de- pend on vadose zone properties. This study assessed the in␣uence of vadose zone thickness (VZT) on the transfer of inor- ganic nutrients (PO34␣, NO␣3 , NH+4 ), dissolved organic carbon (total -DOC- and biodegradable -BDOC-) and bacteria. A ␣eld experiment was conducted on three SIS with a thin vadose zone (b3 m) and three SIS with a thick vadose zone (N10 m). Water samples were collected at three times during a rainy period of 10 days in each in␣ltration basin (stormwater runoff), in the aquifer impacted by in␣ltration (impacted groundwater) and in the same aquifer but upstream of the in␣ltration area (non-impacted groundwater). Inorganic nutrients, organic matter, and dissolved oxygen (DO) were measured on all water samples. Bacterial community structures were investigated on water samples through a next-generation se- quencing (NGS) scheme of 16S rRNA gene amplicons (V5-V6). The concentrations of DO and phosphate measured in SIS-impacted groundwaters were signi␣cantly in␣uenced by VZT due to distinct biogeochemical processes occurring in the vadose zone. DOC and BDOC were ef␣ciently retained in the vadose zone, regardless of its thickness. Bacterial transfers to the aquifer were overall low, but data obtained on day 10 indicated a signi␣cant bacterial transfer in SIS with a thin va- dose zone. Water transit time and water saturation of the vadose zone were found important parameters for bacterial transfers. Most bacterial taxa (N60%) from impacted groundwaters were not detected in stormwater runoff and in non- impacted groundwaters, indicating that groundwater bacterial communities were signi␣cantly modi␣ed by processes as- sociated with in␣ltration (remobilization of bacteria from vadose zone and/or species sorting).