Abstract A shallow CO2 injection experiment was conducted in an unconfined, unconsolidated siliclastic aquifer in western Denmark. The aims were to test injection and sampling systems, confirm the conceptual hydrogeological site model and determine the aquifers potential geochemical response to a larger scale, sustained CO2 injection in order to finalize design of the main release experiment. Food grade CO2 (45 kg in 48 h) was injected at 10 m depth into glacial sand and water chemistry subsequently monitored. Results indicate the injection system effectively delivered CO2 gas into the glacial sand layer where it dissolved and moved with advective flow. Dissolved CO2 was not detected in the aeolian sand (0–5 m depth) indicating prevention of migration due to permeability heterogeneities. Dissolved CO2 in the glacial sand (5–10 m depth) created a plume of depressed pH (5.6–4.7), elevated EC (166–304 μS/cm) and concurrent increases in dissolved ion concentrations. EC was the most effective indicator for presence of dissolved CO2. Ionic concentration changes were generally slight with increases in Al, Ba, K, Na, Mg, Si, Sr and Zn forming the major changes. Water quality was not significantly affected and risks from small scale, short duration CO2 contamination appear minimal for this geological setting.