Hydro-geochemical impact of CO₂ leakage from geological storage on shallow potable aquifers: A field scale pilot experiment

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 (45kg in 48h) was injected at 10m 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 (0-5m depth) indicating prevention of migration due to permeability heterogeneities. Dissolved CO₂ in the glacial sand (5-10m depth) created a plume of depressed pH (5.6-4.7), elevated EC (166-304μS/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.