The 801 m deep artesian borehole Lu-1 in Lucija near Portorož (SW Slovenia) was drilled in 1994 on the coast. The borehole is constructed in flysch in the upper 257 m and continues in lime­stone below that depth. The groundwater has a very distinct chemistry. High concentrations of iron (10.4 mg/l), sulphide (15 mg/l), and chloride (6700 mg/l) are representative showing that mixing of seawater (31%) and groundwater (69%) under highly reducing conditions is present in the confined carbonate aquifer. Groundwater flow is extensive and deep, fed by higher elevation precipitation to the northeast.