In the German-Namibian groundwater exploration project the groundwater potential of the Omaruru River Delta (OMDEL) has to be assessed. The OMDEL aquifer extends 35 km inland up to the Namib Plain at 230 m a.s.l. with less than 50 mm year-1 precipitation. The Omaruru River catchment covers 15 700 km2 and rises to 1450 m a.s.l. with a peak altitude of 2100 m a.s.l. The mountainous catchment area receives 200-450 mm year-1 rain from the east. The main channel is recharged by groundwater throughflow and ephemeral river discharge. The safe yield is estimated to be 4.5 x 106 m3 year-1 but 6.5-8.5 X 106 m3 year-1 groundwater is currently being abstracted. In 1993 the OMDEL dam came into operation to enhance groundwater recharge in order to counterbalance the current rate of over-exploitation. Environmental isotope analyses have begun to study groundwater recharge and mixing between fresh and brackish groundwater, and to improve water budgeting by applying <5180 values of groundwater samples collected along the whole course of the Omaruru River channel.