The steady state regional 3D-hydrogeological model (HM) has been developed for the Baltezers, Rembergi and Baltezers water supply complex which provide the Riga city with drinking water from groundwater resources. However, transient modelling and refinement of the HM grid are necessary, to investigate the artificial infiltration system of the Baltezers well field. The system takes water from the Baltezers Lake and feeds infiltration basins, in order to keep the well field siphons to perform properly when drinking water consumption is large. The water consumption of Riga has decreased almost threefold, since 1995. For this reason, the large capacity of the Baltezers siphon system cannot be exploited in full. Nowadays, the following contra dictionary requirements regarding this well field should be fulfilled: low price of drinking water (no artificial infiltration); high water quality and protection of the well field against contamination (infiltration used); capability of sharp increase of groundwater withdrawal, to compensate possible failure of the supply complex taking water from the Daugava River. In order to solve this optimization problem, the local transient HM has been created, on the base of the existing regional HM. The plane approximation step of local HM is 27.5 metres. Presently, HM uses variable water withdrawals and infiltration rates that can be changed every mouth. The first results are obtained and more precise requirements are formulated to obtain the needed optimal solution regarding exploitation the Baltezers well field.