For the protection of bankside wells and a groundwater recharge an early warning system had to be developed. The monitoring network design is based on sensor measurements only. For this purpose a new submersible spectrometer has been successfully tested for multi-parameter measurements directly in the medium. The developed system can easily be upgraded with other new sensors. Only calibration and validation data are supplied by conventional grab sampling and laboratory analysis.. A conventional testfilter improved by on-line monitoring at 5 sampling sites serves as a reference system. The whole system is equipped with remote control and the internet serves as the control centre of the network. All measurement data from all 9 sites are available in real time on the internet.