This paper presents architecture design and performance evaluation of a back end system used to store, index, manage and visualize water quality measurements collected by sensor networks deployed in rivers, lakes and coastal regions. The embedded communications systems in each sensor node enable ad hoc network operation to relay the measurements to the back end system in charge of managing, processing and providing data to end users and large communities through web interfaces. The focus is on the back end system architecture description and on the evaluation of its performance.