The interaction between groundwater and surface water is one of the main topics of interest in hydrogeology. It is recognized that such interaction affects quantitatively (through exchanged of water volumes leading to complex flow patterns) and qualitatively (by contaminant transport) both water systems. Today, the most common approach for analyzing such interactions is mathematical modeling. This book gives an attempt to analyze the interaction between the aquifer system and the lakes Plumbuita and Fundeni, located in Bucharest City, Romania. For the qualitative assessment of the goroundwater, four hypothetical contaminant transport models (for nitrate which was found with concentrations exceeding the admissible threshold) were developed. The contamination source was considered to be the sewer system that is located at only three meters above the first aquifer system. Such technique, as the one approached for this case of study, is believed to allow the understanding of the communication between the two systems (groundwater and surface water).