Abstract: The dissolved constituents found in groundwater have been of interest for over two centuries. In fact many of the early developments in analytical chemistry were motivated by requests from physicians who were interested in the composition of springs and spas that had presumed therapeutic benefits. More recently, public attention to groundwater chemistry has focused on constituents that are associated with health hazards such as arsenic and hexavalent chromium. In addition to its relevance to human health, water chemistry data provide hydrogeologists with clues to the complex subsurface structures and processes that control groundwater flow and water-rock interactions. A major challenge to interpreting these data is posed by the spatial and temporal variability of measured concentrations. This talk will discuss several case studies in which high resolution sampling and tracer experiments have been used to document the effects of complex flow fields and subsurface reactions on the chemical signatures we observe in groundwater samples.