During the twentieth century, the science of hydrogeology focused on establishing and refining fundamental principles and developing tools to study groundwater flow, well hydraulics, hydrogeochemistry, and contaminant hydrogeology. By the end of the century, the science evolved to assimilate principles and expertise from other disciplines, including surface water hydrology, chemistry, microbiology, geophysics, and ecology. In this chapter, we review seminal achievements in hydrogeology from 1963 to 2013, focusing on work by recipients of the Hydrogeology Division's O.E. Meinzer Award, one of the most prestigious and coveted awards in hydrogeology. The canon of 116 Meinzer Award papers, reports, and books reflects the trends in hydrogeological research since the early 1960s. We also discuss other contributory papers by Meinzer awardees and related work by other scientists, and cover some research areas that have not been recognized by the Meinzer Award. We anticipate that the contributions of future Meinzer awardees will continue to document leadership in hydrogeology, perhaps in areas that have not yet been recognized by the award, including hydrogeoecology and hyporheic processes, submarine groundwater discharge, multilevel slug tests and hydraulic tomography, heat as a groundwater tracer, hydrogeophysics including remote sensing, and regional groundwater hydrology applied to issues of climate change.

That karst aquifers constitute a class with properties distinct from other aquifers has been supported by the work of many researchers. Nonetheless, assessments of aquifers in limestone and dolomite where contamination issues exist continue to be regularly mismanaged by practitioners whose training has focused on the properties of granular aquifers. Incorrect assumptions of pressure and pore-space continuity, erroneous assumptions of isotropy and homogeneity relating to permeability testing, and the inappropriate application of models and computer simulations based upon these assumptions are the principal causes of this mismanagement. The authority conferred by respected consulting firms and sophisticated computer simulations has led to these fundamentally flawed studies being utilized in situations involving contaminants where the public health and welfare must be appropriately protected. Examples of the misuse of assumptions and authority relating to karst aquifers are discussed here.