Introduction The development and application of ground-water models has been an important component of USGS hydrologic investigations since the early 1970s. During the past 35 years, the USGS has developed a wide range of computer models to simulate saturated and unsaturated subsurface flow, solute transport, and chemical reactions. The most widely used of these programs is the MODFLOW model, w...

This latest Perspective on Aquatic Processes focuses on a topic of great value but one that receives little attention—that of the need for water quality monitoring. Research often gets more attention from the public and from funding agencies, but it is monitoring—particularly long term programs—that provide the necessary data to determine trends in assessing ecosystem health. Monitoring program...