Beginning in the late 1990's, the U.S. Geological Survey began to develop analytical methods to detect, at concentrations less than 1 microgram per liter (μg/L), emerging water contaminants such as pharmaceuticals, personal-care chemicals, and a variety of other chemicals associated with various human and animal sources. During 1998-2005, the U.S. Geological Survey analyzed the following Michigan water samples: 41 samples for antibiotic compounds, 28 samples for pharmaceutical compounds, 46 unfiltered samples for wastewater compounds (dissolved and suspended compounds), and 113 filtered samples for wastewater compounds (dissolved constituents only).