Rapid development and population growth are occurring in Webb County in south Texas (fig. 1). Water managers need information on the ground-water resources of the area to address the increased demand for water caused by the development. To help meet this need, the U.S. Geological Survey (USGS), in cooperation with the City of Laredo, began a study in 1997 to assess the ground-water resources of the county. This report describes the quality of ground water determined from water samples collected during 1997-98.