played an important role in sample collection. Linda Zarr contributed her expertise to the data analysis. Scott Hoffman provided valuable graphics support. Kim Otto (retired) provided editorial support. USGS reviewer Kevin Breen provided a particularly insightful review of an early draft of the manuscript. Other USGS reviewers, Lisa Senior, Pat Philips, and Doug Chambers offered valuable suggestions for improving the manuscript. The interest and support of Pat Bowling of the PADEP is greatly appreciated. The authors also wish to extend thanks to the landowners and the wastewatertreatment-plant operators who graciously allowed access to the wells and streams. To all who assisted in any way, the authors convey sincere thanks.

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Chester County encompasses 760 square miles in southeastern Pennsylvania. Groundwater-quality studies have been conducted in the county over several decades to address specific hydrologic issues. This report compiles and describes water-quality data collected during studies conducted mostly after 1990 and summarizes the data in a county-wide perspective.