This dataset contains laboratory standard reference sample concentrations analyzed by RTI Laboratories, the analytical laboratory used by the Gwinnett County, Georgia study. These water-quality-control samples are part of the U.S. Geological Survey (USGS) Standard Reference Samples (SRS) Project inter-laboratory comparison study. RTI Laboratories sample concentrations along with the included inter-laboratory result statistics (for most probable value and standard error) can be used to assess the laboratory’s analytical precision and bias. Sample concentrations are reported for 12 constituents for 13 round-robin sample rounds, in which prepared samples are sent to participating laboratories twice per year, and were conducted between the fall of 2014 and the fall of 2020.