The dataset contains the analytical results for environmental and quality-control replicate sample sets and the computed relative percent differences (RPD) greater than 25 percent for the data collected during the surface-water sampling for the Triangle Area Water Supply Monitoring Project. The data are from samples collected during October 2017 through September 2019. Several study sites contained in this dataset were sampled for other USGS projects during the same time frame. Unless the samples at these sites were collected in conjunction with the Triangle Area Water Supply Monitoring Project, the data for other projects are not included in the dataset.