This part of the Data Release contains the water-quality data compiled and synthesized from various sources (including data from water-quality samples collected by the USGS in 2015 from 26 sites). This collection includes three main tables that contain (1) the list of site locations where water-quality data were collected, and information about those sites, such as the spatial coordinates of a well (Site_information); (2) the list of samples that were collected, and information about those samples, such as the date the sample was collected (Sample_information); and (3) the results of the measured constituent, and information about the result such as the concentration of a constituent that was measured in a sample (Water_quality_results). Additional tables in this collection include analysis completed on the data compiled in the three main tables such as instantaneous load computations computed from compiled dissolved solids concentration and streamflow data (Instantaneous_load_estimates). The collection also includes the results of quality-assurance tests performed on the data (Aggregated_water_quality_QAQC), and various lookup tables that describe codes and acronyms used. The data were used to complete a detailed salinity assessment of the Pecos River from Santa Rosa Lake, New Mexico to the confluence of the Pecos River and the Rio Grande, Texas.