Located in south-central Texas, the Geronimo Creek and Plum Creek watersheds have long been characterized by elevated nitrate concentrations. From April 2015 through March 2016, an investigation was done by the U.S. Geological Survey, in cooperation with the Guadalupe-Blanco River Authority (GBRA) and the Texas State Soil and Water Conservation Board (TSSWCB), to assess nitrate (as nitrogen) concentrations and to document possible sources of elevated nitrate as nitrogen in these two watersheds. Water-quality samples were collected from 20 groundwater, spring, and stream sites across the two watersheds, along with precipitation samples and wastewater treatment plant (WWTP) effluent samples from the Plum Creek watershed. Stream, spring, and groundwater samples were collected from both watersheds during four synoptic sampling events to characterize spatial and temporal variations in water quality and chemical loadings. Water-quality and quantity data from the wastewater treatment plants and stream discharge data were also considered. Samples were analyzed for major ions, selected trace elements, nutrients, stable isotopes of water and nitrogen isotopes. Data collected for this project was used to calculate selected chemical loadings and used in interpretation of sources of nitrate in both watersheds.