An evaluation to define groundwater-quality conditions, with emphasis on the distribution of nitrate concentration, to identify potential sources leading to elevated nitrate concentrations and to estimate the nitrate loads from major sources, was conducted by the U.S. Geological Survey in the South Coast aquifer, Santa Isabel area, between January 2008 and May 2009. The study area covers approximately 5,800 hectares of the south coastal plain of Puerto Rico and 470 hectares in the foothills within the municipality of Santa Isabel. Agriculture is the principal land use in the area and includes the cultivation of diverse truck-farm crops, bioengineered crops for seed production, and until 2007, commercial poultry farms. Nitrate concentrations in groundwater near the center of the South Coast aquifer, Santa Isabel area, have increased from a range of 0.9 to 5.9 milligrams per liter as nitrogen in 1967 to a range of 1.3 to 23.6 milligrams per liter as nitrogen in 2008. Analyses for the stable isotope ratios of nitrogen-15/ nitrogen-14 and oxygen-18/oxygen-16 in nitrate were used to infer the source of nitrate in the aquifer and in the foothills north of the coastal plain. Results obtained from the samples analyses of stable isotope ratios indicate that the high nitrate concentrations in the western and central parts of the study area were in the range typically associated with soil nitrogen.