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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. Potential residual nitrogen loads from areas under cultivation were estimated for the principal crops in the area. The residual load estimates ranged from 25 kilograms of nitrogen per hectare per year in tomato crops to 228 kilograms of nitrogen per hectare per year in banana crops. The potential nitrogen loads from two communities without sanitary sewer systems were estimated to be approximately 101 and 195 kilograms of nitrogen per hectare per year, respectively. Groundwater withdrawals in the South Coast aquifer, Santa Isabel area were estimated to be approximately 64,400 cubic meters per day, of which 52 percent were for agriculture, and 48 percent were for public-water supply. Groundwater flow in the study area was primarily southward from the north towards two cones of depression present within the central and southwestern parts of the study area. The presence of those cones of depression may contribute to an increase in nitrate concentrations within a total area of about 1,300 hectares owing to the recirculation of groundwater with elevated nitrate concentrations that is used to irrigate cultivated lands.

Evaluation of groundwater quality and selected hydrologic conditions in the South Coast aquifer, Santa Isabel area, Puerto Rico, 2008–09