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Evaluation of Hydrologic Conditions and Nitrate Concentrations in the Río Nigua de Salinas Alluvial Fan Aquifer, Salinas, Puerto Rico, 2002-03

By José M. Rodríguez


In cooperation with the



Scientific Investigations Report 2006-5062


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A ground-water quality study to define the potential sources and concentration of nitrate in the Río Nigua de Salinas alluvial fan aquifer was conducted between January 2002 and March 2003. The study area covers about 3,600 hectares of the coastal plain within the municipality of Salinas in southern Puerto Rico, extending from the foothills to the Caribbean Sea. Agriculture is the principal land use and includes cultivation of diverse crops, turf grass, bioengineered crops for seed production, and commercial poultry farms.

Ground-water withdrawal in the alluvial fan was estimated to be about 43,500 cubic meters per day, of which 49 percent was withdrawn for agriculture, 42 percent for public supply, and 9 percent for industrial use. Ground-water flow in the study area was primarily to the south and toward a cone of depression within the south-central part of the alluvial fan. The presence of that cone of depression and a smaller one located in the northeastern quadrant of the study area may contribute to the increase in nitrate concentration within a total area of about 545 hectares by "recycling" ground water used for irrigation of cultivated lands.

In an area that covers about 405 hectares near the center of the Salinas alluvial fan, nitrate concentrations increased from 0.9 to 6.7 milligrams per liter as nitrogen in 1986 to 8 to 12 milligrams per liter as nitrogen in 2002. Principal sources of nitrate in the study area are fertilizers (used in the cultivated farmlands) and poultry farm wastes. The highest nitrogen concentrations were found at poultry farms in the foothills area. In the area of disposed poultry farm wastes, nitrate concentrations in ground water ranged from 25 to 77 milligrams per liter as nitrogen. Analyses for the stable isotope ratios of nitrogen-15/nitrogen-14 in nitrate were used to distinguish the source of nitrate in the coastal plain alluvial fan aquifer.

Potential nitrate loads from areas under cultivation were estimated for the principal crops in the area. The load estimates ranged from 18 kilograms per hectare per year as nitrogen for sorghum crops to 430 kilograms per hectare per year as nitrogen for turf-grass farms. Potential nitrate load from poultry farm wastes and from communities with septic tanks were estimated at about 580 and 47 kilograms per hectare per year as nitrogen, respectively. Results obtained from the analyses of the stable isotope ratios of nitrogen-15/nitrogen-14 in nitrate samples indicated that the high nitrate concentrations are from poultry wastes near the foothills, whereas artificial fertilizers were estimated to contribute between 39 to 97 percent of the total nitrate in the central part of the alluvial fan.





Purpose and Scope

Purpose and Scope



Description of Study Area


Physiographic Features

Land Use

Hydrologic Conditions in the Salinas Area

Hydrogeologic Setting

Ground-Water Flow and Hydraulic Characteristics

Isotopic Composition in Ground Water and Surface Water

Water-Quality Evaluation with Emphasis on Nitrate Concentrations

Common Dissolved Constituents


Identification of Nitrate Sources

Nitrogen Isotopic Characterization of Nitrate Sources

Estimate of Potential Nitrate Load to the Alluvial Aquifer

Summary and Conclusions
Cited References


The citation for this report, in USGS format, is as follows:


Rodríguez, J.M., 2006, Evaluation of hydrologic conditions and nitrate concentrations in the Río Nigua de Salinas alluvial fan aquifer, Salinas, Puerto Rico, 2002-03: U.S. Geological Survey Scientific Investigations Report 2006-5062, 38 p.

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