GEOCHEMICAL CHARACTERIZATION OF STREAM WATER AND SEDIMENT QUALITY IN THE RIO BLANCO WATERSHED, PUERTO RICO

The Rio Blanco watershed begins in El Yunque National Forest (EYNF), a rainforest at the eastern end of Puerto Rico. It’s tributaries flow southward out of the National Forest into coastal lowlands dominated by agriculture. Stream sediment is investigated for phosphorus concentrations and fractionation, while water is analyzed for major cations, anions, nitrate and total phosphorus. Sediment P fractionation is determined using the Psenner method. Stream water major element analyses show samples roughly falling into two groups. Samples from EYNF show lower concentrations of cations with Na concentrations in the 5 to 6 mg/L range while samples from outside EYNF have higher concentrations in the 8 to 19 mg/L range, for example. Anions show similar patterns with SO₄ ( 5.06 to 17.1 mg/L) and NO₃ (0.24 to 1.95 mg/L) concentrations two to four times higher outside EYNF. Total P concentrations were generally low, 0.1 mg/L or less, in most samples. One notable deviation from this pattern is a tributary draining a heavily agricultural area with livestock where total P was found to be 0.21 mg/L. Sediment P fractionation show total sediment P concentrations ranging from 47.9 to 178 mg/kg. The highest concentrations were found in segments were slope was lower or behind flow control structures, resulting in slightly more fines in the less than 2 mm fraction which was used for extractions. Distribution among the various physicochemical phases found variable results. In most instances, the sum of Al associated P and organic matter associated P was dominant, with values ranging from 57.4 to 65.4%. Results show some anthropogenic influence on stream water quality, but less so with respect to sediment quality.