CRACKING THE DISSOLVED INORGANIC MATERIALS CODE: COMPARING TWO PUERTO RICO RIVERS ACROSS SEASONS

Measuring the concentration of inorganic material dissolved in stream water is important for understanding the rate at which drainage basins export mass and the suitability of stream water for agriculture and human consumption.

In southeastern Puerto Rico, we sampled stream water from 19 sites distributed between two adjacent watersheds, the Río Guayanés (68.4 km²; mean annual flow, 2.14 cms) and Río Maunabo (13.9 km²; mean annual flow, 0.56 cms). Both basins are underlain primarily by granitic rocks. Our first round of sampling was in March and the second in June allowing us to replicate water analyses at most sites under different flow conditions. Anion, cation, and silica concentrations were measured at Williams College and at the University of Vermont using AA, ICP, colorimetry, and anion chromatography. We also measured water pH, conductivity, and temperature.

Southeastern Puerto Rico stream water is dominated by Na, SiO₂, and Ca with lesser concentrations of Mg and K. For all but K (1.1 vs 0.9 mg/L), streamflow in March is more dilute by 32 to 49% than streamflow in June (Na, 11.6 vs 17.2 mg/L; Ca; 10.3 vs 14.1 mg/L; SiO₂, 9.3 vs 11.8 mg/ L; Mg, 4.1 vs 5.4 mg/L; March value listed first). Anions are dominated by bicarbonate (69.9 vs 85.8 mg/L) with lesser concentrations of Cl^- (11.7 vs 12.3 mg/L), SO₄^-2 (3.6 vs 3.7 mg/L), NO₃^- (1.9 vs 1.8 mg/L; March value listed first). Total dissolved solids (TDS) averaged 142 mg/L (range, 69-230 mg/L) and were lower in March. Conductivity ranged from 112 to 275 (ms/cm) in March (average = 172) and from 114 to 297 (ms/cm) in June (average = 186). pH was slightly alkaline and similar during both sample rounds (7.6 to 7.5). Conductivity and total dissolved solids are well and linearly related. Water was on average 2.59 degrees C warmer in June than March.

Differences in solute concentrations between March and June appear to be controlled by precipitation differences. More than 10 cm of rain fell in a three-day event in early February. Discharge records show that stream flow after the storm fell gradually over many weeks. May and June had little precipitation. Averaging stream water analyses and using mean annual flow data suggests mass export rates of 216 Mg/km²*yr for the Río Maunabo and 111 Mg/km²*yr for the Río Guayanés; about 50% of the dissolved load likely comes from atmosphere and not rock weathering.