Concentrations and Export of Solutes from Six Mountain Streams in Northwestern Costa Rica

Streamwater chemistry in six streams draining adjacent, undisturbed catchments of volcanic origin in northwestern Costa Rica was monitored between 1990 and 1993. The catchments ranged from 0.36 to 3.2 km² in area, and from 600 to 1300 m in elevation. Rainfall averaged 2.4 m/yr at 600 m but was unmeasured and probably much greater at higher elevations. Runoff among the catchments ranged from 1.4 to 4.3 m/yr, with approximately 84% occurring during the May-December rainy season. Within individual streams, concentrations of NO₃⁻, total dissolved phosphorus (TDP), and dissolved organic carbon (DOC) increased with streamflow while Ca²⁺, Mg²⁺, and Na⁺ decreased with streamflow. K⁺ showed little response to streamflow. Among streams, flow-weighted mean concentrations of NO₃⁻ (0.12-0.30 mg/L), DOC (0.63-1.79 mg/L), and TDP (0.008-0.029 mg/L) were intercorrelated and varied inversely with annual runoff. DOC concentrations were low compared with data from other tropical streams. Annual exports (kg ha⁻¹yr⁻¹) for these solutes were relatively uniform among catchments, suggesting that differences in runoff explain much of the stream-to-stream differences in solute concentrations. Cation exports were also relatively uniform among five of the streams, but were distinctly higher in the remaining stream, which is also enriched in SO₄²⁻ and Cl⁻, and may receive water from a geothermal source.