Chemical loading rates from precipitation in the Colorado Rockies

Abstract

Chemical composition of bulk (= wet + dry) precipitation was measured at two stations in the Colorado Rocky Mountains for each of 150 consecutive weeks. The stations were located at 3000 m elevation approximately 6 km east of the Continental Divide and 40 km north of Denver. Chemical species routinely measured included particulate C, N, P and H; dissolved organic C, P and N; conductance and total alkalinity; SO₄, CI^-, NO3_-3-N, NO2_-N, NH₊₄-N, Ca⁺⁺, Mg⁺⁺, Na⁺, K⁺, H₂PO₄-P and H⁺. Particulates were deposited at a rate of 147 kg/ha/yr; C accounted for about 21% of this. The most important anions were total alkalinity expressed as bicarbonate (281 eq/ha/yr), SO₄ (196 eq/ha/yr) and NO3-N (129 eq/ha/yr). Among cations, Ca⁺⁺ was most important (200 eq/ha/yr) followed by H⁺ (106 eq/ha/yr) and NH4+–N (89 eq/ha/yr). Conductance averaged 16 umho/cm.

The composition of dry fallout was collected separately and analyzed for 70 weeks at a single station. Particulates accounted for 129 of the 169 kg/ha/yr total collected. Total alkalinity was by far the most important dry component and, expressed as HCO3-, accounted for 506 eq/ha/yr of the total 804 eq/ha/yr for anions. About 75% of the total inorganic phosphorous in bulk precipitation was from dry fallout.

The composition of Como Creek precipitation is rather different from that of other sites reported in the literature. At Como Creek, comparatively low levels of SO₄₌ are found simultaneously with comparatively high levels of H+. The surprisingly high levels of observed H+ deposition at the only site in the Rocky Mountain west with a long-term, broad spectrum record signal the need for extended geographic coverage of precipitation chemistry in the western U.S.