SOURCE AND FATE OF METALS IN THE GIBBON RIVER, YELLOWSTONE NATIONAL PARK, USA

The Gibbon R. originates at Grebe Lake and flows nearly 40 km where it combines with the Firehole R. to form the Madison R. Geothermal water from Norris Geyser Basin, Gibbon Geyser Basin, Beryl Spring Area, and West Nymph Creek Thermal Area, water draining from meadows and wetlands, and dilute mountain creeks flow into the Gibbon R. Elevated concentrations of As, B, F, Hg, Li, and SiO₂ leave the park via the Madison R. The purpose of this study was to quantify the chemical contribution of the Gibbon R. to the Madison R. and to determine chemical sources and attenuation.

Synoptic water samples and discharge measurements were obtained from the main stem of the Gibbon R. and its major tributaries under low-flow conditions. Specific conductance of the Gibbon R. upstream and downstream of Norris Geyser Basin and near the confluence with the Firehole R. was 120 µS/cm, 350 µS/cm, and 430 µS/cm, respectively. The discharge was 1.3 m³/s above Norris and increased to 2.5 m³/s near the confluence with the Firehole R. The pH of the Gibbon R. ranged from 6.9 to 7.2 because acidic waters (pH 3) from Norris Geyser Basin (Realgar Cr., Tantalus Cr., and the Gap drainage) and the Hazel Lake Drainage constitute only 9% of the flow. Tantalus Cr., the largest drainage of Norris Geyser Basin, contributes the largest loads of Al, As, B, Cl, Fe, Li, Na, SiO₂ and SO₄ to the Gibbon R. Aluminum remained in solution (0.28±0.04 mg/L) in the Gibbon R. by forming Al-F or possibly Al-organic complexes. Dissolved As from Tantalus Cr. enters the Gibbon R. primarily as As(V) and nearly all of the As flux from Norris flows without attenuation to the Madison R. Although attenuated, iron and Hg behave differently in the Gibbon R. Inflows from Tantalus Cr. and Chocolate Pots increased the instream Fe load in the Gibbon R. The dissolved Fe decreased downstream of both sources indicating oxidation and precipitation which did not affect other trace elements except possibly Hg. The dissolved Hg load substantially increased as the Gibbon R. flowed around Norris Geyser Basin. However, measured inflows from Norris Geyser Basin only accounted for 32% of the dissolved Hg in the Gibbon R. A likely source of dissolved Hg was from diffuse inflows from a large wetlands area west of Norris Geyser Basin. Downstream of Norris Geyser Basin, near Chocolate Pots, the dissolved Hg load decreased by a factor of 4.