Source and fate of inorganic solutes in the Gibbon River, Yellowstone National Park, Wyoming, USA. II. Trace element chemistry

Abstract

The Gibbon River in Yellowstone National Park receives inflows from several geothermal areas, and consequently the concentrations of many trace elements are elevated compared to rivers in non-geothermal watersheds. Water samples and discharge measurements were obtained from the Gibbon River and its major tributaries near Norris Geyser Basin under the low-flow conditions of September 2006 allowing for the identification of solute sources and their downstream fate. Norris Geyser Basin, and in particular Tantalus Creek, is the largest source of many trace elements (Al, As, B, Ba, Br, Cs, Hg, Li, Sb, Tl, W, and REEs) to the Gibbon River. The Chocolate Pots area is a major source of Fe and Mn, and the lower Gibbon River near Terrace Spring is the major source of Be and Mo. Some of the elevated trace elements are aquatic health concerns (As, Sb, and Hg) and knowing their fate is important. Most solutes in the Gibbon River, including As and Sb, behave conservatively or are minimally attenuated over 29 km of fluvial transport. Some small attenuation of Al, Fe, Hg, and REEs occurs but primarily there is a transformation from the dissolved state to suspended particles, with most of these elements still being transported to the Madison River. Dissolved Hg and REEs loads decrease where the particulate Fe increases, suggesting sorption onto suspended particulate material. Attenuation from the water column is substantial for Mn, with little formation of Mn as suspended particulates.

Introduction

The Gibbon River originates at Grebe Lake and flows nearly 40 km to the Firehole River where they combine to form the Madison River. The Madison River is one of four major rivers leaving Yellowstone National Park (YNP). The Gibbon River receives water and solutes from several geothermal sources including Norris Geyser Basin, Chocolate Pots, Gibbon Geyser Basin, Beryl Spring, and Terrace Spring. Knowing the source and fate of geothermal solutes is important because their concentrations are often elevated in rivers receiving geothermal inputs. In Part I of this series (McCleskey et al., 2010), the discharge, concentrations, loads, and geochemical behavior of major solutes in the Gibbon River were described and interpreted. Part I (McCleskey et al., 2010) also contains many details of the study including sampling locations and methods. This paper covers the trace element concentrations, loads, and geochemical interpretations.

Numerous studies on the water and major ion loads in the Madison River exist (e.g., Norton and Friedman, 1985, Friedman and Norton, 1990, Hurwitz et al., 2007); however, there are no comprehensive chemical surveys that include discharge measurements allowing for the identification of sources, loads, and attenuation of trace elements in the Gibbon River. Several trace elements of ecologic importance have elevated concentrations in the Gibbon River, including As and Hg. Geothermal As from YNP causes high As concentrations in the Madison and Missouri Rivers in Montana and Wyoming (Nimick et al., 1998). This is of concern to water managers because some of this water is used for domestic and municipal water supply. Geothermal As in the Gibbon River has been studied previously (Thompson, 1979, Stauffer et al., 1980). Thompson (1979) studied As and F in the Upper Madison River System, including the Gibbon River, and found that Norris Geyser Basin is the primary source of As in the Gibbon River. Working with a small dataset and no discharge measurements, Stauffer et al. (1980) reported that most of the As flux in the Gibbon River drainage basin was precipitated or sorbed prior to the Gibbon River's confluence with the Firehole River at Madison Junction. The diel behavior of Hg has been studied in the Madison River (Nimick et al., 2007), but this study did not identify Hg sources or attenuation within the watershed. The source and fate of other trace metals, including Al, Fe, Mn, Hg, Zn, REEs, Ba, Be, Mo, Sb, Tl, and W, in the Gibbon River have not been previously studied. The purpose of this study was to quantify trace element loads, identify their sources, and identify processes of attenuation and transformation in the Gibbon River. This objective was accomplished by collecting water samples and discharge measurements from the Gibbon River and its major tributaries around Norris Geyser Basin under low-flow conditions in September 2006.