M. Alisa Mast

Processes Controlling the Chemistry of Two Snowmelt‐Dominated Streams in the Rocky Mountains

Time-intensive discharge and chemical data for two alpine streams in the Loch Vale watershed, Colorado, were used to identify sources of runoff, flow paths, and important biogeochemical processes during the 1992 snowmelt runoff season. In spite of the paucity of soil cover the chemical composition of the streams is regulated much as in typical forested watersheds. Soils and other shallow groundwater matrices such as boulder fields appear to be more important in controlling surface-water chemistry than their abundance would indicate. The chemical composition of the major source waters (usually thought of as end-members whose chemical composition is relatively constant over time) changes at the same time that their mixing ratio in streams changes, confounding use of end-member mixing models to describe stream-water chemistry. Changes in the chemical composition of these source waters are caused by the ionic pulse of solutes from the snowpack and the small size of the shallow groundwater reservoir compared to the volume of snowmelt passing through it. The brief hydrologic residence time in the shallow groundwater indicates that concentrations of most dissolved constituents of stream water were controlled by fast geochemical processes that occurred on timescales of hours to days, rather than slower processes such as weathering of primary minerals. Differences in the timing of snowmelt-related processes between different areas of the watershed also affect the stream-water chemical composition. Cirque lakes affect discharge and chemical composition of one of the streams; seasonal control on stream-water NO3 and SiO2 concentrations by diatom uptake in the lakes was inferred. Elution of acidic waters from the snowpack, along with dilution of base cations originating in shallow groundwater, caused episodes of decreased acid-neutralizing capacity in the streams, but the streams did not become acidic.

STRONTIUM 87/STRONTIUM 86 AS A TRACER OF MINERAL WEATHERING REACTIONS AND CALCIUM SOURCES IN AN ALPINE/SUBALPINE WATERSHED, LOCH VALE, COLORADO

Sr isotopic ratios of atmospheric deposition, surface and subsurface water, and geologic materials were measured in an alpine/subalpine watershed to characterize weathering reactions and identify sources of dissolved Ca in stream water. Previous studies have noted an excess of Ca in stream water above that expected from stoichiometric weathering of the dominant bedrock minerals. Mixing calculations based on 87 Sr/ 86 Sr indicate that on an annual basis, 26 67% of Ca export in streams is atmospherically derived, 23 61% is from weathering of plagioclase, and the remainder is from weathering of calcite present in trace amounts in the bedrock. A potential source of error when applying Sr isotopes in catchment studies is determination of the 87 Sr/ 86 Sr of Sr released by mineral weathering, which is complicated by the wide range of mineral isotopic compositions, particularly in older rocks, and the variable rates at which the minerals weather. In this study, base-flow stream chemistry was used to represent the 87 Sr/ 86 Sr of Sr derived from mineral weathering because it effectively integrates the potentially variable isotopic composition of Sr released by weathering in the alpine environment.

Winter fluxes of CO2 and CH4 from subalpine soils in Rocky Mountain National Park, Colorado

Fluxes of CO2 and CH 4 through a seasonal snowpack were measured in and adjacent to a subalpine wetland in Rocky Mountain National Park, Colorado. Gas diffusion through the snow was controlled by gas production or consumption in the soil and by physical snowpack properties. The snowpack insulated soils from cold midwinter air temperatures allowing microbial activity to continue through the winter. All soil types studied were net sources of CO2 to the atmosphere through the winter, whereas saturated soils in the wetland center were net emitters of CH 4 and soils adjacent to the wetland were net CH 4 consumers. Most sites showed similar temporal patterns in winter gas fluxes; the lowest fluxes occurred in early winter, and maximum fluxes occurred at the onset of snowmelt. Temporal changes in fluxes probably were related to changes in soil-moisture conditions and hydrology because soil temperatures were relatively constant under the snowpack. Average winter CO2 fluxes were 42.3, 31.2, and 14.6 mmol m -2 d -• over dry, moist, and saturated soils, respectively, which accounted for 8 to 23% of the gross annual CO2 emissions from these soils. Average winter CH 4 fluxes were -0.016, 0.274, and 2.87 mmol m -2 d -1 over dry, moist, and saturated soils, respectively. Microbial activity under snow cover accounted for 12% of the annual CH 4 consumption in dry soils and 58 and 12% of the annual CH 4 emitted from moist and saturated soils, respectively. The observed ranges in CO2 and CH 4 flux through snow indicated that winter fluxes are an important part of the annual carbon budget in seasonally snow-covered terrains.

Comparison of total mercury and methylmercury cycling at five sites using the small watershed approach

The small watershed approach is well-suited but underutilized in mercury research. We applied the small watershed approach to investigate total mercury (THg) and methylmercury (MeHg) dynamics in streamwater at the five diverse forested headwater catchments of the US Geological Survey Water, Energy, and Biogeochemical Budgets (WEBB) program. At all sites, baseflow THg was generally less than 1ng L(-1) and MeHg was less than 0.2ng L(-1). THg and MeHg concentrations increased with streamflow, so export was primarily episodic. At three sites, THg and MeHg concentration and export were dominated by the particulate fraction in association with POC at high flows, with maximum THg (MeHg) concentrations of 94 (2.56)ng L(-1) at Sleepers River, Vermont; 112 (0.75)ng L(-1) at Rio Icacos, Puerto Rico; and 55 (0.80)ng L(-1) at Panola Mt., Georgia. Filtered (<0.7microm) THg increased more modestly with flow in association with the hydrophobic acid fraction (HPOA) of DOC, with maximum filtered THg concentrations near 5ng L(-1) at both Sleepers and Icacos. At Andrews Creek, Colorado, THg export was also episodic but was dominated by filtered THg, as POC concentrations were low. MeHg typically tracked THg so that each site had a fairly constant MeHg/THg ratio, which ranged from near zero at Andrews to 15% at the low-relief, groundwater-dominated Allequash Creek, Wisconsin. Allequash was the only site with filtered MeHg consistently above detection, and the filtered fraction dominated both THg and MeHg. Relative to inputs in wet deposition, watershed retention of THg (minus any subsequent volatilization) was 96.6% at Allequash, 60% at Sleepers, and 83% at Andrews. Icacos had a net export of THg, possibly due to historic gold mining or frequent disturbance from landslides. Quantification and interpretation of Hg dynamics was facilitated by the small watershed approach with emphasis on event sampling.

Use of stable sulfur isotopes to identify sources of sulfate in Rocky Mountain snowpacks

Stable sulfur isotope ratios and major ions in bulk snowpack samples were monitored at a network of 52 highelevation sites along and near the Continental Divide from 1993 to 1999. This information was collected to better dene atmospheric deposition to remote areas of the Rocky Mountains and to help identify the major source regions of sulfate in winter deposition. Average annual ! # S values at individual sites ranged from #4.0 to #8.2! and standard deviations ranged from 0.4 to 1.6! . The chemical composition of all samples was extremely dilute and slightly acidic; average sulfate concentrations ranged from 2.4 to 12.2! eql

Methane flux in subalpine wetland and unsaturated soils in the southern Rocky Mountains

% and pH ranged from 4.82 to 5.70. The range of ! # S values measured in this study indicated that snowpack sulfur in the Rocky Mountains is primarily derived from anthropogenic sources. A nearly linear relation between ! # S and latitude was observed for sites in New Mexico, Colorado, and southern Wyoming, which indicates that snowpack sulfate in the southern part of the network was derived from two isotopically distinct source regions. Because the major point sources of SO & in the region are coal-red powerplants, this pattern may re#ect variations in the isotopic composition of coals burned by the plants. The geographic pattern in ! # S for sites farther to the north in Wyoming and Montana was much less distinct, perhaps re#ecting the paucity of major point sources of SO & in the northern part of the network. Published by Elsevier Science Ltd.

Environmental characteristics and water quality of hydrologic benchmark network stations in the west-central United States, 1963-95

Methane exchange between the atmosphere and subalpine wetland and unsaturated soils was evaluated over a 15-month period during 1995-1996. Four vegetation community types along a moisture gradient (wetland, moist-grassy, moist-mossy, and dry) were included in a 100 m sampling transect situated at 3200 m elevation in Rocky Mountain National Park, Colorado. Methane fluxes and soil temperature were measured during snow-free and snow-covered periods, and soil moisture content was measured during snow-free periods. The range of mean measured fluxes through all seasons (a positive value represents CH 4 efflux to the atmosphere) were: 0.3 to 29.2 mmol CH 4 m -2 d -1 wetland area; 0. 1 to 1.8 mmol CH 4 m -2 d -1 , moist-grassy area; -0.04 to 0.7 mmol CH 4 m -2 d -1 , moist-mossy area; and -0.6 to 0 mmol CH 4 m -2 d -1 , dry area. Methane efflux was significantly correlated with soil temperature (5 cm) at the continuously saturated wetland area during snow-free periods. Consumption of atmospheric methane was significantly correlated with moisture content in the upper 5 cm of soil at the dry area. A model based on the wetland flux-temperature relationship estimated an annual methane emission of 2.53 mo CH 4 m -2 from the wetland. Estimates of annual methane flux based on field measurements at the other sites were 0.12 mol CH 4 m -2 , moist-grassy area; 0.03 mol CH 4 m -2 , moist-mossy area; and -0.04 mol CH 4 m -2 , dry area. Methane fluxes during snow-covered periods were responsible for 25, 73, 23, and 43% of the annual fluxes at the wetland, moist-grassy, moist-mossy, and dry sites, respectively.

Major-ion chemistry of the Rocky Mountain snowpack, USA

The Hydrologic Benchmark Network was established in 1963 to provide long-term measurements of streamflow and water quality in areas that are minimally affected by human activities. These data were used to study time trends and to serve as controls for separating natural fron1 artificial changes in other strean1s. The network has consisted of as many as 58 drainage basins in 39 States. This report describes the environn1ental characteristics and water quality at 12 benchmark basins in the Western United States. The stations discussed in this report and their physiographic provinces are as follows: Wet Bottom Creek, Arizona, in the Southern Rocky Mountains; Elder Creek, California, in the Pacific Border Province; Merced River and Sagehen Creek, California, and Crater Lake, Oregon, in the Sierra-Cascade Mountains; Big Jacks Creek, Idaho, and Minam River, Oregon, in the Columbia Plateaus; Hayden Creek, Idaho, and Andrews Creek, Washington, in the Northern Rocky Mountains; South Twin River and Steptoe Creek, Nevada, in the Basin and Range; and Red Butte Creek, Utah, in the Middle Rocky Mountains. The information in this report was compiled to aid in the application and interpretation of historical water-quality data collected by the U.S. Geological Survey Hydrologic Benchmark Network program. The Hydrologic Benchmark NetT;vork streams discussed in this report drain either forested areas or grasslands and have land-use activities that include recreational use, timber harvesting, grazing, and scientific research. Surface-water chemistry in the benchmark basins was controlled by the interaction of dilute precipitation with the underlying soils and bedrock, and land-use activities seemed to have a minimal effect on the major-ion and nutrient chemistry of stream water at the Hydrologic Benchmark Network stations. Temporal trends in waterquality constituents were observed at a number of the stations and were attributed to environmental and method-related factors. Trends in base cations and alkalinity at Elder Creek and Sagei.en Creek, California, and Red Butte Creek, Utah, seemed to be associated with an extended period of drought that persisted fron1 the late 1980s through the early 1990s in both regions of the country. Statistically significant upward trends in field pH and downward trends in sulfate that were observed at several of the stations were attributed to changes in field instrumentation and analytical techniques rather than environmental change.

CONTROLS ON SURFACE WATER CHEMISTRY IN THE UPPER MERCED RIVER BASIN, YOSEMITE NATIONAL PARK, CALIFORNIA

During 1993‐97, samples of the full depth of the Rocky Mountain snowpack were collected at 52 sites from northern New Mexico to Montana and analyzed for major-ion concentrations. Concentrations of acidity, sulfate, nitrate, and calcium increased from north to south along the mountain range. In the northern part of the study area, acidity was most correlated (negatively) with calcium. Acidity was strongly correlated (positively) with nitrate and sulfate in the southern part and for the entire network. Acidity in the south exceeded the maximum acidity measured in snowpack of the Sierra Nevada and Cascade Mountains. Principal component analysis indicates three solute associations we characterize as: (1) acid (acidity, sulfate, and nitrate), (2) soil (calcium, magnesium, and potassium), and (3) salt (sodium, chloride, and ammonium). Concentrations of acid solutes in the snowpack are similar to concentrations in nearby wetfall collectors, whereas, concentrations of soil solutes are much higher in the snowpack than in wetfall. Thus, dryfall of acid solutes during the snow season is negligible, as is gypsum from soils. Snowpack sampling o!ers a coste!ective complement to sampling of wetfall in areas where wetfall is dicult to sample and where the snowpack accumulates throughout the winter. Published by Elsevier Science Ltd.

Comparison of snowpack and winter wet-deposition chemistry in the Rocky Mountains, USA: implications for winter dry deposition

Surface water draining granitic bedrock in Yosemite National Park exhibits considerable variability in chemical composition, despite the relative homogeneity of bedrock chemistry. Other geological factors, including the jointing and distribution of glacial till, appear to exert strong controls on water composition. Chemical data from three surface water surveys in the upper Merced River basin conducted in August 1981, June 1988 and August 1991 were analysed and compared with mapped geological, hydrological and topographic features to identify the solute sources and processes that control water chemistry within the basin during baseflow. Water at most of the sampling sites was dilute, with alkalinities ranging from 26 to 77 pequiv. I-. Alkalinity was much higher in two subcatchments, however, ranging from 51 to 302 pequiv. 1-. Base cations and silica were also significantly higher in these two catchments than in the rest of the watershed. Concentrations of weathering products in surface water were correlated to the fraction of each subcatchment underlain by surficial material, which is mostly glacial till. Silicate mineral weathering is the dominant control on concentrations of alkalinity, silica and base cations, and ratios of these constituents in surface water reflect the composition of local bedrock. Chloride concentrations in surface water samples varied widely, ranging from < 1 to 96 pequiv. 1-. The annual volume-weighted mean chloride concentration in the Merced River at the Happy Isles gauge from 1968 to 1990 was 26 pequiv. l-, which was five times higher than in atmospheric deposition (4-5 pequiv. l-), suggesting that a source of chloride exists within the watershed. Saline groundwater springs, whose locations are probably controlled by vertical jointing in the bedrock, are the most likely source of the chloride. Sulphate concentrations varied much less than most other solutes, ranging from 3 to 14 pequiv. 1-. Concentrations of sulphate in quarterly samples collected at the watershed outlet also showed relatively little variation, suggesting that sulphate may be regulated to some extent by a within-watershed process, such as sulphate adsorption.