George P. Ingersoll

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.

Comparison of precipitation chemistry in the Central Rocky Mountains, Colorado, USA

Abstract Volume-weighted mean concentrations of nitrate (NO 3 − ), ammonium (NH 4 + ), and sulfate (SO 4 2− ) in precipitation were compared at high-elevation sites in Colorado from 1992 to 1997 to evaluate emission source areas to the east and west of the Rocky Mountains. Precipitation chemistry was measured by two sampling methods, the National Atmospheric Deposition Program/National Trends Network (NADP/NTN) and snowpack surveys at maximum accumulation. Concentrations of NO 3 − and SO 4 2− in winter precipitation were greater on the western slope of the Rockies, and concentrations of NO 3 − and NH 4 + in summer precipitation were greater on the eastern slope. Summer concentrations in general were almost twice as high as winter concentrations. Seasonal weather patterns in combination with emission source areas help to explain these differences. This comparison shows that high-elevation ecosystems in Colorado are influenced by air pollution emission sources located on both sides of the Continental Divide. It also suggests that sources of nitrogen and sulfur located east of the Divide have a greater influence on precipitation chemistry in the Colorado Rockies.

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

Depth-integrated snowpack chemistrywas measured just prior to maximum snowpack depth during the winters of 1992–1999 at 12 sites co-located with National Atmospheric Deposition Program/National Trend Network (NADP/ NTN) sites in the central and southern RockyMountains, USA. Winter volume-weighted mean wet-deposition concentrations were calculated for the NADP/NTN sites, and the data were compared to snowpack concentrations using the paired t-test and the Wilcoxon signed-rank test. No statisticallysignificant differences were indicated in concentrations of SO4� or NO3 (p > 0:1). Small, but statisticallysignificant differences ( pp0:03) were indicated for all other solutes analyzed. Differences were largest for Ca 2+ concentrations, which on average were 2.3meq l � 1 (43%) higher in the snowpack than in winter NADP/NTN samples. Eolian carbonate dust appeared to influence snowpack chemistrythrough both wet and drydeposition, and the effect increased from north to south. Drydeposition of eolian carbonates was estimated to have neutralized an average of 6.9meq l � 1 and a maximum of 12meq l � 1 of snowpack acidityat the southernmost sites. The good agreement between snowpack and winter NADP/NTN SO 4� and NO3 concentrations indicates that for those solutes the two data sets can be combined to increase data densityin highelevation areas, where few NADP/NTN sites exist. This combination of data sets will allow for better estimates of atmospheric deposition of SO4� and NO3 across the RockyMountain region. Published byElsevier Science Ltd.

Changing Regional Emissions of Airborne Pollutants Reflected in the Chemistry of Snowpacks and Wetfall in the Rocky Mountain Region, USA, 1993–2012

Wintertime precipitation sample data from 55 Snowpack sites and 17 National Atmospheric Deposition Program (NADP)/National Trends Network Wetfall sites in the Rocky Mountain region were examined to identify long-term trends in chemical concentration, deposition, and precipitation using Regional and Seasonal Kendall tests. The Natural Resources Conservation Service snow-telemetry (SNOTEL) network provided snow-water-equivalent data from 33 sites located near Snowpack- and NADP Wetfall-sampling sites for further comparisons. Concentration and deposition of ammonium, calcium, nitrate, and sulfate were tested for trends for the period 1993–2012. Precipitation trends were compared between the three monitoring networks for the winter seasons and downward trends were observed for both Snowpack and SNOTEL networks, but not for the NADP Wetfall network. The dry-deposition fraction of total atmospheric deposition, relative to wet deposition, was shown to be considerable in the region. Potential sources of regional airborne pollutant emissions were identified from the U.S. Environmental Protection Agency 2011 National Emissions Inventory, and from long-term emissions data for the period 1996–2013. Changes in the emissions of ammonia, nitrogen oxides, and sulfur dioxide were reflected in significant trends in snowpack and wetfall chemistry. In general, ammonia emissions in the western USA showed a gradual increase over the past decade, while ammonium concentrations and deposition in snowpacks and wetfall showed upward trends. Emissions of nitrogen oxides and sulfur dioxide declined while regional trends in snowpack and wetfall concentrations and deposition of nitrate and sulfate were downward.

Particulate carbonate matter in snow from selected sites in the south-central rocky mountains

Almtraet--Trends in snow acidity reflect the balance between strong acid inputs and reactions with neutralizing materials. Carbonate dust can be an important contributor of buffering capacity to snow; however, its concentration in snow is difficult to quantify because it dissolves rapidly in snowmelt. In snow with neutral or acidic pH, most calcite would dissolve during sample melting if snow samples were processed using standard techniques. Here a method is described for separating particulate carbonate matter from snow. Snow samples were melted in solutions close to saturation with calcite, decreasing the dissolution rate by a factor of 100-200 compared with natural melting of snow. Particulate matter larger than 0.45 tLm in diameter was then filtered from solution and analysed for carbonate content. Particulate carbonate matter concentrations are reported for 25 sites in the south-central Rocky Mountains. Results are compared with Ca 2+ and H + concentrations and regional trends are evaluated. In Colorado, mean particulate carbonate in snow was 43/~gkg -1 at sampling sites in the southern mountains and only 4 t~g kg- t at sites in the northern mountains. The higher calcite concentrations in the south probably are related to the proximity of sampling sites to major outcrops of limestone. Particulate carbonate at sampling sites in Utah and Wyoming ranged from 3-35 t~g kg-a. The levels of particulate calcite measured in snow samples are sufficient to neutralize an average of 0.4/~eq H + kg-t snow. Strong acid anion concentrations in samples from east of Craig, Colorado, were 30-50% higher than in samples from the Colorado Front Range, but H + concentrations were 400-600% higher east of Craig. Relatively low Ca 2 + concentrations in the samples from east of Craig indicate that the difference in snow acidity was due mostly to lower concentrations of neutralizing materials.