Showing papers by "Diane M. McKnight published in 1990"

Characterization of transport in an acidic and metal-rich mountain stream based on a lithium tracer injection and simulations of transient storage

Abstract: Physical parameters characterizing solute transport in the Snake River (an acidic and metal-rich mountain stream near Montezuma, Colorado) were variable along a 5.2-km study reach. Stream cross-sectional area and volumetric inflow each varied by a factor of 3. Because of transient storage, the residence time of injected tracers in the Snake River was longer than would be calculated by consideration of convective travel time alone. Distributed inflows along the stream were a significant source of in-stream chemical variations. These transport characteristics of the Snake River were established on the basis of the assumption of lithium as an ideally conservative tracer and use of simulations of advection, dispersion, and transient storage. Evaluations of the validity of this combined tracer and simulation approach lend confidence to the estimation of the physical transport parameters, but further development is warranted for methods of onsite transport experimentation in hydrologically complex, chemically reactive environments.

The Chemistry of Iron, Aluminum, and Dissolved Organic Material in Three Acidic, Metal‐Enriched, Mountain Streams, as Controlled by Watershed and in‐Stream Processes

Abstract: Several studies were conducted in three acidic, metal-enriched, mountain streams, and the results are discussed together in this paper to provide a synthesis of watershed and in-stream processes controlling Fe, Al, and DOC (dissolved organic carbon) concentrations. One of the streams, the Snake River, is naturally acidic; the other two, Peru Creek and St. Kevin Gulch, receive acid mine drainage. Analysis of stream water chemistry data for the acidic headwaters of the Snake River shows that some trace metal solutes (Al, Mn, Zn) are correlated with major ions, indicating that watershed processes control their concentrations. Once in the stream, biogeochemical processes can control transport if they occur over time scales comparable to those for hydrologic transport. Examples of the following in-stream reactions are presented: (1) photoreduction and dissolution of hydrous iron oxides in response to an experimental decrease in stream pH, (2) precipitation of Al at three stream confluences, and (3) sorption of dissolved organic material by hydrous iron and aluminum oxides in a stream confluence. The extent of these reactions is evaluated using conservative tracers and a transport model that includes storage in the substream zone.

Phytoplankton Dynamics in Three Rocky Mountain Lakes, Colorado, U.S.A.

Abstract: In 1984 and 1985 seasonal changes in phytoplankton were studied in a system of three lakes in Loch Vale, Rocky Mountain National Park, Colorado. Three periods were evident: (1) A spring bloom, during snowmelt, of the planktonic diatom Asterionellaformosa, (2) a midsummer period of minimal algal abundance, and (3) a fall bloom of the blue-green alga Oscillatoria limnetica. Seasonal phytoplankton dynamics in these lakes are controlled partially by the rapid flushing rate during snowmelt and the transport of phytoplankton from the highest lake to the lower lakes by the stream, Icy Brook. During snowmelt, the A. formosa population in the most downstream lake has a net rate of increase of 0.34 d-l, which is calculated from the flushing rate and from the A. formosa abundance in the inflow from the upstream lake and in the downstream lake. Measurement of photosynthetic rates at different depths during the three periods confirmed the rapid growth of A. formosa during the spring. The decline in A. formosa after snowmelt may be related to grazing by developing zooplankton populations. The possible importance of the seasonal variations in nitrate concentrations were evaluated in situ enrichment experiments. For A. formosa and 0. limnetica populations, growth stimulation resulted from 8- or 16-micromolar amendments of calcium nitrate and sulfuric acid, but the reason for this stimulation could not be determined from these experiments.