2002 Progress Report: Investigations into the causes of diel cycling of heavy metals in streamsEPA Grant Number: R829400E01
Title: Investigations into the causes of diel cycling of heavy metals in streams
Investigators: Gammons, Christopher H. , Hobbs, David , Moore, Johnnie , Nimick, David
Institution: Montana Tech of the University of Montana , United States Geological Survey [USGS]
EPA Project Officer: Chung, Serena
Project Period: October 1, 2001 through September 30, 2003 (Extended to September 22, 2004)
Project Period Covered by this Report: October 1, 2001 through September 30, 2002
Project Amount: $140,000
RFA: EPSCoR (Experimental Program to Stimulate Competitive Research) (2000) RFA Text | Recipients Lists
Research Category: EPSCoR (The Experimental Program to Stimulate Competitive Research)
Recent field research has shown that many metals-including zinc, cadmium, manganese, and arsenic-show large and reproducible diel (24 hour) fluctuations in dissolved concentration in streams draining abandoned mine lands in Montana. The objective of this research project is to examine the chemical processes responsible for these day-night cycles. A multifaceted approach is being used that combines field studies, laboratory experiments, and theoretical modeling.
Field Work. Since May 2002, diel investigations have been conducted on five different streams in Montana: (1) Fisher Creek (near Cooke City, MT); (2) High Ore Creek (near Basin, MT); (3) Butte Metro Storm Drain (Butte, MT); (4) Big Hole River (south of Butte, MT); and (5) Indian Creek (near Townsend, MT). The Fisher Creek study was undertaken in collaboration with the U.S. Geological Survey. Three reaches of Fisher Creek with different pH regimes (pH 3.5, pH 5.5, and pH 7.0) were sampled simultaneously to examine the effect of stream pH on the nature and magnitude of diel cycles in metal concentration. Because this stream has very low biological productivity, there were no 24-hour changes in pH in any of the three reaches. Preliminary results based on in-situ analysis of zinc and ferrous iron showed robust increases in zinc at night (for the reach with circum-neutral pH), and increases in Fe2+ during the day (for the reach with pH ~5.5). The former is attributed to the temperature dependence of adsorption of Zn2+ onto hydrous oxides of iron and aluminum, whereas the latter is attributed to day-time photo-reduction of colloidal ferric hydroxide. If these preliminary results are verified by laboratory analyses (in progress), this will be the first documented example of a stream in which diel metal cycles were induced by temperature changes, with no pH or biological influences.
Work on High Ore Creek has focused on assessing the importance of groundwater-surface water interactions on diel metal cycling. We have concluded that groundwater seeps and springs are the main source of Zn2+ contamination in High Ore Creek, but that the diel cycles in Zn2+ are due to in stream processes (such as adsorption onto organic or inorganic substrates), rather than changes in groundwater or hyporheic water flux. We also have demonstrated how cursory monitoring of Zn2+ levels in a stream such as High Ore Creek may lead to erroneous conclusions if the diel cycling phenomenon is ignored. This is particularly true for synoptic studies, where opposing conclusions may be reached about whether a particular stream is gaining or attenuating heavy metals, depending on whether a sampler works its way upstream or downstream during the day.
Field work on the Big Hole River is in progress, and we have not yet received the results from 2002. However, a similar study in 2001 showed extremely large 24-hour pH swings (from 7.8 to 9.8) in this pristine, but extremely bioproductive stream. Dissolved Mn2+ showed a robust diel fluctuation (increasing five-fold at night), which correlated closely with more subtle variations in Ca2+. Geochemical modeling suggests that both trends could possibly be explained by cyclic dissolution and precipitation of a Mn-rich calcite (Ca-carbonate) on the streambed, or on biofilm surfaces.
Experimental Work. We have completed a set of stream mesocosm experiments to simulate natural conditions in High Ore Creek. Boulders, gravel, water, and biofilms were collected from High Ore Creek and placed in a 20-liter fish tank, with continuous circulation afforded by a 6-inch waterfall. We filled a second tank with stream water and algae only (no boulders or gravel). The tanks were illuminated on a 12-hour cycle with a combination of natural and synthetic light to stimulate algal growth. Results showed a robust and reproducible cycle in dissolved zinc concentration for both tanks that mimicked the natural environment very closely. Zinc concentrations reached a maximum in the morning just before turning the lights on, and decreased to minimum values in the early evening, just before turning the lights off. The fact that zinc cycles were induced in the algae-only experiment underscores the probable significance of biofilms in diel cycling of metals in streams. We performed additional experiments, in which: (1) pH was artificially controlled by acid or base titration, while keeping temperature approximately constant; and (2) temperature was artificially controlled by refrigeration, while keeping pH approximately constant. The results showed that dissolved zinc concentrations are sensitive to changes in both variables, increasing with a decrease in pH or drop in temperature. These results are most easily explained by temperature- and pH-dependent sorption reactions on mineral or biofilm surfaces.
A second generation of "mesocosm" experiments dealing with High Ore Creek, in which we are seeking to determine the relative importance of inorganic versus organic substrates in diel metal cycling is in progress. Inorganic substrates include reddish-brown hydrous oxides of predominantly iron on boulder surfaces, and black hydrous oxides of predominantly Mn and Zn on the bottom of gravel and boulders. We are conducting experiments in which metal partitioning is quantified between aqueous solution and hydrous oxide crusts as a function of changing pH, temperature, and Eh (oxidation state). Organic substrates include autotrophic biofilms (mainly diatoms and green algae) coating gravel and boulder surfaces. Natural biofilms from High Ore Creek have been shown to be extremely rich in zinc, containing a high percentage of Zn by dry mass. Currently, it is not known how much of this zinc is present within the living algal tissues themselves, how much is adsorbed to cell walls, and how much is present as admixed inorganic particles.
Another set of experiments has been conducted in which calcite is sequentially
dissolved and precipitated, in response to cyclic changes in pH, which, in turn,
are controlled by bubbling gas with differing partial pressures of CO2. When
Mn2+ and Zn2+ were artificially added to the system, Mn2+ displayed a pattern
similar to Ca2+, whereas Zn2+ behaved quite differently. These results are tentatively
explained by the incorporation of Mn2+ into a calcium carbonate solid solution,
whereas Zn2+ may form discrete Zn minerals (such as ZnCO3, Zn(OH)2, or hydrozincite).
Much of our efforts in the second year of this project will concentrate on interpreting and documenting the results of experimental and field studies completed (or nearly completed) in Year 1. In addition, a detailed diel investigation will take place off of two reaches along the Clark Fork River (one of the first streams worldwide in which diel metal cycles were documented).
Journal Articles on this Report : 1 Displayed | Download in RIS Format
|Other project views:||All 8 publications||1 publications in selected types||All 1 journal articles|
||Nimick DA, Gammons CH, Cleasby TE, Madison JP, Skaar D, Brick CM. Diel cycles in dissolved metal concentrations in streams:occurrence and possible causes. Water Resources Research 2003;39(9):1247-1264.||