Grantee Research Project Results
Final Report: Photochemical Processes Controlling Manganese Chemistry in Pristine and Contaminated Mountain Streams
EPA Grant Number: R826649Title: Photochemical Processes Controlling Manganese Chemistry in Pristine and Contaminated Mountain Streams
Investigators: McKnight, Diane M. , Hrncir, Duane
Institution: University of Colorado at Boulder , The University of Texas at Dallas
EPA Project Officer: Aja, Hayley
Project Period: October 1, 1998 through September 30, 2001
Project Amount: $351,175
RFA: Exploratory Research - Environmental Chemistry (1998) RFA Text | Recipients Lists
Research Category: Sustainable and Healthy Communities , Land and Waste Management , Air , Safer Chemicals
Objective:
The objectives of the research project were to determine the role of manganese (Mn) photochemistry in controlling the chemical speciation and transport of manganese in surface waters. To focus the research on streams where these photochemical processes might be observable, several streams receiving acid mine drainage (AMD) were studied to determine: (1) the diel variation in the speciation of manganese in a neutral stream with a high concentration of dissolved manganese; and (2) the diel variation in the speciation of dissolved manganese in an acidic stream with abundant iron (hydr)oxides on the streambed. To provide a background for comparison with AMD streams, manganese photochemistry in a pristine stream also was examined to determine the diel variation in the speciation of dissolved manganese in a neutral stream with a low concentration of dissolved manganese.
Manganese is a trace metal that is present in many aquatic environments and can form oxide phases that can sorb other metals, natural organic material such as humic substances, and organic contaminants. One common aspect of iron (Fe) and Mn oxides and humic substances is their lack of chemical definition and their environmental variability. For this reason, it is important to study reactions involving these constituents in field experiments, as well as in more controlled laboratory experiments. Mn(IV) is more soluble than Fe(III) at neutral pH, but the precipitation of manganese oxides can be accelerated by surfaces and microbes. Because of the relative abundance of humic substances to Fe and Mn in solution and in solid phases, interactions between humic substances and Fe and Mn play an important role in the photochemistry of these two metals. Thus, another consideration in studying Mn photochemsitry is that humic substances can be photolyzed, producing reactive photoproducts such as organic radical species.
Summary/Accomplishments (Outputs/Outcomes):
In general, the research showed that Mn photoreduction is a process that contributes to diel variation in Mn speciation and transport in streams that have neutral pH and high Mn concentrations. It was further determined from the experiments that the extent of Mn photoreduction depends upon the pH at the Mn oxide surface and on the form of the Mn oxides on the streambed. In laboratory experiments using natural Mn oxides from the stream, sorption of dissolved humic substances influenced the photoreactivity of Mn oxide surfaces. However, production of hydrogen peroxide by photolysis of dissolved organic carbon (DOC) had no significant effect on Mn speciation because the concentrations of hydrogen peroxide were much lower than the Mn concentrations.
In the first year of the study, advances were made in understanding Mn chemistry in streams by determining that high pH and high Mn concentrations are the environmental conditions that are associated with diel variation due to in-stream biogeochemical processes. The stream scale experiments focused on Lake Fork Creek in Lake County. In Lake Fork Creek, biogeochemical processes in the wetland draining into the stream and in the stream itself resulted in dynamic variations in Mn concentrations. The investigators also studied Mn variations in the Snake River that have high Mn and Fe concentrations and in Deer Creek, a stream that has a neutral pH and low Mn concentrations. It was observed, however, that the Mn concentrations were stable over a 24-hour period in the Snake River, despite varying Fe concentrations due to photochemistry. In Deer Creek, changes in source water during storm events were the main factors causing Mn variations, rather than in-stream processes.
In Lake Fork Creek, the study showed that the dominant process was removal of Mn from the stream by Mn oxidation and precipitation, and that photochemistry of Mn had an influence in the water column speciation. A reactive solute transport model was used to quantify the extent of these reactions using a first order decay reaction to represent precipitation of Mn oxide (Figure 1). These results will be useful in evaluating the potential for trace metal concentrations to vary on a daily basis in acid mine drainage streams and will provide tools for quantitatively assessing Mn chemical reaction rates in these stream systems.
Lake Fork Creek receives mine drainage from an adit (Dinero Tunnel) that originated with the opening of the mine in the late 1800s, and discharges contaminated water into a large wetland, that then drains into the stream (see Figure 1). An annual study of the wetland found that a high mass flow of iron, zinc, and manganese from the wetland occurred throughout the year, with a large spike during spring snowmelt. During summer, the wetland reduced the mass flow from the adit to the stream by over 90 percent for iron, by 65 percent for zinc, and by 25 percent for manganese. The retention in the wetland during summer was caused primarily by precipitation of iron oxides and manganese and zinc sulfides. During autumn, winter, and spring, however, zinc and manganese mass flows to the stream were greater than mass flows from the adit, indicating a seasonal cycle of the wetland acting as a net sink in the summer and then a net source in the winter. Iron mass flow to the stream was less than the iron mass flow from the adit for the entire year. The seasonal shift in the wetland function can be particularly important because winter is the period of low flow and less dilution of excess metal fluxes. Because the wetland studied began receiving mine drainage in the early 1900s, results provide important information concerning seasonal variations in the effectiveness of natural wetlands in retaining metals from mine drainage. Results provide evidence that the long-term functioning of wetlands to improve water quality in mined areas may require planned upgrades to remove accumulated iron oxides. Finally, the magnitude of the spike in Fe, Zn, and Mn fluxes was shown to come from the leaching of the exposed tailings piles and overwhelmed other sources during that period. These tailings piles are common throughout mining districts and could be managed to reduce these snowmelt pulses.
Figure 1. Conservative and first order decay simulations for manganese (total, filterable, and manganous ion) from Site 1 to Site 2 in Lake Fork Creek.
Future work that would advance understanding of the chemistry of manganese in streams should focus on the processes occurring at the rock-water interface. These studies could explore further the role of clay mineralogy and weathering rind development and the role of photosynthesis enhanced manganese oxide precipitation. Because both of these processes have been shown in our study to strongly control manganese transport in Lake Fork Creek, by better understanding the nature of the removal process, it would be possible to apply these results to a broad range of streams. For example, in streams with more abundant periphyton (algae on the streambed) removal through precipitation may be occurring at a more rapid rate than was determined in Lake Fork Creek.
Journal Articles on this Report : 3 Displayed | Download in RIS Format
Other project views: | All 13 publications | 4 publications in selected types | All 3 journal articles |
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Type | Citation | ||
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August EE, McKnight DM, Hrncir DC, Garhart KS. Seasonal variability of metals transport through a wetland impacted by mine drainage in the Rocky Mountains. Environmental Science & Technology 2002;36(17):3779-3786. |
R826649 (Final) |
not available |
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Scott DT, McKnight DM, Voelker BM, Hrncir DC. Redox processes controlling manganese fate and transport in a mountain stream. Environmental Science & Technology 2002;36(3):453-459. |
R826649 (Final) |
not available |
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Scott DT, Gooseff MN, Bencala KE, Runkel RL. Automated calibration of a stream solute transport model: Implications for interpretation of biogeochemical parameters. Journal of the North American Benthological Society 2003;22(4):492-510. |
R826649 (Final) |
not available |
Supplemental Keywords:
photochemical processes, photoreduction, manganese cycling, acid mine drainage, metals, environmental chemistry, precipitation, chemical transport, hydrology, modeling, Colorado, CO, EPA Region 8., Scientific Discipline, Air, Geographic Area, Hydrology, Environmental Chemistry, State, Chemistry, Engineering, Chemistry, & Physics, EPA Region, manganese speciation, fate and transport, mountain streams, acid mine drainage, stream ecosystems, chemical transport modeling, chemical kinetics, Region 8, photochemical processes, heavy metalsProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.