Zinc and Copper Isotopes as Tracers of Anthropogenic Contamination in Lake Sediments, Seattle, Washington - 2010

Mining & Minerals - Evaluations / Wastes & Environmental Issues

Borrok, and 4 co-authors, covered the following topics at a 2010 National Monitoring Conference:

Why Zn and Cu isotopes?
Zn and Cu isotope systematics,
Case study of sediment core from Lake Ballinger, Washington:

o           Study location and history of metal inputs

o           Metal accumulation rates

o           Isotopic results

o           Resolving metal sources

o           Conclusions and implications

The global atmospheric emissions of Zn and Cu are on the rise despite the fact that fluxes for other trace metals like Pb have decreased over the last 2 decades. The sources of environmental Zn and Cu pollution are numerous (metal refining, automotive, agricultural, power generation, etc. It is critical to find new environmental tracers that can fingerprint and track Zn and Cu pollution. Zn is emitted to the atmosphere in greater quantities than any trace metal worldwide (Pacyna and Pacyna, 2001).

• Natural rock and water samples display only a small variation in δ⁶⁶Zn (Mostly between +0.1‰ to +0.4‰).
• Zn isotopes are only weakly fractionated by natural processes.
• Zn isotopes are fractionated substantially during high- temperature combustion processes (i.e., evaporation and condensation reactions).
• Zn metal boils at 907ËšC.
• Natural rock and water samples display a large variation in δ⁶⁵Cu, which is largely attributable to redox processes.
• Cu isotopes are fractionated substantially by natural processes involving oxidation and reduction.
• Cu isotopes are fractionated weakly during high- temperature combustion processes (i.e., evaporation and condensation reactions).
• Cu metal boils at 2,567ËšC.
• Located in a highly urbanized area,
• Within the range of Tacoma smelter contaminant plume,

A previous study of Lake Ballinger core collected in 1998 indicated that Zn and Cu contamination was on the rise despite closure of the Tacoma smelter a decade earlier, see Mahler et al. (2006) Trends in metals in urban and reference lake sediments across the United States, 1970 to 2001. Environ. Tox. Chem. 25, 1698-1709.

Tacoma smelter:

• Pb and Cu smelting and a major domestic supplier of As.
• Urbanization – automobile emissions, tire wear, waste burning, power generation, fertilizers, industrial inputs, storm water, etc.
•  Tacoma Smelter (closed 1985).  Now the Commencement Bay-Nearshore Tideflats Superfund Site.
• In 2007 - Collected 50 cm free-fall gravity core from Lake Ballinger.
• Core was sectioned into 2 cm intervals from 0-20 cm depth and 3 cm intervals from 20-50 cm depth and prepared for multiple analyses.
• Stream-bed sediments from Hall Creek were collected from three locations.
• Ages were estimated using the ²¹⁰Pb profile and applying the constant rate of supply model (¹³⁷Cs was used as a check).

Key Zn and Cu isotope results:

• The δ⁶⁶Zn and δ⁶⁵Cu for the pre-smelting period (i.e., natural metal inputs) are easily distinguished from anthropogenic metal sources.
• The δ⁶⁶Zn and δ⁶⁵Cu of sediments during the rapid urbanization period are similar to those from the smelting period, suggesting that the added metal input came from remobilization of historically contaminant soils.
• The “light” δ⁶⁶Zn found in the post-smelting sediments and in Hall Creek (0.02 ± 0.06‰ 2σ) is likely from urban runoff.
• The δ⁶⁵Cu for the most recent post-smelting sediments is “heavy”. SSD work suggests that this is related to changes in Cu-organic complexation during diagenesis,
• Possible sources are numerous and include galvanized steel, automobile emissions, tire wear, oil leakage, power plants, and fertilizers.

The authors suspected tire wear was an important contributor because tire rubber contains about 1 wt% Zn and Lake Ballinger is situated between two major highways:

• Zn isotopes for five different tires ranged from -0.1‰ to +0.14 ‰ and averaged +0.04 ‰ ± 0.2 ‰.
• The δ⁶⁶Zn of tires overlaps with the δ⁶⁶Zn of the urban Zn source
  
  Thapalia, A., Borrok, D.M., Van Metre, P., Musgrove, M. (2010) Zn and Cu isotopes as tracers of anthropogenic contamination in a sediment core from an urban lake. ES&T 44, 1544-1550. and https://scholarworks.utep.edu/cgi/viewcontent.cgi?article=2360&context=open_etd  

Also see: https://hal.archives-ouvertes.fr/hal-01466186/document  

The authors concluded that:

• Zn (and probably to a lesser extent Cu) isotopes can be useful for evaluating contaminant trends in sediment core.
• These metal isotopes can resolve anthropogenic and natural metal sources. 
• In some cases they can be used to distinguish among the different sources of urban pollution.

CURRENT PROJECTS:

• Using Zn isotopes to identify fertilizer contributions in river systems.
• Assessment of Zn isotopic fractionation during coal and coal + tire combustion.
• Using Zn isotopes to elucidate metal attenuation mechanisms in soils.