18. Geochemical, Hydrologic, and Modeling Methods for Resolving Natural and Anthropogenic Sources of Metals from Weathering of Mineral Deposits

The western United States has a relatively long mining history, the visible effects of which include surface disturbances (mine workings, waste-rock and mill-tailings dumps, etc.) and, in some areas, degraded stream-water chemistry. Weathering of the solid mine-waste material may lead to increased transport of some heavy metals, either in solution or by physical processes such as suspended sediment and wind transport. In most if not all of these areas, no pre-mining baseline geochemical data exist, hampering remediation efforts by governmental agencies and other groups. Despite this lack of data, it may be possible to estimate pre-mining baseline geochemical conditions in streams through a combination of spatially dense field sampling accompanying in-stream tracer tests, and sensitive geochemical and isotopic analyses from samples taken in reaches of watersheds carefully chosen to reflect the least amount of mining impact. This Mendenhall research opportunity is designed to take advantage of in-stream tracer methods and advanced numerical modeling techniques developed by USGS personnel, combined with new analytical chemistry facilities housed in the USGS Denver laboratories. For an overview of existing work by the research supervisors, please see Kimball and others (2002) and Wanty and others (2004).

The postdoctoral research will focus on one or more small catchments that have been affected by historic mining. Following initial assessments of the area(s), an in-stream tracer test with synoptic sampling will be conducted (cf. Kimball and others, 2002) to quantitatively evaluate all stream inputs. The samples collected during the synoptic sampling will be analyzed by the usual ICP-MS and ICP-AES methods. The work described thus far is similar to that which has been performed by USGS researchers in many mining-affected catchments. The new work, which will be the focus of this research opportunity, will be to conduct additional chemical and isotopic analyses, and to modify existing strategies by which the tracer-synoptic data are numerically modeled.

The additional analyses will be performed using a Finnegan Element ICP-MS and Nu Instruments Plasma HR multi-collector ICP-MS. The former instrument is capable of performing extremely sensitive analyses for total concentrations of elements. This capability lowers our detection levels from traditional methods by one to three orders of magnitude for many elements. Taking advantage of this analytical technology, the successful candidate will be able to evaluate element concentrations and ratios that have not been examined because their limited mobility in natural systems left them mostly below analytical detection limits. The multi-collector ICP-MS allows for sensitive determination of isotope ratios, even at parts per billion-levels of detection for many elements (cf. Zhu et al., 2002). We expect that examination of previously undetectable levels of some elements, combined with the isotopic ratio measurements, will open up new research avenues and lead to new capabilities to resolve different natural and anthropogenic sources of solutes to streams. In a more general sense, it should be possible to use these new geochemical capabilities to understand local catchment-scale hydrology in greater detail.

Catchment modeling will follow previous studies, but with a new twist: the geochemical and isotopic data will be used to formulate strategies to determine whether given tributaries are likely to be of natural or anthropogenic origin. The model should then be able to account for these input types separately. A first-order approach to this problem might be to: a) calibrate a reactive solute transport model (such as OTEQ) so that it reproduces the pH and metal concentrations associated with existing conditions (synoptic data from tracer-dilution test); b) using information from the chemical and isotopic analyses, replace all mining-influenced sources with sources thought to represent natural background; and c) rerun the model to provide estimates of pre-mining water quality. This approach is an extension of that used for the analysis of remedial alternatives (Runkel and Kimball, 2002). This overall approach will add an element of certainty to the determination of likely pre-mining baseline geochemistry by combining the possibility of isotopic mass-balance calculations with chemical mass balances. Possible additional work that potential candidates might consider would be to analyze other media (stream sediments, tree rings, etc.) to back up the water data.

This Mendenhall research opportunity combines elements of field, laboratory, and modeling investigations. Successful candidates will have a Ph.D. in geochemistry, but chemistry or environmental sciences majors might also be strong candidates.

REFERENCES

Kimball, B.A., Runkel, R.L., Walton-Day, K., and Bencala, K.E. 2002, Assessment of metal loads in watersheds affected by acid mine drainage by using tracer injection and synoptic sampling: Cement Creek, Colorado, USA: Applied Geochemistry v. 17, p. 1183-1207.

Runkel, R.L., and B.A. Kimball, 2002, Evaluating remedial alternatives for an acid mine drainage stream: Application of a reactive transport model: Environmental Science and Technology, v. 36, no. 5, p. 1093-1101.

Wanty, R.B., Verplanck, P.L., Kimball, B.A., Tuttle, M.L., Runkel, R.L., and Berger, B.R., 2004, Resolving natural and anthropogenic sources of solutes to a watershed with historic mining, in Wanty, R.B., and Seal, R.R. II, eds., Proceedings of the 11th International Symposium on Water-Rock Interaction: Lisse, The Netherlands, Taylor and Francis,.

Zhu, X.K., and others, 2002, Mass fractionation processes of transition metal isotopes: Earth and Planetary Science Letters, v. 200, p. 47-62.

Proposed Duty Station: Denver, CO

Areas of Ph.D.: Geology, geochemistry, hydrology, environmental science

Qualifications: Applicants must meet one of the following qualifications: Research Geologist, Research Chemist

(This type of research is performed by those who have backgrounds for the occupations stated above. However, other titles may be applicable depending on the applicant's background, education, and research proposal. The final classification of the position will be made by the Personnel specialist.)

Research Advisor(s): Richard Wanty, (303) 236-1819, rwanty@usgs.gov; Briant Kimball (801) 908-5047, bkimball@usgs.gov; Philip Verplanck, (303) 236-1902 or (303) 541-3080, plv@usgs.gov; Robert Runkel (303) 236-4882, eunkel@usgs.gov; Ian Ridley, (303) 236-5558, iridley@usgs.gov; Paul Lamothe, (303) 236-1923, plamothe@usgs.gov

Personnel Office contact: Kathleen Scheich, (303) 236-9581, kscheich@usgs.gov

Summary of Opportunities