Tools to Define Pre-Mining Water-Quality Restoration Targets
Mineral Creek watershed in southwestern Colorado has natural and mining-related sources of contamination. In the background of this photo is a large naturally occurring seep that discharges acidic, metal-rich water to a tributary of Mineral Creek. Natural sources of contamination like this one make it difficult to determine appropriate water-quality standards for cleanup actions. Photo Credit: David A. Nimick, USGS
U.S. Geological Survey scientists and their colleagues have developed a variety of tools to estimate the pre-mining water quality of streams in watersheds with a legacy of past mining activities. The scientists are applying these tools to help land managers faced with defining realistic water-quality restoration targets for watersheds that may have been affected by natural weathering of mineral deposits before mining ever began. Many streams in these watersheds are contaminated by both natural and mining-related sources of toxic metals and acidity. The lack of historic information on pre-mining water quality and the current contributions of contamination from both natural and mining-related sources make it difficult to establish appropriate water-quality standards for cleanup activities.
The scientists have developed and are applying the following set of tools:
- A computer model that can simulate pre-mining water quality — Data on the water quality of natural and mining-related inputs to the stream are collected along a section of the stream subject to remediation. Once the model is adjusted, or calibrated, to the data for current stream conditions, the mining-related inputs are removed from the model. The revised model is then used to estimate pre-mining water quality. In some cases, the estimated pre-mining concentration of metals is significantly greater than the water-quality standards established for streams in watersheds with no mineral deposits.
- A geochemical approach that compares the ratio of iron and copper in old and new sediments cemented by iron precipitates (ferricretes) — Ferricrete deposits that were formed in contact with stream water before mining preserve a record of pre-mining water quality in the stream. Therefore, a comparison of modern ferricretes in mining-affected and natural areas to ancient ferricretes can be used to estimate within reasonable bounds what the pre-mining water quality of the streams might have been.
Deposit of iron-cemented stream gravel (ferricrete) with embedded wood fragments that can be age dated using radiocarbon to determine the age of the ferricrete deposit. Knowing the age of the ferricretes helps scientists determine if the associated enrichment of metals in streams occurred before or after mining in the watershed started. Photo Credit: David A. Nimick, USGS
- The use of isotopes of iron and zinc to track natural and mining-related sources of metals to streams — Studies of the isotopic composition of the iron and zinc found in natural and mining-related sources of contamination can help to differentiate between natural sources, such as wetlands, bogs, and seeps, in mineralized watersheds, which often have different isotopic composition than the mineral-rich deposits that are mined for metals. These differences in isotopic composition can be used to track and identify the magnitude of natural and anthropogenic contributions of metals to streams.
- Comparisons of the water quality of streams in mineralized watersheds with and without mining — USGS studies of streams in mineralized watersheds that have not been mined are increasing the knowledge base on the occurrence and causes of natural acidic conditions in streams. This knowledge base is then used to compare the water quality of mined and un-mined watersheds with similar characteristics. The results of the comparison can be used to estimate pre-mining water quality in the mined watershed.
Combining the use of these tools brings multiple lines of evidence to defining pre-mining water-quality conditions in streams and increases confidence in the cleanup targets based on these analyses. These tools are being applied to watersheds in Montana and Colorado, where land managers and environmental regulators are developing cleanup standards for remediation activities in historic mining districts.
Kimball, B.A., Runkel, R.L., Wanty, R.B., Philip L. Verplanck, P.L., 2010, Reactive solute-transport simulation of pre-mining metal concentrations in mine-impacted catchments Redwell Basin, Colorado, USA: Chemical Geology, v. 269, p. 124-136, doi:10.1016/j.chemgeo.2009.05.024.
Nimick, D.A., Gurrieri, J.T., and Furniss, G., 2009, An empirical method for estimating instream pre-mining pH and dissolved Cu concentration in catchments with acidic drainage and ferricrete: Applied Geochemistry, v. 24, no. 1, p. 106-119, doi:10.1016/j.apgeochem.2008.11.007.
Verplanck, P.L., Nordstrom, D.K., Bove, D.J., Plumlee, G.S., and Runkel, R.L., 2009, Naturally acidic surface and ground waters draining porphyry-related mineralized areas of the Southern Rocky Mountains, Colorado and New Mexico: Applied Geochemistry, v. 24, no. 2, p. 255-267, doi:10.1016/j.apgeochem.2008.11.014.
Runkel, R.L., 2010, One-Dimensional Transport with Equilibrium Chemistry (OTEQ)--A reactive transport model for streams and rivers: U.S. Geological Survey Techniques and Methods Book 6, Chapter B6, 101 p.
Runkel, R.L., Kimball, B.A., Walton-Day, K., and Verplanck, P.L., 2007, A simulation-based approach for estimating pre-mining water quality--Red Mountain Creek, Colorado: Applied Geochemistry, v. 22, no. 9, p. 1899-1918, doi:10.1016/j.apgeochem.2007.03.054
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