Crustal Imaging and Characterization Team
Kathleen S. Smith, James F. Ranville, Paul J. Lamothe, Katie Walton-Day, and Brian P. Jackson
Kathleen S. Smith is a research geochemist with the USGS, Denver, CO
James F. Ranville is an associate professor in the Department of Chemistry and Geochemistry, Colorado School of Mines, Golden, CO
Paul J. Lamothe is a research chemist with the USGS, Denver, CO
Katie Walton-Day is a research hydrologist with the USGS, Denver, CO
Brian P. Jackson is an environmental analytical chemist in the Department of Earth Sciences, Dartmouth College, Hanover, NH
This information is excerpted from:
The upper Arkansas River south of Leadville, Colorado, contains deposits of fluvial tailings from historical metal-mining operations. These sulfide-bearing deposits are possible non-point sources of acid and metal contamination to surface- and ground-water systems. We conducted a pilot study to evaluate the use of stable-metal isotopes to help ascertain metal retention and release mechanisms that influence metal transport from the deposits to shallow ground water. To accomplish this, we excavated an intact core from a small fluvial tailings deposit and performed laboratory column experiments to examine the amount of metals leaching through the core under different geochemical conditions. Deionized water was continuously applied to the top of the core, and effluent was collected from the bottom of the core for a period of about 2 months. The core was sequentially leached under unsaturated, partially saturated, and fully saturated conditions to simulate changing water-table levels within the deposit. Reducing conditions developed upon partial and complete saturation of the core. During all leaching phases, core effluents were acidic (pH 2.8-3.5) and contained elevated metal concentrations. During a portion of the unsaturated leaching phase, stable-metal isotope spikes (111Cd, 65Cu, 54Fe, 207Pb, and 68Zn) were applied to the top of the core along with conservative tracers. None of the isotope spikes exhibited breakthrough with simultaneously added conservative tracers during the unsaturated leaching phase. However, some of the isotope spikes did break through when reducing conditions developed (during partial- and complete-saturation conditions). By comparing the behavior of metal-isotope spikes with total-metal concentrations in the effluent from the core, we were able to gain insights into geochemical conditions that might promote release of particular metals from the fluvial tailings deposits to the shallow ground-water system at our field site. Hence, use of stable-metal isotopes facilitated the determination of different metal-attenuation processes, metal-release processes, and metal sources in the fluvial tailings deposit in response to changing geochemical conditions.
The objective of our study was to evaluate the use of stable-metal isotopes to examine processes that control metal transport from the fluvial tailings deposit to the underlying shallow ground water. We excavated an intact 20-centimeter-diameter (60 cm in length) core of the deposit from the bank of a distributary channel that cuts through the deposit. We sequentially leached the core in the laboratory under unsaturated, partially saturated, and fully saturated conditions for a period of about two months, and collected the effluent at various times. This procedure was intended to simulate the impact of water infiltration through the deposit (seasonally) and into the shallow ground-water system. We employed stable-metal isotope spikes to identify processes within the core that control metal retention and release in response to changing geochemical conditions.
Our study site is located along the upper Arkansas River approximately 13 km south of Leadville, Colorado and is about 0.1 km2 in size. The local ground water beneath the fluvial tailings deposit exhibits reduced pH and elevated metal concentrations (Walton-Day and others, 2000; Smith and others, 1999, 2000).
Fluvial tailings deposits along the upper Arkansas River are generally overbank and pointbar deposits containing fine-grained mixtures of tailings and other sediment. Material from the deposits is compositionally heterogeneous. At our study site, the top of the fluvial tailings deposit commonly consists of a fine-grained pyrite-rich layer, the middle portion of the deposit is clay-rich with sand and silt lenses, and the bottom contains an organic-rich layer underlain by a sand and gravel shallow aquifer. The dominant minerals are quartz, feldspar, and mica.
A 20-centimeter-diameter 60-centimeter-length core was excavated intact from the bank of a distributary channel that cuts through the fluvial tailings deposit. A clear polymethylmethacrylate tube was placed on top of the bank. The fluvial tailings around the tube were slowly excavated and the tube pushed down to encase the remaining material. The process was repeated until the shallow sand and gravel aquifer was reached (approximately 60 cm of overlying material). The bottom of the tube was fitted with a polyvinylchloride (PVC) cap and the joint sealed with silicone cement. The cap contained sampling ports designed to separately sample water draining along the interface between the cored material and the inner edge of the tube (side-wall flow, edge port), and water draining through the center part of the core, making full contact with the core material (center port). A perforated acrylic plate separates the cored material from the cap, and polypropylene cloth on top of the plate prevents fine material from passing from the cored material into the cap. The cap was filled with clean quartz sand to eliminate any void space.
Three saturation conditions were used for the core leaching experiments, beginning with unsaturated, followed by partially saturated (lower half of the core flooded), and ending with fully saturated (entire core flooded). Effluent was collected at varying times, and sample volume, pH, and specific conductance were measured. Each effluent analysis represents a composite value over the time period between collection times. A total of 87 sequential effluent samples were collected over the 57-day period of the leaching experiment.
A mixed isotope spike solution was introduced during the unsaturated flow phase (at about 265 hours). The spike solution consisted of 111Cd, 65Cu, 54Fe, 207Pb, and 68Zn, with Na and ClO4 added as conservative tracers. The mixed isotope solution was continuously dripped on the top of the core from 265 hours until 373 hours into the experiment. At 373 hours, deionized water was again continuously dripped on the top of the core.
At 565 hours, partial saturation conditions were introduced by allowing water in the core to back up from the bottom to flood up to about 15 cm from the top of the core (the water level was just below the pyritic zone of the fluvial tailings). This partially saturated condition is intended to simulate rising of the underlying shallow ground water table into the fluvial tailings deposit. At about 912 hours, a 207Pb isotope solution was introduced until 1,016 hours into the experiment. At 1,016 hours, a conservative tracer consisting of Na and ClO4 was introduced and continuously dripped into the partially saturated core.
Complete saturation of the core was initiated at 1,152 hours into the experiment and was maintained for about 9 days. The core was drained at 1,369 hours into the experiment.
The figure shows the isotope ratios for the stable metal isotope spikes added to the core. The tracers, including ClO4, Na, and Li, all exhibited breakthrough shortly after they were introduced into the core. In contrast, none of the stable metal isotope spikes exhibited breakthrough during the unsaturated phase of the leaching experiment, which indicates that the spiked metals were being retained in the core. Iron exhibits some erratic behavior. When partial saturation conditions are introduced (along with subsequent anoxic conditions and reductive dissolution of iron phases), Cd and Zn isotope ratios indicate that the spikes are partially released from the core. This partial release of the spiked Cd and Zn is likely a result of the reductive dissolution of iron phases with which the metals were associated. In contrast, Cu and Pb spikes do not show any release from the core. Also, total concentrations of Cd and Zn in the core effluent show little variation when anoxic conditions develop (during the partial and full saturation phases of the experiment).
These contrasts between the behavior of total metal concentrations and isotope ratios in the core effluents highlight the utility of stable metal isotopes to distinguish between different metal release and retention processes. The behavior of Cd and Zn indicates that they become mobile under anoxic conditions. Hence, during seasonal fluctuations of the shallow water table beneath the fluvial tailings deposit, Cd and Zn would be expected to be more mobile when the water table is high. The behavior of Cu indicates that Cu is strongly retained under both oxic and anoxic conditions and therefore would not be expected to be very mobile in the fluvial tailings deposit system.
Ranville, J.F., Smith, K.S., Lamothe, P.J., Jackson, B.P., and Walton-Day, K., 2003, Use of an intact core and stable-metal isotopes to examine leaching characteristics of a fluvial tailings deposit, in Proceedings of the Sixth International Conference on Acid Rock Drainage (ICARD 6), Cairns, North Queensland, Australia, July 14-17, 2003, p. 1101-1104. PDF file of ICARD 2003 paper [66 KB].
Smith, K.S., Ranville, J.F., Lamothe, P.J., and Jackson, B.P., 2003, Use of stable isotopes to examine metal-attenuation and release processes in a fluvial tailings deposit, in Proceedings of the Joint Conference of the 9th Billings Land Reclamation Symposium (BLRS) and the 20th Annual Meeting of the American Society of Mining and Reclamation (ASMR), Billings, Montana, June 3-6, 2003, p. 1186-1203. PDF file of Billings/ASMR paper [377 KB].
Ranville, J.F., Smith, K.S., Lamothe, P.J., Jackson, B.P., and Walton-Day, K., 2003, Use of an intact core and stable-metal isotopes to examine leaching characteristics of a fluvial tailings deposit [abs.]: Sixth International Conference on Acid Rock Drainage (ICARD 6), Cairns, North Queensland, Australia, July 14-17, 2003. (Abstract and poster) Available online at http://www.shop.ausimm.com.au/paperdetails.php?PaperID=832.
Smith, K.S., Ranville, J.F., Lamothe, P.J., and Jackson, B.P., 2003, Use of stable isotopes to examine metal-attenuation and release processes in a fluvial tailings deposit [abs.]: Joint Conference of the 9th Billings Land Reclamation Symposium (BLRS) and the 20th Annual Meeting of the American Society of Mining and Reclamation (ASMR), Billings, Montana, June 3-6, 2003. (Abstract and presentation) Meeting abstracts are available online at http://ces.ca.uky.edu/asmr/ASMR%20Proceedings/PROCEEDINGS.pdf [PDF file, 572 KB].
Walton-Day, K., Smith, K.S., and Ranville, J.F., 2001, Control of iron and trace-metal mobility in an alluvial aquifer affected by acidic rock drainage: Geological Society of America Abstracts with Programs, v. 33, no. 6, p. 281. View Walton-Day GSA abstract.
Smith, K.S., Walton-Day, K., and Ranville, J.F., 2001, Considerations of observational scale when evaluating the effect of, and remediation strategies for, a fluvial tailings deposit in the Upper Arkansas River Basin, Colorado, USA: Earth System Processes - Global Meeting, Edinburgh, Scotland, June 24-28, 2001.
Walton-Day, Katherine, Rossi, F.J., Gerner, L.J., Evans, J.B., Yager, T.J., Ranville, J.F., and Smith, K.S., 2000, Effects of fluvial tailings deposits on soils and surface- and ground-water quality, and implications for remediation—Upper Arkansas River, Colorado, 1992-1996: U.S. Geological Survey Water-Resources Investigations Report 99-4273, 100 p.
Smith, K.S., Walton-Day, Katherine, and Ranville, J.F., 2000, Evaluating the effects of fluvial tailings deposits on water quality in the Upper Arkansas River Basin, Colorado: Observational scale considerations, in Proceedings of the Fifth International Conference on Acid Rock Drainage (ICARD 2000), Denver, Colorado, May 21-24, 2000, Volume II: Littleton, Colorado, Society for Mining, Metallurgy, and Exploration, Inc., p. 1415-1424. PDF file of ICARD 2000 paper [311 KB].
Smith, K.S., Walton-Day, Katherine, and Ranville, J.F., 1999, Considerations of observational scale when evaluating the effect of, and remediation strategies for, a fluvial tailings deposit in the Upper Arkansas River Basin, Colorado, in Morganwalp, D.W., and Buxton, H.T., eds., U.S. Geological Survey Toxic Substances Hydrology Program—Proceedings of the Technical Meeting, Charleston, South Carolina, March 8-12, 1999—Volume 1 of 3—Contamination from Hardrock Mining: U.S. Geological Survey Water-Resources Investigations Report 99-4018A, p. 131-138. PDF file of Toxics paper [235 KB].
Smith, K.S., Ranville, J.F., Lamothe, P.J., Meier, A.L., and Walton-Day, Katherine, 1999, Metal leaching through a fluvial tailings deposit along the upper Arkansas River, Colorado, in Proceedings of the Sixth International Conference on Tailings and Mine Waste '99, Fort Collins, Colorado, January 24-27, 1999: Rotterdam, A.A. Balkema, p. 627-632. PDF file of Tailings & Mine Waste '99 paper [104 KB].
Smith, K.S., Sutley, S.J., Briggs, P.H., Meier, A.L., Walton-Day, Katherine, Ranville, J.F., and Jerz, J.K., 1998, Trends in water-leachable lead from a fluvial tailings deposit along the upper Arkansas River, Colorado, in Proceedings of the Fifth International Conference on Tailings and Mine Waste '98, Fort Collins, Colorado, January 26-29, 1998: Rotterdam, A.A. Balkema, p. 763-768. PDF file of Tailings & Mine Waste '98 paper [80 KB].
Smith, K.S., Jerz, J.K., Ranville, J.F., Walton-Day, Katherine, Sutley, S.J., Meier, A.L., and Briggs, P.H., 1997, Geochemical characterization of a fluvial tailings deposit along the Arkansas River, Colorado, USA, in Wanty, R.B., Marsh, S.P., and Gough, L.P., eds., Proceedings of the Fourth International Symposium on Environmental Geochemistry, Vail, Colorado, October 5-10, 1997: U.S. Geological Survey Open-File Report 97-0496, p. 86.
Walton-Day, Katherine, Jerz, J.K., Ranville, J.F., Evans, J.B., and Smith, K.S., 1996, Effects of fluvial tailings deposits on the quality of surface and ground water at a site in the upper Arkansas River basin, Colorado: Geological Society of America 1996 Annual Meeting, Denver, Colorado, October 28-31, 1996, Abstracts with Programs, v. 28, no. 7, p. A-466.
Kathleen Smith
Box 25046 MS 964D Denver Federal Center
Denver, CO 80225-0046
Phone: (303) 236-5788
Email: ksmith
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