Grantee Research Project Results
Final Report: Geochemical, Biological and Economic Effects of Arsenic and Other Oxyanions on a Mining Impacted Watershed
EPA Grant Number: R825289Title: Geochemical, Biological and Economic Effects of Arsenic and Other Oxyanions on a Mining Impacted Watershed
Investigators: Miller, Glenn C. , Anderson, Susan L. , Miller, Watkins W. , Shaw, Douglass , Papelis, Lambis , Hershey, Ron , Tyler, Scott
Institution: University of Nevada - Reno , University of California - Davis
EPA Project Officer: Packard, Benjamin H
Project Period: January 1, 1997 through December 31, 1999 (Extended to December 31, 2000)
Project Amount: $767,805
RFA: Water and Watersheds Research (1996) RFA Text | Recipients Lists
Research Category: Watersheds , Water
Objective:
The objective of this research project is to investigate how large scale precious metals mining affects the Humboldt River watershed, which presently produces over half of the U.S. production of gold. Specifically, we are investigating the long-term chemistry and environmental risk of large pit lakes that will form after the mines close, long-term drainage from cyanidization heaps, biological effects of arsenic, and economic effects of long-term alteration of the watershed.
Summary/Accomplishments (Outputs/Outcomes):
The results of each of the major objectives are discussed below.
Determination of As Mobility in Precious Metals Mining Pit Lakes (Ron Hershey, Lambis Papelis). Presently, more than 30 open pit mines in Nevada are operating below the water table. When mining is completed and dewatering ceases, groundwater will flow into these pits and create standing bodies of water. Combined, these pit lakes will contain more than an estimated one trillion liters of water; however, very few pit lakes presently exist and the basis for predicting long-term pit-lake water quality is mostly hypothetical. Several factors that control pit-lake water quality include their morphometry, limnology, and pit-wall geochemistry. These factors may be characteristically different from naturally occurring lakes. A review of water quality of existing and currently filling pit lakes suggests that previous geochemical modeling efforts underestimate the amount of sulfate (an indicator of oxidation). The source of sulfate is not well understood at present, although there are indications that large amounts of air are being advectively transported into the wall rock as the water tables are lower around the pit mines, sometimes by over 500 meters. The introduced oxygen is consumed by pyrite oxidation, which remains on the rock surfaces until washed into the pit lake as the water table recovers.
The 117 m deep former Anaconda pit lake in Yerington, NV, has been filling for 22 years and has provided an ideal location to investigate the limnological and geochemical processes controlling pit-lake water quality. The Anaconda mine exploited a porphyry-copper deposit hosted by quartz monzonite with oxidized and reduced ore zones. Water-quality sampling and vertical profiling of physical parameters show the pit lake is similar to other monomitic lakes in the region where thermal stratification develops during the summer months. Dissolved oxygen profiles show the lake is oxygenated throughout the year and does not become anoxic at depth during thermal stratification. The pit-lake water-quality is consistent throughout the lake with no development of a chemocline. The pit-lake water is slightly alkaline with total dissolved solids of 650 mg/L. Arsenic concentrations in the pit lake are low (<0.011 mg/L), while Se concentrations (0.11 - 0.13 mg/L) are above aquatic wildlife standards. As the lake level has risen, water quality has not varied; however, Cu concentrations decreased from 0.18 mg/L in 1994 to 0.011 mg/L in 1998.
Chlorophyll-a concentrations in the pit lake are very low and typical of oligotrophic or low biomass lakes. In April 1998, chlorophyll-a was <1.0 g/L but was twofold higher in September 1998 at 2.1 g/L. In August 2000, maximum chlorophyll-a concentrations had not increased (1.5 g/L). In April 1998, two major taxa dominated the phytoplankton community, unicellular green algae (Chlorophyta) of the genus Oocystis, and a filamentous cyanobacterium genus Oscillatoria. In September, the phytoplankton community was more diverse. Oocystis and Oscillatoria were dominant in surface waters while large euglenids and smaller chlorophytes also were present. Based on community composition, phytoplankton productivity in the Anaconda pit lake may be limited by the availability of phosphorous. Also, tolerance to Cu and Se found in high concentrations in the water column may play a role in controlling phytoplankton community composition.
Sediments from 15 and 37 m depth are composed of quartz, plagioclase, and calcite. The presence of calcite in shallow sediments, not present in wall rocks, shows that calcite is precipitating in the shallow water column as indicated by thermodynamic calculations. Sediments from 100 m are sand-sized grains composed of quartz and plagioclase with minor amounts of smectite clay from feldspar weathering. Deep sediments appear to originate mostly from sloughing of alluvial overburden from the upper pit walls. Analysis of 100-m sediments did not show significant amounts of trace metals including Cu or labile minerals such as calcite.
The main process controlling decreasing Cu concentrations from the pit lake is complexation with particulate organic matter and uptake by phytoplankton and/or bacterioplankton. The high Cu present in wall rocks above the lake surface and the unchanging lake concentrations of other major constituents suggests that Cu continues to be readily available for dissolution by inflowing ground water. Cu forms stable complexes with organic compounds and cyanobacteria produce strong Cu-binding organic ligands. Also, polypeptides secreted by cyanobacteria can effectively complex Cu ions reducing toxic effects. The predominance of cyanobacteria in the epilimnion of the pit lake suggests that Cu complexing to biogenic ligands is an important mechanism of Cu removal. There is a strong correlation between Cu concentrations found in sediments and the amount of total organic carbon (TOC). TOC and Cu concentrations in sediments are greatest at 37 m and least at 100 m. These data suggest that Cu is being sequestered in organic matter in the shallow sediments.
Geochemical modeling simulated input of various mixtures of Walker River water and up gradient groundwater from the Singatse Range that were then equilibrated with atmospheric gases and reacted with wall-rock minerals. Modeling results show that most of the water in the pit lake is derived from the Walker River; however, modeled TDS and trace metal concentrations are lower than observed. These results suggest that evaporative concentration of pit-lake waters may be an important process; however, additional simulations evaporating up to 50 percent of the pit-lake volume do not produce the observed TDS concentrations. These simulations suggest that there are more chemical processes to consider or the geochemical modeling codes used are inadequate.
Dissolution of Constituents from Heaps and Waste Rock Dumps (Scott Tyler, Wally Miller, Glenn Miller and Dave Decker). Between 1 and 2 billion tons of precious metals heaps will have been extracted with cyanide and closed during the next 5-10 years in Nevada. From modeling and field data, it is now apparent that these heaps will release water in response to rain or snow melt infiltration at the surface. This drainage water contains a variety of constituents, including arsenic, antimony, selenium, sulfate, and vanadium. The results to date from modeling heap and waste rock dumps indicate that the mass flux of these constituents in the drainage water can be significant over very large time scales of 10s to 100s of years. Our research has focused on the physics and chemistry of oxyanion transport through reactive rock with the goal of improving our ability to estimate the long-term water quality impacts of these operations.
An important component of the subsurface phase of the research is the development of predictive models to estimate both the flux and concentration of transition and rare earth oxyanions produced from mining operations. In particular, we are focused on oxyanion production from typical heap leach piles and waste rock dumps, both during mining activities and following mine closure. For this effort, a fully coupled unsaturated flow and transport solver, combined with a geochemical model, have been adapted to heap leach and waste rock dump environments. UNSATCHEM is an unsaturated flow and reactive transport numerical model that was developed by the U.S. Department of Agriculture (USDA) at the U.S. Soil Salinity Laboratory. The modifications to UNSATCHEM include the addition of oxygen transport, the dissolution of solid phase arsenic (Arsenopyrite), the dissolution of pyrite, the precipitation of iron hydroxide, and pH-dependent adsorption of arsenate and arsenite aqueous oxyanion species.
To determine the sorption isotherm behavior of arsenic oxyanions onto mine waste material, an experimental campaign was conducted to produce the data necessary for the development of an isotherm useful to the numerical flow and transport modeling effort. A series of batch reactor experiments was conducted using rock collected from two Carlin-type heap leach facilities. The results show that the two heap samples exhibit strong pH-dependent adsorption behavior for both arsenate and arsenite oxyanion species. The sorption behavior is significantly different for arsenate and arsenite on both rock types studied. At high pH (>9), arsenate is desorbed, whereas at increasingly lower pH, arsenate readily adsorbs. Arsenite, on the other hand, does not begin to significantly adsorb until pH > 8. This behavior has been reported elsewhere in the soils literature. The primary adsorption sites are iron oxyhydroxides based on the observed adsorption behavior.
To incorporate both arsenate and an arsenite transport into the numerical flow and transport model, it is necessary to simulate the redox chemistry that controls the arsenic speciation chemistry. We have simplified the redox chemistry by using oxygen concentration as a surrogate redox switch. The rate-limited dissolution chemistry for pyrite is included in the model as part of the redox chemistry modification suite: pyrite dissolution consumes oxygen, oxygen is transported via a diffusive/advective process, and arsenic is speciated based on the aqueous oxygen concentration. At low oxygen concentrations due to the presence of pyrite, reduced conditions exist and arsenic is present only as arsenite. Conversely, if pyrite is not present, oxidizing conditions exist and arsenic is present only as arsenate.
The reactive transport model that we have developed is being used to explore the primary mechanisms that control arsenic production rates from heaps under meteoric conditions. The sorption chemistry developed from the batch reaction experiments indicate that both rock types studied have very large capacities to adsorb both arsenate and arsenite oxyanionic species. This observation, coupled with the low infiltration rates observed in Nevada's desert climate, result in extremely long times for any significant reduction in the mass of arsenic adsorbed onto the rock. In addition, the model has revealed that the significant processes affecting arsenic mobility are related to the geochemistry of the heap leach facility material. In particular, the redox and pH chemistry associated with pyrite oxidation control the speciation and adsorption behavior of arsenic and, thus, the transport of arsenic oxyanion species. In turn, pyrite oxidation is controlled by oxygen diffusion rates. In addition, the reactive transport model has shown that the current practice of incorporating a fluid-flux cover onto a heap leach facility will have a significant positive effect on reducing the total mass of arsenic released from the heap leach structure; however, this same fluid-flux cover will have a negligible positive effect on reducing the concentration of arsenic produced in the leachate water from the heap. This result is due to arsenic concentration being controlled by geochemistry coupled with the fact that the fluid-flux cover has no effect on the geochemistry of the heap. Thus, it is increasingly apparent that completely halting the flows of contaminated water from the heap will be very difficult, and that the water quality of the leachate, which is very often poor quality, is unlikely to improve for several decades.
Effects of Arsenic on Aquatic Organisms (Susan L. Anderson). Extensive research exists regarding the toxicity of metals (including arsenic) to aquatic invertebrates; however, there has been little consideration of the potential synergies between metals and ultraviolet (UV) radiation; despite considerable debate on this topic in human health research. Ultraviolet radiation is nearly ubiquitous in the natural environment, but it is generally overlooked as a confounding variable in toxicological assessments. This project evaluated synergies between arsenic and solar UV radiation using the crustacean, Ceriodaphnia dubia. Both laboratory (with simulated solar radiation and outdoor (with natural solar radiation) factorial experiments were performed with two intensities of UV (low and high) and four arsenic concentrations (0, 1, 1.25, and 1.5 mg/L). The laboratory experiments were multigenerational examining survival and fecundity effects. The combination of high UV+1.5 mg/L arsenic adversely impacted survival, whereas high UV+0 mg/L arsenic and low UV+1.5 mg/L arsenic treatments did not. These results suggest synergism. This pattern was consistent for all three generations. Fecundity effects were not consistent across generations, and arsenic seemed to have a greater impact than UV. Outdoor experiments were limited to assessing survival. Exposures in September 1999 resulted in a pattern similar to that in the laboratory exposure. High UV+1.5 mg/L arsenic had diminished survival as compared to high UV+0 mg/L arsenic and low UV+1.5 mg/L arsenic. These results indicate a synergistic effect between arsenic and UV exposure is possible under ambient conditions and within a relatively narrow dose range. The mechanism of this effect is unknown, but could include additive genotoxic or oxidative stress. The findings point to the importance of using realistic UV exposures when determining criteria for protection of aquatic life.
Economic Impacts (Douglass Shaw and Noelwah Netusil). Due to the lack of quantitative information available on the potential impacts of mining on surface and groundwater quality, the goals of this project were modified to examine the human behaviors and values for water flow changes accompanying the mine dewatering. With this as the topic, it became clear that existing data from the U.S. Geological Survey (USGS) on past annual flows in the river could be used, and surveys could be developed to explore the importance of those flows as they might change in the future.
Agriculture in the Humboldt River Basin currently is more important than recreational activities because of small population centers near the Basin that generate recreational outings. Simply put, very few people currently visit the Basin for purposes of fishing or riparian-based recreation. This may change in the future. Preservation of the existing watershed was explored in a contingent valuation survey experiment; however, we learned that few outside of the Basin had heard of or cared about resources within the Basin. In addition, we learned that the survey environment in Nevada, which is rather low in literacy and educational levels, but has a large amount of junk mail in circulation, is quite a difficult one in which to operate.
Despite these obstacles, the researchers were able to analyze recreational use on the main downstream reservoir on the river and its relationship to flows and water quantity using existing data. We also analyzed this same recreational use and it relationship to agricultural uses downstream using a programming approach. Finally, the willingness to pay that Basin residents have to maintain "dewatering" flows or, failing that, to enhance recreation opportunities at the resulting pit lakes also were estimated.
Opportunities for future research are abundant. These include the need to: (1) quantify the relationships upstream relating to groundwater pumping; (2) quantify the relationships that eventually are understood between water quality and human and ecological activities; (3) explore values for protection of the watershed; and (4) further explore values for pit lake water quality.
Journal Articles on this Report : 10 Displayed | Download in RIS Format
Other project views: | All 37 publications | 19 publications in selected types | All 10 journal articles |
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Decker DL, Tyler SW. Evaluation of flow and solute transport parameters for heap leach recovery materials. Journal of Environmental Quality 1999;28(2):543-555. |
R825289 (1999) R825289 (Final) |
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Hansen LJ, Whitehead JA, Anderson SL. Solar UV radiation enhances the toxicity of arsenic in Ceriodaphnia dubia. Ecotoxicology 2002;11(4):279-287. |
R825289 (Final) |
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Huszar E, Shaw WD, Englin J, Netusil N. Recreational damages from reservoir storage level changes. Water Resources Research 1999;35(11):3489-3494. |
R825289 (Final) |
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Huszar EJ, Netusil NR, Shaw WD. Contingent valuation of some externalities from mine dewatering. Journal of Water Resources Planning and Management-ASCE 2001;127(6):369-377. |
R825289 (Final) |
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Karr JR. Defining and measuring river health. Freshwater Biology 1999;41(2):221-234. |
R825289 (Final) R825284 (Final) |
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Karr JR, Chu EW. Sustaining living rivers. Hydrobiologia 2000;422-423(0):1-14. |
R825289 (Final) R825284 (Final) |
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Kimberling DN, Karr JR, Fore LS. Measuring human disturbance using terrestrial invertebrates in the shrub-steppe of Eastern Washington (USA). Ecological Indicators 2001;1(2):63-81. |
R825289 (Final) R825284 (Final) |
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Lambert DK, Shaw WD. Agricultural and recreational impacts from surface flow changes due to gold mining operations. Journal of Agricultural and Resource Economics 2000;25(2):533-546. |
R825289 (Final) |
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Shaw WD, Eiswerth M, Huszar E. Environmental damages from gold mining (English translation from Catalan). Revista Econ¢mica de Catalunya. Colegio de Economistas de Catalunya 2000;41:70-78. |
R825289 (Final) |
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Tsukamoto TK, Miller GC. Methanol as a carbon source for microbiological treatment of acid mine drainage. Water Research 1999;33(6):1365-1370. |
R825289 (Final) |
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Supplemental Keywords:
media, soils, acid mine drainage, leachate, heaps, cyanide, pit lakes, arsenic, arsenate, arsenite, toxics, metals, sulfates, selenium, environmental chemistry, biology, hydrology, geology, risk, risk assessment, ecological effects, mines, gold, precious metals, decision making, nonmarket valuation, contingent valuation, survey, public good, willingness-to-pay, conservation, environmental assets, cost benefit, modeling, monitoring, analytical, column studies., RFA, Scientific Discipline, Geographic Area, Water, Water & Watershed, Geochemistry, Arsenic, State, Economics, Ecological Risk Assessment, Biology, Watersheds, fate and transport, epidemiology, human health effects, genotoxic research, pit lakes, mine tailings, geochemical efffects, Nevada (NV), economic effects, aquatic ecosystems, water quality, arsenic exposure, biological effects, mining impacted watershedProgress 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.