LaDonna M. Choate

Predicting toxic effects of copper on aquatic biota in mineralized areas by using the Biotic Ligand Model

The chemical speciation of metals influences their biological effects. The Biotic Ligand Model (BLM) is a computational approach to predict chemical speciation and acute toxicological effects of metals on aquatic biota. Recently, the U.S. Environmental Protection Agency incorporated the BLM into their regulatory waterquality criteria for copper. Results from three different laboratory copper toxicity tests were compared with BLM predictions for simulated test-waters. This was done to evaluate the ability of the BLM to accurately predict the effects of hardness and concentrations of dissolved organic carbon (DOC) and iron on aquatic toxicity. In addition, we evaluated whether the BLM and the three toxicity tests provide consistent results. Comparison of BLM predictions with two types of Ceriodaphnia dubia toxicity tests shows that there is fairly good agreement between predicted LC50 values computed by the BLM and LC50 values determined from the two toxicity tests. Specifically, the effect of increasing calcium concentration (and hardness) on copper toxicity appears to be minimal. Also, there is fairly good agreement between the BLM and the two toxicity tests for test solutions containing elevated DOC, for which the LC50 is 3-to-5 times greater (less toxic) than the LC50 for the lower-DOC test water. This illustrates the protective effects of DOC on copper toxicity and demonstrates the ability of the BLM to predict these protective effects. In contrast, for test solutions with added iron there is a decrease in LC50 values (increase in toxicity) in results from the two C. dubia toxicity tests, and the agreement between BLM LC50 predictions and results from these toxicity tests is poor. The inability of the BLM to account for competitive iron binding to DOC or DOC fractionation may be a significant shortcoming of the BLM for predicting site-specific water-quality criteria in streams affected by iron-rich acidic drainage in mined and mineralized areas. Additional

Integrating bioavailability approaches into waste rock evaluations

The presence of toxic metals in soils affected by mining, industry, agriculture and urbanization, presents problems to human health, the establishment and maintenance of plant and animal habitats, and the rehabilitation of affected areas. A key to managing these problems is predicting the fraction of metal in a given soil that will be biologically labile, and potentially harmful (‘bioavailable’). The molecular form of metals and metalloids, particularly the uncomplexed (free) form, controls their bioavailability and toxicity in solution. One computational approach for determining bioavailability, the biotic ligand model (BLM), takes into account not only metal complexation by ligands in solution, but also competitive binding of hardness cations (Ca 2+ ,Mg 2+, ) and metal ions to biological receptor sites. The more direct approach to assess bioavailability is to explicitly measure the response of an organism to a contaminant. A number of microbial enzyme tests have been developed to assess the impact of pollution in a rapid and procedurally simple way. These different approaches in making bioavailability predictions may have value in setting landuse priorities, remediation goals, and habitat reclamation strategies. Additional

LEACHATE AND ENZYME BIOASSAY TOXICITY ASSESSMENT TESTS AT THE TIP TOP MINE, A MARGINALLY IMPACTED SITE

Over the past three years, a decision tree has been developed to rank mine waste sites for potential environmental impacts. This approach relies on simple leach tests to determine the chemical composition and toxicity of water in contact with mining wastes. When the pH of the leachate solutions is less than 5, the toxicity of the water is certain. However, when the pH of the leachate solutions is greater than 5, lower concentrations of toxic metals make toxicity assessment uncertain and a simple “in-vitro” test is necessary. These methods were used to evaluate a mine site that is marginally impacted. The Tip Top Mine in Gamble Gulch, Colorado is a high mountain site where the stream upstream of the mine is pristine and downstream of the influx of acid rock drainage, the aquatic ecosystem is marginally impacted. Aquatic toxicity assessments, made using a microbial enzyme bioassay, were conducted to determine the impact of contaminants on the stream. All tests show that the stream water upstream of the adit inflow is unimpacted. However, the stream downstream of the inflow shows concentrations of Al, Cu and Zn that are only slightly higher than acute aquatic toxicity limits. Leaching tests on stream sediment samples taken at the adit entrance show concentrations of contaminants that are also higher than toxicity limits. Simple enzyme bioassay tests, using metals sensitive bacteria, were conducted to establish the toxic response of the sediment leachate. The preliminary results show that leachate water upstream of the adit is not toxic and downstream, the leachate solution is marginally toxic. Duplicate leach tests and enzyme bioassay tests were conducted to determine the reproducibility of these approaches. Additional

Using enzyme bioassays as a rapid screen for metal toxicity

Mine tailings piles and abandoned mine soils are often contaminated by a suite of toxic metals, which were released in the mining process. Traditionally, toxicity of such areas has been determined by numerous chemical methods including the Toxicity Characteristic Leachate Procedure (TCLP) and traditional toxicity tests using organisms such as the cladoceran Ceriodaphnia dubia. Such tests can be expensive and time-consuming. Enzymatic bioassays may provide an easier, less costly, and more time-effective toxicity screening procedure for mine tailings and abandoned mine soil leachates. This study evaluated the commercially available MetPLATE  enzymatic toxicity assay test kit. The MetPLATE  assay uses a modified strain of Escherichia coli bacteria as the test organism. Toxicity is defined by the activity of -galactosidase enzyme which is monitored colorometrically with a 96-well spectrophotometer. The study used water samples collected from North Fork Clear Creek, a mining influenced water (MIW) located in Colorado. A great benefit to using the MetPLATE  assay over the TCLP is that it shows actual toxicity of a sample by taking into account the bioavailability of the toxicants rather than simply measuring the metal concentration present. Benefits of the MetPLATE  assay over the use of C. dubia include greatly reduced time for the testing process (~2 hours), a more continuous variable due to a greater number of organisms present in each sample (100,000+), and the elimination of need to maintain a culture of organisms at all times.

Acid neutralizing capacity and leachate results for igneous rocks, with associated carbon contents of derived soils, Animas River AML site, Silverton, Colorado

Mine planning efforts have historically overlooked the possible acid neutralizing capacity (ANC) that local igneous rocks can provide to help neutralize acid- mine drainage. As a result, limestone has been traditionally hauled to mine sites for use in neutralizing acid drainage. Local igneous rocks, when used as part of mine life-cycle planning and acid mitigation strategy, may reduce the need to transport limestone to mine sites because these rocks can contain acid neutralizing minerals. Igneous hydrothermal events often introduce moderately altered mineral assemblages peripheral to more intensely altered rocks that host metal-bearing veins and ore bodies. These less altered rocks can contain ANC minerals (calcite-chlorite-epidote) and are referred to as a propylitic assemblage. In addition, the carbon contents of soils in areas of new mining or those areas undergoing restoration have been historically unknown. Soil organic carbon is an important constituent to characterize as a soil recovery benchmark that can be referred to during mine cycle planning and restoration. This study addresses the mineralogy, ANC, and leachate chemistry of propylitic volcanic rocks that host polymetallic mineralization in the Animas River watershed near the historical Silverton, Colorado, mining area. Acid titration tests on volcanic rocks containing calcite (2 - 20 wt %) and chlorite (6 - 25 wt %), have ANC ranging from 4 - 146 kg/ton CaCO3 equivalence. Results from a 6-month duration, kinetic reaction vessel test containing layered pyritic mine waste and underlying ANC volcanic rock (saturated with deionized water) indicate that acid generating mine waste (pH 2.4) has not overwhelmed the ANC of propylitic volcanic rocks (pH 5.8). Sequential leachate laboratory experiments evaluated the concentration of metals liberated during leaching. Leachate concentrations of Cu-Zn-As-Pb for ANC volcanic rock are one-to-three orders of magnitude lower when compared to leached solution from mine waste used in the kinetic reaction vessel test. This finding suggests that mine waste and not ANC rock may generate the majority of leachable metals in a field scenario. The organic carbon content of naturally reclaimed soils derived from weathering of propylitically-altered andesite was determined in catchments where ANC studies were initiated. Soils were found to have total carbon concentrations (TOC) that exceed global average soil TOC abundances by as much as 1.5 - 5 times. These data support an environmental management system involving use of ANC rocks as part of life-cycle mine planning to reduce post-mine closure acid mitigation measures. Carbon contents of undisturbed soils in mined catchments can possibly be used to validate post-reclamation success and help quantify carbon sequestration for CO2 emission offset trading as carbon markets mature.

Determining the toxicity potential of mine-waste piles

When assessing the environmental impact from mining operations, an immediate question arises about potential impact and toxicity of mine-waste piles. This question is particularly difficult to assess for waste piles on abandoned mine lands in the western United States and coal-waste piles in the eastern United States. In many of these situations, there is no water in direct contact with the piles, except during meteorological events, yet it appears that the pile has caused significant ecological disturbance. For the past several years, scientists at the Colorado School of Mines and the U.S. Geological Survey have been studying the toxicity potential of waste-rock piles. Simple and practical methods have been developed for determining the potential of a waste- rock pile to cause significant contamination. For example, quick inexpensive field leaching tests have been developed that offer an evaluation of acid and trace-metal release from mine-waste material. Additionally, two-dimensional hydrologic and erosion models might be used to assess acid and metal sources and sinks. Such methods are presented for evaluating mine-waste piles from watershed scale, site scale, and microscopic scale, using geophysical, geochemical, and mineralogical methods. Current methods used to determine bioaccessibility and bioavailability of metals from wastes, such as extraction techniques, are described and assessed. Case studies with field and laboratory data illustrate these methods. These applications are used as the basis for a simple decision tree that has been developed to assess the potential impact of a waste-rock pile, and the scientific background that serves as the basis for decisions. Workshop Time: 8:30 am – 4:00 pm, June 1, 2003 Workshop Organizers: Dr. Thomas R. Wildeman Dr. Kathleen S. Smith Dept. of Chemistry & Geochemistry U.S. Geological Survey Colorado School of Mines M.S. 973, Denver Federal Cntr. Golden, CO 80401 Denver, CO 80225-0046 Phone: 303-273-3642 Phone: 303-236-5788 E mail: twildema@mines.edu E mail: ksmith@usgs.gov