ANTICIPATING POTENTIAL ENVIRONMENTAL EFFECTS OF AQUEOUS METALS IN MINERALIZED AREAS USING THE BIOTIC LIGAND MODEL

The chemical speciation of dissolved metals influences their potential uptake and toxicity to aquatic biota. The Biotic Ligand Model (BLM) is a computational approach to predict metal speciation and acute metal toxicity to aquatic biota. Geoscientists can use the BLM as a link between geochemical information and estimates of potential toxicological effects on aquatic organisms in streams. Recently, the U.S. Environmental Protection Agency (USEPA) incorporated the BLM into their regulatory ambient water-quality criteria for Cu. We compared results from laboratory Cu toxicity tests (using “Ceriodaphnia dubia”) with BLM predictions for simulated test waters in order to evaluate the ability of the BLM to accurately predict the effects of hardness and concentrations of dissolved organic carbon (DOC) and Fe on the acute aquatic toxicity of Cu. Iron was added to different test waters to simulate stream water typical of mineralized areas. Generally, comparison of BLM predictions with laboratory toxicity tests in Fe-free water shows that there is good agreement between the predictions and the laboratory toxicity tests. When Fe is added to the test waters, Cu bioavailability may be affected in a number of ways. First, added Fe coprecipitates with a portion of the DOC, which is demonstrated by the loss of some of the measured DOC and Fe from solution. The added Fe may also bind to a portion of the remaining DOC, thereby interfering with Cu binding. It may also be possible that, after some DOC was removed by coprecipitation with Fe, the remaining DOC has lower Cu-binding capacity than does the original DOC. One or more of these scenarios can result in lower Cu binding by the remaining DOC, which results in more bioavailable Cu and greater Cu toxicity to “C. dubia.” We observed greater Cu toxicity in laboratory tests that contained added Fe in the test waters. The recent revision to the USEPA Cu water-quality criteria discusses this issue, but does not require users to monitor for Fe or to include Fe effects in BLM simulations. The effects of Fe on Cu toxicity demonstrated in our laboratory tests will be compared with BLM predictions with and without Fe included in the computations. These comparisons highlight the importance of considering Fe when attempting to anticipate potential toxic effects of metals in mineralized areas.