No abstract

Current water-quality criteria for metals typically are derived from toxicity tests with the metal dissolved in clean laboratory water. Estimating the toxicity of iron from such tests, however, is extremely difficult because of the complex solubility and toxicity characteristics of the ferrous and ferric forms of the metal in freshwater. Consequently, a criterion for dissolved iron in freshwater derived from standard laboratory bioassays may not accurately describe the actual bioavailability and toxicity of this metal. A new approach is necessary to adequately protect aquatic life from the direct (toxic) and indirect (physical) negative effects of iron. We present a novel methodology to derive bioassessment-based benchmarks for total iron. This approach involves the use of quantile regression to model the decline in maximum abundance of taxa along a gradient of increasing iron concentrations. The limiting function (e.g., 90th quantile) is used to project the iron concentration associated with a selected reduction in maximum number of organisms (e.g., 20%). The projected declines in abundance of aquatic organisms are interpreted within the larger context of biological responses to increasing levels of stress (i.e., a biological condition gradient). Projections of iron concentration associated with multiple levels of reduction are selected to establish acceptable levels of change in the various tiers of a biological community. The bioassessment-based benchmarks that we establish for total iron (0.21 and 1.74 mg/L) are based on the assumption that if ecological effects-based criteria for total iron are derived and applied, the structure and function of the aquatic community will be protected.

This study evaluated the relationship between ambient sulfate concentrations and acute selenate toxicity to freshwater aquatic life. Previous studies indicated that increasing sulfate concentrations reduced selenate bioconcentration and toxicity. However, these studies generally were not conducted in a manner that was conducive to their use in deriving a water quality criterion. We compiled results from previous studies and generated additional data to help define a selenate-sulfate relationship for acute toxicity. Selenate toxicity was determined in standardized test waters with varying sulfate concentrations using Ceriodaphnia dubia, Gammarus pseudolimnaeus, Hyalella azteca, and Pimephales promelas as the test organisms. Analysis of test results indicated that a significant relationship does exist between acute selenate toxicity and ambient sulfate concentrations. Data from these tests and previous studies were combined to develop a statistical relationship sufficiently robust to derive a sulfate-dependent water quality criterion for selenate. The relationship is similar to those commonly derived between divalent metals and hardness to adjust water quality criteria.

The U.S. Environmental Protection Agency (EPA) has developed a “Seven‐Day Fathead Minnow Larval Survival and Growth Test” for measuring the toxicity of effluents. This study evaluated the ease and performance and the intra‐ and interlaboratory variability of this 7‐d test using two reference toxicants (sodium pentachlorophenate and potassium dichromate), two refinery effluents, and one low‐volume internal waste stream from an electric power plant. Ten different laboratories representing academia, private industry, contractors, the state EPA, and the U.S. EPA participated in this study. The reference toxicants were tested twice during two separate test periods, and each effluent and waste stream was tested once. Using EPA's criteria for a successful test, 90% of the 140 planned tests were completed as valid tests. The intralaboratory variability of the survival (LC50) and growth (IC50) results for tests conducted concurrently (spatial variability), expressed as the coefficient of variation (C.V.), ranged between 5.9 and 15.6% and between 9.1 and 26.1%, respectively. The intralaboratory variability of the survival (LC50) and growth (IC50) results for tests conducted at two different times (temporal variability), expressed as C.V., ranged between 14.5 and 25.3% and between 17.6 and 22.7%, respectively. The interlaboratory variability of the survival (LC50) and growth (IC50) results, expressed as C.V., ranged between 24.1 and 43.7% and between 22.4 and 88.0%, respectively. Overall, the variability of the 7‐d fathead minnow test was similar to the variability observed with other toxicity tests and with analytical chemistry measurements of environmental samples.

This study evaluated the relationship between ambient sulfate concentrations and acute selenate toxicity to freshwater aquatic life. Previous studies indicated that increasing sulfate concentrations reduced selenate bioconcentration and toxicity. However, these studies generally were not conducted in a manner that was conducive to their use in deriving a water quality criterion. We compiled results from previous studies and generated additional data to help define a selenate-sulfate relationship for acute toxicity. Selenate toxicity was determined in standardized test waters with varying sulfate concentrations using Ceriodaphnia dubia, Gammarus pseudolimnaeus, Hyalella azteca, and Pimephales promelas as the test organisms. Analysis of test results indicated that a significant relationship does exist between acute selenate toxicity and ambient sulfate concentrations. Data from these tests and previous studies were combined to develop a statistical relationship sufficiently robust to derive a sulfate-dependent water quality criterion for selenate. The relationship is similar to those commonly derived between divalent metals and hardness to adjust water quality criteria.