Comparison of two modeling frameworks for unifying the toxicant-affected percentage of aquatic species, individuals, and time into a single metric.

To address time-variable exposure to toxicants, this work compares simple and complex approaches to unifying the affected percentage of aquatic species, individuals, and time into a single metric. The simple approach uses only information on the probability distribution of exposure concentrations, a species sensitivity distribution (SSD) of chronic values, and the distribution of tolerance within species. The complex approach involves time-series simulation with a kinetics-based toxicity model coupled with a population model for each species in the SSD. Unlike the simple approach, this takes into account the exposure duration needed to elicit toxicity, differing sensitivities of life stages within a species, differing effects on survival versus reproduction, and species differences in their model population's response to press disturbance and recovery time from pulse disturbance. The probability distribution approach indicated that, for SSD assemblages challenged with moderately variable toxicant concentrations exceeding the aquatic life criterion a few percent of the time, most of the predicted aggregate effect is usually experienced by the most sensitive 10% of species and individuals. The kinetics-population simulation approach indicated that, for time-variable exposure (but not for constant exposure), the severity of the population effect depended on the type of effect and life stage affected. The results from both approaches suggest that moderately time-variable exposure is best viewed as a fluctuating press disturbance, not as a sporadic pulse disturbance. Integr Environ Assess Manag 2022;18:1364-1374. © 2021 SETAC.

Best Practices for Derivation and Application of Thresholds for Metals Using Bioavailability-Based Approaches.

The primary goal of the present study is to provide a broad view of best practices for evaluating bioavailability models for metals for use in the protection of aquatic life. We describe the state of the science regarding 1) the evaluation and selection of ecotoxicity data, 2) the selection of bioavailability models for use in normalization, and 3) subsequent application of bioavailability models. Although many examples of normalization steps exist worldwide, a scheme is proposed to evaluate and select a model that takes account of its representativeness (water chemistry and taxonomic coverage of the ecotoxicity data set) and validation performance. Important considerations for a suitable model are the quantity of inputs needed, accuracy, and ease of use, all of which are needed to set protective values for aquatic life and to use these values to evaluate potential risks to organisms in receiving waters. Although the end results of different model application approaches may be broadly similar, the differences in these application frameworks ultimately come down to a series of trade-offs between who needs to collect the data and use the bioavailability model, the different requirements of spatial scales involved (e.g., regional vs site-specific values), and model predictiveness and protectiveness. Ultimately, understanding the limits and consequences of these trade-offs allows for selection of the most appropriate model and application framework to best provide the intended levels of aquatic life protection. Environ Toxicol Chem 2019;39:118-130. © 2019 SETAC.

Application of a Fixed Monitoring Benchmark Approach to Evaluate Attainment of Time-Variable Water Quality Criteria: Copper Biotic Ligand Model as a Case Study.

In 2007, the Biotic Ligand Model (BLM) became the basis for the US Environmental Protection Agency (USEPA) freshwater water quality criteria (WQC) for Cu. Applying the BLM typically results in time-variable WQC, which are not unique to the BLM; they result from any criteria approach that depends on water chemistry (e.g., ammonia criteria or hardness-based equations for metals). However, widespread use of the BLM has renewed interest in developing an approach that considers variability when setting permit limits or benchmarks. To aid in establishing these benchmarks, we developed a fixed monitoring benchmark (FMB) approach: a probability-based method that incorporates time variability in BLM-predicted instantaneous water quality criteria (IWQC) and instream Cu concentrations. The FMB approach provides benchmarks that can be used to simplify implementation of time-variable WQC. Although it appears reasonable to apply this approach to derive a site-specific regulatory limit, the FMB does not technically represent a limit above which aquatic effects are expected. Rather, it represents a fixed concentration intended to yield the same level of protection as time-variable IWQC, which rely upon toxic unit (TU) distribution; each TU is calculated for a single sample using the Cu concentration and IWQC for this sample. The distribution of TUs for a particular site is used to estimate the probability that instream Cu concentrations are below associated IWQC. Our results suggest that Cu variability and corresponding IWQC, and their degree of correlation, indicate the magnitude of the FMB relative to the IWQC distribution. The FMB approach determines a maximum Cu distribution such that the resulting WQC exceedance frequency is consistent with the level of protection that is intended for the applicable water quality standard (WQS). This approach makes use of time-variable BLM-based WQC in regulatory contexts wherein a single benchmark is consistent with past practices and established implementation methods. Integr Environ Assess Manag 2018;14:722-735. © 2018 SETAC.

Influence of water hardness and sulfate on the acute toxicity of chloride to sensitive freshwater invertebrates.

Total dissolved solids (TDS) represent the sum of all common ions (e.g., Na, K, Ca, Mg, chloride, sulfate, and bicarbonate) in freshwater. Currently, no federal water quality criteria exist for the protection of aquatic life for TDS, but because the constituents that constitute TDS are variable, the development of aquatic life criteria for specific ions is more practical than development of aquatic life criteria for TDS. Chloride is one such ion for which aquatic life criteria exist; however, the current aquatic life criteria dataset for chloride is more than 20 years old. Therefore, additional toxicity tests were conducted in the current study to confirm the acute toxicity of chloride to several potentially sensitive invertebrates: water flea (Ceriodaphnia dubia), fingernail clams (Sphaerium simile and Musculium transversum), snail (Gyraulus parvus), and worm (Tubifex tubifex), and determine the extent to which hardness and sulfate modify chloride toxicity. The results indicated a significant ameliorating effect of water hardness (calcium and magnesium) on chloride toxicity for all species tested except the snail; for example, the 48-h chloride median lethal concentration (LC50) for C. dubia at 50 mg/L hardness (977 mg Cl(-) /L) was half that at 800 mg/L hardness (1,836 mg Cl(-) /L). Conversely, sulfate over the range of 25 to 600 mg/L exerted a negligible effect on chloride toxicity to C. dubia. Rank order of LC50 values for chloride at a given water hardness was in the order (lowest to highest): S. simile < C. dubia < M. transversum < G. parvus < T. tubifex. Results of the current study support the contention that the specific conductivity or TDS concentration of a water body alone is not a sufficient predictor of acute toxicity and that knowledge of the specific ion composition is critical.

Protectiveness of species sensitivity distribution hazard concentrations for acute toxicity used in endangered species risk assessment.

A primary objective of threatened and endangered species conservation is to ensure that chemical contaminants and other stressors do not adversely affect listed species. Assessments of the ecological risks of chemical exposures to listed species often rely on the use of surrogate species, safety factors, and species sensitivity distributions (SSDs) of chemical toxicity; however, the protectiveness of these approaches can be uncertain. We comprehensively evaluated the protectiveness of SSD first and fifth percentile hazard concentrations (HC1, HC5) relative to the application of safety factors using 68 SSDs generated from 1,482 acute (lethal concentration of 50%, or LC50) toxicity records for 291 species, including 24 endangered species (20 fish, four mussels). The SSD HC5s and HCls were lower than 97 and 99.5% of all endangered species mean acute LC50s, respectively. The HC5s were significantly less than the concentrations derived from applying safety factors of 5 and 10 to rainbow trout (Oncorhynchus mykiss) toxicity data, and the HCls were generally lower than the concentrations derived from a safety factor of 100 applied to rainbow trout toxicity values. Comparison of relative sensitivity (SSD percentiles) of broad taxonomic groups showed that crustaceans were generally the most sensitive taxa and taxa sensitivity was related to chemical mechanism of action. Comparison of relative sensitivity of narrow fish taxonomic groups showed that standard test fish species were generally less sensitive than salmonids and listed fish. We recommend the use of SSDs as a distribution-based risk assessment approach that is generally protective of listed species.

The biotic ligand model: a historical overview.

During recent years, the biotic ligand model (BLM) has been proposed as a tool to evaluate quantitatively the manner in which water chemistry affects the speciation and biological availability of metals in aquatic systems. This is an important consideration because it is the bioavailability and bioreactivity of metals that control their potential to cause adverse effects. The BLM approach has gained widespread interest amongst the scientific, regulated and regulatory communities because of its potential for use in developing water quality criteria (WQC) and in performing aquatic risk assessments for metals. Specifically, the BLM does this in a way that considers the important influences of site-specific water quality. This journal issue includes papers that describe recent advances with regard to the development of the BLM approach. Here, the current status of the BLM development effort is described in the context of the longer-term history of advances in the understanding of metal interactions in the environment upon which the BLM is based. Early developments in the aquatic chemistry of metals, the physiology of aquatic organisms and aquatic toxicology are reviewed first, and the degree to which each of these disciplines influenced the development of water quality regulations is discussed. The early scientific advances that took place in each of these fields were not well coordinated, making it difficult for regulatory authorities to take full advantage of the potential utility of what had been learned. However, this has now changed, with the BLM serving as a useful interface amongst these scientific disciplines, and within the regulatory arena as well. The more recent events that have led to the present situation are reviewed, and consideration is given to some of the future needs and developments related to the BLM that are envisioned. The research results that are described in the papers found in this journal issue represent a distinct milestone in the ongoing evolution of the BLM approach and, more generally, of approaches to performing ecological assessments for metals in aquatic systems. These papers also establish a benchmark to which future scientific and regulatory developments can be compared. Finally, they demonstrate the importance and usefulness of the concept of bioavailability and of evaluative tools such as the BLM.