Development of Fluoride Protective Values for Aquatic Life Using Empirical Bioavailability Models

Abstract

The derivation of protective values for aquatic life can be enhanced by the development and use of bioavailability models. Recent advances to metals bioavailability modeling are applicable to other analyte groups and should be widely considered. We conducted a meta‐analysis of the available aquatic toxicity literature for fluoride to evaluate the utility of hardness, alkalinity, and chloride as toxicity‐modifying factors (TMFs) in empirical bioavailability models of freshwater taxa. The resulting optimal multiple linear regression model predicting acute fluoride toxicity to the invertebrate Hyalella azteca included all three TMFs (observed vs. predicted 50% lethal concentrations, R2 = 0.88) and the optimal model predicting toxicity to the fish Oncorhynchus mykiss included alkalinity and hardness (R2 = 0.37). At >20 mg/L chloride, the preliminary final acute values for fluoride were within 1 order of magnitude and ranged from approximately 18.1 to 56.3 mg/L, depending on water chemistry. Sensitivity of H. azteca to low‐chloride conditions increased model uncertainty when chloride was <20 mg/L. Because of limited toxicity data, chronic bioavailability models were not developed, and final chronic values were derived using an acute‐to‐chronic ratio (ACR) approach. Accounting for TMFs, the geometric mean ACR was 5.4 for fish and invertebrate taxa (n = 6). The present assessment highlights the need to expand bioavailability modeling to include inorganic anions, particularly fluoride, and demonstrates that existing promulgated protective values for fluoride are likely overly conservative. More toxicological studies are recommended to further refine multivariate empirical bioavailability models for inorganic anions. Environ Toxicol Chem 2022;41:396–409. © 2021 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Response to Letter to the Editor on Parker et al., 2022, “Development of Fluoride Protective Values for Aquatic Life Using Empirical Bioavailability Models”

“…The relationship between hardness and alkalinity in natural waters and the empirical evidence that alkalinity is a TMF for fluoride suggest that a model interaction term between these water quality parameters may be best suited to explain observed variability in fluoride toxicity. The limited sample size and limited variability in hardness-to-alkalinity ratios in available fluoride toxicity data precluded the ability to incorporate interaction terms into the MLR models developed by Parker et al (2022). This further supports the need for additional research on the role of alkalinity in modifying fluoride toxicity, which was recommended in our study.…”

“…These studies suggest that the addition of bicarbonate to systems in solubility equilibrium with fluorite will drive the precipitation of calcite and dissolution of fluorite according to Le Châtelier's principle, thereby increasing fluoride concentrations (and likely bioavailability) in solution. These studies also highlight the relationship between hardness and alkalinity, which is depicted for surface waters across the United States in Supporting Information, Figure S3 of Parker et al (2022). The relationship between hardness and alkalinity in natural waters and the empirical evidence that alkalinity is a TMF for fluoride suggest that a model interaction term between these water quality parameters may be best suited to explain observed variability in fluoride toxicity.…”

“…The letter emphasizes that safe concentrations of fluoride lower than those derived by Parker et al (2022) have been previously derived for various Nearctic freshwater invertebrates that are considered relatively sensitive. However, the low application factor for Nearctic caddisfly fluoride toxicity was developed using nonstandard methods to derive chronic effects estimates by extrapolating acute toxicity data far beyond the test durations and concentrations studied.…”

“…Under this study design, it is no surprise that alkalinity was not explicitly identified as a TMF for fluoride because differentiating the toxicity-modifying effects of alkalinity and hardness was not possible. Table 1 of Parker et al (2022) illustrates that alkalinity has a significant effect on fluoride's 50% lethal concentrations (LC50) for Hyalella azteca toxicity tests evaluated by Pearcy et al (2015). Our discussion expounds on key considerations regarding the role of alkalinity in ameliorating or modifying fluoride toxicity and recommends further evaluation of the interaction between hardness and alkalinity in MLR models and the importance of alkalinity as a TMF for freshwater organisms.…”

“…This response is intended to address the points that were raised in the Letter to the Editor prepared by Dr. Camargo (2022) regarding Parker et al (2022). We appreciate the critical review but contend that elements raised in the Letter to the Editor were explicitly considered in our analysis.…”