The United States Environmental Protection Agency's (U.S. EPA) current ambient water quality criteria (AWQC) for zinc in freshwater are hardness-based and were last updated in 1995. The acute and chronic freshwater toxicity databases have since expanded substantially and the U.S. EPA's minimum phylogenetic diversity requirements for chronic zinc toxicity are now met (an acute:chronic ratio was previously required). Additionally, several acute and chronic biotic ligand models (BLMs) for zinc have since been developed and validated for freshwater organisms. Using the expanded toxicity database and existing BLMs, we developed a unified zinc BLM that could efficiently predict both acute and chronic toxicity over a wide range of zinc bioavailabilities. The unified BLM, developed by objectively averaging the biotic ligand binding constants for zinc (Zn(2+)) and competing cations (Ca(2+), Mg(2+), Na(+), H(+)) from existing BLMs, performed better in predicting toxicity to a diverse set of organisms than any individual existing BLM. Performance of the unified BLM was further improved by optimizing the biotic ligand binding constant for the ZnOH(+) species. The updated freshwater zinc toxicity database and unified BLM were then used to estimate the fifth percentiles of the acute and chronic species sensitivity distributions following the U.S. EPA guidelines for AWQC development.

The influence of dissolved natural organic matter (NOM) source on copper toxicity was investigated with larval fathead minnows (Pimephales promelas) in reconstituted moderately hard water. Ninety-six-hour static renewal toxicity tests were conducted to investigate an assumption of the biotic ligand model (BLM) that NOM source does not need to be considered to adequately predict copper toxicity. The nine different NOM isolates used in these toxicity tests were chemically well-characterized substances that were obtained by reverse osmosis as part of an NOM typing project based in southern Norway. Three median lethal concentration (LC50) values were estimated for toxicity tests conducted with each NOM, at nominal dissolved organic carbon (DOC) concentrations of 2, 5, and 10 mg/L. Tests also were conducted in dilution waters in which no NOM was added. Regression analyses were conducted to compare NOM-specific (specific NOM source) LC50s versus DOC concentration relationships to each other, as well as to the overall LC50 versus DOC concentration relationship. Statistical differences were found regarding the effects of NOM source on copper toxicity. Similar analyses were conducted with humic acid (HA) concentrations and spectral absorbance, and differences in the effect of NOM source on copper toxicity were similarly concluded. These results do not support the assumption that copper toxicity can be adequately predicted by utilizing DOC concentration, regardless of NOM source. Evaluation of relationships between LC50 values and other NOM characteristics revealed that despite significant differences due to NOM source on copper toxicity, DOC and HA concentrations were the most effective parameters in explaining variability in LC50 values. When BLM-predicted LC50 values were compared to observed LC50 values, predicted values showed reasonable agreement with observed values, but some deviations occurred due to NOM source and DOC concentration.

The hardness-based regulatory approach for Cu prescribes an extrapolation of the toxicity-versus-hardness relationship to low hardness (< or =50 mg/L as CaCO3). Hence, the objective of the present research was to evaluate the influences of water quality on acute Cu toxicity to larval fathead minnow (Pimephales promelas) in low-hardness surface waters. Seasonal water sampling was conducted at 24 sites throughout South Carolina, USA, to determine the site-specific influences of soft surface-water conditions on acute Cu toxicity. Concurrent toxicity tests in laboratory water, matched for hardness and alkalinity (modified method), also were conducted to allow calculation of water-effect ratios (WERs). In addition, tests were conducted at recommended hardness levels (recommended method) for comparison of WER methodology in soft water. Surface-water conditions (average+/-standard deviation, n = 53) were hardness of 16+/-8 mg/L as CaCO3, alkalinity of 18+/-11 mg/L as CaCO3, and dissolved organic carbon of 6+/-4 mg/L. Dissolved Cu 48-h median lethal concentration (LC50) values varied nearly 45-fold across the dataset and greater than four-fold at individual sites. Spatial (p < 0.0001) and seasonal (p = 0.026) differences among LC50 values were determined for eight sites that had multiple toxicity results for one year. All modified WERs were greater than 1.0, suggesting that the site waters were more protective of Cu toxicity than the matched laboratory water. Some WERs generated using recommended methods were less than 1.0, suggesting limited site-specific protection. Based on these observations, extrapolation of the hardness-based equation for Cu at 50 mg/L or less as CaCO3 would adequately protect fathead minnow populations in soft surface waters. The WER results presented here demonstrate the inconsistency between hardness-based criteria and the methodology for deriving site-specific water-quality criteria in low-hardness waters.

We studied biotic ligand model (BLM) predictions of the toxicity of nickel (Ni) and zinc (Zn) in natural waters from Illinois and Minnesota, USA, which had combinations of pH, hardness, and dissolved organic carbon (DOC) more extreme than 99.7% of waters in a nationwide database. We conducted 7‐day chronic tests with Ceriodaphnia dubia and 96‐hour acute and 14‐day chronic tests with Neocloeon triangulifer and estimated median lethal concentrations and 20% effect concentrations for both species. Toxicity of Ni and Zn to both species differed among test waters by factors from 8 (Zn tests with C. dubia) to 35 (Zn tests with N. triangulifer). For both species and metals, tests with Minnesota waters (low pH and hardness, high DOC) showed lower toxicity than Illinois waters (high pH and high hardness, low DOC). Recalibration of the Ni BLM to be more responsive to pH‐related changes improved predictions of Ni toxicity, especially for C. dubia. For the Zn BLM, we compared several input data scenarios, which generally had minor effects on model performance scores (MPS). A scenario that included inputs of modeled dissolved inorganic carbon and measured Al and Fe(III) produced the highest MPS values for tests with both C. dubia and N. triangulifer. Overall, the BLM framework successfully modeled variation in toxicity for both Zn and Ni across wide ranges of water chemistry in tests with both standard and novel test organisms. Environ Toxicol Chem 2021;40:3049–3062. © 2021 SETAC. This article has been contributed to by US Government employees and their work is in the public domain in the USA.

The majority of metal toxicity data available for freshwater organisms have been generated in laboratory water at pH > 6.5 and hardness > 50 mg/L as CaCO3. Extrapolation of these results to soft surface waters (i.e., hardness < or = 40 mg/L as CaCO3), similar to predominant conditions in the southeastern United States, may prove challenging. For example, South Carolina has surface waters that average 20 mg/L as CaCO3, and exist at extremes of 1 and 600 mg/L as CaCO3. This research characterized the acute toxicity of Cu to Daphnia magna in waters with low hardness and low pH. The 48-h total Cu median lethal concentrations were related to water hardness over a hardness range of 8 to 51 mg/L as CaCO3. Although toxicological differences existed between water hardness of 7 and 20 mg/L as CaCO3 (p = 0.0001), differences in pH (range 5.5-8.5) did not influence acute Cu toxicity. Results of these laboratory studies will provide the data needed to more accurately predict organism response to Cu in waters with low pH and low hardness.