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Since the early 2000s, biotic ligand models and related constructs have been a dominant paradigm for risk assessment of aqueous metals in the environment. We critically review 1) the evidence for the mechanistic approach underlying metal bioavailability models; 2) considerations for the use and refinement of bioavailability-based toxicity models; 3) considerations for the incorporation of metal bioavailability models into environmental quality standards; and 4) some consensus recommendations for developing or applying metal bioavailability models. We note that models developed to date have been particularly challenged to accurately incorporate pH effects because they are unique with multiple possible mechanisms. As such, we doubt it is ever appropriate to lump algae/plant and animal bioavailability models; however, it is often reasonable to lump bioavailability models for animals, although aquatic insects may be an exception. Other recommendations include that data generated for model development should consider equilibrium conditions in exposure designs, including food items in combined waterborne-dietary matched chronic exposures. Some potentially important toxicity-modifying factors are currently not represented in bioavailability models and have received insufficient attention in toxicity testing. Temperature is probably of foremost importance; phosphate is likely important in plant and algae models. Acclimation may result in predictions that err on the side of protection. Striking a balance between comprehensive, mechanistically sound models and simplified approaches is a challenge. If empirical bioavailability tools such as multiple-linear regression models and look-up tables are employed in criteria, they should always be informed qualitatively and quantitatively by mechanistic models. If bioavailability models are to be used in environmental regulation, ongoing support and availability for use of the models in the public domain are essential. Environ Toxicol Chem 2019;39:60-84. © 2019 SETAC.

As part of a SETAC Pellston Workshop, we evaluated the potential use of metal tissue residues for predicting effects in aquatic organisms. This evaluation included consideration of different conceptual models and then development of several case studies on how tissue residues might be applied for metals, assessing the strengths and weaknesses of these different approaches. We further developed a new conceptual model in which metal tissue concentrations from metal-accumulating organisms (principally invertebrates) that are relatively insensitive to metal toxicity could be used as predictors of effects in metal-sensitive taxa that typically do not accumulate metals to a significant degree. Overall, we conclude that the use of tissue residue assessment for metals other than organometals has not led to the development of a generalized approach as in the case of organic substances. Species-specific and site-specific approaches have been developed for one or more metals (e.g., Ni). The use of gill tissue residues within the biotic ligand model is another successful application. Aquatic organisms contain a diverse array of homeostatic mechanisms that are both metal- and species-specific. As a result, use of whole-body measurements (and often specific organs) for metals does not lead to a defensible position regarding risk to the organism. Rather, we suggest that in the short term, with sufficient validation, species- and site-specific approaches for metals can be developed. In the longer term it may be possible to use metal-accumulating species to predict toxicity to metal-sensitive species with appropriate field validation.

Critical tissue copper (Cu) residues associated with adverse effects on embryo-larval development were determined for the Mediterranean mussel (Mytilus galloprovincialis) and purple sea urchin (Strongylocentrotus purpuratus) following laboratory exposure to Cu-spiked seawater collected from San Diego Bay, California, USA. Whole body no-observed-effect-residues (NOER) were similar, with means of 21 and 23 l gg � 1 dw, for M. galloprovincialis and S. purpuratus, respectively. Mean whole body median effect residues (ER50) were 49 and 142 l gg � 1 dw for M. galloprovincialis and S. purpuratus, respectively. The difference in ER50s between species was reduced to a factor of

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.

Chemical speciation controls the bioavailability and toxicity of metals in aquatic systems and regulatory agencies are recognizing this as they develop updated water quality criteria (WQC) for metals. The factors that affect bioavailability may be quantitatively evaluated with the biotic ligand model (BLM). Within the context of the BLM framework, the ‘biotic ligand’ is the site where metal binding results in the manifestation of a toxic effect. While the BLM does account for the speciation and complexation of dissolved metal in solution, and competition among the free metal ion and other cations for binding sites at the biotic ligand, it does not explicitly consider either the physiological effects of metals on aquatic organisms, or the direct effect of water chemistry parameters such as pH, Ca2+and Na+ on the physiological state of the organism. Here, a physiologically-based model of survival time is described. In addition to incorporating the effects of water chemistry on metal availability to the organism, via the BLM, it also considers the interaction of water chemistry on the physiological condition of the organism, independent of its effect on metal availability. At the same time it explicitly considers the degree of interaction of these factors with the organism and how this affects the rate at which cumulative damage occurs. An example application of the model to toxicity data for rainbow trout exposed to silver is presented to illustrate how this framework may be used to predict survival time for alternative exposure durations. The sodium balance model (SBM) that is described herein, a specific application of a more generic ion balance model (IBM) framework, adds a new physiological dimension to the previously developed BLM. As such it also necessarily adds another layer of complexity to this already useful predictive framework. While the demonstrated capability of the SBM to predict effects in relation to exposure duration is a useful feature of this mechanistically-based framework, it is envisioned that, with suitable refinements, it may also have utility in other areas of toxicological and regulatory interest. Such areas include the analysis of time variable exposure conditions, residual after-effects of exposure to metals, acclimation, chronic toxicity and species and genus sensitivity. Each of these is of potential utility to longer-term ongoing efforts to develop and refine WQC for metals.

The biotic ligand model (BLM) of acute metal toxicity to aquatic organisms is based on the idea that mortality occurs when the metal-biotic ligand complex reaches a critical concentration. For fish, the biotic ligand is either known or suspected to be the sodium or calcium channel proteins in the gill surface that regulate the ionic composition of the blood. For other organisms, it is hypothesized that a biotic ligand exists and that mortality can be modeled in a similar way. The biotic ligand interacts with the metal cations in solution. The amount of metal that binds is determined by a competition for metal ions between the biotic ligand and the other aqueous ligands, particularly dissolved organic matter (DOM), and the competition for the biotic ligand between the toxic metal ion and the other metal cations in solution, for example, calcium. The model is a generalization of the free ion activity model that relates toxicity to the concentration of the divalent metal cation. The difference is the presence of competitive binding at the biotic ligand, which models the protective effects of other metal cations, and the direct influence of pH. The model is implemented using the Windermere humic aqueous model (WHAM) model of metal-DOM complexation. It is applied to copper and silver using gill complexation constants reported by R. Playle and coworkers. Initial application is made to the fathead minnow data set reported by R. Erickson and a water effects ratio data set by J. Diamond. The use of the BLM for determining total maximum daily loadings (TMDLs) and for regional risk assessments is discussed within a probabilistic framework. At first glance, it appears that a large amount of data are required for a successful application. However, the use of lognormal probability distributions reduces the required data to a manageable amount.

The biotic ligand model (BLM) was developed to explain and predict the effects of water chemistry on the acute toxicity of metals to aquatic organisms. The biotic ligand is defined as a specific receptor within an organism where metal complexation leads to acute toxicity. The BLM is designed to predict metal interactions at the biotic ligand within the context of aqueous metal speciation and competitive binding of protective cations such as calcium. Toxicity is defined as accumulation of metal at the biotic ligand at or above a critical threshold concentration. This modeling framework provides mechanistic explanations for the observed effects of aqueous ligands, such as natural organic matter, and water hardness on metal toxicity. In this paper, the development of a copper version of the BLM is described. The calibrated model is then used to calculate LC50 (the lethal concentration for 50% of test organisms) and is evaluated by comparison with published toxicity data sets for freshwater fish (fathead minnow, Pimephales promelas) and Daphnia.

The OECD is currently harmonizing procedures for aquatic hazard identification of substances. Such a system already exists in Europe where it is recognized that special consideration must be given to sparingly soluble metals and metal compounds (SSMMCs) because standard hazard testing procedures designed for organic chemicals do not accommodate the characteristics of SSMMCs. Current aquatic hazard identification procedures are based on persistence, bioaccumulation, and toxicity (PBT) measurements. Persistence measurements typically used for organic substances (biodegradation) do not apply to metals. Alternative measurements such as complexation and precipitation are more appropriate. Metal bioaccumulation is important in terms of nutritional sufficiency and potential food chain transfer and toxicity. Unlike organic substances, metal bioaccumulation potential cannot be estimated using log octanol-water partition coefficients. Further, bioaccumulation and bioconcentration factors are often inversely related...

The United States Environmental Protection Agency (USEPA) has established nationally applicable water quality criteria (WQC) for metals that are designed to be protective of aquatic life. However, in some instances these criteria may be over-protective as a result of natural, site-specific differences in water quality characteristics. These differences affect metal speciation and bioavailability, fundamental considerations in assessing toxicity. Laboratory studies completed during recent years have advanced the current understanding of metal chemistry in aquatic systems, including the formation of organic and inorganic metal complexes and sorption to particulate organic matter. Parallel investigations have led to an improved understanding of the physiological basis of why metals are toxic to aquatic organisms. These studies have, in combination, led to an improved understanding of how site-specific water chemistry affects bioavailability, and how metals exert toxicity at the organism site of action, at the biotic ligand in the context of the model to be described. The biotic ligand model provides a quantitative framework for assessing metal toxicity over a range of hardness, pH and dissolved organic carbon (DOC) levels. The chemical equilibrium sub-model incorporates metal-biotic ligand interactions to compute metal accumulation at the site of action (e.g., the gill of a fish) as a function of water quality. The toxicity sub-model uses this computed accumulation level as the basis for successfully predicting observed variations in toxicity associated with changes in water quality. The results highlight the potential utility of this approach to provide an alternative means of developing site-specific permit limits and WQC. The ability of the model framework to quantitatively assess the effects of hardness, pH and DOC on toxicity, in comparison to current WQC for metals that typically vary with hardness alone, is a significant advance in understanding how site-specific conditions affect the toxicity of metals.

Power generation and desalination plants have used water-cooled copper alloy condensers since the 1960s. During the late 1970s some copper discharges associated with these condensers were implicated as a cause of adverse aquatic impacts and water quality criteria (WQC) exceedances. Given recent advances in monitoring and impact assessment techniques, it was therefore decided to complete an updated review of the aquatic impacts of copper alloy condenser discharges. In conducting such a review, it is necessary to accurately define both the exposure and effect levels of concern, and to consider bioavailability. The evaluation of copper exposure levels requires use of clean sampling and analytical techniques. A review of historical data showed that some previously reported copper data probably overstated actual levels due to failure to use these techniques. Further, the regulatory effect levels of concern, the WQC, have often been found to be overly protective relative to the level of protection intended by regulatory guidelines. Thus, WQC refinements have been introduced, including: a revision of the saltwater copper criterion, a recommendation to use dissolved copper to assess compliance, and development of procedures to set site-specific WQC. Considering the latter, the biotic ligand model, an evaluative tool that considers copper bioavailability, is introduced as a computational alternative to setting bioassay-based site-specific criteria. Based on the preceding considerations, bioavailable copper exposure levels are lower than previously believed, effect levels of concern (applicable site-specific WQC) have increased, and the desired level of protection has been achieved more frequently than previous analyses have indicated.

Copper is used as an antimicrobial agent in building materials such as algae-resistant roofing shingles and treated wood products for decks, fences, and utility poles used in urbanized areas. Releases from these materials may pose risks to aquatic and terrestrial organisms. Copper exposures in surface water, sediment, and soil were estimated for a hypothetical urban setting using the TREX watershed model. Drainage and soil characteristics were based on an existing watershed. Urban landscape characteristics were developed from data regarding housing densities and copper use in building materials. This setting provides a spatially distributed, upper-bound assessment scenario. Release rates from algae-resistant shingles and treated wood were defined based on surface area and rainfall. Simulations for the urban landscapes were performed for a 10-year period. Simulation results were used to evaluate exceedences of benchmark concentrations for water, sediment, and soil. For algae-resistant shingles, exposures did not exceed benchmarks in any media. For treated wood, exposures did not exceed sediment and soil benchmarks, and surface water benchmarks were exceeded on 2 days in 10 years. Based on this analysis, copper use as an antimicrobial agent in algae resistant shingles and treated wood is not expected to pose significant adverse environmental risks on an individual use basis.

Acute silver toxicity studies were conducted with and without food for four common freshwater test species: Daphnia magna, Ceriodaphnia dubia, Pimephales promelas (fathead minnow-FHM), and Oncorhynchus mykiss (rainbow trout-RBT) in order to generate acute-to-chronic ratios (ACR). The studies were conducted similarly (i.e., static-renewal or flow-through) to chronic/early-life stage studies that were previously performed in this laboratory. The acute toxicity (EC/LC50 values) of silver without food ranged from 0.57 μg dissolved Ag/l for C.dubia to 9.15 μg dissolved Ag/l for RBT. The presence of food resulted in an increase in EC/LC50 values from 1.25× for RBT to 22.4× for C. dubia. Invertebrate food type was also shown to effect acute silver toxicity. Food did not affect EC/LC50s or ACRs as greatly in fish studies as in invertebrate studies. ACRs for both invertebrate species were

Most bioaccumulation models in current use were originally developed for organic chemicals and subsequently applied to metals. As a result, they do not typically incorporate metal-specific features that affect how metals are taken up, metabolized, detoxified, and eliminated. This review describes models that are used to evaluate metal accumulation, from waterborne and dietary exposure, by aquatic organisms. It considers whole-body models (including single-compartment and multicompartment variations), physiologically based pharmacokinetic models (providing an explicit representation of individual organs), and food chain or food web models. A representative model framework and example application are described for each model. Variations are also discussed. Metal bioaccumulation models have not typically been developed to predict effects on aquatic organisms. However, several that were developed for this purpose are described herein. These include damage–repair models, biotic ligand models, physiologically based mechanistic models, and intracellular speciation models. While some effects models incorporate relatively unique metal-specific features, they have primarily been applied to waterborne exposures, and received limited use for predicting effects resulting from both waterborne and dietary exposures. Studies that facilitate the refinement of both bioaccumulation and effects models, and the unification of such models, will likely benefit both the scientific and regulatory communities.

The purpose of this review paper is to present the technical basis for establishing sediment quality criteria using equilibrium partitioning (EqP). Equilibrium partitioning is chosen because it addresses the two principal technical issues that must be resolved: the varying bioavailability of chemicals in sediments and the choice of the appropriate biological effects concentration. The data that are used to examine the question of varying bioavailability across sediments are from toxicity and bioaccumulation experiments utilizing the same chemical and test organism but different sediments. It has been found that if the different sediments in each experiment are compared, there is essentially no relationship between sediment chemical concentrations on a dry weight basis and biological effects. However, if the chemical concentrations in the pore water of the sediment are used (for chemicals that are not highly hydrophobic) or if the sediment chemical concentrations on an organic carbon basis are used, then the biological effects occur at similar concentrations (within a factor of two) for the different sediments. In addition, the effects concentrations are the same as, or they can be predicted from, the effects concentration determined in water- only exposures. The EqP methodology rationalizes these results by assuming that the partitioning of the chemical between sediment organic carbon and pore water is at equilibrium. In each of these phases, the fugacity or activity of the chemical is the same at equilibrium. As a consequence, it is assumed that the organism receives an equivalent exposure from a water-only exposure or from any equilibrated phase, either from pore water via respiration, from sediment carbon via ingestion; or from a mixture of the routes. Thus, the pathway of exposure is not significant. The biological effect is produced by the chemical activity of the single phase or the equilibrated system. Sediment quality criteria for nonionic organic chemicals are based on the chemical concentration in sediment organic carbon. For highly hydrophobic chemicals this is necessary because the pore water concentration is, for those chemicals, no longer a good estimate of the chemical activity. The pore water concentration is the sum of the free chemical concentration, which is bioavailable and represents the chemical activity, and the concentration of chemical complexed to dissolved organic carbon, which, as the data presented below illustrate, is not bioavailable. Using the chemical concentration in sediment organic carbon eliminates this ambiguity. Sediment quality criteria also require that a chemical concentration be chosen that is sufficiently protective of benthic organisms. The final chronic value (FCV) from the U.S. Environmental Protection Agency (EPA) water quality criteria is proposed. An analysis of the data compiled in the water quality criteria documents demonstrates that benthic species, defined as either epibenthic or infaunal species, have a similar sensitivity to water column species. This is the case if the most sensitive species are compared and if all species are compared. The results of benthic colonization experiments also support the use of the FCV. Equilibrium partitioning cannot remove all the variation in the experimentally observed sediment- effects concentration and the concentration predicted from water-only exposures. A variation of approximately a factor of two to three remains. Hence, it is recognized that a quantification of this uncertainty should accompany the sediment quality criteria. The derivation of sediment quality criteria requires the octanol/water partition coefficient of the chemical. It should be measured with modern experimental techniques, which appear to remove the large variation in reported values. The derivation of the final chronic value should also be updated to include the most recent toxicological information.

A model of sediment oxygen demand is presented that relates the sediment oxygen demand (SOD) to the extent of oxidation of dissolved methane and ammonia generated in the anaerobic zone of the sediment of lakes and streams. The fluxes of dissolved methane and ammonia from the sediment to the overlying water, as well as methane and nitrogen gas fluxes that escape as bubbles, are included in the model. The three model parameters—the dissolved methane mass transfer coefficient and the two oxidation rate parameters—are estimated from laboratory and field data sets. The effect of overlying water dissolved oxygen and temperature is examined. The importance of the gas fluxes and their quantitative relationship to SOD is established. Any field program that includes the measurement of SOD should also include the measurement of the nitrogen and methane fluxes as well. The model is limited to freshwater sediments since the oxidation of sulfides is not included. The contribution from the respiration of benthic macro f...

Laboratory toxicity studies and a caged bivalve field study complemented by computer modeling were conducted to investigate the significance of effects on aquatic organisms due to dietary exposure to metals. The lab studies were performed with saltwater and freshwater organisms. The saltwater studies provided partial support for previous research that showed dietary effects could occur when the food source, a mono-algal culture, was loaded with Ag, Cu, Ni or Zn at dissolved metal concentrations below than the current ambient water quality criteria (WQC). However, in contrast to the earlier studies, continuous 7-day exposures to Ag rather than a 4-hour pulse exposure were required to elicit a response by Acartia tonsa. The effects concentrations were also much higher than in the earlier studies and slightly higher than the proposed chronic water quality criteria (WQC) for Ag. The three other metals were only tested under a continuous-exposure regime and yielded similar test results to Ag, though the dissolved concentrations in which the algae were cultured were below the current chronic saltwater WQC. In the freshwater studies performed with Ceriodaphnia dubia exposed to dietary Cu and Ag, reproductive impairment was not consistent among repeat tests. Variations in reproductive vitality of the C. dubia might have contributed to the variability in adverse responses by the organisms. Questions remain regarding the applicability of the saltwater and freshwater results to field conditions, where mitigating factors such as a more nutritional mixed-algal diet or increased metal complexation capacity could decrease the amount of the bioavailable form of the metals, thereby decreasing exposure and effects. This title belongs to WERF Research Report Series ISBN: 9781843397656 (eBook)

The biotic ligand model (BLM) is a computational tool that may be used to predict toxic effect levels of metals, including Ag. The BLM considers the effect of site-specific water quality characteristics on Ag speciation and bioavailability. This provides a basis for extrapolating from laboratory waters, used for most Ag toxicity tests, to natural waters, where the bioavailability and toxicity of Ag may differ. The original version of the BLM for the acute toxicity of Ag was reviewed by the USEPA Science Advisory Board (SAB) in 1999 and they recommended the completion of additional Ag chemistry and toxicity studies to provide data to test and refine the model. This project was completed to satisfy these recommendations. A chemistry component was completed to improve the representation of Ag speciation in natural waters and to characterize the important effect of sulfide on Ag bioavailability. These investigations added to the information that was available to characterize Ag chemistry and provided a basis for updating the earlier version of the Ag BLM. The BLM was also calibrated to laboratory toxicity data that were generated for a Ag-sensitive invertebrate, Ceriodaphnia dubia , a cladoceran that had previously been tested to only a limited degree. The studies were carried out in laboratory waters in which the chemistry was systematically varied to provide a basis for calibration of the BLM to the toxicity data for C. dubia . Toxicity tests were also performed with both Pimephales promelas and C. dubia in natural water samples that were well characterized chemically, with the data then used to test the model. While the C. dubia BLM results were relatively successful at simulating the observed effect levels the P. promelas results highlighted the need for further testing and refinement of the Ag BLM as it was applied to this fish species. Physiological tests were performed to explore the apparent increase in sensitivity of P. promelas in some of the test waters that were relatively low in ionic strength. It was determined that acclimation to such low ionic strength waters, in the absence of Ag, did not significantly reduce their sensitivity to Ag. Rather, it appeared that any benefit that may have been realized by acclimation, such as an enhanced ability to upregulate ion uptake rates in low ionic strength waters, was apparently offset by a concomitant increase in the rate at which Ag was accumulated. The Ag BLM was modified to account for the effect of low ionic strength water on the sensitivity to Ag of > 4-day old P. promelas . This led to an improved ability of the BLM to predict P. promelas effect levels in the natural waters. Physiological tests were also performed to investigate the apparent species-specific protection against Ag toxicity that is provided by chloride. Investigations with P. promelas suggested that the differences in degree of protection may be related to differences in how Ag affects the ionoregulation of Na+ and Cl− by different fish species. Although questions remain with regard to chloride, it appears that the BLM is able to predict this response reasonably well, particularly for the relatively sensitive

The chronic (early life stage [ELS]) and short-term chronic (STC) toxicity of silver (as silver nitrate) to fathead minnows (FHM) was determined concurrently in flow-through exposures (33 volume additions/d). Paired ELS (approximately 30 d) and STC (7 d) studies were conducted with and without the addition of 60 mg/L Cl (as NaCl). The paired studies in unamended water were later repeated using standard flow conditions (9 volume additions/d). The purpose of the paired studies was to determine if short-term chronic endpoints can be used to predict effects in ELS studies. For each experiment, a "split-chamber" design (organisms were held in a common exposure chamber) allowed the direct comparison between short-term and chronic exposures. It appeared that the chronic toxicity of silver was mitigated to some extent by NaCl addition. The maximum acceptable toxicant concentration for growth in the ELS study was 0.53 microg dissolved Ag/L under standard flow conditions. Early life stage and STC endpoints in all three studies typically agreed within a factor of two. Whole-body sodium and silver concentrations measured in individual fathead minnows during these studies showed an increase in silver body burdens and a decrease in sodium concentration. These results indicate that the STC study could be used as a surrogate test to estimate chronic toxicity and that the mechanism of chronic silver toxicity may be the same as for acute toxicity.

An important final step in development of an acute biotic ligand model for silver is to validate predictive capabilities of the biotic ligand model developed for fish and invertebrates. To accomplish this, eight natural waters, collected from across North America, were characterized with respect to ionic composition, pH, dissolved organic carbon, and sulfide. Tests were conducted with the cladoceran Ceriodaphnia dubia (48-h static) and the fish Pimephales promelas (96-h static renewal) to determine the concentrations causing lethality to 50% of the organisms (LC50s) for silver in each of these waters. Overall, the biotic ligand model adequately predicted silver toxicity to C. dubia; however, in some cases, predicted LC50 values exceeded measured values. The accuracy of the biotic ligand model predictions was less convincing for silver toxicity to P. promelas with pronounced problems in low-ionic strength waters. Another issue was the use of acclimated organisms in toxicity studies because the biotic ligand model has been developed with the use of a mix of studies with acclimated and nonacclimated test organisms of varying ages and sizes. To evaluate whether effects of acclimation to test waters influence biotic ligand model predictions, a subset of the natural waters were also tested with P. promelas that had been acclimated to the natural water for 7 d before testing. These experiments revealed no differences in toxicity between acclimated and nonacclimated P. promelas. To determine the influence of organism size, which has been previously correlated to Na(+) turnover and acute silver toxicity across multiple species, Na(+) and Cl(-) influx rates were measured in P. promelas of different sizes. Our results show that Na(+) and Cl(-) influx rates were inversely related to fish mass and positively correlated with silver sensitivity.

T current water quality standards (WQS) in China were issued in 2002. 1 According to the surface water environmental function classification and protection target, all surface waters are divided into five classes. Class I waters generally have the most stringent WQS and Class V the least stringent. The WQS have played an important role in China. However, they need to be updated to better reflect the latest advances in research, and regional water quality characteristics of Chinese waters. Chinese regulators are planning to begin revisions to the national WQS by the end of year 2012. Because there is currently no scientific research being directed at the development of water quality criteria (WQC) or WQS that are specific to China, a common approach is to build upon the successful WQC experiences of other countries. 2 To some extent, current WQS for some chemicals might under-protect or overprotect freshwater aquatic species. 3 The reason is that many WQS fail to consider how site-specific water chemistry influences the bioavailability and toxicity of chemicals to aquatic life. So, it is imperative to carry out WQC research to support the updating of WQS in China. A bioavailability-based method should be used to harmonize the approach for setting WQS for metals in China with the current understanding of how water quality influences the effects of metals on aquatic organisms. Efforts to incorporate the influence of water chemistry on metal bioavailability into the derivation of updated WQC are ongoing throughout the world. One approach for doing so is to use the biotic ligand model (BLM). The BLM allows metal− organism interactions to be taken into account and, given sitespecific information on water chemistry, to evaluate the dissolved metal concentration associated with a critical level of metal accumulation that is toxic to an organism. The BLM serves as a powerful tool for predicting metal toxicity because it accounts for the concurrent influences of several environmental factors that alter site-specific metal bioavailability to an organism. The influence of water chemistry on metal bioavailability and effects has been reflected in updated regulations in several countries. For example, the U.S. Environmental Protection Agency (EPA) has already adopted a BLM-based WQC for copper. 4 The copper-BLM has been applied to several organisms at different trophic levels. Incorporation of the copper-BLM into EPA’s copper-WQC is based on an acute toxicity BLM. The European Union (EU) has launched a European Union Water Framework Directive. WQC for copper and other metals that have been developed for use within this regulatory framework are typically based on site-specific effect levels that are evaluated by means of chronic BLMs. To date, the BLM has not been widely used in WQC research in China, and only a few studies have attempted to use BLMs to predict metal toxicity in surface waters. Therefore, it is necessary for Chinese scientists to begin to apply currently available BLMs for metals to Chinese waters and to conduct the research that will be needed to implement updated WQC for metals, especially for nickel, copper and zinc, over concentration ranges that reflect water quality characteristics of Chinese waters. There are several reasons why it will be a great challenge to apply BLMs to Chinese waters. First, the physicochemical properties of Chinese surface waters are somewhat unique. As one example, eutrophication is a serious problem in some inland lakes and rivers, such as Lake Taihu, which means concentrations of dissolved organic carbon (DOC) and other parameters will likely be elevated. There are also other regional differences in water chemistry (e.g., pH and hardness), and these geochemical boundaries need to be considered when using the BLM as well. Given the relatively “clean”, less eutrophic characteristics of EU and U.S. freshwaters, it must be determined if BLM parameter values evaluated for concentration ranges of environmental constituents in those settings

The mitigating effect of increasing hardness on metal toxicity is reflected in water quality criteria in the United States over the range of 25–400 mg l −1 (as CaCO 3 ). However, waters in the arid west of the US frequently exceed 400 mg l −1 hardness, and the applicability of hardness–toxicity relationships in these waters is unknown. Acute toxicity tests with Ceriodaphnia dubia were conducted at hardness levels ranging from approximately 300 to 1200 mg l −1 using reconstituted waters that mimic two natural waters with elevated hardness: (1) alkaline desert southwest streams (Las Vegas Wash, NV), and (2) low alkalinity waters from a CaSO 4 -treated mining effluent in Colorado. The moderately-alkaline EPA synthetic hard water was also included for comparison. Copper toxicity did not consistently vary as a function of hardness, but likely as a function of other water quality characteristics (e.g., alkalinity or other correlated factors). The hardness equations used in regulatory criteria, therefore, may not provide an accurate level of protection against copper toxicity in all types of very hard waters. However, the mechanistic Biotic ligand model generally predicted copper toxicity within ±2 X of observed EC 50 values, and thus may be more useful than hardness for modifying water quality criteria.

The Biotic Ligand Model has been previously developed to explain and predict the effects of water chemistry on the toxicity of copper, silver, and cadmium. In this paper, we describe the development and application of a biotic ligand model for zinc (Zn BLM). The data used in the development of the Zn BLM includes acute zinc LC50 data for several aquatic organisms including rainbow trout, fathead minnow, and Daphnia magna. Important chemical effects were observed that influenced the measured zinc toxicity for these organisms including the effects of hardness and pH. A significant amount of the historical toxicity data for zinc includes concentrations that exceeded zinc solubility. These data exhibited very different responses to chemical adjustment than data that were within solubility limits. Toxicity data that were within solubility limits showed evidence of both zinc complexation, and zinc-proton competition and could be well described by a chemical equilibrium approach such as that used by the Zn BLM.

Operation of a leather tannery site adjacent to Tannery Bay (Michigan) has resulted in elevated concentrations of chromium in the bed sediments. Of concern is the potential resuspension of the sediments and their subsequent migration from the bay to the adjacent river. Evaluating the stability of these contaminated sediments is a necessary ingredient for developing sediment remediation strategies. An integrated hydrodynamic and sediment transport model was used to assess model response to (i) maximum probable flows, (ii) variable wind forcing, and (iii) variable resuspension potential parameters. A hydrodynamic model was used to determine current velocities and circulation patterns. This information was used to compute the bed shear stresses, which, in turn, controls sediment resuspension and deposition. Velocities predicted by the model were relatively low in the inner region of the bay indicating that the volume of sediment resuspended and transported to the adjacent river would comprise a small percentage of the contaminated sediments in the bay, even under the most extreme conditions considered. The sediment transport simulations were used to predict water column concentrations of dissolved chromium, which were found to be lower than the acute water quality criterion.

An extension of the simultaneously extracted metals/acid-volatile sulfide (SEM/AVS) procedure is presented that predicts the acute and chronic sediment metals effects concentrations. A biotic ligand model (BLM) and a pore water-sediment partitioning model are used to predict the sediment concentration that is in equilibrium with the biotic ligand effects concentration. This initial application considers only partitioning to sediment particulate organic carbon. This procedure bypasses the need to compute the details of the pore-water chemistry. Remarkably, the median lethal concentration on a sediment organic carbon (OC)-normalized basis, SEM*(x,OC), is essentially unchanged over a wide range of concentrations of pore-water hardness, salinity, dissolved organic carbon, and any other complexing or competing ligands. Only the pore-water pH is important. Both acute and chronic exposures in fresh- and saltwater sediments are compared to predictions for cadmium (Cd), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn) based on the Daphnia magna BLM. The SEM*(x,OC) concentrations are similar for all the metals except cadmium. For pH = 8, the approximate values (micromol/gOC) are Cd-SEM*(xOC) approximately equal to 100, Cu-SEM*(x,OC) approximately equal to 900, Ni-SEMoc approximately equal to 1,100, Zn-SEM*(x,OC) approximately equal to 1,400, and Pb-SEM*(x,OC) approximately equal to 2,700. This similarity is the explanation for an empirically observed dose-response relationship between SEM and acute and chronic effects concentrations that had been observed previously. This initial application clearly demonstrates that BLMs can be used to predict toxic sediment concentrations without modeling the pore-water chemistry.

A time variable model has been used to analyze a field scale experiment in which a flooded limestone quarry was dosed with equal quantities of Lindane and DDE. The water column and sediment chemical concentrations were monitored for one year after the initial dosing. The markedly different physical-chemical characteristics of the test chemicals provided an interesting contrast for model application. A simplified time variable model of partitioning chemicals is cast in the form of analytical solutions for the total, dissolved and particulate concentrations of chemical in completely mixed, interactive, water column and sediment compartments. The model formulation incorporates chemical decay and transport mechanisms of particulate and diffusive exchange between water column and sediment. The Lindane and DDE model calibrations based on data from the first year after dosing are presented. A preliminary verification of the model, obtained by projecting DDE levels in the water and sediment five years after initial dosing, when the quarry was revisited, is also shown. The model results underscore the significance of chemical partitioning on chemical fate and highlight the importance and utility of a modeling framework which incorporates realistic mechanisms of water column and sediment interaction.

Calculations are often performed to relate the effect of specific wastewater discharges on dissolved oxygen conditions in receiving waters. The calculation procedure involves the application of a mathematical water quality model containing various rate coefficients which must be evaluated from data or estimated. The atmospheric reaeration rate coefficient is particularly important in this regard and is often estimated by a variety of predictive equations. The purpose of this paper is to illustrate the significance of the re-aeration coefficient and the differences in estimates provided by the various predictive equations on the oxygen balance and resulting practical decision making. It is shown that projected water quality and required wastewater treatment are highly sensitive to estimated reaeration coefficients. Judicious use of the predictive equations and field measurements for gas transfer rates are encouraged.