Effluent toxicity testing methods have been well defined, but for the most part, these methods do not attempt to segregate the effects of active ionic concentrations and ion imbalances upon test and species performances. The role of various total dissolved solids in effluents on regulatory compliance has emerged during the last few years and has caused confusion in technical assessment and in permitting and compliance issues. This paper assesses the issue of ionic strength and ion imbalance, provides a brief summary of applicable data, presents several case studies demonstrating successful tools to address toxicity resulting from salinity and ion imbalance, and provides recommendations for regulatory and compliance options to manage discharges with salinity/ion imbalance issues. Effluent toxicity resulting from inorganic ion imbalance and the ion concentration of the effluent is pervasive in permitted discharge from many industrial process and municipal discharges where process streams are concentrated, adjusted, or modified. This paper discusses procedures that use weight-of-evidence approaches to identify ion imbalance toxicity, including direct measurement, predictive toxicity models for freshwater, exchange resins, mock effluents, and ion imbalance toxicity with tolerant/ susceptible text species. Cost-effective waste treatment control options for a facility whose effluent is toxic because of total dissolved solids (TDS) or because of specific ion(s) are scarce at best. Depending on the discharge situation, TDS toxicity may not be viewed with the same level of concern as other, more traditional, toxicants. These discharge situations often do not require the conservative safety factors required by other toxicants. Selection of the alternative regulatory solutions discussed in this paper may be beneficial, especially because they do not require potentially expensive or high-energy-using treatment options that may be ineffective control options. The information presented is intended to provide a better understanding of the role of ion imbalance in aquatic toxicity testing and to provide various recommendations that should be considered in addressing these issues.

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Abstract-Effluent toxicity testing methods have been well defined, but for the most part, these methods do not attempt to segregate the effects of active ionic concentrations and ion imbalances upon test and species performances. The role of various total dissolved solids in effluents on regulatory compliance has emerged during the last few years and has caused confusion in technical assessment and in permitting and compliance issues. This paper assesses the issue of ionic strength and ion imbalance, provides a brief summary of applicable data, presents several case studies demonstrating successful tools to address toxicity resulting from salinity and ion imbalance, and provides recommendations for regulatory and compliance options to manage discharges with salinity/ion imbalance issues. Effluent toxicity resulting from inorganic ion imbalance and the ion concentration of the effluent is pervasive in permitted discharge from many industrial process and municipal discharges where process streams are concentrated, adjusted, or modified. This paper discusses procedures that use weight-of-evidence approaches to identify ion imbalance toxicity, including direct measurement, predictive toxicity models for freshwater, exchange resins, mock effluents, and ion imbalance toxicity with tolerant/ susceptible text species. Cost-effective waste treatment control options for a facility whose effluent is toxic because of total dissolved solids (TDS) or because of specific ion(s) are scarce at best. Depending on the discharge situation, TDS toxicity may not be viewed with the same level of concern as other, more traditional, toxicants. These discharge situations often do not require the conservative safety factors required by other toxicants. Selection of the alternative regulatory solutions discussed in this paper may be beneficial, especially because they do not require potentially expensive or high-energy-using treatment options that may be ineffective control options. The information presented is intended to provide a better understanding of the role of ion imbalance in aquatic toxicity testing and to provide various recommendations that should be considered in addressing these issues.

Some female killifish (Fundulus heteroclitus) from Montauk, N.Y., produce eggs that are much more resistant to the craniofacial, cardiovascular, and skeletal effects of 0.05 ppm methylmercury than eggs of other females. The following relationships were observed in studying 128 batches of eggs from 85 females fertilized by 35 males. Batches of eggs with a high percentage of noncleaving eggs (NCEs) tended to be most susceptible. Tolerance was also associated with the relation of the date to the lunar spawning cycle and with the number of dorsal fin rays in the females; fish that had ten fin rays produced no unaffected batches, whereas those with eleven or twelve rays produced eggs ranging from very tolerant (unaffected) to very susceptible. A weaker correlation with female weight was seen. Because of the importance of the percentage of NCEs in determining the tolerance of a batch of eggs, the data were reanalyzed after removing all those batches with high rates of (>35 percent) NCEs. This revealed a strong association of tolerance with the stickiness of the eggs in that batches which were very sticky tended to be more resistant. Correlations were also seen with female length and fecundity. No correlations were seen with any measured traits of the males.

In March 1984, the U.S. Environmental Protection Agency (EPA) issued a significant change in procedures regulating toxic materials in effluents through the National Pollutant Discharge Elimination System (NPDES). Concurrent with this toxicity‐based effluent control policy, the EPA established the marine/estuarine component of the Complex Effluent Toxicity Testing Program (CETTP). The CETTP was established to provide reliable, sensitive and environmentally meaningful test protocols that could be used to detect toxic industrial and municipal effluents within the NPDES. Five toxicity test methods have been developed and validated for the program since 1984 using a marine plant (Champia parvula), two invertebrate species (Arbacia punctulata and Mysidopsis bahia) and two fish species (Cyprinodon variegatus and Menidia beryllina). The laboratory precision test results for the methods were acceptable; coefficients of variation for all methods were less than 54%, averaging 34%. Numerous field tests were conducted using these methods and the results indicate that tests on receiving waters (in which effluent concentrations could be estimated through controlled dye studies) accurately reflect the toxicity of the effluents measured directly. Receiving water impacts, when observed, were generally near‐field in nature. The test methods developed are sensitive, efficient, reliable and environmentally relevant measures of effluent toxicity and offer promise for reducing adverse impacts from point‐source discharges in near‐coastal waters.