The Canadian Council for Ministers of the Environment (CCME) has recently revised the Water Quality Guideline for chloride (from 230 to 120 mg/L for chronic exposure), applicable to freshwater aquatic life receptors (CCME 2011; Canadian Water Quality Guidelines – Chloride Ion. Scientific Criteria Document. Canadian Council for Ministers of the Environment, Winnipeg. PN 1460, ISBN 978-1-896997-77-3 PDF). As part of this, two additional key factors were identified that may influence the derivation of a chloride guideline for aquatic life: 1) water hardness; and, 2) cation co-exposure. Some studies have shown a relatively strong influence of water hardness on toxic response to salinity exposure, while other studies did not find hardness, as a coexposure agent, responsible for a difference in toxic response. CCME (2011) indicated there was a need for the generation of more toxicity data incorporating aspects of hardness. In addition, CCME (2011) indicated that further research would be of value for assessing differences in toxic responses for various species observed as a function of the cation(s) present.