The toxicity test results (acute and chronic) for the studies commissioned by BC Environment for water hardnesses of 25, 100 and 250 mg/L CaCO3 are presented in graphical form on the following page. The Microtox IC50 values have not been included as the results were the highest recorded among the tests conducted. Data from literature sources that met the BC Protocol requirements for primary and/or secondary data are also plotted on this graph.
The graphical presentation illustrates the general trend of increased manganese concentration with increased water hardness. The exception was the 96 hour acute LC50 test for rainbow trout as discussed in Section 4.2.5. The graph also illustrates the trends in acute data versus chronic data. The levels of manganese at which adverse effects were observed increased with increasing hardness more quickly for the acute tests than for the chronic tests. This pattern is what would be expected as a higher level of exposure without adverse effects would be anticipated for a shorter term (acute) exposure versus a longer term (chronic) exposure.
For purposes of applying the BC Protocol to derive water quality criteria, the data have been separated into acute and chronic categories as presented in Sections 4.3.1 and 4.3.2. This will allow determination of separate acute and chronic guidelines for the protection of freshwater aquatic life.