Values of EC50 (Table 7) provided more detailed information than
the results from undiluted samples. They allowed the differences in
toxicity between A1, A2 and the rest of the samples to be distinguished.
Samples A1 and A2 showed very high acute toxicity, corresponding
with their proximity to the mine and the fact that they had not undergone
dilution with other surface waters, hence the levels of As and
metals measured in these waters were very high.