Scientists investigated heavy metal pollution in two different rivers: River Avon and River Kennet. They isolated bacteria from water samples and tested their tolerance to copper ions. The findings are: for River Avon, 42 species were resistant to copper and 15 were killed by copper; for River Kennet, 19 species were resistant to copper and 38 were killed by copper. Identify which river is likely to have the highest level of copper pollution and explain your answer using this data.