I'm not sure what is meant by "natural water". I would guess that the presence of acid rain in bodies of water would have a significant effect on the degree of metal solubility. Are the properties mentioned related to animal or human toxicity, or something else?
Natural water meant, water having pH nearly neutral. The pH of Northern Indian Rivers are 6.8-8.5. So, I need to clear some confusion regarding the role of this pH on dissolving of elements.