It is true that different water supplies will have different levels of contaminants (lead, arsenic, etc) but can all be within EPA limits. Switching to a water supply with a higher level of contamination will increase exposure. The medical study seems to look at the percentage of children below 5g/dL before and after the switch. It goes from 2% to 4%. So with the old water supply, a certain percentage of children were already being exposed to elevated levels of lead. Switching to a water source with higher lead levels will push more children who are being exposed to lead through other sources to above the 5g/dL mark. However, this would seem to indicate that the primary source of lead for these children above 5g/dL is something other than the water.