GRE General Test: RC-435298 GRE Reading Comprehension

In the 1980's billions of batteries were thrown into landfills. There is growing concern that as the batteries deteriorate, the toxic metals they contain will seep into the groundwater and contaminate it. That concern is unfounded, however, for studies of groundwater near large landfills used in the 1950's and then closed showed little, if any, such contamination.
Which of the following, if true, most seriously weakens the argument above?