Some of the monitoring wells, intended to record background or naturally occurring levels of metals near NRG plants, are located in areas where coal ash is buried, making it more difficult to measure how the waste is affecting groundwater that flows away from the sites, said Jennifer Cassel, an attorney with EarthJustice, another group behind the report. Even in those cases, Cassel said, the company’s testing shows higher levels of metals in groundwater flowing through the ash pits compared with concentrations found in background wells.