MOORESVILLE Gastonia has joined Charlotte in expressing concern about the possible impacts of Duke Energy's Riverbend power plant on Mountain Island Lake, the major water source they share.

In comments prepared before a state hearing Tuesday night on discharge permits for Riverbend and two other Duke plants, Gastonia questioned the source of arsenic and mercury found in sludge left over after treating its drinking water.

Arsenic and mercury are among the potentially toxic elements found in coal ash, which Duke stores at Riverbend and its Allen plant on Lake Wylie and Marshall plant on Lake Norman. The three plants pipe an average of 23 million gallons a year from their ash basins into the lakes.

Charlotte-Mecklenburg officials also want the state to give more oversight to Duke's discharges. Twice this year, county staff detected arsenic above state standards and once found high levels of zinc near Riverbend's discharge pipe.

Also Tuesday, environmental advocates urged state regulators to reduce the temperature of warm water Duke's plants are allowed to release into the lakes. Duke says the water does no harm to aquatic life. Advocates say they've seen fish kills and that millions of gallons evaporate because of the water's warmth.