In a plant producing cast iron / engine segments, an agent detects suffocating vapors in the wastewater treatment plant, notes a leakage of hydrochloric acid on the seal of a pump and closes an upstream valve; 300 l of HCl spilled into a drip tray and then into a subsoil tank used as a detention retention basin. Firefighters neutralize HCl with soda in a neighboring tank. The pipe breaks at the bottom of the tank, 1000 l of soda flows before clogging of the breach. An exothermic reaction damages the retention tank, new flows take place. A plug had formed downstream of the pump. Air pollution remained limited (80 m).