DOE later found water intruding into at least 14 single-shell tanks and that one of them had been leaking about per year into the ground since about 2010. In 2012, DOE discovered a leak also from a double-shell tank caused by construction flaws and corrosion in the bottom, and that 12 double-shell tanks have similar construction flaws. Since then, the DOE changed to monitoring single-shell tanks monthly and double-shell tanks every three years, and also changed monitoring methods. In March 2014, the DOE announced further delays in the construction of the Waste Treatment Plant, which will affect the schedule for removing waste from the tanks. Intermittent discoveries of undocumented contamination have slowed the pace and raised the cost of cleanup.