The most significant challenge at Hanford is stabilizing the of high-level radioactive waste stored in 177 underground tanks. By 1998, about a third of these tanks had leaked waste into the soil and groundwater. , most of the liquid waste had been transferred to more secure double-shelled tanks; however, of liquid waste, together with of salt cake and sludge, remains in the single-shelled tanks. DOE lacks information about the extent to which the 27 double-shell tanks may be susceptible to corrosion. Without determining the extent to which the factors that contributed to the leak in AY-102 were similar to the other 27 double-shell tanks, DOE cannot be sure how long its double-shell tanks can safely store waste. That waste was originally scheduled to be removed by 2018. , the revised deadline was 2040. Nearby aquifers contain an estimated of contaminated groundwater as a result of the leaks. , of radioactive waste is traveling through the groundwater toward the Columbia River. This waste is expected to reach the river in 12 to 50 years if cleanup does not proceed on schedule. The site includes of solid radioactive waste.