Hanford workers have finished installing a 70,000-square-foot "umbrella" over soil contaminated by what may have been the largest leak of radioactive waste from Hanford's underground tanks.

The cap is a temporary measure to keep rain and snow melt from driving contamination deeper into the ground. But eventually, as leak-prone tanks are emptied of radioactive waste, the Department of Energy is expected to identify a way to clean up or otherwise permanently protect the public and environment from the remains of the spill.

In the meantime, CH2M Hill Hanford Group and DOE are looking at ways to reduce the risk of contamination spreading toward the ground water. That could include building up to five of the temporary caps over contaminated sites, but CH2M Hill also is investigating some less costly options to protect spill sites.

The initial test cap was built at Hanford's oldest group of tanks, T Farm. It's home to Tank T-106, built during World War II to hold waste from T Plant, which chemically separated plutonium from irradiated fuel rods for the nation's nuclear weapons program.

In 1973 the tank leaked an estimated 115,000 gallons of waste, including uranium, chromium nitrate and radioactive technetium, which moves easily with water. The plume has spread about 75 yards around the tank and 90 feet below it to within about 100 feet of the ground water.

The area already had been surrounded with berms and curbs to keep water from running into the low point where tanks were buried. Workers have built up the soil in the center of the octagon-shaped cap to about 2 feet tall in its center using soil placed on a conveyor belt to carry it from an uncontaminated area over a wire fence and into the tank farm.

The dirt layer was covered with a synthetic fabric, then sprayed with polyurea plastic. It's similar to the substance used to line pickup beds, but the quarter-inch layer of plastic will be more chemically resistant and longer wearing. It's designed to be tough enough to allow trucks to drive across it as work continues to empty Hanford's 149 single-shell tanks.

The cap covers not only T-106, but also part of nine other tanks with capacities of up to 530,000 gallons. That required building it around numerous risers that provide access to the underground tanks and around monitoring stations for checking moisture movement.

The cap is "heavily instrumented," said Moses Jaraysi, CH2M Hill vice president for environmental programs. Sensors in the soil will provide data to see how dry the cap keeps the soil through different seasons for the next two years.

The sensors may not see a season with weather as bad as the one in which workers built the cap, however. Work started in October with plans to finish in a month or two rather than this spring. But an unusually cold and snowy winter plus days of high wind slowed work to spray on the plastic.

Although final costs of the project are not in, it will be more than the $2 million estimated, according to CH2M Hill. Not only did bad weather add to the cost, but earthwork also was more costly than estimated and more plastic was needed after it soaked into the fabric layer.

However, changes to the design of the soil portion of the cap and the use of a felt fabric that absorbs less spray should bring the cost of additional caps in line with original estimates, Jaraysi said.

The cap is intended to last at least 25 years and eventually can be cut and moved out of the way for soil cleanup.

However, CH2M Hill also is looking at some less costly options for keeping water off key spill sites in the tank farm. That could include bringing in large tents that are sturdy enough to withstand the Mid-Columbia winds, Jaraysi said.