An innovative interim surface barrier was constructed as a demonstration project at the Hanford Site's TY Tank Farm. The purpose of the demonstration barrier is to stop rainwater and snowmelt from entering the soils within the tank farm and driving contamination from past leaks and spills toward the ground water. The interim barrier was constructed using a modified asphalt material with very low permeability developed by MatCon{reg_sign}. Approximately 2,400 cubic yards of fill material were added to the tank farm to create a sloped surface that will gravity drain precipitation to collection points where it will be routed through buried drain lines to an evapotranspiration basin adjacent to the farm. The evapotranspiration basin is a lined basin with a network of perforated drain lines covered with soil and planted with native grasses. The evapotranspiration concept was selected because it prevents the runoff from percolating into the soil column and also avoids potential monitoring and maintenance issues associated with standing water in a traditional evaporation pond. Because of issues associated with using standard excavation and earth moving equipment in the farm a number of alternate construction approaches were utilized to perform excavations and prepare the site for the modified asphalt
