Nineteen Year Water Balance at an Engineered Surface Barrier.

The evapotranspiration (ET) surface barriers or covers have been used to isolate underlying waste zone and to protect the environment. ET barriers rely on two natural processes to control water movement into the underlying waste zone: (1) water is stored in the soil, which acts as a natural reservoir for precipitation, and (2) water is released from soil to the atmosphere via natural evaporation from the soil surface and transpiration from the plant community. The Prototype Hanford Barrier (PHB) was constructed in 1994 with an area of 2.5 ha on an existing waste site and its performance was monitored from 1994 to 2013. This barrier is a field-scale evapotranspiration (ET) barrier atop a capillary break.  The basic concept of an ET barrier is to store all precipitation in the ET barrier (or layer) where it can be released later via soil evaporation and plant transpiration such that drainage is always less than the design criterion.  In this design, the ET layer consists of a 2-m-thick layer of silt loam.  The 19-year monitoring results show that the silt loam layer was recharged in the winter season (from November to March) and the ET process was sufficiently strong to release the summer precipitation and the stored water in the 2-m-thick barrier to the atmosphere even before the end of the summer season (from April to October). Based on the monitoring results, empirical relationships were established among precipitation, barrier water storage, and ET. The findings are useful for predicting water storage and ET under different precipitation conditions and in the design of future barriers.