Investigation of liquid instrusion in 241-BX-103

Although SST 241-BX-103 is classified as being interim stabilized and intrusion prevented, FIC readings have indicated the tank's surface level to be increasing in a 'step like' fashion even after the completion of intrusion prevention activities. This letter report discusses the finding of an investigation conducted to (i) determine if liquid intrusion (rather than other potential causes such as a change in the waste's physiochemical properties, instrumentation error, etc.) was responsible for the increase in the surface level and if so, (ii) recommend a likely course of action to pursue if the tank's supernate volume is exceeding the interim stabilization criterion. Engineering judgement (supplemented by analysis of historical tank data, tank video and photographs) was used to determine the extent to which the rise in the surface level could be attributed to various types of liquid intrusion and/or other phenomena. It was concluded that the rise in BX-103's surface level was indeed real (and most likely caused by liquid intrusion), and that the tank's supernate volume was exceeding the interim stabilization criterion of 5000 gallons. However the tank's surface level has remained fairly constant since August 1993. It was recommended that tank BX-103 be left 'as is' for the moment and continued being monitored. Pumping of BX-103 was recommended only under one of the following circumstances: (i) the tank leaks, (ii) the tank is once again subject to significant liquid intrusion in the future, or (iii) it is decided that all supernate be removed from a SST prior to retrieval of the tank's solid waste