Identification of Application-Specific Critical Inputs for the 1991 Johnson and Ettinger Vapor Intrusion Algorithm

摘要

At sites where soil or ground water contains chemicals of concern, there is the potential for chemical vapors to migrate through the subsurface to nearby basements, buildings, and other enclosed spaces. The 1991 Johnson and Ettinger algorithm and subsequent refinements are often used to assess the significance of this pathway and to establish target cleanup levels. To facilitate its use, the U.S. EPA distributes spreadsheets programmed with the 1991 Johnson and Ettinger algorithm. These user-friendly spreadsheets make the equations more accessible; however, the U.S. EPA spreadsheets require a large number of inputs (> 20), and as a result, relationships between model inputs and outputs are not well understood and users are not able to identify and focus on the critical inputs. The U.S. EPA spreadsheets also allow users to inadvertently enter inconsistent and unreasonable sets of input values, and these often lead to unreasonable outputs. The objective of this work, therefore, is to help users develop a better understanding of the relationships between inputs and outputs so that they can identify critical inputs and also to ensure reasonableness of inputs and outputs. The 1991 Johnson and Ettinger algorithm is introduced, and differences between it and its U.S. EPA spreadsheet implementation are identified. Next, results from a parametric analysis of the algorithm are used to create a flowchart-based approach for identifying the application-specific critical inputs. Use of the flowchart-based approach is then illustrated and validated through comparison with the results of a more traditional sensitivity analysis for four scenarios. Recommendations are also given for the reformulation of inputs to minimize misapplication of the algorithm and the spreadsheets, and reasonable ranges for reformulated input values are discussed.