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Unless limited by rule, an owner or operator may choose to prepare a Tier 3 site assessment as an alternative to completion of a Tier 2 Site Cleanup Report or Corrective Action Design Report. Prior to conducting a Tier 3 site assessment, a workplan must be submitted to the department for approval. The workplan should follow an outline similar to a scientific experiment and should include an introduction, description of methods and models to be used, discussion of risk classification, summary and references.
The Tier 3 workplan may include a proposal for additional site assessment, the use of probabilistic evaluations, or more sophisticated chemical fate and transport models. Calibration of the IDNR Tier 2 model can also be considered with justification. Following approval of the workplan, a Tier 3 assessment report will be required within a reasonable time designated by the department.
The Tier 3 assessment report must include a recommendation for site classification as high risk, low risk or no action required. If a corrective action is required, the Tier 3 report must provide an outline of possible corrective actions technologies and a recommendation for implementation of a remediation technology which is consistent with the standards and policies underlying the department's risk classification and corrective action response rules.
The most commonly used three-dimensional (3D) computer software packages, Groundwater Modeling System (GMS) and Visual MODFLOW, plus the two-dimensional (2D) software package, BIOPLUME III were selected to be evaluated for the purpose of the Iowa RBCA Tier 3 analysis. The 3D packages contain the well-established groundwater modeling codes, MODFLOW, MT3D, MT3DMS, and RT3D. The specific features of each of the packages are described and compared. In addition, general data requirements and modeling procedures to use these codes and packages are discussed.
A comparison of the three software packages suggests that GMS is superior to Visual MODFLOW and BIOPLUME III because (1) GMS does everything Visual MODFLOW and BIOPLUME III do and more, and (2) GMS is better documented and more users’ friendly. Other factors may have to be considered when selecting a software package. The license prices and training costs for GMS and Visual MODFLOW are similar and excellent technical support are provided. The software BIOPLUME III is free but no technical support and training course are provided by the software developer. All three software can be run on a Pentium PC with 32 MB RAM and SVGA monitor but only GMS has a UNIX version which can be run on a work station.
This document aims to provide guidance for consultants who contemplate using a numerical modeling program such as Visual MODFLOW or GMS. The guidance stresses how one should approach a modeling project and how to report the results. The need to include in a report all details of the multi-step modeling process is treated. Three examples of numerical modeling of petroleum releases are provided from the western Iowa towns of Climbing Hill, Ida Grove, and Sioux City. The examples are not complete modeling reports of the type described in the text, but are included to show the degree of detail needed to accomplish such a project and to demonstrate validity to IDNR.
Guidelines to Determine Site-Specific Parameters for Modeling the Fate and Transport of Monoaromatic Hydrocarbons in GroundwaterThe objective of the guidelines report is to provide guidelines for determining site-specific parameters to model the fate and transport of dissolved groundwater pollutants. Ground water professionals will find this report helpful in cases where tier-3 assessments need to be conducted, and when additional site characterization is required to inform a choice about corrective action or remediation system design. This report will emphasize modeling the fate and transport of benzene. This decision is based on the facts that benzene is the most toxic of the soluble components of gasoline, is more mobile than the other aromatic hydrocarbons, and is subject to the strictest cleanup standards because of its carcinogenic properties. Thus, benzene concentrations often determine the need for remedial action at LUST sites. Nevertheless, the fundamental principles and procedures discussed herein are applicable to other groundwater contaminants commonly associated with petroleum product releases.