A Hydrogeologic Database for Ground-Water Modeling

A new hydrogeologic database, the HGDB, was developed from a national survey of National Water Well Association (NWWA) members. The database contains general hydrogeologic information from 400 field site investigations across the country and detailed statistical summaries of five ground-water parameters: hydraulic conductivity, seepage velocity, hydraulic gradient, saturated thickness, and depth to top of aquifer. The HGDB was developed to verify and expand statistical distributions used in a Monte Carlo ground-water model developed by EPA for land disposal regulation (Federal Register, 1986,1988). The database structure is a unique application of the aquifer classification method used in the NWWA's DRASTIC system (Aller et al., 1987). Respondents were asked to classify their aquifers as one of 111 different DRASTIC hydrogeologic settings, and 12 groupings of settings were analyzed to produce statistical distributions of hydrogeologic data based on site geology and geomorphology. Three examples of the hydrogeologic groupings are coastal beaches; alluvial basins, valleys and fans; and outwash settings. The HGDB can be used for several purposes. First, the HGDB results indicate that the EPA's distributions of seepage velocity and hydraulic conductivity used in the land disposal model are sound. These are the most important hydrogeologic parameters in the model. The HGDB goes a step further, and provides a set of statistical distributions that can be used to make the land disposal regulations more site-specific than the national approach now being used. Finally, the HGDB data can be used for general site characterization and for educational purposes. The database is available as a detailed written report and spreadsheet file from the American Petroleum Institute, and is contained in a graphical computerized decision support system for ground-water modeling called OASIS. The HGDB serves as a framework for organizing hydrogeologic information from different site investigations and can be expanded easily beyond the 400 sites now in the database.