DRASTIC Hydrogeologic Settings Modified for Fractured Till: Part 1. Theory

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The ground water vulnerability assessment model, DRASTIC, has been modified to better evaluate the effect of fractured till. In the mid-1980s, the Ohio Department of Natural Resources (ODNR) Division of Water began statewide, county-by-county mapping of the potential for ground water pollution. Eventually it was recognized that the original DRASTIC methodology needed to be modified to incorporate the concept of double-block porosity and preferential flow through Ohio’s fractured glacial tills. Glacial till was eventually recognized as a unique vadose zone media, and different ratings were assigned to the various till lithologies. It was determined that thin, weathered, highly-fractured tills should be more highly rated by increasing the rating of “R” Net Recharge and “I” Impact of the vadose zone media, where appropriate. In rare instances, the ratings of very thin soils (“S” Soil media) were modified to reflect the nature of underlying parent materials. In contrast, extremely thick sequences of unweathered till were given lower ratings for “R” Net Recharge and “I” Impact of the vadose zone media. DRASTIC maps have been completed for 76% of the 88 counties in Ohio. With the advent of Geographic Information Systems (GIS) applications, compiling a county DRASTIC map has become faster, and publication costs have been significantly reduced. GIS provides the tools to review and quickly modify historical mapping efforts that predate the fracture modification. This paper reviews the history of DRASTIC mapping in Ohio, presents the theory of modifications for fractures, and includes some discussion of Ohio regulatory applications.


Author Institution: Bennett & Williams Environmental Consultants Inc., Columbus, OH
Author Institution: Dept. of Food, Agricultural and Biological Engineering, The Ohio State University, Columbus, OH
Author Institution: Ohio Dept. of Natural Resources, Division of Water, Columbus, OH
Author Institution: Bennett & Williams Environmental Consultants Inc., Columbus, OH



The Ohio Journal of Science, v106, n2 (April, 2006), 45-50.