ESTIMATING AND COMMUNICATING UNCERTAINTY FOR A 3-D GEOLOGIC MAP IN EAST-CENTRAL ILLINOIS

A 3-D geologic map of the Quaternary sediments above bedrock in an area of East-Central Illinois was developed as part of a long-term groundwater management strategy. Initial use of the 3-D map was to delineate aquifers and guide parameterization decisions for a groundwater flow model. An uncertainty assessment was made to support future management decisions and provide insight on the reliability of the groundwater flow model. Goals of this assessment were to estimate uncertainty and overall reliability of the map units, and to identify specific uncertainty issues relevant to the flow modeling results, using methods that maximized reliance on geologic insights gained in the mapping process. Three sources of uncertainty were evaluated and estimates made of their combined effect on uncertainty and reliability. These sources included: errors and imprecision within input data; sediment and map unit complexities; and variations in the spatial distribution of the input data.

Data error analysis found some errors in borehole land-surface elevation values and errors/imprecision in some drillers’ lithologic descriptions to be significant enough to impact the local accuracy of the maps. However, these sources of error were known to the geologists during mapping, were partially accommodated in the definition of the map units, and integrated into the maps in a way that was difficult to separate. Results from evaluations of sediment and map unit complexities and spatial distribution analysis were considered together to provide a rating of the reliability of the maps and a quantitative estimate of uncertainty of map unit distributions. Uncertainty was estimated for areas of typical data density, and for areas where data density was lower and, therefore, uncertainty proportionally higher. Cartographic solutions were applied to communicate estimated uncertainty and reliability on various lithostratigraphic and interpretive maps.