USE OF HISTORICAL MAPS TO EVALUATE AN URBAN SINKHOLE INCIDENT

Greene County, Missouri sits atop a 1000+ foot thick sequence of essentially flat-lying carbonate sedimentary rocks. Home to Springfield, Missouri's third largest city, the county has documented over 1400 sinkholes in its roughly 675 square mile area. Although the Mississippian-aged Burlington-Keokuk Limestone is the surficial unit over about two-thirds of the total county area, over 90% of the known sinkholes are mapped as occurring in the Burlington-Keokuk Limestone. With approximately 300,000 residents, many businesses and homeowners have first-hand experience with sinkholes and subsidence. This experience results in frequent calls to university and government geoscientists requesting evaluation of the subsidence and an assessment of ongoing risk.

In one recent incident, initial field examination of a sinkhole was inconclusive in determining a probable reason as to why the sink formed where it did and therefore, an inability to provide meaningful assessment of ongoing risk. However, discussions with the homeowner regarding seepage into the home's foundation and basement level prompted additional analysis of the situation. During additional investigation, it was discovered that a freshwater spring known to the homeowner was marked on historic USGS topographic maps of the area, but had been omitted from the most recent USGS topographic maps (apparently when "urbanized area" shading had been added to the map to reflect the urban growth of the residential subdivision). Development of the sinkhole and the home's seepage issues appeared to be directly related to the location of the spring and a more meaningful assessment of ongoing risk was provided to the homeowner.

This project illustrates the value of historical information during the evaluation of sinkholes and other karst features. Details of the field investigation, the map investigation, and ongoing site activities will be presented and described.