Although sinkhole development in susceptible areas cannot be completely prevented, policy makers and the public can use geologic maps that delineate karst features to develop strategies that can minimize or avoid property damage and personal injuries.
Defining the Problem
Sinkholes, which abound in the Frederick Valley in west-central Maryland, impact urban growth and development (Fig. 1). Sinkholes form in carbonate areas as the dissolving and weakening of bedrock cause it to collapse. Activities, such as quarrying, which alter surface drainage and lower the water table also can decrease ground strength and exacerbate the sinkhole hazard. It is important to assess potential risks of infrastructure damage and personal injury due to sinkhole formations.
Figure 1: Sinkholes in collapsed parking area, Frederick, Maryland. Sinkholes form in carbonate areas as the dissolving and weakening of bedrock cause it to collapse. Credit: D.K. Brezinksi
The Geologic Map
The geologic map of the Frederick Valley shows the presence of two limestone formations, the Frederick and Grove (Fig. 2). At the formational level, there is no discernible difference in sinkhole proclivity between the two units. When the two formations are subdivided into lithologically distinct parts, the increased detail allows the geologic map to be used as a predictive tool for potential sinkhole development.
Figure 2: On this geologic map of the Frederick, Maryland area, note the prevalence of sinkholes (circles with hachures) in the Lime Kiln Member of the Frederick Formation. Credit: D.K. Brezinski
Applying the Geologic Map
The map of bedrock units and sinkholes demonstrates the correlation between sinkhole distribution and rock type. Table 1 shows that most sinkholes are present in the upper part of the Frederick Formation, contradicting the long-held belief that the Grove Formation is the unit most susceptible to sinkhole development in the Frederick Valley. An increased level of detail in both the lithologic description and the mapping practices facilitates the evaluation of the susceptibility of these geologic units to sinkhole formation. By subdividing and mapping units as precisely as possible, and accurately locating sinkholes with a Global Positioning System (GPS), geologists are able to develop a new tool, the susceptibility index (SI), that portrays the relative sinkhole propensity for each geologic unit (Fig. 3). Planners and developers can use the SI as a comparative tool to evaluate the likelihood of sinkhole occurrence in areas considered for development.
Table 1: Number of sinkholes occurring in different rock types as revealed by recent geologic mapping. Credit: Brezinski, Reger, and Baum, Data: Brezinski and Reger, 2002
Sinkholes and other karst features represent one of the most widespread and underevaluated geologic hazards in carbonate terrains. Geologic maps are the principal tools for displaying and conveying data important to understanding reasons for sinkhole distribution. Although sinkhole development in susceptible areas cannot be completely prevented, policy makers and the public can use geologic maps that delineate karst features to develop strategies that can minimize or avoid property damage and personal injuries.
Brezinski, D.K., and Reger, J. P., 2002. Stratigraphy-karst relationships in the Frederick Valley of Maryland, in Kuniansky, E.L. (ed.), U.S. Geological Survey Karst Interest Group Proceedings, Shepherdstown, West Virginia, August 20-22, 2002. U.S. Geological Survey Water Resources Investigations Report 02-4174, p. 59-65.
Southworth et. al., 2007, Geologic map of the Frederick 7.5-minute quadrangle, Maryland. Maryland Geological Survey, scale 1:24,000 (digital).
Case study authors: David K. Brezinski, James P. Reger, and Gerald R. Baum (Maryland Geological Survey)
Case study from: Thomas, W.A. 2004. Meeting Challenges with Geologic Maps, p. 36-37. Published by the American Geosciences Institute Environmental Awareness Series. Click here to download the full handbook.