This report summarizes the results of a two year project conducted by the Minnesota
Geological Survey to map the Upper Cambrian St. Lawrence Formation and investigate its
hydrologic properties in the Twin Cities Metropolitan area (TCMA). Funding was provided by
the Minnesota Department of Health. Final products are a map delivered in electronic format that
can be used with Arcview 3.2 GIS software, and this informal report.
Our hydrogeologic study indicates that the St. Lawrence Formation commonly has a
moderate to high horizontal hydraulic conductivity across all of the study area. In conditions of
shallow burial beneath younger bedrock it is most similar in the development of secondary pores
and measured hydraulic properties to fractured carbonate rock aquifers. Discrete intervals with
secondary pores have a high horizontal hydraulic conductivity whereas rock between these
intervals are orders of magnitude lower in conductivity.
The properties of the St. Lawrence Formation in a vertical direction are not as wellunderstood,
but available data are consistent with the traditional classification of the formation as
an aquitard. However, the integrity of the formation as an aquitard in a vertical direction,
particularly under conditions of shallow burial such as where it is uppermost bedrock, has not
been rigorously tested, and may be markedly variable across the TCMA. This is chiefly because
vertical fractures play an important role in determining aquitard integrity, and the distribution of
such fractures is poorly understood. Suggestions for additional research that might lead to a
better understanding of aquitard integrity are made at the end of this report.
Minnesota Geological Survey (MGS) Open File Report 06-04 (pdf report) and Open
File Report 06-05 (ArcView apr and files for map and map pdf)
Runkel, A.C.; Mossler, J.H.; Tipping, R.G.; Bauer, E.J..
A Hydrogeologic and Mapping Investigation of the St. Lawrence Formation in the Twin Cities Metropolitan Area.
University of Minnesota.
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