FRACTURE STRATIGRAPHY LINKED TO DEPTH-DISCRETE MULTILEVEL BOREHOLE MONITORING IN LOWER PALEOZOIC BEDROCK OF THE CENTRAL MIDCONTINENT, NORTH AMERICA

Recent fracture mapping in exposures of relatively undeformed lower Paleozoic bedrock in the central midcontinent of North America has led to the recognition of stratigraphically controlled, and thus predictable, preferential termination horizons (PTHs) that can impede vertical groundwater flow. Concurrently, we have monitored boreholes with multilevel systems (MLSs) constructed across stratigraphic intervals equivalent to those studied in outcrop. MLSs with meter (or less) spacing of depth-discrete monitoring provide hydrogeologic data (e.g., hydraulic head, hydraulic conductivity and chemistry) at the resolution necessary to recognize flow-related characteristics dictated by the fracture patterns expressed in outcrops.

One project targets the Ordovician Platteville Formation, an ~8 m thick, fractured carbonate rock with widespread contamination in Minnesota’s Twin Cities metropolitan area. We collected temperature and pressure data at discrete (<1m) intervals in several boreholes, some at distances less than 500 meters from an outcrop where fractures are characterized in three dimensions. Inflections in hydraulic head profiles from these boreholes correspond to PTHs identified in the outcrop. This information is combined with other borehole data (e.g., geophysical logs, ambient borehole flow and chemistry) to produce a conceptual model of the Platteville hydrogeologic system.

A key outcome was identification of predictable low permeability layers within the Platteville Formation that can hinder vertical transport of contaminants. The presence of these layers means that conventional techniques for monitoring and remediating contaminant plumes are not as effective as presumed. For example, conventional sampling of water and measurement of hydraulic head elevation in monitoring wells open to as little as a few meters of the Platteville can be representative of a blend of two or more distinct water chemistries from intervals with hydraulic head elevations that can differ by meters. The relevance of our results applies to all fractured sedimentary rock aquifers and aquitards. We therefore encourage greater application of more rigorous site characterization techniques such as the inexpensive and efficient methods we used to improve conceptual site models and long-term monitoring.