APPLYING SUBSURFACE STRUCTURAL AND STRATIGRAPHIC ANALYSIS TO ASSESS CONTAMINATION HAZARDS FOR REGIONAL AQUIFERS

Migration of fluids are commonly handled differently in subfields within the geosciences. In petroleum systems, bedrock evaluation, isopach stratigraphic models, and fault throw estimation are key tools for assessing reservoir quality. Because of the similarities between petroleum systems and groundwater/well-head protection investigations, there is significant opportunity to retask tools and approaches between these two specialties.

In southwest Missouri, horizontal to subhorizontal strata are offset by small displacement (throw ranging from 3-55 meters) normal faults. Stratigraphic units also contain karst heterogeneities in carbonate units, which dominate the stratigraphy. The Mississippian Northview Formation contains shale with minor interbedded siltstone that varies between 0 and 27 meters and acts as a regional confining unit for the underlying Ozark aquifer. In recent years, the connection and potential for surficial contaminants to infiltrate the Ozark aquifer through the Northview Formation along major normal faults has been of concern to state and local officials.

To assess the risk of migration through the Ozark Confining Unit along faults, we present (a) isopach thickness maps constructed from publicly available well data and (b) maximum fault throw models based on geologic maps and field investigations. Using these two map products, a fault migration risk map was created by subtracting the maximum fault throw from the Northview Formation thickness map to identify areas where fault offset is insufficient to allow downward migration through faults (Northview thickness exceeds fault throw), and therefore there is a low risk of contamination into the Ozark aquifer from the surface or near surface. High risk areas are identified by zones where Northview thickness does not exceed maximum fault throw and, therefore, surficial contaminants could conceivably travel downward along fault conduits into lower stratigraphic units. We propose this decision-making tool may improve site-by-site evaluation of the potential for vertical migration via faults and allow for more focused remediation techniques to fit the actual risks.