17 D1. Caves, Karst, and the Hydrogeology of Fractured Bedrock Aquifers

Monday, 18 May 2020
1:30 PM - 5:30 PM
Lake Superior Ballroom MN

USING TRACERS AND BOREHOLE CHARACTERIZATION TO UNDERSTAND FLOW AND RECHARGE IN CRYSTALLINE ROCK

Paper number: 17-2
Program: Technical Sessions
Time: 1:35 PM – 1:55 PM

Authors

Abstract

In Minnesota, glacial and sedimentary aquifers provide the majority of groundwater to end users. In areas of northern Minnesota, where glaciogenic deposits are thin and patchy, the principle aquifers are crystalline rocks that generally have low matrix porosity and permeability. Recent characterization of wells completed in 2.7 Ga Archean schist and granite-rich migmatite near Ely, Minnesota using a combination of stable isotope sampling, continuous conductivity monitoring, and downhole well investigations sheds light on recharge characteristics and groundwater flow. Collection of discrete stable isotope samples, snowpack samples, and continuous conductivity began prior to springtime thaw. Following peak snowmelt, precipitous drops in conductivity with corresponding distinct pulses of more negative δD and δ18O showed recent snowmelt recharge. Downhole well characterization at three wells onsite showed steeply dipping fracture apertures of approximately 15 mm or narrower that were commonly sealed with secondary minerals; below approximately 46 meters (150 feet) fracture density was diminished.

In 1.1 Ga Mesoproterozoic igneous bedrock near Finland, Minnesota robust environmental investigations by the United States Air Force began in 1989 (MWH Americas, 2006). Dye tracing and fractured rock characterization beyond what is typically conducted at sites in Minnesota found significant fracture zones in the upper 15 to 24 meters (50 to 80 feet) of bedrock with apertures ranging from 0.1 to 6 mm (MWH Americas, 2006). Fracture density and aperture width decreased with depth. Additional fracture zones were found at geologic contacts and faults (MWH Americas, 2006). Dye tracing showed that contaminants travel downward into a shallow fractured rock system and move laterally to a series of seep locations along the midsection of Lookout Mountain.

In each of these investigation areas, multifaceted characterization has shown that shallow fractures in crystalline rock play a key role in groundwater recharge, flow, and contaminant transport.

MWH Americas, Inc., 2006. Final Remedial Investigation Report – Former Finland Air Force Station, Finland, Minnesota, 8056 p.

Geological Society of America Abstracts with Programs. Vol. 52, No. 5

doi: 10.1130/abs/2020NC-347082

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