HYDROGEOLOGY IN KARST AND FRACTURED ROCKS: AN OLD PROBLEM REQUIRING THE HELP OF DEEP THOUGHT

Investigations in fractured-rock settings involve delineating groundwater basins. This assumes that all discharge locations can be located nearby, which could be wishful thinking. Injected tracers have been typically used for sites that are difficult to monitor by using wells alone, typically only at the local scale. If actual basin dimensions are at the regional scale it is essential to understand the geology and enhance this with hydrological, geochemical and isotopic data, i.e., use multiple tracers. For example, the Middle Ordovician (upper Knox Group) unconformity was a karst surface, that was buried and is now again subaerially exposed. These Lower Paleozoic sediments extend across the entire US midcontinent, and still do at depth today. Evidence of fresh groundwater is abundant in these carbonates and large segments of this deep (1.5 - 4 km) flow system are still active. Discharge is likely to be at considerable depth in the Mississippi River valley. From Oak Ridge, TN, that is some basin to delineate! In the U.S. Rocky Mountains, when investigating abandoned gold and silver mines (a sort of karst) except for some of the discharges, the challenge is finding pathways and monitoring flowing groundwater. Mine workings make up a tiny fraction of overall porosity, just like caves in carbonates. Using multiple tracers is essential. Interestingly, there is also evidence of deep flow here (in granitic gneiss, quartz monzonite). Many of these discharging waters also have meteoric signatures despite having isotopic U-series fingerprints of deep flow (low U conc, AR ~ 1) through the mountains. Deep circulation of meteoric water, at a variety of groundwater velocities in so many settings (many non carbonate) should force a rethink of geologically recent surface-driven weathering, include dissolution (?) and thus development of deep preferential flow paths. Surely this requires considerable geological time? Worthington (1991) shows flowpaths in carbonate aquifers must form very early. Lowe and Waters, (2014) suggest that karstification originates far back in the geological history of the rock at depth. Caves in Carboniferous rocks in South Wales have have MVT Pb-Zn veins in walls, coastal outcrops are covered by fragments of Triassic rocks, and suggest again, paleokarst reborn.