DEEP IMPACTED GROUNDWATER SOURCES: COAXING OUT SIGNALS USING MULTIPLE TRACERS

Fractured rocks present challenges when investigating contaminated groundwater deep in these systems. Work at abandoned mine sites in the Rocky Mountains using injected tracers reveals both shallow and deep(?) groundwater pathways with rapid (> 10 m to 100 m/day) flow components. Even though very useful, they can often be difficult to recover in mine portal discharges. They are also subject to permafrost or snow/ice storage. These tracers do of course typically migrate rapidly, but along the steepest natural gradients to springs, presumably via the shallow weathered bedrock. However, injected tracers alone cannot evaluate the inaccessible (impacted) deeper groundwater. It is obvious that multiple tracers are needed to do this, because, unlike many hazardous waste sites where sources are known, mine sites have mostly hidden ore bodies and impacted groundwater sources that discharge and impact the environment. Many of these settings are often assumed to contain only ancient groundwater at depth. These rocks weather incongruently (e.g., feldspars to clays) producing a saprolite, altered upper bedrock and relatively insoluble deeper bedrock. A simple assumption would be that the deeper bedrock would be less subjected to alteration or weathering, but fractures have a low surface area coefficient and alteration tends to extend as well as widen them producing a positive feedback loop that promotes development of longer, deeper more complete flow paths. Thest were possibly formed early in the landscape and have remained intact by discharging abundant meteoric water. Using uranium-series disequilibrium and other isotopic methods suggests that some deep pathways could contain mixed rapid and slow-flow components, and suggest both "young" and "older" deep ground waters discharge during the snowmelt. It may be that geologically old but connected groundwater systems in hard rocks are common.