GEOLOGIC CONTROLS ON CIRCULATION DEPTHS, GEOCHEMICAL KINETICS, AND RESIDENCE TIMES OF MOUNTAIN GROUNDWATER SYSTEMS (Invited Presentation)

Mountain groundwater processes reflect their tectonic history and geologic setting. Uplift and volcanism impact the occurrence, distribution, and interconnectivity of porosity with increasing spatial scale and depth in mountain bedrock aquifers. This, in turn, affects the overall storage that is available in mountain bedrock aquifers. These stores of groundwater are essential to sustaining flow to springs and baseflow in streams. Changes in these groundwater systems can have adverse affects on mountain aquatic ecosystems and communities. Despite the importance of mountain groundwater, questions regarding the geologic controls on mountain groundwater processes remain unresolved. For example, field estimates of groundwater circulation depths in mountain systems remain lacking. As a consequence, there is very little data available on circulation depths to: 1) critique, assess, and refine hydrogeological models, 2) inform and improve geochemical models, and 3) improve our understanding of residence times and response times of mountain groundwater systems. Circulation depths are commonly described using relative terms such as “shallow” or “deep” since, in the majority of cases we do not have sufficient data to accurately describe these depths. This poses additional problems with biologists and microbiologists since relative descriptions of circulation depths may not always be sufficient to define critical relationships between depth dependent processes (e.g., geochemical kinetics, temperature/geothermal heat transfer, and residence times) and the structure of biological and microbiological communities. Here I present a synthesis of recent research on mountain groundwater systems in NM, CO, AZ, CA, and NV. Springs were used in each of the studies to better understand mountain groundwater processes including: geochemical kinetics and spatial trends in geochemical evolution, scaling of residence times, and circulation depths. The following questions are addressed: How do groundwater circulation depths vary with geologic and tectonic setting? How do circulation depths impact geochemical kinetics and residence times of groundwater? How do these findings affect our evolving conceptual models of mountain groundwater systems? Important knowledge gaps will be identified and discussed.