Collaborative Research: Geophysical characterization of a karst aquifer using dynamic recharge events

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Karst aquifers are important water resources, providing water for up to a quarter of the world?s population. These aquifers are complex hydrogeologic systems, where flow and transport predominantly occur through preferential flow paths or conduits that range in size from cm-scale openings to passages much larger than required for human access (caves). Despite their hydrologic importance, the location of karst conduits and characteristics of the larger aquifer are typically poorly constrained. To address these problems, we will monitor recharge-induced responses that arise as water flows into the subsurface in a karst aquifer in Florida using geophysical instrumentation to characterize the conduits, subsurface flow, and the larger karst system. The noninvasive remote sensing investigation will generate knowledge for direct societal benefit, including tools to improve the creation of karst hazards maps that highlight areas susceptible to sinkhole formation, the identification of preferential flow paths relevant to contaminant transport, and the development of a methodology to determine subsurface flow for water supply considerations. We will develop a temporary, interactive traveling exhibit in collaboration with the National Cave and Karst Research Institute that will frequent visitor centers near the field site in Florida and in New Mexico to increase public engagement and scientific literacy about karst hydrogeology and environmental seismology. The project will also contribute to the education of the next generation of the scientific workforce through the involvement of two graduate students at New Mexico Tech, an undergraduate student at the University of Florida, two undergraduate student interns through the IRIS undergraduate student intern program, and two early career scientists and a mid-career female scientist in an EPSCoR state. Finally, collected datasets will be incorporated into lecture material and homework sets of undergraduate and graduate courses at New Mexico Tech, which is a Hispanic-serving institution.A previous pilot study demonstrated the generation of seismic signals during artificial recharge experiments and a natural recharge event in a karst aquifer. These and other geophysical signals are caused by processes during flow through karst aquifers and includes pressure pulses generated as recharge enters conduits with full pipe flow, pore pressure changes in fractures and the rock matrix, mass loading induced subsidence due to changes in water mass, and turbulent interaction of flow with the wall rocks. To capitalize on the information content provided by these signals, we will use simultaneous, large-scale observations of recharge events at two co-located geophysical sensor networks to characterize the conduit network, flow processes within this network, and material properties of the larger karst system. Seismometers, tiltmeters, and other instruments will be deployed at the Santa Fe River Sink-Rise system in Florida to observe karst recharge events over a two-year period. This field site has a well-constrained conduit network, and thus it permits verification of the interpreted geophysical signals that arise from hydrologic processes. This transformative project will not only enable delineation of the karst conduit network, but will do so while providing a regionally integrated analysis of the karst aquifer flow system based on deformation observations on the timescales between fractions of a second to months. As recharge activates new flow paths, geophysical monitoring enables extensive 3D characterization of the dynamics and evolution of flow during recharge-induced changes in the aquifer. Furthermore, the signals will help to determine the architecture in the subsurface between the conduits and the surface, advancing knowledge of critical zone environments, for example, by specifically determining soil and regolith thicknesses and depths to the soil-rock interface. The young field of environmental seismology encompasses studies of a range of Earth surface processes, and this project will use and expand the respective methods to transform the understanding of karst aquifers across the full frequency range of deformation and develop techniques in preparation for a future karst critical zone observatory.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.