John L. Wilson, New Mexico Institute of Mining and Technology
This project will attempt to measure, for the first time, hyporheic exchange at the margin of a karst conduit (cave below the water table). Hyporheic exchange in surface streams is a relatively new field, but related publications are growing exponentially as the implications for flow contaminant transport, residence time distributions, biogeochemistry and biology are realized. One definition of the hyporheic zone is the subsurface volume where water enters from the stream, travels through the subsurface, and returns to the stream. Hyporheic zones in streams exist at a variety of nested spatial and temporal scales whether there is net gain, net loss, or even no net exchange along the course. The fundamental fluid dynamics of karst conduits are not different from streams, and karst conduits have the same features that cause hyporheic flow in fluvial systems. This project will investigate whether, as with streams, hyporheic exchange occurs in karst conduits, regardless of larger-scale net exchange with the aquifer. Karst-conduit hyporheic exchange, and related biogeochemical processing, then becomes a fundamental component of the karst water cycle. The project will be conducted in a phreatic conduit field site in the unconfined Floridan Aquifer; additionally there will be a nearby air-filled analog site that was exposed to similar processes at some time in the geologic past. Cores will be taken from the phreatic and analog sites. Porosity, permeability, and petrophysical measurements will be used a proxy indicators of past conduit hyporheic flow. The core holes at the phreatic site will be equipped with multilevel samplers and injectors, and a series of dye traces will be conducted to observe karst-conduit hyporheic flow. Modeling will be used throughout the project, first to help plan the field work and then to synthesize the data. Results from the data and models will give insights into the temporal and spatial scales of hyporheic flow. The project will end with modeling as a predictive tool to generalize our results.
Karst aquifers supply water to 25% of the United States, and almost all water to some regions, e.g. 90% of Florida's population. New mathematical models and field studies will be used to illuminate a previously unrecognized process, karst hyporheic exchange at the margin of a flowing karst conduit, in which the conduit and surrounding karst rock matrix exchange water with consequences for karst chemistry and biology. As does hyporheic exchange in surface streams, karst hyporheic exchange may impact water supply, water quality, and ecology, and will be of interest to environmental agencies and interest groups at the local, state and federal level.
