DEVELOPMENT AND IMPLEMENTATION OF TRACERS FOR USE AT THE FIELD MACROSCALE TO ELUCIDATE AQUIFER FLOW PATHS AND STORAGE RESERVOIRS AND QUANTIFY AQUIFER HYDRAULIC PROPERTIES

Tracers can be used independently or in combination to assess aquifer flow and storage dynamics, and to quantify hydraulic properties at the field macroscale. This can include: 1. conservative chemical tracers such as bromide and chloride, 2. dyes like Fluorescein and Rhodamine, 3. contaminants, whether natural or human induced, such as trace metals, 4. organic compounds such as pesticides, 5. microorganisms including bacteria and viruses, 6. stable isotopes, and 7. colloidal materials, such as clays or human made microspheres, among others. Each tracer has distinct physical, chemical and biological properties and characteristics that help resolve individual components of the flow system when considered individually, and become a much more powerful tool when evaluated as a tracer set. Over the years, we have used most of these at sites within the shallow karst aquifer in the Ozarks Plateaus Physiographic Province. Here, we discuss the use of tracers in categories 1, 2, 3, 5 and 7 as tools to elucidate karst aquifer flow and storage components, and as the basis for calculation of aquifer hydraulic properties. We have shown for multiple sites, including the Savoy Experimental Watershed, that the primary dye tracers Fluorescein and Rhodamine behave similarly to the conservative chloride in these shallow karst aquifers. This allows greater flexibility in the type of tracer used and the detection mechanism required. The dyes and conservative tracers are excellent for determining maximum advective flow velocities. From the breakthrough curves we can estimate hydraulic conductivity and storage along the primary flow paths. However, these tracers provide little insight for mechanisms of storage for microorganisms or aquifer colloidal materials. Tagged clays and bacteria are tracers that help us understand short- and long-term storage within multiple reservoirs of the karst system, including the vadose zone, epikarst, primary flow conduits and aquifer matrix. Comparison of these curves for these non-conservative tracers with the conservative tracers provides data from which hydraulic properties can be estimated for each of these separate compartments. Completing this is essential in order to successfully populate and link flow models that are required to fully characterize these complex hydrogeologic systems.