CONCEPTUALIZING FLOW INTERACTION AND CONDUIT GEOMETRY IN NEAR-SURFACE KARST BEDROCK USING A QUANTITATIVE DYE TRACING METHOD

Dye trace studies are valuable in understanding groundwater flow in karstic aquifers. This is especially true in regions where flow is highly anisotropic between groundwater inputs (sinkholes and losing streams) and outputs (springs). Karst systems also tend to be characterized by high groundwater velocities and short infiltration times. Because of this, surface and subsurface flow in karst systems are strongly linked. Typically, dye traces are used to establish point-to-point flow paths by injecting dye into an input and placing activated carbon packets at nearby springs to capture dye. For this study, using a previously established flow connection, the use of a dye injection pump and automated water samplers allowed for a quantitative method for understanding karst conduit geometry.

In the Ward Branch watershed in Springfield, Missouri, dye traces implementing a quantitative approach have provided new insight into local near-surface karst groundwater flow. The Ward Branch stream is a classic example of a karst stream, being recharged by springs and gaining sections while also containing losing sections which become dry for much of the year. Two dye traces were performed in summer 2011 using a quantitative dye trace method. Rhodamine WT dye was pumped at a constant rate into a stream sink with known groundwater connections to two nearby springs. Two 24-bottle autosamplers were installed at the two springs to collect spring water samples. By using the quantitative constraints placed on the dye trace: concentration of injected dye, rate of dye injected, spring discharges, and concentration of dye in spring water a model of the karst conduit system can be conceptualized. This also allows for dye recovery to be calculated as a ratio between dye-discharge out of the springs and dye-discharge into the losing stream, as opposed to the traditional method of the ratio between total mass of dye recovered to total mass of dye injected. Treating these dye traces as mass balance calculations (both dye and water) gives insight into the conduit system by showing which flow paths act as open or closed systems indicated by dye concentration variations between the two springs and dye loss in the overall system.