Chattahoochee-Apalachicola Rivers Water Quality Sampling a Lagrangian Sampling Project
Fuller, Robert C.
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The field work portion of this project began September 22, 2012, with an expected completion date of November 10, 2012. The field work will involve traveling by canoe down the entire Chattahoochee-Apalachicola River system from the source spring near Chattahoochee Gap to the Gulf of Mexico, collecting water quality data, documenting illegal incursions into the river channel, and gathering such other information as may seem to be valuable. As of September 30, 2012, 89.4 miles of the river system has been covered. What makes this project unique and of particular value will be the Lagrangian design of the observations. The purpose of a Lagrangian sampling scheme is to follow an initial mass or “parcel” of water as it moves through its containing channel, tracking changes to the water’s constituents over space and time. Hydraulic modeling work done by others was used to make initial estimates of average river velocities along the length of the system, which were used to calculate doses of a tracking dye sufficient to be detected but not so large as to violate EPA guidelines. Rhodamine WT dye was chosen for tracking and it was detected using a fluorometer. The concept of an initial water mass is used in recognition of the fact that a small mass of water emerging from the source spring will be increasingly dispersed as the mass moves downstream due to the mixing within the channel and the variability of water velocity across the channel. Because of this, it is easier to think of trying to follow the centroid of the dispersing mass than it is to think of predicting the likely position of a single molecule that emerges from the source. Due to low rhodamine WT doses used, the dye is re-dosed at roughly 25 to 50 kilometer intervals along the system.