REPRODUCIBILITY OF GRANULAR ACTIVATED CARBON FOR DYE TRACER DETECTION

The Edwards Aquifer Authority is interested in determining the reliability of using granular activated carbon (GAC) for groundwater dye tracer studies. Understanding the effectiveness of GAC dye receptors is important for conducting future studies designed to better manage the Edwards Aquifer as well as other karst aquifers. This study attempts to evaluate some of the variables related to the use of GAC for tracer testing.

Granular activated carbon (GAC) is commonly used to adsorb various organic materials in liquid or air, as well as fluorescent dyes used for tracing of groundwater flow paths in karst terrains. GAC offers a semi-quantitative method to passively detect dyes in water in wells, springs, and streams. Packets, made of woven nylon containing GAC, called “bugs”, are placed in locations where dye may appear (Smart and Simpson, 2002). The packets are left for a period of days to weeks and then recovered for processing and analysis. To test whether a bug has been exposed to dye, the GAC is placed in a solution of alcohol and a base (usually KOH) for 1 hour. This causes the dye to be extracted from the carbon and the eluent is then analyzed using a luminescence spectrometer (Perkin Elmer LS 50B) to determine the absence or presence of dyes. The LS 50B is capable of separating dyes commonly used for water tracing. There are many variables associated with the use of GAC to detect dyes and there are some uncertainties related to the reproducibility of results. Four dyes (Uranine, Rhodamine WT, Sulforhodamine B, and Eosine) were flushed through bugs containing approximately 100g of GAC for five hours using a stirring plate. The charcoal was then dried, homogenized, and divided into ten samples of 10g each. Once the dye was extracted from the GAC, it was analyzed with the luminescence spectrometer. Results for the bugs containing Uranine, Rhodamine WT, and Sulforhodamine B showed desired reproducibility. Eosin curves showed weak dye detection as well as inconsistent curve structure. All bugs, other than the GAC containing Uranine, showed some type of contaminant in the lower 490 nm range, but the origin of the contaminant is unknown. Statistical analysis of the peaks of the Uranine fluorescence curves show good reproducibility based on the consistency of peak shape between each of the samples.