Development of hydrogel tracer beads and comparative tracer tests to better understand contaminant fate and transport in karst systems

Abstract

Development of hydrogel tracer beads and comparative tracer tests to better understand contaminant fate and transport in karst systems Amanda Laskoskie Karst aquifers can be highly susceptible to contamination due to a close connection to the land surface, lack of filtration through a soil zone, and rapid transmission along solutionallyenhanced flowpaths. These factors also allow for aquifer contamination by direct injection of immiscible organic compounds in the form of non-aqueous phase liquids (NAPLs). The fate and transport of NAPLs in karst aquifers is poorly understood. Tracer tests allow for qualitative characterization of hydraulic flow in the surface and subsurface, but their results may not be applicable to the movement of NAPL for which density plays a critical role. Particulate tracers have been used to predict sediment transport but they fail to mimic the range of possible NAPL densities. Therefore, this research focused on the development and field testing of hydrogel tracer beads (HTBs), a non-toxic tracer made of calcium alginate (a derivative of marine algae), to better represent the behavior of NAPLs. The density of the HTBs can be readily modified to match different types of NAPLs. Sinking-and buoyant-HTBs were released during a preliminary field test at Hazel Run, in Bruceton Mills, WV. The buoyant-HTBs traveled the fastest and were recovered at the collection site, while the sinking-HTBs beads settled to the channel bed. Comparative tracer tests using fluorescein and buoyant-HTBs were completed at Buckeye Creek Cave near Lewisburg, WV and Rhine Creek in Terra Alta, WV. Despite the two systems being very different, the field tests in both systems demonstrated that the buoyant-HTBs had a greater velocity and had a lower mean transit time than did the fluorescein. These results are similar to what other comparable research has found-that particulate tracers travel faster than dissolved solutes. Based on the comparative tracer tests, light NAPLs (LNAPLs) may travel faster than dissolved tracers and travel times determined from solute tracers may not accurately reflect the appropriate time to collect water samples.

Hydrogel tracer beads as potential proxies for non-aqueous phase liquids in karst: Development of an optical quantification method for fluorescent beads

“…The tracer tests were conducted in the laboratory (Experimental Stream Facility) as well as in the field (Buckeye Creek Cave). In both of these sites, the systems have an air-water interface, but other than that, the sites are not directly comparable to Even with all of the differences between both sites, Laskoskie (2013) found that the buoyant HTBs traveled faster than the solute tracer.…”

“…Since density plays a critical role on the movement of NAPLs, the transport of commonly used dissolved tracers such as fluorescent dyes and salts or non-dissolved tracers such as microspheres is not applicable to that of NAPLs. Therefore, in order to better predict the behavior of NAPLs, Laskoskie (2013) developed a particle tracer named hydrogel tracer beads (HTBs). These are millimetersized particles formed by cross-linked sodium alginate polymers that can be made with different fluorescent pigments and density-modifying additives.…”

“…Qualitative tests have been conducted with buoyant and non-buoyant HTBs (Laskoskie, 2013). In those tests the buoyant HTBs were transported quickly on the water surface, whereas non-buoyant HTBs sank close to their release point, which suggests that the transport of HTBs is partly controlled by density.…”

“…Hydrogel beads are millimeter-sized particles that can be formed by cross-linked polymers such as alginate, agarose and chitosan. They are generally used for drug entrapment and delivery applications because they can hold a significant volume of liquid (Bajpai and Sharma, 2004), but they have also been developed and tested as tracers by Laskoskie (2013). HTBs are easily-made made in the laboratory at a relatively low cost.…”

“…For this study, only buoyant HTBs were used. These were made using the procedure described in Laskoskie (2013). The solution used to make the HTBs consisted of 3 g sodium alginate (Sigma Aldrich, St. Louis, MO) per 100 g of water, and 1 g of fluorescent pigment In each test, the HTBs were released together with a salt tracer.…”