U.S. GEOLOGICAL SURVEY
Scientific Investigation Report 2004-5190
A natural-gradient ground-water tracer test was designed and conducted in a tidal freshwater wetland at West Branch Canal Creek, Aberdeen Proving Ground, Maryland. The objectives of the test were to characterize solute transport at the site, obtain data to more accurately determine the ground-water velocity in the upper wetland sediments, and to compare a conservative, ionic tracer (bromide) to a volatile tracer (sulfur hexafluoride) to ascertain whether volatilization could be an important process in attenuating volatile organic compounds in the ground water. The tracer test was conducted within the upper peat unit of a layer of wetland sediments that also includes a lower clayey unit; the combined layer overlies an aquifer. The area selected for the test was thought to have an above-average rate of ground-water discharge based on ground-water head distributions and near-surface detections of volatile organic compounds measured in previous studies. Because ground-water velocities in the wetland sediments were expected to be slow compared to the underlying aquifer, the test was designed to be conducted on a small scale.
Ninety-seven ¼-inch-diameter inverted-screen stainless-steel piezometers were installed in a cylindrical array within approximately 25 cubic feet (2.3 cubic meters) of wetland sediments, in an area with a vertically upward hydraulic gradient. Fluorescein dye was used to qualitatively evaluate the hydrologic integrity of the tracer array before the start of the tracer test, including verifying the absence of hydraulic short-circuiting due to nonnatural vertical conduits potentially created during piezometer installation. Bromide and sulfur hexafluoride tracers (0.139 liter of solution containing 100,000 milligrams per liter of bromide ion and 23.3 milligrams per liter of sulfur hexafluoride) were co-injected and monitored to generate a dataset that could be used to evaluate solute transport in three dimensions. Piezometers were sampled 2 to 15 times each, from July 1998 through September 1999, to assess background conditions and monitor tracer movement. During the test, 644 samples were analyzed for fluorescein, 617 samples were analyzed for bromide with an ion-selective electrode, 213 samples were analyzed for bromide with colorimetric methods, and 603 samples were analyzed for sulfur hexafluoride, including samples collected prior to tracer injection to determine background concentrations. Additional samples were analyzed for volatile organic compounds (96 samples) and methane (37 samples) to determine the distribution of these contaminants and the extent of methanogenic conditions within the tracer array; however, these data were not used for the analysis of the test.
During the tracer test, the fluorescein dye, bromide, and sulfur hexafluoride were transported predominantly in the upward direction, although all three tracers also moved outward in all directions from the injection point, and it is likely that some tracer mass moved beyond the lateral edges of the array. An analysis of the tracer-test data was performed through the use of breakthrough curves and isoconcentration contour plots. Results show that movement of the fluorescein dye, a non-conservative tracer, was retarded compared to the other two tracers, likely as a result of sorption onto the wetland sediments. Suspected loss of tracer mass along the lateral edges of the array prevented a straightforward quantitative analysis of tracer transport and ground-water velocity from the bromide and sulfur-hexafluoride data. In addition, the initial density of the bromide/sulfur hexafluoride solution (calculated to be 1.097 grams per milli2 Ground-Water Tracer Test, West Branch Canal Creek, Aberdeen Proving Ground, MD liter) could have caused the solution to sink below the injection point before undergoing dilution and moving back up into the array. For these reasons, the data analysis in this report was performed largely through qualitative methods.
The mass of bromide and sulfur hexafluoride tracers within the array was estimated from the mean concentrations during each of four major sampling episodes that took place between 103 and 375 days after tracer injection. Assuming an effective porosity of 0.40, the estimated mass of bromide ranged from about 68 percent (during the first major sampling episode) to as little as 4 percent of the mass that was initially injected, whereas the estimated mass of sulfur hexafluoride ranged from about 51 percent to 3 percent of the initial mass. These masses would be larger, however, if a higher porosity was assumed; for example, the mass of bromide estimated for the first major sampling episode would be 94 percent if an effective porosity of 0.55 was assumed. A comparison of bromide and sulfur hexafluoride concentrations relative to their respective injected concentrations indicates that a smaller proportion of the injected sulfur hexafluoride moved up into the tracer array compared to bromide. Breakthrough curves of bromide and sulfur hexafluoride concentrations with time showed differences between the two tracers in most parts of the array. In several instances, large concentrations of one tracer were not matched by large concentrations of the other. Breakthrough curves and isoconcentration contour plots indicate that the bromide tracer generally moved away from the injection point more efficiently than sulfur hexafluoride. The movement of the tracers coupled with the loss of tracer mass throughout the test shows qualitatively that there is a slight northward horizontal component to the ground-water flow in the area of the tracer array in addition to vertical flow.
Diffusion and advection are thought to be the major processes responsible for the tracer movement, based on the predominantly upward movement of the conservative tracers, coupled with the outward movement in all directions from the injection point. Decreases in concentration as the tracers moved upwards through the array were likely caused by dilution. Sorption was evaluated and eliminated as a potential source of mass loss or retardation for sulfur hexafluoride. Because the wetland sediments were saturated to land surface throughout the tracer test, potential losses of sulfur hexafluoride due to volatilization from the surface of the water table into an unsaturated zone could not be evaluated. Part of the sulfur hexafluoride could have volatilized into ambient bubbles of marsh gases at depth; this process could explain the early presence of sulfur hexafluoride in a few upper-level sampling points (because of gas-bubble rise) and the retardation of some of the sulfur hexafluoride (because of gas-bubble trapping) in the lower levels of the tracer array. The tracer movement observed during this test can provide insights into the transport of volatile organic compounds in the wetland sediments at West Branch Canal Creek. Because the volatile organic compounds that are the contaminants of interest are nonconservative, sorption would be expected to retard their movement, though to a lesser degree than was observed for fluorescein, which has a higher octanol-water partitioning coefficient than the contaminants of interest. Dilution, which is thought to be a factor in reducing the tracer concentrations, could also affect volatile organic compounds in the wetland sediments; however, the higher proportion of daughter compounds detected in the shallow levels of the array (6 to 24 inches below land surface) compared to the deeper levels indicates that biodegradation is also acting to transform these compounds and reduce their mass. Finally, if volatile organic compounds are able to volatilize into bubbles of biogenic marsh gases at depth, their transport could be affected by potential acceleration or retardation due to gas-bubble rise or trapping, as was hypothesized with the sulfur hexafluoride.
Purpose and scope
Description of study area
Design of the ground-water tracer test
Selection of the tracer-test site
Design and installation of the tracer-test array
Selection, preparation, and injection of tracers
Bromide and sulfur hexafluoride
Sampling and analytical methods
Sulfur hexafluoride analysis
Volatile organic compound and methane analysis
Results and evaluation of the chemical analyses
Fluorescein-dye, bromide, and sulfur hexafluoride data
Volatile organic compound and methane data
Quality assurance of the data
Split samples and spiked split sample
Analysis of the tracer movement
Bromide breakthrough curves
Sulfur hexafluoride breakthrough curves
Isoconcentration contour plots
Sulfur hexafluoride concentrations
Comparison of tracers
Summary and conclusions
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