Eos, Vol. 81, No. 44, October 31, 2000
Thomas Nalepa (right), a biologist with the Great Lakes Environmental Research Laboratory of the National Oceanic and Atmospheric Administration, points out mussels trawled from Lake Michigan. Photo by Randy Showstack.
the texture of the sand rather than analyzing its composition. While the exact makeup of the lake's dune sand differs with each handful, the sand aver ages 87-94% quartz, 10-18% feldspar, 1-3% mag netite, with traces of garnet, calcite, ilmenite, hornblende, and epidote. As it turns out, that mineral composition, as well as the sand's clean and fairly uniform grain shape, have made it ideal for use in pro ducing metal castings in foundries in Michi gan and out of state. Once the metal cools, the sand is easily removed. Miners and developers hold that most of the region's dunes are not being mined, and that mining generally improves land by leveling it off for housing that might circle around a newly dug-out lake. Conservationists, however, say that the sand dunes—a natural feature of the land—are dis appearing at an alarming rate. Since the pas sage of Michigan's Sand Dune Protection and Management Act of 1976, an average of 2.26 million metric tons of sand have been mined each year, according to the Lake Michigan Fed eration. In an April 1999 report, "Vanishing Lake Michigan Sand Dunes:Threats from Min ing," the federation also noted that sand is cheap, with the cost of a ton averaging about U.S. $5-10. Cabala said that while there are
Unraveling Complex Hydrogeologic Systems Using Field Tracer Tests PAGES 514-515 Tracking the movement of underground contaminants is vital to protecting public health and the environment worldwide. Scien tific efforts using field tracer techniques to solve contaminant migration problems are rapidly evolving to fill critical information gaps and provide confirmation of laboratory data and numerical models. Various chemical tracers are being used to formulate and evaluate alternative conceptual hydrogeologic models; namely, to constrain hydraulic properties of geologic systems, iden tify sources of groundwater, flow paths, and rates, and determine mechanisms that affect contaminant transport. Naturally occurring elements and environmental isotopes from atmospheric and underground nuclear testing can make excellent tracers. In addition, charac terizing sites of future waste disposal, such as the potential high-level nuclear waste reposito ry at Yucca Mountain, requires new and inno vative techniques like injecting surrogate tracers that simulate potential contaminants and shed light on mechanisms that could control future contaminant migration. Earth scientists at the 2000 AGU Spring Meeting reported on these recent advances. In the underground test facility at Busted Butte, Nevada, near Yucca Mountain, field-scale transport properties of the unsaturated zone were investigated by injecting a mixture of reactive and non-reactive tracers into non-
welded tuffs. Results from tests using fluorescein dye tracer injected continuously into horizontal boreholes for several months and illuminated by UV light during mineback and overcoring operations, emphasized the importance of cap illary forces in the tuffs. Even at relatively high injection flow rates (10-50ml/h),the downward migration of tracers was significantly attenuated by fracture-matrix interactions (imbibition and matrix diffusion) and heterogeneities (lithologic boundaries). In contrast,the interbedded basalts and sediments at the Idaho National Engineering and Environmental Laboratory provide evidence of rapid, long-range horizon tal transport. In response to seasonal ponded infiltration, water and contaminants could move farther than one km in less than three months within the unsaturated zone. Tracer tests in saturated fractured volcanic rocks at Yucca Mountain using reactive and non-reactive tracers suggest matrix diffusion (the exchange of mass between relatively mobile fracture fluid and relatively immobile matrix fluid) may be an important mechanism for reducing radionuclide concentrations and attenuating radionuclide transport rates in frac tured tuffs. Laboratory experiments were run in parallel with the field tests to investigate trans port parameter scaling issues and the applicabil ity of laboratory-derived transport data to fieldscale predictions. In general, the laboratory data predicted less tracer sorption than observed in the field, and more matrix diffusion. In fractured granitic schist at Mirror Lake, New Hampshire,
currently only 5,000 permitted acres of sand dunes in the state, mining losses over the past 100 years are unknown, and mining activities actually have increased since the Sand Dune Act took effect. Foundries already have learned to re-use sand up to five times, rather than landfill it after one use. However, a bill in the Michigan state legislature—the Sand Dune Mining Act (SB 1311)—would ban all mining in critical dune areas by 2006 and force foundries to find new sources—such as inland sand—for their castings. While the bill may not move during this election year, bill sponsor State Senator Gary Peters from the metropolitan Detroit area has promised to reintroduce it early next year. On a bright October afternoon, the dune sand glitters in one's hands, as does the water on the shells of mussels trawled from the lake bottom. While the Sun shines on both of these resources, scientists try to understand their impact on the future of Lake Michigan. For further information, visit the NOAA Web site: http://wwwglerl.noaa.gov/, and Lake Michigan Federation Web site: http://www. lakemichigan.org/. Randy Showstack,
Staff Writer
tracers of varying diffusive properties produced identical late-time breakthrough. This suggests that advective, and not diffusive, processes may control transport of the injected tracers. Lower effective porosity in the schist (as compared to tuffaceous rock) and shorter tracer residence times may partially explain the differences between the findings at Mirror Lake and those at Yucca Mountain. An investigation at the Nevada Test Site (NTS) sampled previously injected tracers using a pumping well for 87 days at a rate of 120 gallons per minute (654 cubic meters per day). Similar to the tracer test results at Yucca Moun tain, evidence of matrix diffusion in fractured volcanics was indicated by differences in solute transport behavior. Microsphere tracer breakthrough curves dis played double peaks, suggesting remobilization during flow transients.The NTS tracer test results indicated that effective porosity values are higher than values found in the literature. Inter pretation of experiments in fractured rocks requires a better understanding of hetero geneities. Radioisotopes also provide reactive and non-reactive tracers from NTS underground nuclear device tests. Several radionuclides sorbed onto colloids have been detected in groundwater samples taken from fractured aquifers at significant distances from these tests. In a sedimentary aquifer in Germany, humic colloids migrated unretarded 25 km in 15,000 years, which attest to their stability Several presenters discussed the use of natu ral isotopic variations in groundwater to evalu ate contaminant transport. Differences in the strontium isotope ratios between process water and regional groundwater provide an effective tracer for evaluating wastewater infiltration
Eos, Vol. 81, No. 44, October 31,2000 from the Hanford tanks located in Washington State. Stable isotope and fluid inclusion studies on fracture minerals provide indicators of paleohydrology at Benken,a potential nuclear waste repository site in Switzerland. Specifical ly, minerals above and below the Opalinus Clay indicate intrusion of low salinity fluids, possibly of meteoric origin. In summary, scientific efforts using tracer techniques to solve contaminant migration problems are rapidly evolving and resulting in the merging of geology hydrology, and geo chemistry disciplines. Results of field tracer tests are filling critical information gaps by
addressing data needs and providing confir mation of laboratory data and numerical models. Field tracer test data at various scales are needed to enhance confidence in performance assessment models used to quantify transport of sorbing radionuclides. Additional and repeated tracer studies using different hydraulic configurations and multi ple tracers are warranted to evaluate spatial and temporal scaling phenomena and hetero geneities of geologic materials. For further information about the AGU ses sions, visit the AGU Web site at http://www. agu.org/meetings/waissmOO.html.
The AGU 2000 Spring Meeting took place in May in Washington, D.C.
Authors William L. Dam and Thomas Nicholson U.S. Nuclear Regulatory Commission For more information, contact William L.Dam, Division of Waste Management, Office of Nuclear Material Safety and Safeguards, U.S. Nuclear Regulatory Commission, M.S.T7C6, Washington, D.C, 20555, USA; E-mail:
[email protected].
Experiment Will Examine Gravity Waves in the Middle Atmosphere PAGE 517 Over the last 4 decades, the study of high-fre quency gravity waves with periods from min utes to several hours in the stratosphere, mesosphere, and lower thermosphere has been a leading area of research in both mete orology and aeronomyThis interest has been natural, since gravity wave motions often domi nate the variance observed in the upper mesospheric and thermospheric wind field, and since the vertical transport of mean flow momentum by gravity waves is now thought to play a key role in maintaining global-scale cir culation throughout the middle atmosphere. Part of the spectrum of the gravity waves propagating upward through the mesosphere will reach F-layer heights, where the gravity waves account for temporal and spatial variability in electron density Electron density, in turn, can affect aspects of radio wave propa gation. Gravity waves in the equatorial F-region may also trigger large-scale ionospheric insta bilities, which account for the "spread-F" phe nomenon. Late next year, investigators involved in the Darwin Area Wave Experiment (DAWEX) will make detailed coordinated observations of the gravity wave field in the mesosphere and thermosphere with unprecedented areal coverage, and will attempt to relate these observations to a particularly well-defined and intense tropospheric source. The importance of gravity waves in the middle atmosphere has been recognized by interna tional scientific organizations in recent years. Both the steering committee of the Equatorial Processes Including Coupling (EPIC) program of the Scientific Committee on Solar-Terrestrial Physics, and the scientific steering group of the Stratospheric Processes and their Role in Cli mate (SPARC) program of the World Climate Research Programme have advocated the development of coordinated international field experiments to examine the generation and propagation of middle atmospheric gravity waves with a focus on low latitudes and convective sources.The deployment of resources from research groups in several countries should allow an experiment to transcend the limitations
of most previous gravity wave studies, which have typically been restricted to one type of observation from a single geographical location. One initiative that has emerged in response to this interest from EPIC and SPARC is a plan for a field experiment to examine gravity wave generation and propagation in the region near Darwin, Australia (12°S, 131°E), in Austral spring 2001.The geography of the region is shown in Figure 1. In the Austral winter, there is little convection in Northern Australia. However, in the late premonsoon period, from mid-November to midDecember, continental deep convection is generally limited, but intense convection over the Tiwi Islands near Darwin develops each aftemoon.This diurnal convection, known locally as "Hector," is generally initiated by interaction of convectively-driven cold pools and sea breeze fronts and builds to a bundle of intense thunder storms that reaches the tropopause in mid-after noon. Hector then moves offshore to the west, where it dissipates by mid-evening. The Hector thunderstorms represent some of the most intense and penetrative convection observed anywhere in the world, with updrafts as strong as 40 m/s and cloud tops to 20 km (more typically 17-18 km).The interesting char acter of convection in the area led to Darwin being chosen as the site for the Maritime Conti nent Thunderstorm Experiment (MCTEX) cam paign conducted in November and December 1995, which helped to establish an understand ing of the dynamics of the initiation and devel opment of the Hector convection. The goal of the DAWEX is to characterize the wave field in the middle atmosphere over North ern Australia prior to the onset of the Hector convection, and then during the active Hector period.The locations of some of the principal instruments tentatively committed to DAWEX are shown in Figure l.The tropospheric convection over the islands will be monitored by the Australian Bureau of Meteorology Research Centre C-band polarized Doppler radar. Informa tion on the wind and temperature fields up to about 35 km will be provided by balloon sound ings, which are to be conducted several times per day at Darwin and two nearby locations by
15°S
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Fig. I. (a) The Northern Australian region that is the focus of DAWEX. The Tiwi Islands are heavily shaded. The solid circles show the approximate fields of view of optical instruments that are likely to be deployed at Katherine (K) and Wyndham (W) that will use airglow emissions to image the wave field near 90-km altitude. The dotted circle shows the field of view of an imag er likely to be deployed at Katherine that will image emissions from about 220-km altitude. The dashed circle shows the field of view of the weather radar at Darwin (D) when operated in storm surveillance mode, (b) The AustraliaIndonesia region showing the fields of view of all the airglow imagers to be deployed during DAWEX.
scientists from the Japanese Radio Science Cen ter for Space and Atmosphere (RASC). Winds in the mesosphere and lower thermos phere will be monitored continuously by a medium frequency radar to be deployed by the University of Adelaide and located at Katherine, which is about 250 km south of
