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91WR00241 - WATER RESOURCES RESEARCH 27,NO.5 PAGES VOL...

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WATER RESOURCES RESEARCH, VOL. 27, NO. 5, PAGES 895-910, MAY 1991 Large-Scale Natural Gradient Tracer Test in Sand and Gravel, Cape Cod, Massachusetts 1. Experimental Design and Observed Tracer Movement DENIS R. LEBLANC, • STEPHEN P. GARABEDIAN,1 KATHRYN M. HESS, • LYNN W. GELHAR, 2 RICHARD D. QUADRI• • KENNETH G. STOLLENWERK, 3 AND WARREN W. WOOD 4 A large-scale natural gradient tracer experiment was conducted on Cape Cod, Massachusetts, to examine the transport and dispersion ofsolutes in a sand and gravel aquifer. The nonreactive tracer, bromide, and the reactive tracers, lithium and molybdate, were injected as a pulse in July 1985 and monitored in three dimensions as they moved as far as 280 m down-gradient through anarray of multilevel samplers. The bromide cloud moved horizontally at a rate of 0.42 m per day. It also moved downward about 4 m because of density-induced sinking early in the test and accretion of areal recharge from precipitation. After 200 m oftransport, the bromide cloud had spread more than 80 m in the direction of flow, but was only 14 m wide and 4-6 m thick. The lithium and molybdate clouds followed the same pathas the bromide cloud, but their rates of movement were retarded about 50% relative to bromide movement because of sorption onto the sediments. INTRODUCTION During the past decade, a focus of groundwater research has been the development of quantitative models to predict contaminant transportin aquifers. Thesepredictive toolsare needed in the investigation and cleanup of contaminated subsurface systems. However, there is a growing awareness that the transport and fate of contaminants are greatly affected by the natural heterogeneity that characterizes aquifers [Anderson, 1987]. A key research question is how to incorporate this heterogeneity into quantitative analyses of flow and transport. In particular, we would like to know how to find the appropriate average physical and chemical prop- erties that govern this transport and how to assess the reliability of predictions based on models using these effec- tive, or average, properties [Gelhat, 1986]. Several theories account for the effects of heterogeneity of aquifer properties on flow and transport by treating hetero- geneity as a stochastic process. This work has been partic- ularly useful in understanding field-scale dispersion of sol- utes in groundwater IDagan, 1982, 1984; Gelhar andAxness, 1983; Neuman et al., 1987]. The stochastic theories relate macrodispersion observed in the field to variations in veloc- ity caused largely by the spatial variability of hydraulic conductivity. Stochastic analysis has also been used to examine the effectsof chemical heterogeneity of the sedi- ments on macrodispersion of reactive solutes [Garabedian et al., 1988; Valocchi, 1989]. Although these theories have changed the way in which we conceptualize flow and transport in heterogeneous aqui- fers, the practical application to field problems is still the subject of debate [MoIz et al., 1988]. The pioneering work at the Borden site in Ontario [Sudicky et aI., 1983;Mackay et aI., 1U.S. Geological Survey,
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