Macro-Dispersion Experiment (MADE) site in Alabama
Overview and Site Description
Boggs et al., 1992. Water Resources Research, 28(12):3281-3291
Field Study of Dispersion in a Heterogeneous Aquifer 1. Overview and Site Description
J. Mark Boggs, Steven C. Young, Lisa M. Beard, Lynn W. Gelhar, Kenneth R. Rehfeldt, E. Eric Adams Engineering Laboratory, Tennessee Valley Authority, Norris, Tennessee
Results are presented for a large-scale natural gradient tracer experiment conducted in a heterogeneous alluvial aquifer at a site near Columbus, Mississippi. The study was initiated with a 48-hour pulse injection of 10 m3 of groundwater containing bromide and three organic tracers (pentaflourobenzoic acid, o-trifluoromethylbenzoic acid, and 2,6-diflourobenzoic acid). Over a 20-month period, seven comprehensive samplings of the tracer plume were performed at approximately 1- to 4-month intervals using an extensive three-dimensional sampling well network. The dominant feature of the tracer plume that evolved during the study was the highly asymmetric concentration distribution in the longitudinal direction. This asymmetry was produced by accelerating groundwater flow along the plume travel path that, in turn, resulted from an approximate 2-order-of-magnitude increase in the mean hydraulic conductivity between the near-field and far-field regions of the site. The Columbus study is distinct from previous natural gradient experiments because of the extreme heterogeneity of the aquifer, the large-scale spatial variations in groundwater velocity, and the extensive set of hydraulic conductivity measurements for the aquifer.
Keywords: 1872 Hydrology; Transport 1830 Groundwater 1881 Water quality
Water Resour. Res. 92WR01756 Vol. 28 , No. 12 , p. 3281-3291
Boggs JM, Beard LM, Long SE, McGee MP, MacIntyre WG, Antworth CP, Stauffer TB. Database for the second macrodispersion experiment (MADE-2). Tech. Rep. TR-102072, Electric Power Res. Inst., Palo Alto, California. 1993.
The second experiment (MADE-2) was conducted between June 1990 and September 1991 in an attempt to further develop a field database for validating geochemical models at the site. In the MADE-2 experiment, tritium and several organic dissolved compounds were used and tritiated water served as the conservative reference tracer.
Hydrogeologic Characterization of the MADE Site
Abstract: In an investigation of field-scale dispersion of nonreactive solutes in a saturated heterogeneous aquifer at the Macrodispersion Experiment (MADE) site, several measurement techniques were used to characterize hydraulic conductivity variations. The borehole flowmeter technique proved to be the most effective and economic method for providing hydraulic conducting measurements for modeling the transport of contaminants in groundwater.
Background: Dispersion or spreading is one of the key transport processes influencing the extent of inorganic chemical waste migration and environmental concentrations. To accurately interpret the tracer plume-dispersion characteristics measured at sites, detailed information on hydraulic conductivity variability is essential. In addition, the transport component of hydrochemical codes requires input on the spatial variation of hydraulic conductivity for predicting advective transport. EPRI therefore engaged in this study at the MADE site in Mississippi to develop methods to more accurately predict the transport of inorganic chemicals leached from solid-waste disposal facilities.
Objective: To measure the spatial variability of hydraulic conductivity at the MADE site; to test direct and indirect techniques for measuring hydraulic conductivity.
Approach: Researchers studied direct methods of measuring the spatial variability of hydraulic conductivity (slug, logging, double-packer, borehole flowmeter, large-scale aquifer pump, and laboratory permeameter tests). They also studied indirect methods, including soil grain size analyses and geophysical surveys at the MADE site. The spatial details of hydraulic conductivity were examined using 2187 flowmeter measurements obtained from 52 fully penetrating groundwater wells. Several statistical parameters were then determined to help characterize the physical properties of the aquifer, and the various methods of measuring hydraulic conductivity were evaluated in terms of effectiveness and cost.
Results: Of the many hydraulic conductivity measurement techniques evaluated, the borehole flowmeter technique proved to be the most effective and economic means of characterizing the heterogeneity of the MADE site aquifer. Regarding spatial variability, statistical parameters indicated that the estimate for the variance of log hydraulic conductivity was 4.5, and correlation scales were 12.0 m in the horizontal and 1.5 m in the vertical. Despite the large number of measurements used in the computations, the uncertainty in the estimate of covariance was quite large--95% confidence interval of 3.4 to 5.6. A spatial trend was detected in the hydraulic conductivity data. Removal of this trend prior to computing the statistical parameters resulted in a variance of log hydraulic conductivity of 2.8 and correlations of 5.3 m in the horizontal and 0.7 m in the vertical.
EPRI Perspective: Spatial variability of hydraulic conductivity in an aquifer is the major contributor to solute spreading. Efforts to understand the process of spreading and to develop prediction methods require the ability to measure hydraulic conductivity distributions. The successful testing of the borehole flowmeter technique gives hope that aquifer heterogeneity characterization can be performed effectively and inexpensively. With this measurement ability, the application of three-dimensional advective transport models coupled with solute transport algorithms becomes a feasible approach to simulating the spread of solutes in groundwater. The alternative is to develop stochastic predictive methods. The merits of these approaches are currently being evaluated in EPRI project RP2485-15. Details on the borehole flowmeter technique can be found in report EN-6511; estimates of dispersivity are presented in report EN-6405.
Keywords: Groundwater, Dispersions, Transport, Hydrodynamics, Solid Waste Disposal, Leachate Migration
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List Price: (U.S. Dollars) $ 2,500