Observations of the spatial variability of hydraulic conductivity at a tracer test site, located atColumbus Air Force Base in Mississippi, are presented. Direct measurements of hydraulic conductivity of the heterogeneous alluvial aquifer at the site were made using borehole flowmeter logging, slug tests, and a laboratory permeameter to test undisturbed soil cores. Indirect methods estimating hydraulic conductivity were also evaluated, including soil grain size analyses, surface geophysical surveys, and mapping of sediment facies. The spatial covariance of the 2187 hydraulic conductivity values obtained with the borehole flowmeter method was examined. The log hydraulic conductivity variance (cr•2n •c) and the horizontal and vertical correlation scales (An and X,) of 4.5, 12.8 m, and 1.6 m, respectively, were estimated assuming second-order stationarity of the conductivity field. The covariance parameters are uncertain with bounding values that are 24-76% above or below the estimate. Covariance parameters estimated with more limited nonfiowmeter data were within the same magnitude as those obtained using the extensive flowmeter data, suggesting that data from a variety of methods may be used to provide approximate values of the autocovariance parameters. Nonstationarity of the In K field was examined by removing three-dimensional polynomial trend surfaces and calculating variograms of the residuals. Significantly lower estimates for cr•2n K, An and A• of 2.7, 4.8 m, and 0.8 m, respectively, were obtained from the third-order log conductivity residuals. After trend removal, the bounding parameter values differ 15-44% from the estimated values. Accounting for unsteady flow and the uncertainty in the covariance parameters of the third-order log conductivity residuals, the calculated longitudinal and horizontal transverse macrodispersivities ranged from 1.5 m to 1.8 m and 0.3 m to 0.6 m, respectively.

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 m 3 of groundwater containing bromide and three organic tracers (pentaftourobenzoic acid, o-trifiuoromethylbenzoic 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.

The declining mass balance trend observed for bromide and three fluorobenzoate tracers during a natural gradient experiment at a site near Columbus, Mississippi, prompted investigations related to the reactivity of the tracers and the representativeness of the tracer samples obtained from multilevel samplers. A laboratory soil column study indicated that adsorption of bromide during the field experiment was of the order of 20% and that up to 10% of the three fluorobenzoates was adsorbed. The presence of iron oxides and kaolinitc in the alluvial aquifer combined with the low groundwater pH of 4.8 produced geochemical conditions conducive to adsorption of the artionic tracers. Multilevel samplers (MLS) used in the field experiment were evaluated by comparing bromide concentrations from the MLS with water samples extracted from adjacent soft cores. Vertically averaged bromide concentrations for the MLS were 21% lower than those for the cores. A matrix diffusion process in conjunction with a natural tendency for preferential sampling from permeable regions in the heterogeneous alluvial sediments is proposed as an explanation for the apparent bias in the MLS sample concentrations. This process is shown to be qualitatively consistent with the tracer mass balance observed during the natural gradient experiment. (PFBA), trifluoromethylbenzoic acid (TFBA), and 2,6difluorobenzoic acid (DFBA). Transport of the tracers was monitored over a period of approximately 20 months using an extensive three-dimensional array of multilevel sampling points. The analysis of the spatial moments of the tracer concentration measurements reported by Adams and Gelhar [this issue] indicated the anomalous trend in the tracer mass balances shown in Figure 1. The mass recoveries for all tracers generally showed declining trends over time, beginning with initial overestimates of mass followed by mass underestimates during the later stages of the experiment. Several possible causes for the mass balance trend were investigated, including (1) analytical error associated with tracer concentration measurements, (2) tracer retention in the vadose zone resulting from water table fluctuations, (3) the method of tracer mass estimation, (4) variations in aquifer porosity, (5) incomplete spatial sampling of the plume, (6) tracer adsorption, and (7) sampling bias associated with the multilevel samplers. The contributions of analytical error and vadose zone retention of tracers to the Copyfight 1992 by the American Geophysical Union. Paper number 92WR01759. 0043-1397/92/92WR-01759505.00 mass imbalance were found to be negligible by Boggs et al. [this issue]. Questions regarding tracer mass estimation methods, porosity variations, and plume truncation are addressed by Adams and Gelhar [this issue].In this paper we describe a series of investigations designed to determine the extent to which tracer adsorption and sampling bias contributed to the anomalous tracer mass balance trends observed during the natural gradient test. A laboratory column experiment was per...

Results are presented for numerical simulations of ground water flow and physical transport associated with a natural gradient tracer experiment conducted within a heterogeneous alluvial aquifer of the Natural Attenuation Study (NATS) site near Columbus, Mississippi. A principal goal of NATS is to evaluate biogeochemical models that predict the rate and extent of natural biodegradation under field conditions. This paper describes the initial phase in the model evaluation process, i.e., calibration of flow and physical transport models that simulate conservative bromide tracer plume evolution during NATS. An initial large-scale flow model (LSM) is developed encompassing the experimental site and surrounding region. This model is subsequently scaled down in telescopic fashion to an intermediate-scale ground water flow model (ISM) covering the tracer-monitoring network, followed by a small-scale transport model (SSM) focused on the small region of hydrocarbon plume migration observed during NATS. The LSM uses inferred depositional features of the site in conjunction with hydraulic conductivity (K) data from aquifer tests and borehole flowmeter tests to establish large-scale K and flow field trends in and around the experimental site. The subsequent ISM incorporates specified flux boundary conditions and large-scale K trends obtained from the calibrated LSM, while preserving small-scale K structure based on some 4000 flowmeter data for solute transport modeling. The configuration of the ISM-predicted potentiometric surface approximates that of the observed surface within a root mean squared error of 0.15 m. The SSM is based on the dual-domain mass-transfer approach. Despite the well-recognized difficulties in modeling solute transport in extremely heterogeneous media as found at the NATS site, the dual-domain model adequately reproduced the observed bromide concentration distributions. Differences in observed and predicted bromide concentration distributions are attributed to aquifer heterogeneity at the decimeter (dm) and smaller scales. The calibrated transport parameters for the SSM (i.e., 1:7 for the ratio of mobile-to-total porosity; 2.5 x 10(-3) day-1 for the mass-transfer coefficient; 1 m for longitudinal dispersivity; and 0.1 m for transverse dispersivity) are consistent with separate numerical simulations of two earlier tracer experiments at the site. The multiscale modeling approach adopted in this study permits the incorporation of both large-scale geologic features important for flow simulation and small-scale heterogeneities critical for transport simulation. In addition, the dual-domain transport model provides a foundation for multispecies reactive transport modeling studies of natural attenuation of hydrocarbons during NATS.