Numerical simulation of a natural gradient tracer experiment for the natural attenuation study: Flow and physical transport

Results are presented for numerical simulations of ground water flow and ph ysical transport associated with a natural gradient tracer experiment condu cted within a heterogeneous alluvial aquifer of the Natural Attenuation Stu dy (NATS) site near Columbus, Mississippi. A principal goal of NATS is to e valuate biogeochemical models that predict the rate and extent of natural b iodegradation under field conditions. This paper describes the initial phas e in the model evaluation process, i.e., calibration of flow and physical t ransport models that simulate conservative bromide tracer plume evolution d uring NATS, An initial large-scale flow model (LSM) is developed encompassi ng the experimental site and surrounding region. This model is subsequently scaled down in telescopic fashion to an intermediate-scale ground water fl ow model (ISM) covering the tracer-monitoring network, followed by a small- scale transport model (SSM) focused on the small region of hydrocarbon plum e migration observed during NATS, The LSM uses inferred depositional featur es of the site in conjunction with hydraulic conductivity (K) data from aqu ifer tests and borehole flowmeter tests to establish large-scale K and flow field trends in and around the experimental site. The subsequent ISM incor porates specified flux boundary conditions and large-scale K trends obtaine d from the calibrated LSM, while preserving small-scale K structure based o n some 4000 flowmeter data for solute transport modeling. The configuration of the ISM-predicted potentiometric surface approximates that of the obser ved 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 diffic ulties in modeling solute transport in extremely heterogeneous media as fou nd at the NATS site, the dual-domain model adequately reproduced the observ ed bromide concentration distributions. Differences in observed and predict ed bromide concentration distributions are attributed to aquifer heterogene ity at the decimeter (dm) and smaller scales. The calibrated transport para meters 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 se parate numerical simulations of two earlier tracer experiments at the ate. The multiscale modeling approach adopted in this study permits the incorpor ation of both large-scale geologic features important for flow simulation a nd small-scale heterogeneities critical for transport simulation. In additi on, the dual-domain transport model provides a foundation for multispecies reactive transport modeling studies of natural attenuation of hydrocarbons during NATS.