Numerical Simulation of a Natural Gradient Tracer Experiment for the Natural Attenuation Study: Flow and Physical Transport

Abstract

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.

Time and space nonlocalities underlying fractional-derivative models: Distinction and literature review of field applications

“…In response to the positively skewed tritium plumes which cannot be described by the classical ADE with a coarse-scale flow field [2], researchers have attempted to simulate the tracer plumes using many different methods in the last decade (see the review by Molz et al [107] and Zhang et al [148]). The commonly accepted conclusion is that the classical ADE with a fine-scale (such as decimeter-scale) flow field [51,93,125,152,153,155], or the dual-domain approach with a similarly fine resolution of flow field [53,69,75,107], may describe the anomalous plumes at the MADE site. However, the practicality of the first method is questionable due to limitations of current characterization techniques in differentiating between small-scale heterogeneities.…”

Space‐fractional advection‐dispersion equations with variable parameters: Diverse formulas, numerical solutions, and application to the Macrodispersion Experiment site data

“…[36] The plausible hydrogeological explanation for the ''nonideal'' transport behavior observed at the MADE site supports three types of modeling methods, which include the following: (1) the second-order ADE with a fine-scale velocity field capturing both the multiscale flow paths and relatively immobile, trapping zones [Zheng and Jiao, 1998;Eggleston and Rojstaczer, 1998;Zheng and Gorelick, 2003;Zinn and Harvey, 2003;Liu et al, 2004]; (2) the dualdomain approach with a relatively coarse (upscaled) flow field [Harvey and Gorelick, 2000;Feehley et al, 2000;Julian et al, 2001]; and (3) novel nonlocal techniques, such as the CTRW method [Berkowitz and Scher, 1998] and the fADE method , which directly capture the anomalous transport process, using a very coarse, or even a constant, mean transport velocity. An exceptional case of the first method was discussed recently by Barlebo et al [2004], who showed that the second-order ADE with a simplified zonal conductivity (K) field can also characterize the positively skewed plume at the MADE site.…”

Highlighting the implications of selenium dispersion from disposal of Kahota Industrial Triangle area, Islamabad, Pakistan using three-dimension solute transport model

“…Review of the literature indicates that modeling of different kinds of contaminant has been studied by several researchers. Among them most prominent includes Wilber (1980), Copoulos and Sehayek (1986), Hutson and Wagenet (1989), Shifang (1991), Hooshmand (1992), Zheng and Bennett (1997), Feehley et al (2000), Holder et al (2000), Barth et al (2001), Boutt et al (2001), Elci et al (2001, Mehl and Hill (2001), Julian et al (2001), Shamrukh et al (2001), Faidi et al (2002), Zheng and Wang (2002), Andrews and Neville (2003), Bakker (2003), Brown and Glynn (2003), Conan et al (2003), Harrar et al (2003), Langevin (2003), Prommer et al (2003), Zheng and Gorelick (2003), Liu et al (2004), and Wang and Bright (2004). Risk assessments of the subsurface environment often involve the use of quantitative contaminant fate and transport models.…”