Contaminant-transport models results

Contaminant Transport Modeling. A major difficulty in the calibration of any two-dimensional contaminant transport model is relating the two-dimensional simulated plume to the real three-dimensional plume. A model based on Equation 4 can simulate two dimensions in cross section or areal view. An areal view was selected for the problem considered here. Use of a two-dimensional areal view model implies that the contaminant is uniformly spread out through the entire saturated thickness of the aquifer. However, in the field the aldicarb plume is only around 10 feet (3m) thick while the aquifer is around 70 feet (21 m) thick. Moreover, the concentration data were collected from wells having 3 ft (0.91 m) well screens and hence are representative of only a small fraction of the total aquifer thickness. It was decided to calibrate the model to concentrations representative of the center of the plume vertically. That is, the model was calibrated to maximum measured concentrations in each well nest. As a result, the loading rate to the model is inflated over probable field values. The model assumes the load to the model is distributed over the full aquifer thickness, when in the field the zone of maximum concentration is probably no more than 3 feet thick. Therefore, the probable loading rate in the field is roughly 3/70 or 4% of that used to calibrate the model. 

We have argued that molecular-scale understanding of the structure and composition of mineral surface-aqueous solution sorption complexes is vital to development of robust reactive contaminant transport models. This argument suggests that significant error may be expected in predicted environmental behavior in the absence of such knowledge. In an attempt to test this claim, we have used the results of our XAFS studies of Co(II) sorbed by alumina to refine a quasi-thermodynamic uptake model, then used the model to evaluate the sensitivity of predicted Co(II) partition coefficients to the choice of reactions included in the sorption model. 

Similarly, contaminant concentrations in rivers or streams can be roughly assessed based on rate of contaminant introduction and dilution volumes. Estuary or impoundment concentration regimes are highly dependent on the transport mechanisms enumerated. Contaminants may be localized and remain concentrated or may disperse rapidly and become diluted to insignificant levels. The conservative approach is to conduct a more in-depth assessment and use model results or survey data as a basis for determining contaminant concentration levels. 

Modeling results of subsurface pressure gradients were used to simulate subsurface soil gas velocity throughout the unsaturated zone profile. Figure 15 shows vertical profiles of unsaturated-zone air velocities for 12-hr time periods for August and October 1996. Results show that subsurface airflow is almost never zero, as is assumed in a diffusion-only transport model. Air-phase solute transport models based solely on diffusion would therefore not be able to accurately predict contaminant flux from the subsurface. 

Chrysikopoulos CV.Voudrias EA, Fyrillas MM (1994) Modeling of contaminant transport resulting from dissolution of nonaqueous phase liquid pools in saturated porous media. Transp Porous Media 16 125-145... 

Laboratory experiments, transport modeling, field data, and engineering cost analysis provide complementary information to be used in an assessment of the viability of an MNA approach for a site. Information from kinetic sorption/ desorption experiments, selective extraction experiments, reactive transport modeling, and historical case analyses of plumes at several UMTRA sites can be used to establish a framework for evaluation of MNA for uranium contamination (Brady et al, 1998, 2002 Bryan and Siegel, 1998 Jove-Colon et al, 2001). The results of a recent project conducted at the Hanford 100-N site provided information for evaluation of MNA for a °Sr plume that has reached the Columbia River (Kelley et al, 2002). The study included strontium sorption-desorption studies, strontium transport and hydrologic modeling of the near-river system, and evaluation of the comparative costs and predicted effectiveness of alternative remediation strategies. 

The basic thermodynamic principles for describing flow and contaminant transport in the vadose zone are well established, but the complexity of the processes results in different conceptualisations of flow and contaminant transport in current models. The modelling and characterisation of solute fate and transport in soils is complicated by the space-time variability of the underlying processes in particular in the vadose zone (Fig. 1). In addition, any experimental technique is operational at a certain scale, which is not necessarily the scale at which the process can reasonably be described, neither the scale at which a prediction is needed. The expressions of the solute fate and transport are therefore often considered as scale-dependent, and scaling is needed to model and characterise the transport processes at the larger spatial and temporal scales. 

To better quantify the mobility of substances, indicators that can be calculated from the model results have been developed. Indicators for long-range transport potential are the spatial range [26,27], the characteristic travel distance (CTD) [33], the Great Lakes Transport Efficiency (GLTE) [34], and the Arctic contamination potential [35]. In the following two sections, we describe how such indicators have been adapted for transformation products. 

Natural attenuation is controlled by numerous processes, which include sorption, intraparticle diffusion as weU as biological and chemical degradation. In order to be able to quantify respectively predict the fate and transport of contaminants, appropriate models that are able to deal with the complexity and interactions of the involved processes need to be developed. Due to insufficient information on the spatial distribution of transport parameters in the subsurface, stochastic methods are a preferred alternative to deterministic approaches. In the present paper a one-dimensional Lagrangian streamtube model is used to describe the reactive transport of acenaphthene as a sample organic compoimd at field scale. As the streamtube model does not consider the heterogeneity of hydrogeochemical parameters but only hydraubc heterogeneity, model results from the streamtube model are compared in a Monte Carlo approach to results of a two-dimensional Eulerian model. 

The semi-infinite models of Palermo [1] or van Genuchten [2] are accurate predictions of contaminant migration and resulting concentrations only imtil near steady-state conditions are reached and the influence of the conditions at the upper boimdary can no longer be ignored. The time required to achieve steady state can be estimated from the relationships below. A separate relationship is provided for advectively dominated transport and diffusion... 

Steefel et al. ([23] and references therein) noted that the approach does not account for pH, competitive ion effects or oxidation-reduction reactions. As a consequence, values may vary by orders of magnitude from one set of conditions to another. Chen [25] also highlighted these limitations by comparing numerical modeling results of contaminant transport using a multi-component coupled reactive mass transport model and a based transport model. The conclusion from this work was that values vary with location and time and this variation could not be accounted for in the model. 

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