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A case study is presented that implements two numerical models for simulating a 30-year PAT operation conducted at a large contaminated site for which high-resolution data sets are available. A Markov chain based stochastic method is used to conditionally generate the realizations with random distribution of heterogeneity for the Tucson International Airport Area (TIAA) federal Superfund site. The fields were conditioned to data collected for 245 boreholes drilled at the site. Both MT3DMS and the advanced random walk particle method (RWhet) were used to simulate the PAT-based mass removal process. The results show that both MT3DMS and RWhet represent the measured data reasonably, with Root Mean Square Error (RMSE) less than 0.03. The use of fine grids and the total-variation-diminishing method (TVD) limited the effects of numerical dispersion for MT3DMS. However, the effects of numerical dispersion were observed when compared to the simulations produced with RWhet using a larger number of particles, which provided more accurate results with RMSE diminishing from 0.027 to 0.024 to 0.020 for simulations with 1, 20, and 50 particles. The computational time increased with more particles used in the model, but was still much less than the time required for MT3DMS, which is an advantage of RWhet. By showing the results using both methods, this study provides guidance for simulating long-term PAT systems. This work will lead to improve understanding of contaminant transport and plume persistence, and in turn will enhance site characterization and site management for contaminated sites with large plumes.

The purpose of this study is to evaluate the impact of heterogeneity on the long-term performance of a large pump-and-treat (PAT) system that has been in operation for 30 years at a site located in Tucson, AZ. A 3D numerical model was developed. Three different concentrations were examined: composite concentration in the influent to the treatment plant, resident concentration in the aquifer, and concentration for downgradient boundary discharge. The time scales needed for concentrations measured in these ways to reach the Maximum Contaminant Levels (MCLs) are significantly different, with ∼125 years required for treatment-plant influent compared to ∼225 years for downgradient boundary discharge and >>227 years (total simulated time) for the resident concentration in the aquifer. These large time scales, compared to 36 years for a hypothetical homogeneous system, demonstrate the significant impacts of permeability heterogeneity on remediation at this site. The possibility of closure of the site was investigated by examining the mass discharge from the site boundary and the concentration rebound after simulating shutdown of the PAT system. The results of this study provide insight on evaluation of closure potential for large, complex contamination sites and a reference on selecting performance metrics for site management.

Military training exercises can result in deposition of energetic residues on range soils, which ultimately can contaminate groundwater with munitions constituents. Column experiments followed by HYDRUS-1D modeling were conducted to evaluate dissolution and transport of energetic constituents from the new insensitive munitions (IM) formulations IMX-101, a mixture of 3-nitro-1,2,4-triazol-5-one (NTO), nitroguanidine (NQ), and 2, 4-dinitroanisole (DNAN), and IMX-104, a mixture of NTO, 1,3,5-hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and DNAN. NTO and DNAN are emerging contaminants associated with the development of insensitive munitions as replacements for traditional munitions. Flow interruption experiments were performed to investigate dissolution kinetics and sorption non-equilibrium between soil and solution phases. The results indicated that insensitive munitions compounds dissolved in order of their aqueous solubility, consistent with prior dissolution studies conducted in the absence of soil. Initial elution of the high concentration pulse of highly soluble NTO and NQ was followed by lower concentrations, while DNAN had generally lower and more constant concentrations in leachate. The sorption of NTO and NQ was low, while RDX, 1,3,5,7-octahydro-1,3,5,7-tetranitrotetrazocine (HMX, an impurity in technical grade RDX), and DNAN all exhibited appreciable sorption. DNAN transformation was observed, with formation of amino-reduction products 2-ANAN (2-amino-4-nitroanisole) and 4-ANAN (4-amino-2-nitroanisole). HYDRUS-1D model, incorporating one-dimensional advective-dispersive transport with particle dissolution and first-order solute transformation was used to simulate the measured breakthrough curves. Optimized dissolution parameters varied widely but were correlated between compounds in the same formulation. Determined adsorption coefficients generally agreed with values determined from batch and column studies conducted with pure NTO and DNAN, while mass-loss rate coefficients were in better agreement with ones from batch than column studies possibly due to suppression of microbial transformation during elution of high concentrations of explosives. Even in the low organic matter soils selected in this study DNAN experienced significant retardation and transformation, indicating potential for its natural attenuation.

A framework for developing predictive models for PFAS physical–chemical properties and mass-partitioning parameters is presented. The framework is based on the objective of developing tools that are of sufficient simplicity to be used rapidly and routinely for initial site investigations and risk assessments. This is accomplished by the use of bespoke PFAS-specific QSPR models. The development of these models entails aggregation and curation of measured data sets for a target property or parameter, supplemented by estimates produced with quantum–chemical ab initio predictions. The application of bespoke QSPR models for PFAS is illustrated with several examples, including partitioning to different interfaces, uptake by several fish species, and partitioning to four different biological materials. Reasonable correlations to molar volume were observed for all systems. One notable observation is that the slopes of all of the regression functions are similar. This suggests that the partitioning processes in all of these systems are to some degree mediated by the same mechanism, namely hydrophobic interaction. Special factors and elements requiring consideration in the development of predictive models are discussed, including differences in bulk-phase versus interface partitioning processes. [ABSTRACT FROM AUTHOR]