Alan J. Rabideau

Short communication: ISOFIT - A program for fitting sorption isotherms to experimental data

Isotherm expressions are important for describing the partitioning of contaminants in environmental systems. ISOFIT (Isotherm Fitting Tool) is a software program that fits isotherm parameters to experimental data via the minimization of a weighted sum of squared error (WSSE) objective function. ISOFIT supports a number of isotherms, including several dual-mode isotherms that combine Freundlich, Langmuir, and Polanyi expressions with a linear partitioning term. To minimize the WSSE objective function, ISOFIT utilizes a hybrid optimization procedure that combines particle swarm optimization with Levenberg-Marquardt nonlinear regression. An initial swarm optimization step identifies promising solutions while circumventing local minima and the follow-on regression step provides local refinement and facilitates the calculation of numerous regression statistics. To demonstrate ISOFIT and evaluate its performance, the program was applied to a readily available sorption dataset and benchmarked against results generated using the MS Excel solver package.

Field performance of air-sparging system for removing TCE from groundwater

The removal of volatile organic compounds from groundwater by air sparging (AS) is well-established, although reliable methods for predicting the time required to reach site closure have not been established. To develop an improved understanding of mass transfer processes that limit AS performance, two extended controlled shutdowns of an operating AS system were performed. Monitoring of TCE concentrations in source zone groundwater indicated “tailing” and “rebound” behavior similar to that observed for pump-and-treat systems. A simple two-compartment model provided a reasonable description of the 3-year AS history, using parameters calibrated from data collected during the first shutdown period. Comparison of the calibrated rate constants with parameters estimated from laboratory soil columns suggests that, for the study site, aqueous diffusion to discrete air channels has a stronger influence on system performance than rate-limited desorption. Predictions based on the calibrated model indicated that rest...

Transport of sequentially decaying reaction products influenced by linear nonequilibrium sorption

Predicting the migration of sequentially degrading reaction products is important for the accurate analysis of subsurface treatment walls and other environmental systems. Recently, it has also been realized that consideration of rate-limited sorption may be necessary to quantify the transport of contaminants in reactive treatment walls. In this paper, analytical and semianalytical solutions are developed for the one-dimensional transport of a parent compound and three sequentially decaying reaction products influenced by nonequilibrium sorption. The solutions are based on the Laplace transformation technique and incorporate a different sorption distribution coefficient and mass transfer rate for each reaction product. Two types of exit boundary conditions are considered: semi-infinite and finite zero-gradient. The developed solutions are used to simulate the migration of trichloroethylene and its reaction products in a zero-valent iron treatment wall. The effect of nonequilibrium sorption on the time to reach steady state is investigated, and it is shown that a zero-valent iron treatment wall may exhibit a significant transient period. However, concentrations reach 80% of the steady state values within the first 100 pore volumes. It is also shown that neglecting dispersion may lead to nonconservative predictions of the effluent contaminant concentrations.

Approximate evaluation of contaminant transport through vertical barriers

Abstract Containment of groundwater contamination using physical barriers can be an important element of a subsurface remediation program. This work presents simple analytical tools for predicting the performance of barriers in terms of the steady-state contaminant flux across the barrier, the duration of the transient period following barrier installation, and the time-dependent contaminant concentration distribution within the barrier. The analytical expressions are developed from approximate boundary layer (BL) solutions to the advective–dispersive equation subject to conservative fixed concentration boundary conditions. Critical ranges of important dimensionless quantities are identified for use in barrier performance assessment, for both steady-state and transient conditions. Comparative calculations made with the BL equations and more exact semi-analytical solutions are used to characterize the accuracy and applicability of the BL approach.

Comparison of soluble and immobilized acetate for removing Pb from contaminated soil

Five lead (Pb) contaminated soils were used in a laboratory and modeling study to examine the effects of soluble and immobilized acetate on Pb removal from a contaminated soil as a function of pH. Soluble acetate was added as sodium acetate; immobilized acetate was added in the form of a cation exchange resin. For comparative purposes, Pb adsorption with no acetate also was measured as a function of pH. A surface complexation modeling framework was used to interpret experimental data. Experimental results showed the cation exchange resin was much more effective than soluble acetate in removing Pb from soils due to a strong affinity of the resin for Pb. In addition, concentrations of soluble Pb in resin/soil slurry were very low, minimizing the pollution threat if discharged. As deduced from modeling studies, soluble acetate performed poorly compared to the resin, in part, due to adsorption of the soluble PbAc(+) complex. The effectiveness of both soluble and immobilized acetate was diminished below pH 4 as a result of competition by H(+) for acetate. Modeling results based on resin affinity for Pb compare well with experimental data for resin/soil mixtures, suggesting that Pb partitioning in resin/soil mixtures may be predicted reasonably well if soil/Pb and resin/Pb partitioning are known. Thus, the modeling approach may be used as a screening tool to determine the performance of alternative resins.

NIGHTHAWK—A program for modeling saturated batch and column experiments incorporating equilibrium and kinetic biogeochemistry

NIGHTHAWK is a computer program for simulating solute transport and transformations in the saturated subsurface. Supported biogeochemical processes include microbial-mediated degradation, inorganic redox chemistry, aqueous-phase equilibrium speciation, mineral precipitation and dissolution, and several sorption processes including surface complexation, ion exchange, and linear/nonlinear isotherms. The primary niche for the code is in supporting laboratory column and batch experiments, and as a screening tool when working with field data. In such a role and relative to existing approaches, the code offers an increased level of sophistication with respect to sorption processes, while not requiring the computational effort or unnecessary dimensionality of field-scale modeling codes.

Background Concentrations of Polycyclic Aromatic Hydrocarbon (PAH) Compounds in New York State Soils

Using information from a variety of published studies, a data set was assembled with approximately 200 surface soil samples collected from urban locations across New York State (NYS) not directly influenced by known sources of contamination. Statistical characteristics for 17 polycyclic aromatic hydrocarbon (PAH) compounds were examined and compared with draft NYS soil cleanup objectives that had been developed using risk-based and rural background considerations. For the carcinogenic PAH compounds, approximately 12–40% of samples exceeded cleanup objectives proposed for residential land use, but few samples exceeded the less stringent standards proposed for commercial and industrial properties. Qualitative comparisons with a recent study of NYS rural soils indicated substantial differences in background PAH levels between urban and rural locations. These findings motivate further research into the best manner for incorporating background information into soil cleanup objectives for urban areas.

Sorbing Vertical Barriers

This chapter presents an overview of vertical barrier technologies that use sorbing materials to remove contaminants from groundwater. Two classes of system are considered: 1) low-permeability earthen barriers, in which sorbing additives are used to reduce the diffusive flux of organic contaminants, and 2) high-permeability treatment walls designed to remove contaminants under advection-dominated natural groundwater conditions. The focus of the discussion is on the performance assessment of strongly sorbing barrier materials using laboratory tests. Emphasis is placed on the design and analysis of column studies to characterize the barrier sorption capacity and the appropriate formulation of mathematical models to extrapolate long-term barrier performance. Two case studies are considered: the amendment of soil-bentonite slurry walls with an organic-rich additives, and the use of natural zeolite to remove strontium-90 from groundwater.

Natural zeolite permeable treatment wall for removing Sr-90 from groundwater.

Experimental and modeling studies were completed to investigate the potential performance of a sorbing permeable treatment wall (PTW) comprised of natural zeolite for removal of strontium-90 (Sr-90) from groundwater at the West Valley Demonstration Project (WVDP) near Buffalo, NY. Multiple column tests were performed at the University at Buffalo (UB) and WVDP for periods ranging from 6 months to 2 years; UB columns were supplied with synthetic groundwater referenced to anticipated field conditions, while radioactive groundwater obtained on site was used for the WVDP columns. The primary focus was on quantifying the competitive cation reactions among five cations (Na(+), K(+), Ca(2+), Mg(2+), Sr(2+)) and Sr-90 with data obtained from the column studies used to estimate Gaines-Thomas (GT) selectivity coefficients. The resulting six-solute transport model provided flexibility to explore the influence of PTW parameters on long-term PTW performance, including variations in Sr-90 concentrations and groundwater geochemistry. The natural zeolite PTW is a viable method for in situ removal of Sr-90 from groundwater and potentially applicable to other sites contaminated by Sr-90.

Parameterizing sorption isotherms using a hybrid global-local fitting procedure

Predictive modeling of the transport and remediation of groundwater contaminants requires an accurate description of the sorption process, which is usually provided by fitting an isotherm model to site-specific laboratory data. Commonly used calibration procedures, listed in order of increasing sophistication, include: trial-and-error, linearization, non-linear regression, global search, and hybrid global-local search. Given the considerable variability in fitting procedures applied in published isotherm studies, we investigated the importance of algorithm selection through a series of numerical experiments involving 13 previously published sorption datasets. These datasets, considered representative of state-of-the-art for isotherm experiments, had been previously analyzed using trial-and-error, linearization, or non-linear regression methods. The isotherm expressions were re-fit using a 3-stage hybrid global-local search procedure (i.e. global search using particle swarm optimization followed by Powells derivative free local search method and Gauss-Marquardt-Levenberg non-linear regression). The re-fitted expressions were then compared to previously published fits in terms of the optimized weighted sum of squared residuals (WSSR) fitness function, the final estimated parameters, and the influence on contaminant transport predictions - where easily computed concentration-dependent contaminant retardation factors served as a surrogate measure of likely transport behavior. Results suggest that many of the previously published calibrated isotherm parameter sets were local minima. In some cases, the updated hybrid global-local search yielded order-of-magnitude reductions in the fitness function. In particular, of the candidate isotherms, the Polanyi-type models were most likely to benefit from the use of the hybrid fitting procedure. In some cases, improvements in fitness function were associated with slight (<10%) changes in parameter values, but in other cases significant (>50%) changes in parameter values were noted. Despite these differences, the influence of isotherm misspecification on contaminant transport predictions was quite variable and difficult to predict from inspection of the isotherms.