Evaluating an Analytical Model to Predict Subsurface LNAPL Distributions and Transmissivity from Current and Historic Fluid Levels in Groundwater Wells: Comparing Results to Numerical Simulations

Abstract

This column reviews the general features of PHT3D Version 2, a reactive multicomponent transport model that couples the geochemical modeling software PHREEQC-2 (Parkhurst and Appelo 1999) with three-dimensional groundwater flow and transport simulators MODFLOW-2000 and MT3DMS (Zheng and Wang 1999). The original version of PHT3D was developed by Henning Prommer and Version 2 by Henning Prommer and Vincent Post (Prommer and Post 2010). More detailed information about PHT3D is available at the website http://www.pht3d.org.The review was conducted separately by two reviewers. This column is presented in two parts. PART I by C.A.J. Appelo IntroductionPHT3D is a computer code for general reactive transport calculations, coupling MODFLOW/MT3DMS for transport and PHREEQC for chemical reactions. It was developed by Henning Prommer in the 1990s and has been applied by him and his coworkers to various groundwater problems of practical interest. The resulting publications (http://www.pht3d.org/pht3d public.html) show an impressive applicability of the code and illustrate the underlying understanding of quite complicated interactions (e.g., Prommer and Stuyfzand 2005; Prommer et al. 2008 Prommer et al. , 2009). In the original version, transport is calculated during a time step, an input file is written for PHREEQC for calculating reactions such as ion exchange and precipitation or dissolution of minerals, and these steps are repeated for subsequent time steps until finished. This loose coupling has the advantage that updates of the master programs can be installed without much effort. A disadvantage is that the calculation of the chemical reactions needs to be initialized time and again for each cell in the model, which adds another time-consuming step to calculations that are already computer-intensive. Another disadvantage is that surface complexation reactions need to be calculated first using the water composition from the previous time step and then reacted with the changed water concentrations. This procedure was not implemented in the original version of PHT3D, and surface complexation reactions could not be calculated.Prommer and Post recently released the second version of PHT3D that resolves the shortcomings and works very well. The improvement is owing firstly to the implementation of total-variation-diminishing (TVD) scheme that MT3DMS uses for calculating advective and dispersive transport (Zheng and Wang 1999). Secondly, it is because PHREEQC is now being used for storing the chemical data of the model, including the chemical activities and the composition of surface complexes from the previous time step. In addition, the procedure to transport total oxygen and hydrogen has been adapted from PHAST (PHAST is the 3D reactive transport model developed by Parkhurst et al. 2004, based on HST3D and PHREEQC). This enables the user to obtain the redox state of the solution without having to transport individual redox concentrations of the elements (e.g., C being distributed over carbon-dioxide, C(4), and methane, C...

A Screening Model to Predict Entrapped LNAPL Depletion

“…These authors have demonstrated that a high LNAPL saturation can sustain high concentrations over a time scale that could span decades, while a low saturation would completely deplete the LNAPL within a few years. The difficulty in obtaining reliable values of may hamper the accurate prediction of LNAPL depletion using our approach; however, reliable estimates of this parameter may be derived from the floating phase thickness in monitoring wells, as demonstrated by Jeong and Chaberneau [8] and Lenhard et al [12]. Alternatively, Gatsios et al [16] and Teramoto et al [49] demonstrated that may be estimated by the laser-induced fluorescence method via empirical models.…”

“…The parameter A may be estimated using , as previously demonstrated by Helder and Celia [50], Culligan et al [51], and Brusseau et al [52]; however, for practical purposes, this parameter may be optimized manually by trial-and-error during model calibration. The difficulty in obtaining reliable values of S nw may hamper the accurate prediction of LNAPL depletion using our approach; however, reliable estimates of this parameter may be derived from the floating phase thickness in monitoring wells, as demonstrated by Jeong and Chaberneau [8] and Lenhard et al [12]. Alternatively, Gatsios et al [16] and Teramoto et al [49] demonstrated that S nw may be estimated by the laser-induced fluorescence method via empirical models.…”

“…When the LNAPL remains mostly as an immiscible fluid in the capillary fringe, the volatilization in the air/LNAPL interface represents the prevailing mechanism of LNAPL depletion with respect to benzene, ethylbenzene, toluene, and xylenes (BTEX) [1][2][3][4][5]; however, as the water level fluctuates in response to the cyclical alternation of the dry and rainy seasons, there is a continuous redistribution of the LNAPL because it migrates to lower aquifers as water levels decrease [6][7][8][9][10]. When the water level rises, LNAPL is retained by capillary force in the saturated zone, a phenomenon known as entrapment [4][5][6][7][8][9][10], which has been demonstrated in numerous previous studies (e.g., [11][12][13][14][15][16]). When entrapment occurs, LNAPL is entrapped by the capillarity in the saturated zone; in contrast, when the water level drops, LNAPL is released, and previously isolated ganglia clump together to gain mobility.…”

Testing an Analytical Model for Predicting Subsurface LNAPL Distributions from Current and Historic Fluid Levels in Monitoring Wells: A Preliminary Test Considering Hysteresis

“…An assumption is made in the model that the predictions are not dependent on whether the historical highest or lowest fluid level elevations in a well occur first. Numerical simulations suggested that this assumption is reasonable [14].…”

Simulações da migração de plumas dissolvidas de compostos BTEX geradas por LNAPL trapeado

“…The tests indicate that the proposed methodology is a promising alternative to predict the behavior of dissolved plumes over time contemplating the condition of depletion of the source zone. mento (FARR et al, 1990, MARINELLI & DURNFORD, 1996, KEMBLOWSKI & CHIANG, 1990, JEONG & CHARBENEAU, 2014LENHARD et al, 2018). A condição de trapeamento foi demonstrada por incontáveis trabalhos, incluindo Pede (2009), Suthersan et al (2015), Flores et al (2016, Gatsios et al (2018) e Isler et al (2018).…”