Mibitrans: A python package for modelling subsurface contaminant transport and natural attenuation decision support

Flow and solute transport in groundwater are primarily controlled by the hydraulic conductivity (K) field. Variations in the K field greatly influence subsurface flow rates and solute migration and dispersion. Due to the importance of aquifer heterogeneity, several approaches have been proposed in the literature for modelling K field spatial distribution based on borehole data. In this work, we evaluate two methods to generate spatial realisations of the K field of a heterogeneous aquifer. Results are analysed in terms of the connectivity of high-K cells, and solute transport results are compared and discussed. The methodology consists of three main steps. First, borehole PSD data are used to characterise hydrofacies and generate 3D stochastic realisations of those hydrofacies using the Markov-Chain/Transition Probability approach (MC/TP). Resultant realisations are used to fill a 3D grid. Then, two methods are used to generate hydraulic conductivity fields by assigning a K value to each cell on those grids: (1) using the geometric mean of each hydrofacies (GMEAN) and (2) using a value sampled from the KDE probability functions of each hydrofacies (KDE). Two different porosity scenarios are considered (high porosity, HP; and low porosity, LP). Finally, solute transport estimates were computed using MODFLOW and MT3DMS.Connectivity analysis of the stochastic realisations shows a higher degree of connectivity of high-K cells on the GMEAN mode than in the KDE. The latter leads to overall higher average K values, but also to slower flow regions with lower K values than the GMEAN.Solute transport runs result in slower travel times and lower peak concentrations on the KDE realisations than in the GMEAN case. Breakthrough curves at different observation points show that, when concentrations fall after peaking (for a time-limited input pulse), both GMEAN and KDE curves tend to converge. The field-scale longitudinal dispersivity implied from the ensemble probability plumes is similar between the two K field realisation approaches modelled, but the vertical dispersivity is higher in the KDE realisations. The high porosity scenario shows higher dispersivities and considerably longer travel times.Results show that, for the same porosity scenario, contaminant plumes behave similarly for the KDE and GMEAN approaches at longer times and distances from the source. This suggests that, on the studied site, the small-scale heterogeneity has a reduced effect on the on the long term, field scale macrodispersivity and solute transport behaviour.

Application of a reactive transport model for interpreting non-conservative tracer experiments: The Rio Maior case-study

The present study is concerned with the modeling of conservative as well as non-conservative solute transport in ground water. The model is based on an operator split approach which uses an Eulerian frame work with finite volume method for the advective transport and fully implicit central difference method for the dispersive transport. This formulation helps in accurately simulating both highly advective and dispersive transport cases with less restriction on the grid size and time step. The numerical solution is compared with exact as well as the approximate analytical solution and the maximum error as percentage of peak concentration is presented for different values of decay constant at varying distances. It is found that finite volume method provides accurate solutions of both conservative and non-conservative solutes for both advection and dispersion dominated situations.

Pressure pulsing technology is an innovative method that has been developed with the aim of overcoming preferred flow paths associated with remediation techniques that rely on the injection of reagents. Numerical and field experiments were conducted to assess how pressure pulsing affects groundwater flow and solute transport during reagent injection. A series of field experiments were performed at two field sites where a monitoring network designed to capture the breakthrough of solutes delivered from an injection well was installed. Pressure pulsing and conventional injection methods were used at each site. One site was comprised of fine sand with low heterogeneity, while the other was moderately heterogeneous with discrete layers varying from fine sand to silt. The data suggest that breakthrough was more uniform for the pressure pulsing injections; however, this difference was minor and complicated by sorption of some of the tracers employed. The groundwater flow and solute transport modeling exercise simulated the rapid boundary pressure modulation that occurs in association with pressure pulsing. Two-dimensional (2D) simulations revealed that repeated sudden onset of injection cessation produces brief periods of gradient reversal and the development of a mixing zone near the injection well. The spatial extents of this mixing zone were found to be highly dependent upon the hydraulic diffusivity of the medium, with medium heterogeneity and pulsing frequency playing secondary roles. Three-dimensional (3D) numerical simulations were used to benchmark the observations from one of the field sites. The results from the modeling effort showed that solute breakthrough from a pressure pulsing injection is more dispersed relative to a conventional injection as a result of the mixing zone phenomenon; however, we were unable to directly observe this mixing zone using the instrumentation deployed at the two field sites.

Numerical models for solving reactive solute transport in groundwater are often based on standard Eulerian methods, such as finite elements (FE). In this work, we combine the moving mesh Eulerian Lagrangian Localized Adjoint Method (ELLAM) with the direct substitution approach (DSA) and the sequential non‐iterative approach (SNIA) to accurately solve advection dominated problems including chemical reactions. Performances of ELLAM are evaluated in comparing SNIA_ELLAM and DSA_ELLAM to SNIA_FE and DSA_FE for (1) kinetic reactions with first‐order decay, Monod biodegradation, and interphase mass transfer problems and (2) equilibrium reactions with linear and nonlinear sorption problems. DSA_ELLAM (respectively SNIA_ELLAM) is shown to be more efficient and accurate than DSA_FE (respectively SNIA_FE). For the test cases with kinetics, SNIA_ELLAM is highly accurate and efficient. However, for equilibrium reactions, it induces numerical diffusion and DSA_ELLAM reveals the more efficient and accurate method. © 2007 American Institute of Chemical Engineers AIChE J, 2007

The U.S. Department of Energy (DOE) and the U.S. Department of Army (DA) are conducting an evaluation to identify the appropriate response action to address groundwater contamination at the Weldon Spring Chemical Plant (WSCP) and the Weldon Spring Ordnance Works (WSOW), respectively. The two areas are located in St. Charles County, about 48 km (30 rni) west of St. Louis. The groundwater operable unit (GWOU) at the WSCP is one of four operable units being evaluated by DOE as part of the Weldon Spring Site Remedial Action Project (WSSRAP). The groundwater operable unit at the WSOW is being evaluated by the DA as Operable Unit 2 (OU2); soil and pipeline contamination are being managed under Operable Unit 1 (OU1). Remedial activities at the WSCP and the WSOW are being conducted in accordance with the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA). Consistent with DOE policy, National Environmental Policy Act (NEPA) values have been incorporated into the CERCLA process. A remedial investigation/feasibility study (RI/FS) work plan summarizing initial site conditions and providing site hydrogeological and exposure models was published in August of 1995 (DOE 1995). The remedial investigation (RI) and baseline risk assessment (BRA) have also recently been completed. The RI (DOE and DA 1998b) discusses in detail the nature, extent, fate, and transport of groundwater and spring water contamination. The BRA (DOE and DA 1998a) is a combined baseline assessment of potential human health and ecological impacts and provides the estimated potential health risks and ecological impacts associated with groundwater and springwater contamination if no remedial action were taken. This feasibility study (FS) has been prepared to evaluate potential options for addressing groundwater contamination at the WSCP and the WSOW. A brief description of the history and environmental setting of the sites is presented in Section 1.1, key information relative to the nature and extent of contamination is presented in Section 1.2, and the results of the BRA are summarized in Section 1.3. The objective of this FS is discussed in Section 1.4, and preliminary remediation goals are identified in Section 1.5. The organization of the remaining chapters of this FS is outlined in Section 1.6.

<p>Statistical analysis of development and distribution of fault fissures at 3720 m in the first mining region of Pulang Copper Mine, SW China was performed. Strongly heterogeneous hydro-geological numerical models influenced by fault fissures were set up. Effects of developmental distribution cracks on solute transport in 65° and 307°directions were simulated. Rock mass with strong permeability water-conducting fissure was found favorable for solute transport, water-blocking fault was found to inhibit solute transport. The direction of fracture development was found consistent with direction of groundwater flow, which would be beneficial for solute transport along water-flow direction. When direction of fracture development intersects with that of groundwater flow and the strong lens affected by fracture changes direction of groundwater flow, direction of solute transport is changed.</p>

It is important for establishment of safety assessment techniques of geological disposal to understand groundwater flow and solute transport accurately. Therefore, we are positioning to confirm an applicability of the techniques in realistic environment as a crucial issue in R&D. We have attempted and planed some relevant studies as below: - A methodology to integrate activities from site investigations to evaluation of solute transport was examined. We have carried out groundwater flow analysis on a regional scale using geological and hydrological information from surface-based investigations at the Horonobe area, and also solute transport analysis based on the information of the trajectory analysis. - We have carried out a preliminary simulation of groundwater flow and salinity concentration distribution using information on climatic and sea-level changes, and evolution of geological structures considering the impacts of natural events and processes. Consequently, we could outline the impacts of natural events and processes on geological environment including hydrogeology, hydrochemistry and their evolutions. - We have been planning to develop and apply a methodology of groundwater flow and solute transport analysis to the shallow part, the Horonobe coastal area and around the URL. These techniques would become a basis for future site specific safety assessment in Japan.

Efficient simulation of groundwater solute transport using the multipoint flux approximation method with arbitrary polygon grids

Few analytical or semi-analytical models simulating the transport of sequentially decaying reaction products affected by nonequilibrium sorption in the groundwater have been presented considering decay or degradation reaction occurring exclusively in the dissolved phase in the literature. However, the process of decay in the sorbed phase, which is important for the transport of decaying contaminants, has been neglected in previously developed analytical models. This study is thus designed to develop a novel semi-analytical model for simulating the multispecies transport of decaying contaminants subject to a nonequilibrium sorption process simultaneously coupled in both the dissolved and sorbed phases. For this purpose, a set of first-order reversible kinetic sorption reaction equations that respectively represent the nonequilibrium sorption processes between the dissolved and sorbed phases, are coupled to a set of advection-dispersion equations, to illustrate the decay process which occurs in both the dissolved and the sorbed phases. By including the decay in the sorbed phase both the parent sorbed concentration and daughter sorbed concentration coexist in a set of first-order reversible kinetic sorption reaction equations, which absolutely complicate the theoretical derivation of the analytical solution. Recursive analytical solutions are derived to account for the concentration distribution of arbitrary transformation products with the aid of the Laplace transform and generalized integral transform. The correctness of the solutions is confirmed through a comparison of our newly derived recursive analytical solution with an existing model which considers an equilibrium sorption process. The newly developed recursive analytical solution is then applied to investigate how the decay in the sorbed phase affects the nonequilibrium transport of a four-member radionuclide decay chain. The result clearly predicts a lower radioactivity concentration of the first nuclide (238Pu), with decay in the sorbed phase, than the simulated results obtained using a model without decay in the sorbed phase. For other daughter elements (234 U, 230 Th, 226 Ra) of the radionuclide decay chain, neglect of decay in the sorbed phase leads to overestimation of the radioactivity concentrations. The result of large differences found in dissolved radioactivity concentration between the decay and no decay in sorbed phase may alter the decision of health risk assessment in the performance of radioactive waste disposal site. The implication is that the decay reactions of contaminants in the sorbed phase are important mechanisms that should be taken into accounts for accurately simulating and assessing the nonequilibrium multispecies transport of decaying contaminants.

A solute transport model representing a 2.3-sq mi area surrounding and downgradient from a municipal landfill site in the Town of Brookhaven, N.Y. was used to simulate migration of a conservative solute (chloride) in the upper glacial aquifer. Aquifer values used in the model were: hydraulic conductivity, 200 ft/day; effective porosity, 0.30; longitudinal dispersivity, 100 ft; transverse dispersivity, 20 ft. Average concentration of chloride was set at 875.0 mg/L in leachate and 10 mg/L in recharge and in ambient groundwater. Entry of leachate into the aquifer was assumed to have begun in 1977. Chloride concentrations in the simulated plume after 6 years of travel matched reasonably well the chloride data collected in October-December 1982. After 12 years of travel, the simulated plume extended 6,200 ft and was 2,600 ft wide. Maximum predicted concentration at the site boundary was 160 mg/L. Additional simulations were made to test the model 's ability to predict the effect of several remedial strategies on the movement of solutes. These included capping the landfill with an impermeable surface, removal of contaminated groundwater through four recovery wells, and a combination of the first two actions.

This study examines a new hybrid three‐dimensional computational solution for groundwater solute transport in a layered aquifer. In this study, the horizontal length scale of the computational domain, referring to the nature geometry, is assumed much larger than the vertical length scale. Thus the computation of solute transportation in the horizontal direction is more complex than the computation in the vertical direction. Using the Dupuit‐Forchheimer approximation of groundwater flow as a reference, this study employs the vertical/horizontal splitting (VHS) concept to divide the three‐dimensional solute transport equations into depth averaged (two‐dimensional) and vertical variation (one‐dimensional) equations. The two‐dimensional equations are processed via the so‐called finite analytic method, while the one‐dimensional equations are solved analytically. The three‐dimensional solute transport is effectively reduced to a hybrid three‐dimensional model by combining the two‐dimensional computations and the one‐dimensional analytical solutions. The resultant hybrid three‐dimensional method is capable of large computational time steps and large spatial grids and is thus suitable for simulating solute transport in large‐scale sites of layered aquifers with negligible spatial variation of both thickness and hydraulic properties within each layer, where traditional numerical schemes fall short of satisfying the requirements of both accuracy and efficiency. Nevertheless, the use of this method is necessarily restricted when (1) the site is extremely heterogeneous and thus cannot be treated as layers or (2) the flows in the aquifer and in the aquitard are highly three‐dimensional. This method is verified by analytical solutions as well as by the numerical model MT3D.

18.2 - Groundwater Quality Monitoring in Landfill Areas

Convective-dispersive transport of solutes involved in sequential first-order decay reactions

A finite element formulation is developed for the simulation of groundwater flow and solute transport in multilayer systems of several aquifers and aquitards. This formulation is general, flexibile, and capable of taking full advantage of the nature of flow in such multilayer systems. A fully three‐dimensional spatial representation can be performed for certain aquifers or for the entire flow system if needed. Those parts of the system that require three‐dimensional spatial discretization are handled effectively by combining two‐dimensional basis functions in the x‐y space and one‐dimensional basis functions in the z space. Furthermore, the formulation has a desirable option to perform one‐dimensional representation of flow and transport in aquitards and areal representation of flow and transport in certain aquifers that do not require the fully three‐dimensional discretization. When the one‐dimensional representation of an aquitard is used, coupling of adjacent aquifer and aquitard layers is handled using a convolution integral approach. A general solution strategy is also developed to allow systematic time stepping and cost‐effective matrix handling schemes. For those parts of the system that require three‐dimensional discretization, an algorithm referred to as the alternate sublayer and line sweep procedure is presented for decomposing three‐dimensional matrix equations. This algorithm can accomodate several thousand nodal unknowns without requiring excessive core storage and CPU time. Four examples of transient flow and transport problems are provided. These examples show verification of numerical results and demonstrate that the present finite element models are far more cost‐effective than earlier three‐dimensional models that are based on the conventional Galerkin approach and direct matrix solution algorithms.