Peter Engesgaard

A geochemical transport model for redox‐controlled movement of mineral fronts in groundwater flow systems: A case of nitrate removal by oxidation of pyrite

A one-dimensional prototype geochemical transport model was developed in order to handle simultaneous precipitation-dissolution and oxidation-reduction reactions governed by chemical equilibria. Total aqueous component concentrations are the primary dependent variables, and a sequential iterative approach is used for the calculation. The model was verified by analytical and numerical comparisons and is able to simulate sharp mineral fronts. At a site in Denmark, denitrification has been observed by oxidation of pyrite. Simulation of nitrate movement at this site showed a redox front movement rate of 0.58 m yr−1, which agreed with calculations of others. It appears that the sequential iterative approach is the most practical for extension to multidimensional simulation and for handling large numbers of components and reactions. However, slow convergence may limit the size of redox systems that can be handled.

Modelling the fate of oxidisable organic contaminants in groundwater

Subsurface contamination by organic chemicals is a pervasive environmental problem, susceptible to remediation by natural or enhanced attenuation approaches or more highly engineered methods such as pump-and-treat, amongst others. Such remediation approaches, along with risk assessment or the pressing need to address complex scientific questions, have driven the development of integrated modelling tools that incorporate physical, biological and geochemical processes. We provide a comprehensive modelling framework, including geochemical reactions and interphase mass transfer processes such as sorption/desorption, non-aqueous phase liquid dissolution and mineral precipitatation/dissolution, all of which can be in equilibrium or kinetically controlled. This framework is used to simulate microbially mediated transformation/degradation processes and the attendant microbial population growth and decay. Solution algorithms, particularly the split-operator (SO) approach, are described, along with a brief resume of numerical solution methods. Some of the available numerical models are described, mainly those constructed using available flow, transport and geochemical reaction packages. The general modelling framework is illustrated by pertinent examples, showing the degradation of dissolved organics by microbial activity limited by the availability of nutrients or electron acceptors (i.e., changing redox states), as well as concomitant secondary reactions. Two field-scale modelling examples are discussed, the Vejen landfill (Denmark) and an example where metal contamination is remediated by redox changes wrought by injection of a dissolved organic compound. A summary is provided of current and likely future challenges to modelling of oxidisable organics in the subsurface.

Modelling of transport and biogeochemical processes in pollution plumes: literature review and model development

Abstract A literature survey shows how biogeochemical (coupled organic and inorganic reaction processes) transport models are based on considering the complete biodegradation process as either a single- or as a two-step process. It is demonstrated that some two-step process models rely on the Partial Equilibrium Approach (PEA). The PEA assumes the organic degradation step, and not the electron acceptor consumption step, is rate limiting. This distinction is not possible in one-step process models, where consumption of both the electron donor and acceptor are treated kinetically. A three-dimensional, two-step PEA model is developed. The model allows for Monod kinetics and biomass growth, features usually included only in one-step process models. The biogeochemical part of the model is tested for a batch system with degradation of organic matter under the consumption of a sequence of electron acceptors. A second paper [J. Hydrol. 256 (2002) 230–249], reports the application of the model to a field study of biogeochemical transport processes in a landfill plume in Denmark (Vejen).

Modelling of transport and biogeochemical processes in pollution plumes: Vejen landfill, Denmark

A biogeochemical transport code is used to simulate leachate attenuation, biogeochemical processes, and development of redox zones in a pollution plume downstream of the Vejen landfill in Denmark. Calibration of the degradation parameters resulted in a good agreement with the observed distribution in the plume of a number of species, such as dissolved organic carbon (DOC), Fe2+, NO3−, HCO3−, SO42−, CH4, and pH. The simulated redox zones agree with observations confirming that the Fe-reducing zone played an important role in the attenuation of the DOC plume. Effective first-order rate constants for every redox zone were determined giving DOC half-lives ranging from 100 to 1–2 days going from the methanogenic to the aerobic zone. The order of decrease in DOC half-lives from the anaerobic to the aerobic zone corresponds to findings at other landfills.

Numerical analysis of biological clogging in two-dimensional sand box experiments

Two-dimensional models for biological clogging and sorptive trace transport were used to study the progress of clogging in a sand box experiment. The sand box had been inoculated with a strip of bacteria and exposed to a continuous injection of nitrate and acetate. Brilliant Blue was regularly injected during the clogging experiment and digital images of the tracer movement had been converted to concentration maps using an image analysis. The calibration of the models to the Brilliant Blue observations shows that Brilliant Blue has a solid biomass dependent sorption that is not compliant with the assumed linear constant Kd behaviour. It is demonstrated that the dimensionality of sand box experiments in comparison to column experiments results in a much lower reduction in hydraulic conductivity (factor of 100) and that the bulk hydraulic conductivity of the sand box decreased only slightly. However, in the central parts of the clogged area, the observations and simulations clearly show a complex picture of flow diverting the injected nutrients around the clogged area as fingers. The calibration of the model demonstrates that the physical and microbiological processes (advection, dispersion, attachment-detachment, growth-decay) are all needed to capture the progress of clogging.

Large‐Scale Dispersion in a Sandy Aquifer: Simulation of Subsurface Transport of Environmental Tritium

Large-scale dispersion in a sandy unconfined aquifer in Denmark was studied by simulating subsurface transport of environmental tritium. Subsurface transport included transport in a moderately deep unsaturated zone and in a relatively long cross section of the aquifer. The tritium data from the site enabled a four-step modeling analysis comprising (1) estimation of tritium content in the infiltration water, (2) transport in the unsaturated zone, (3) estimation of flux-averaged tritium concentration in the recharge water, and (4) transport in the groundwater zone. The groundwater model simulations were sensitive to the longitudinal and transverse dispersivity parameters, αL and αr, as a set of parameters, but a model sensitivity analysis showed that it was not possible to identify a unique set of parameter values. A likely range of variation for the two parameters could be identified: (αL, αT); ∈ [(1 m, 0.005 m); (10 m, 0.0 m)] the two parameters being interdependent in that an increase in αL results in a decrease in αT and vice versa. The reported dispersivities represent a scale of 1000 m, the approximate travel distance from the water table to the observation wells. If the estimated αL can be regarded as being of intermediate reliability following earlier defined criteria, the range or the representative set of values then represent the largest scale of earlier reported values. Including our range of αL in the set of reported dispersivities suggests that αL does not increase indefinitely with scale.

Contaminant Transport at a Waste Residue Deposit: 1. Inverse Flow and Nonreactive Transport Modeling

An application of an inverse flow and transport model to a contaminated aquifer is presented. The objective of the study is to identify physical and nonreactive flow and transport parameters through an optimization approach. The approach can be classified as a statistical procedure, where a flow and transport simulation model is combined with nonlinear least squares multiple regression. The U.S. Geological Survey method of characteristics model is used to simulate flow and transport, and the optimization part is solved using a Levenberg-Marquardt algorithm. The sensitivity of the optimization approach to steady state versus transient flow conditions and to the amount of hydraulic and solute data used is investigated. The flow parameters, transmissivity and leakage factor, are estimated simultaneously with the transport parameters: source strength, porosity, and longitudinal dispersivity. This paper is the first in a two-paper series describing contaminant transport at a waste residue site. In the second paper, reactive transport at the site is investigated.

Contaminant Transport at a Waste Residue Deposit: 2. Geochemical Transport Modeling

Contaminant transport in an aquifer at an incinerator waste residue deposit in Denmark is simulated. A two-dimensional, geochemical transport code is developed for this purpose and tested by comparison to results from another code. The code is applied to a column experiment and to the field site. The ongoing geochemical processes and any unknown geochemical parameters are obtained through simulation of the column experiment that used soil and leachate from the field site. The application of the code to the field site, which has been monitored for more than 15 years, use this geochemical information along with the flow and nonreactive transport parameters obtained by the inverse modeling procedure described in the first paper [Sonnenborg et al., this issue] of this two-paper series. The simulation results of the site model are compared with several measured component breakthroughs at monitoring wells. Contamination was first controlled by transport, and later by transport and ion exchange. In both the column and field site simulations the code is used to identify the controlling transport processes, physical or geochemical (ion exchange and mineral precipitation), and to estimate the involved parameters (primarily ion exchange selectivity coefficients).

Development of preferential flow in bioclogging of porous media

Bioclogging in the subsurface has been noted to occur in a number of situations, typically in engineered systems such as in bioremediation and artificial recharge, see [1,2] for a presentation of earlier studies on bioclogging. Recent experimental findings by [2–5] have shown that the growth of bacteria can create structures that may change over time and that growth can cause the development of preferential flow. Preferential flow is here loosely defined as the change in flow from being more or less uniform to structured non-uniform flow. Clogging, therefore, does not necessarily lead to very erratic changes in the flow field, but can develop heterogeneous structures that can promote flow in certain directions. [2] and [4] showed how preferential flow paths can develop in 2D sand box experiments and recently [5] carried out similar experiments that indicated the development of finger-like flow structures. In this paper we shall focus on describing and modelling a column experiment where flow apparently developed from being uniform to what can be characterized as preferential flow behaviour. The data and modelling results suggest that changes in the flow regime must have occurred during the clogging experiment.

Modeling transport and biogeochemical processes in dense landfill leachate plumes

A 3D density-dependent flow and biogeochemical transport model was developed and applied to study the effects of a dense leachate plume on secondary redox reactions at a Norwegian landfill site.