Biogeochemical transport modelling of natural and ...

Land Contamination & Reclamation, 8 (3), 2000 DOI 10.2462/09670513.572

© 2000 EPP Publications

Biogeochemical transport modelling of natural and enhanced remediation processes in aquifers H. Prommer, G.B. Davis and D.A. Barry

Abstract PHT3D, a three-dimensional biogeochemical tool for natural and enhanced remediation of organic compounds in saturated porous media, is presented. The model is based on four modules: a three-dimensional transport model accounting for advective-dispersive transport and three modules accounting for (bio)chemical reactions. These comprise a NAPL (non aqueous phase liquids) dissolution module, a geochemical equilibrium package and a biodegradation module. The modules are coupled via an operator-splitting method. Model capabilities are demonstrated by way of an illustrative simulation for a dissolved hydrocarbon plume undergoing natural biogeochemical attenuation. Keywords: biodegradation, biogeochemical modelling, BTEX, groundwater, NAPL dissolution, operator splitting, PHREEQC, PHT3D

INTRODUCTION

influenced by co-contaminants. Heron et al. (1995) have demonstrated changes in mineral speciation and reduction capacity within an aquifer contaminated with landfill leachates. Thullner and Schäfer (1997) have reported significant sediment-water interactions for in situ remediation of chlorinated aromatics via injection of oxygen. They found that a large fraction of the oxygen injected was consumed by oxidation of pyrite. Traditional contaminant transport and fate models, which do not explicitly model secondary geochemical reactions, would not be capable of describing such mineral precipitation or dissolution.

Due to its cost-effectiveness, natural remediation has become the preferred remediation technique for a wide range of dissolved organic contaminants in groundwater. Partial or complete degradation of BTEX (benzene, toluene, ethylbenzene, xylene) and other (e.g. chlorinated solvents) compounds has been demonstrated by Barbaro et al. (1992), Semprini et al. (1995), Johnson et al. (1996), Borden et al. (1997), Rügge et al. (1998), Davis et al. (1999) and many others. Numerical modelling has evolved as an important step in analysing, demonstrating and predicting natural attenuation (e.g. Essaid et al. 1995; Lu et al. 1999). Processes involved in natural remediation, in most cases, are complex interactions between physical transport and (bio)geochemical reactions. For example, degradation rates of chlorinated compounds such as PCE and TCE are heavily dependent on the redox environment generated or

In order to quantify such processes, modelling tools providing extensive geochemical capabilities are needed. Furthermore, field-scale modelling of advection-dominated cases (e.g. Davis et al. 1999) where multiple species/components are transported and interact chemically, requires numerical schemes which can handle advective transport efficiently without producing either excessive numerical dispersion nor oscillations, i.e. negative concentrations. Here, an existing model, PHTRAN (Prommer et al. 1999a), has been extended to a modular three-dimensional biogeochemical model by replacing the one-dimensional simulator for advective-dispersive transport with MT3DMS (Zheng and Wang 1998). This paper gives a brief description of PHT3D and demonstrates its capabilities.

Received May 2000; accepted May 2000 Authors H. Prommer1, G.B. Davis2 and D.A. Barry1 1. Contaminated Land Assessment and Remediation Research Centre, The University of Edinburgh, Edinburgh EH9 3JN, UK 2. Centre for Groundwater Studies, CSIRO Land and Water, Private Bag, P.O. Wembley, WA 6014, Australia

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Land Contamination & Reclamation / Volume 8 / Number 3 / 2000

REACTIVE TRANSPORT MODEL

port is solved by the transport module MT3DMS for ntot entities:

PHT3D couples, via an operator-splitting technique (Yeh and Tripathi 1989; Barry et al. 1996; Steefel and MacQuarrie 1996), interphase mass transfer from a multi-component NAPL phase, three-dimensional advective-dispersive transport of multiple organic compounds and inorganic components, a biodegradation module and a geochemical equilibrium module (PHREEQC, Parkhurst 1995). Advective-dispersive transport is carried out separately for each organic compound. The appropriate governing equation is (in indical notation): ∂ ∂C = ∂x i ∂t

  D ∂C  ij ∂x j 

  − ∂ (v C ) + r + r , dis deg  ∂x i i 

(1)

where C is the aqueous concentration of the organic compound, vi is the pore velocity in direction xi, Dij is the hydrodynamic dispersion coefficient tensor, rdis is a source rate due to mass transfer from a residual NAPL phase and rdeg is a sink rate for mass loss due to biodegradation. Reactive transport of inorganic chemicals is described by

∂C ∂  ∂C  ∂ (viC ) + rmin,ir , = Dij − ∂t ∂xi  ∂x j  ∂xi

(2)

where rmin,ir is a source/sink rate due to mineral precipitation/dissolution and mineralisation of degraded organic compounds. Inorganic chemical species are not transported separately. Instead, using the concept of components (Westall et al. 1976), hydrological transport is carried out for total aqueous component concentrations Cj (Yeh and Tripathi 1989; Engesgaard and Kipp 1992):

Cj = cj +

∑Yijs si ,

(3)

i =1,ns

where cj is the concentration of the component j in the complexed species i, ns is the number of complexed species in dissolved form, Y ijs is the stoichiometric coefficient of the jth aqueous component in the ith complexed species and si is the concentration of the ith complexed species. The (local) redox-state, pe, is modelled by transporting chemicals/components in different redox states separately, while the pH is modelled from the (local) charge balance. In total, hydrological trans218

ntot = norg + ne,nre +

∑n

i =1, ne , re

rs,i

,

(4)

where ne,nre is the number of chemical elements occurring in only one redox state, ne,re is the number of elements occurring in multiple redox states, nrs,i is the appropriate number of redox states of the ith element and norg is the number of organic compounds. A partial redox-equilibrium approach (McNab and Narasimhan 1994; Brun et al. 1994; Postma and Jakobsen 1996) is employed to incorporate slow degradation kinetics into a geochemical equilibrium reaction framework. In this approach, reactions of inorganic chemicals are governed by the local equilibrium assumption (LEA) while the computation of the reaction/degradation rates of organic compounds, rdeg, is accounted for by the biodegradation module. It simulates microbial growth and decay of multiple microbial groups. The former depends on the presence/concentrations of organic substrates, the presence/concentrations of appropriate aqueous and solid electron acceptors (Monod kinetics) and on additional inhibition terms which are included, e.g., to account for diffusional limitations of microbial growth. The biodegradation module links the organic and the inorganic aquifer geochemistry by computing for each organic compound the mass degraded during a time step, ∆t. The geochemical equilibrium module is used to quantify rmin,ir from the mass being transformed to inorganic species/components (mineralisation) and the simultaneous equilibration with mineral phases. Kinetically controlled interphase mass transfer from the NAPL phase, i.e. rdis, is modelled according to Raoult’s law. ILLUSTRATIVE SIMULATION PROBLEM The model capabilities are demonstrated for a three-dimensional simulation of a generic aquifer. The spatial extent of the model domain is 270 m (length) × 36 m (width) × 8 m (thickness). However, due to symmetry, the modelled width was only 18 m. In this model aquifer, a BTEX plume originating from a NAPL source undergoes natural attenuation in a sequence of different redox conditions (aerobic, nitrate-, iron- and sulphate-reducing conditions). The groundwater is assumed to be initially uncontaminated and in geochemical equilibrium. A schematic illustration of the model set-up is shown in Figure 1. The hydraulic system (computed with MODFLOW) has been chosen such that both recharge and upstream inflow each con-

Biogeochemical transport modelling of natural and enhanced remediation processes in aquifers

Figure 1. Schematic illustration of the simulation problem

tribute about 50% of the flux at the outflow end. Oxygen, nitrate and sulphate are soluble electron acceptors. Additional oxidation capacity is provided by goethite (FeOOH). Other minerals included in the simulations are magnetite (Fe3O4) and pyrite (FeS2), both potential end products of degradation reactions. Note that magnetite is a mixed Fe(II)/Fe(III) compound. Its inclusion in the simulation reflects findings that such compounds might be an important end product of iron reduction (e.g. Roden and Zachara 1996). The recharge water is

assumed to have the same chemical composition as the uncontaminated groundwater (see Table 1). The contamination source is, for simplicity, modelled as an immobile NAPL phase located near the water table close to the upstream end of the model domain. The multi-component NAPL consists of six organic compounds including the BTEX compounds. Bacterial activity was simulated for three different bacterial groups: facultative aerobes/denitrifying bacteria, in addition to iron-reducing and sulphate-reducing bacte-

Table 1. Background concentrations of the uncontaminated aquifer in the simulation Aqueous inorganic component*

Background and initial concentrations Cin (mg L-1)

Organic compound, mineral

Background and initial concentrations Cin (mg L-1)

pH

7.68

Benzene

0.0

pe

12.86

Toluene

0.0

O(0)

3.9

Ethylbenzene

0.0

N(V)

31.0

Xylenes

0.0

N(III)

0.0

Propylbenzene

0.0

N(0)

0.0

Trimethylpentane

0.0

N(-III)

0.0

S(VI)

134.4

S(-II)

0.0

Fe(II)

0.0

Fe(III)

0.0

C(IV)

111.6

Ca

69.2

Mg

24.8

Na

166.8

K

4.9

Cl

277.2

Goethite

* Values in parentheses indicate valence state

219

140.0

Magnetite

0.0

Pyrite

0.0


Figure 2. Concentrations of selected aqueous components, minerals and bacterial groups after 800 days simulation time. Cross sections along the plumes are shown for the plume centres; cross sections across the plumes are shown for x = 52.5 m (33 m downstream of the centre of the NAPL source). Note that only half plumes are shown on the right side.

compounds migrate downstream while being either partly or completely degraded by a series of biogeochemical reactions. Due to their higher (multi-component) solubility, only the BTEX compounds reach significant concentrations in the dissolved phase (see benzene and toluene plot in Figure 2, other compounds not shown). Driven by aquifer recharge, the plume centre moves downward toward the base of the aquifer with increasing distance. After two years, the mass of dissolved toluene peaks as dissolution from the NAPL phase and degradation of dissolved toluene reach equilibrium (Figure 3). No such equilibrium is reached for benzene within the model domain as benzene was assumed to degrade only under aerobic conditions and

ria. The total time simulated was 2000 days. Dispersivities of 0.5 m, 0.05 m and 0.01 m were assumed for the longitudinal, horizontal transversal and vertical transversal dispersion, respectively. The third-order total-variation-diminishing (TVD) scheme (Zheng and Wang 1998) was employed for solving advective transport. It largely suppresses numerical diffusion while being mass conservative.

RESULTS Organic Compounds Once dissolved from the NAPL source, the organic 220


Figure 3. Integrated total dissolved contaminant mass; sw = width of source; results of cases with sw = 4.5 were scaled by the factor 1.5 m/4.5 m.

not enough oxygen was available. This can be seen in Figure 3 where the simulated total mass of dissolved toluene and benzene in the aquifer are plotted. Results are shown for two 3D cases with two different NAPL source widths, sw (1.5 m and 4.5 m) and a 2D case (width 1.5 m). For comparison, the appropriate non-reactive cases where the biodegradation and geochemical equilibrium modules (but not the NAPL dissolution module) were switched off are also plotted. The difference between the 2D and 3D plots demonstrate the influence of transversal hydrodynamic mixing on the total (dissolved) contaminant mass. In the toluene case with a source width of 1.5 m, the total mass is only approximately 65% of the mass of the comparable 2D simulation. The benzene plots of the integrated masses from the non-reactive cases show a slow decrease of total mass after the maximum has been reached. This indicates that the benzene dissolution rate is decreasing due to a change of the mole fraction of benzene within the NAPL mix.

milieu (see pe plot in Figure 2). Electron acceptors are used sequentially, leading locally to a complete depletion of oxygen, nitrate and goethite (Figure 2) and a partial depletion of sulphate. The zone depleted in oxygen has the largest extent (both along the plume direction and laterally), followed by nitrate and goethite (Figure 2). However, as a consequence of aquifer recharge (includes recharge of oxygen), the frontal end of the zone depleted in oxygen does not travel as fast as the zone enriched in inorganic carbon. Aerobic degradation and reduction of nitrate cause a decrease in pH while it increases under iron- and sulphate-reducing conditions. In the model scenario presented, a low pH zone is created only at the front of the reactive zone (Figure 2), whereas in the rest of the reactive zone an increased pH is found. Magnetite is the dominant iron species where goethite has been reduced, however, some reduced iron reacts with the sulphide produced in the sulphate-reducing zone and precipitates as pyrite (Figure 2).

Inorganic aquifer geochemistry The mineralisation of the organic compounds leads to a zone (i.e., a plume) enriched in inorganic carbon (see C(IV) plot in Figure 2) and a reduced geochemical

Bacterial activity The plots for the simulated bacterial concentrations indicate the locations of ongoing degradation and the appropriate dominating reduction reactions. As can be 221


seen in the appropriate plots for aerobes/denitrifying and for iron-reducing bacteria (Figure 2), the bacterial activity related to locally completely depleted components/minerals is confined to the fringes of these zones. However, as oxygen and nitrate are replenished from upstream, the contamination source zone is the most active zone while the iron-reducing zone travels downstream. Sulphate-reducing bacteria dominate within the inner plume core where all other electron acceptors except sulphate have been depleted.

a gasoline-contaminated aquifer. Water Resour. Res., 33, 1105-1115. Brun, A., Engesgaard, P. and Frind, E.O. (1994) A coupled microbiology-geochemistry transport model for saturated groundwater flow. In: Transport and reactive processes in aquifers (eds. T.H. Dracos and F. Stauffer), pp. 457-462. A.A. Balkema. Davis, G.B., Barber, C., Power, T.R., Thierrin, J., Patterson, B.M., Rayner, J.L. and Wu, Q. (1999) The variability and intrinsic remediation of a BTEX plume in anaerobic sulphate-rich groundwater. J. Contam. Hydrol., 36, 265-290.

CONCLUSIONS

Engesgaard, P. and Kipp, K.L. (1992) A geochemical transport model for redox-controlled movement of mineral fronts in groundwater flow systems: A case of nitrate removal by oxidation of pyrite. Water Resour. Res., 28, 2829-2843.

The modelling framework for a three-dimensional biogeochemical transport and fate model has been presented. Model capabilities were demonstrated for an illustrative case in which BTEX contamination undergoes natural attenuation in a sequence of biogeochemical reactions. Interactions between the degradation of the organic compounds, the inorganic aquifer geochemistry and the microbial environment were discussed. Besides the type of biogeochemical problem shown in this paper, the model presented is capable of handling a wide range of other pollution types, including purely inorganic problems (Prommer et al. 1999b, 2000). Current work is focusing on implementation and testing the geochemical module PHREEQC-2 (Parkhurst and Appelo 2000) to also allow for arbitrary kinetic reactions of inorganic species, minerals and surfaces.

Essaid, H.I., Bekins, B.A., Godsy, E.M., Warren, E., Baedecker, M.J. and Cozarelli, I.M. (1995) Simulation of aerobic and anaerobic biodegradation processes at a crude-oil spill site. Water Resour. Res., 31, 3309-3327. Heron, G., Bjerg, P.L. and Christensen, T.H. (1995) Redox buffering in shallow aquifers contaminated by leachate. In: Intrinsic Bioremediation. Bioremediation 3(1) (eds. Hinchee, R.E., Wilson, J.T. and Downey, D.C.), pp. 143-152. Battelle Press, Columbus, Ohio. Johnson, J.J., Borden, R.C. and Barlaz, M.A. (1996) Anaerobic biodegradation of hazardous organics in groundwater downgradient of a sanitary landfill. J. Contam. Hydrol., 23, 263-283. Lu, G., Clement, T.P., Zheng, C. and Wiedemeier, T.H., (1999) Natural attenuation of BTEX compounds: model development and field-scale application. Ground Water, 37, 707-717.

ACKNOWLEDGEMENTS This work was partly funded by the Centre for Groundwater Studies, CSIRO. Our thanks go also to Chunmiao Zheng, who provided the source code of MT3DMS at an early stage.

Parkhurst, D.L. (1995) Users guide to PHREEQC – A computer program for speciation, reaction-path, advective-transport and inverse geochemical calculations. Technical Report 4227, US Geol. Survey Water-Resources Investigations Report.

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