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A biogeochemical model of contaminant fate and transport in river waters and sediments

A biogeochemical model of contaminant fate and transport in river waters and sediments
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  See discussions, stats, and author profiles for this publication at: A biogeochemical model of contaminant fateand transport in river waters and sediments  Article   in  Journal of Contaminant Hydrology · November 2009 DOI: 10.1016/j.jconhyd.2009.11.001 · Source: PubMed CITATIONS 24 READS 244 3 authors , including: Some of the authors of this publication are also working on these related projects: A flexible tool for hydraulic and water quality performance assessment of stormwater greeninfrastructure   View projectArash MassoudiehThe Catholic University of America 73   PUBLICATIONS   378   CITATIONS   SEE PROFILE All content following this page was uploaded by Arash Massoudieh on 06 January 2017. The user has requested enhancement of the downloaded file. All in-text references underlined in blue are added to the srcinal documentand are linked to publications on ResearchGate, letting you access and read them immediately.  This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institutionand sharing with colleagues.Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third partywebsites are prohibited.In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further informationregarding Elsevier’s archiving and manuscript policies areencouraged to visit:  Author's personal copy A biogeochemical model of contaminant fate and transport in river watersand sediments Arash Massoudieh ⁎ , Fabián A. Bombardelli, Timothy R. Ginn Department of Civil and Environmental Engineering, University of California, Davis, One Shields Ave., Engineering III, Davis, CA 95616, United States a r t i c l e i n f o a b s t r a c t  Article history: Received 31 December 2008Received in revised form 30 September 2009Accepted 13 November 2009Available online 20 November 2009 A quasi-two-dimensional model is presented for simulating transport and transformation of contaminant species inriver watersand sediments, taking intoaccount theeffect of both bioticand abiotic geochemical reactions on the contaminant fate and mobility. The model considersthe downstream transport of dissolved and sediment-associated species, and the mass transferwith bed sediments due to erosion and resuspension, using linked advection – dispersion – reaction equations. The model also couples both equations to the reactive transport within bedsediment phases. This is done by the use of a set of vertical one-dimensional columnsrepresenting sediment layers that take into account the reactive transport of chemicals, burial,sorption/desorption to/from the solid phase, and the diffusive transport of aqueous species.Kinetically-controlled reversible solid-water mass exchange models are adopted to simulateinteractions between suspended sediments and bulk water, as well as the mass exchangebetween bed sediments and pore water. An innovative multi-time step approach is used tomodel the fully kinetic nonlinear reaction terms using a non-iterative explicit method. Thisapproachenablesthemodeltohandlefastandnear-equilibriumreactionswithoutasigni fi cantincreaseincomputationalburden.Attheend,twodemonstrationcasesaresimulatedusingthemodel, including transport of a sorbing, non-reactive trace metal and nitrogen cycling, both inthe Colusa Basin Drain in the Central Valley of California.© 2009 Elsevier B.V. All rights reserved. Keywords: SedimentContaminant transportRiverBiogeochemical reactionModeling 1. Introduction Contaminationofwaterbodiesisaproblemofcontinually-growing concern worldwide. Due to the high af  fi nity of manycontaminants with solid matter, sediment can serve as arepository of pollutants, and consequently long-term sourceof contaminants in water bodies. During the past decadesresearchers have taken two major diverging pathways inmodeling sediment-facilitated contaminant fate and trans-port in aquatic systems. One path has focused mainly on thetransport of both dissolved and sediment-bound species byphysical processes governed by the hydrodynamics of thewaterbody,includingdepositionandresuspension,whilethesedimentbedhasbeentreatedasaboundary(Thomannetal.,1991;Shrestha,1996;ShresthaandOrlob,1996; Jietal.,2002; Perianez, 2002; Zagar et al., 2007 and references therein).Although the range of dimensionality and the physicalprocesses considered in these models has been generallybroad, ranging from one-dimensional models assumingequilibrium between sediment and aqueous phases as wellas two- and three-dimensional models using multiple siterepresentationofsolid-phasemassexchange,biogeochemicalprocesses in the bed sediments affecting the speciation of contaminants have been largely over-simpli fi ed in suchmodels.The second approach has placed focus mainly on thebiogeochemical processes within the sediments, while usingsimpli fi ed sediment deposition and resuspension models tolink to a usually completely mixed water column. In thesemodels mainly one-dimensional columns representing bedsediments have been used, and the effect of deposition and  Journal of Contaminant Hydrology 112 (2010) 103 – 117 ⁎  Corresponding author. Civil Engineering, The Catholic University of America, Washington, DC 20815, United States. Tel./fax: +1 202 319 5671. E-mail address: (A. Massoudieh).0169-7722/$  –  see front matter © 2009 Elsevier B.V. All rights reserved.doi:10.1016/j.jconhyd.2009.11.001 Contents lists available at ScienceDirect  Journal of Contaminant Hydrology  journal homepage:  Author's personal copy resuspension of sediments has been considered as the upperboundaryconditionofthesystem(e.g.,Furreretal.,1990;vanCappellen et al., 1993; Boudreau, 1996; Soetaert et al., 1996;van Cappellen and Gaillard, 1996; van Cappellen and Wang,1996). These models have been primarily used for simulatingnutrient cycling or carbon diagenesis processes in coastal ormarine sediments. Either of these approaches has someadvantages and drawbacks on their own.In the  fi rst approach, the benthic sediment chemicalprocesses that can affect the speciation  —  and thereforemobility, toxicity, bioavailability, and in some cases biodeg-radation of contaminants  —  are overlooked. For instance, formany metallic contaminants, the mobility of constituents is afunction of the redox condition of the species in thesediments, which in turn is a function of the availability of other major ambient constituents, pH, redox potential andthe activity of certain kinds of bacteria. Temporal and spatialvariations in any of these factors can cause a change in thespeciation of metal contaminants, which in turn can changetheir bioavailability or mobility and cause their release orcontainment in a part of the system. Furthermore, thesebiogeochemical processes can affect the transformation orbiodegradation of nutrients and organic contaminants in thesediments. Clearly, ignoring these processes can signi fi cantlyaffect the usefulness of any modeling effort on contaminantfate and transport.In the second approach, the main focus has been onbiogeochemical processes within the sediments. Highlydetailed biogeochemical models involving kinetic, equilibri-um or mixed reaction networks including dissolution,precipitation, andsurfacecomplexation havebeen developedto predict the fate of species in the sediments (van Cappellenet al., 1993; Boudreau, 1996; van Cappellen and Wang, 1996;Berg et al., 2003). These models have been mainly one-dimensional in nature, without any lateral distinctions, andhave used predetermined top boundary conditions in termsof both concentration of species and sedimentation ratesbased on available data or estimated constant or periodicaltime series (van Cappellen and Gaillard, 1996; Meysmanet al., 2005; Canavan et al., 2007; Sengor et al., 2007). Although this modeling approach may be appropriate forrelatively calm water bodies such as lakes or deep oceans, inmore dynamic systems such as rivers, estuaries, or wetlands,highly spatially and temporally variable sedimentation – resuspension rates can signi fi cantly contribute to the dynam-ics of constituent concentrations. Thus, ignoring theseprocesses cause these models to be less representative of the river systems when the sediment dynamics have asigni fi cant impact on the cycling of contaminants.The main goal of this work is to forge a linkage betweensediment dynamics and biogeochemical processes in bothwaterbodyandbedsediments.Aone-dimensionalhorizontalriver reach is coupled with a vertical representation of bedsediments, yielding a quasi-two-dimensional model. Trans-port of contaminants in both dissolved and sediment-associated forms is considered, and the diffusive massexchange through the sediment – water interface, as well asdeposition and resuspension, are taken into consideration. Amoving-coordinate transformation is used to address burialand exposure due to sedimentation and erosion at the bedsediment – water column interface.The paper is organized as follows: First, the modeling of physical processes in multiple phases, including the contam-inant transport as a result of advection – dispersion, resuspen-sion and sedimentation of particles and mass exchangebetween solid and aqueous phases are described. In thesecond part, the multicomponent reactions modeling ap-proach to simulate chemical transformation of contaminantsis described;  fi nally two demonstration simulation cases arepresented and discussed. 2. Model development and solution technique  2.1. Physical and generic biogeochemical process formulation Thecomputationaldomainofthemodelincludestheriverreachandacertainmaximumdepthofthebenthicsediments.The river reach is represented by a one-dimensional (1-D)model in the stream-wise dimension (Fig. 1) and thetransport processes such as advection – dispersion, depositionand resuspension of sediments and mass exchange of dissolved and sediment-associated constituents are consid-ered. The transport in the bed sediments is represented byconsidering the variation of concentration versus depth, andits dynamics due to vertical transport processes such asdiffusion and bio-dispersion are taken into account. Horizon-tal transport processes in the bed sediments are neglected.This is a reasonable assumption due to the fact that thehorizontal scale in the bed sediments is several orders of magnitude larger than the vertical scale. The couplingbetween the benthic component and the overlying water isconducted by considering the  fl uxes of solids as well asdissolved and particulate-bound chemicals as a result these fl uxes, diffusive mass exchange through the diffusive bound-ary layer and resuspension as source terms in the massbalance equations governing the transport in the watercolumn. These  fl uxes are considered as to boundary condi-tions for the benthic sediments.  2.1.1. Two-phase transport in the river reach Species in the water body are allowed to occur indissolved and (suspended) particulate-bound phases. Themass exchange between the two phases is represented by a fi rst-order kinetic rate model. Mass exchange with benthicsediments for dissolved species is assumed to be through aboundary layer as a linear kinetic exchange process also,whereas for particle-bound phase, species mass exchange isconsidered to result from erosion and deposition. Tocomplete the overall mass balance, the effect of upward fl ow generated due to sediment consolidation is additionallytaken into account. For each species involved, a pair of coupled 1-D advection – dispersion equations, one for dis-solved species and one for particle-associated species, areused: ∂ ð C  i Þ ∂ t   +  v ∂ C  i ∂  x  = 1  A ∂∂  x D h  A ∂ C  i ∂  x   +  k b P  A ð c  i ð 0 Þ − C  i Þ − C  s k r ð K  D C  i − S  i Þ  +  R i  +  q in  A  ð C  i ; in − C  i Þ +  k at ð C  i ; at − C  i Þ − P  Au f  θ 0 c  i ð 0 Þð 1 Þ 104  A. Massoudieh et al. / Journal of Contaminant Hydrology 112 (2010) 103 – 117   Author's personal copy ∂ C  s S  i ∂ t   +  v ∂ C  s S  i ∂  x  = 1  A ∂∂  x D s  A ∂ C  s S  i ∂  x   +  P  AEr s i ð 0 Þ − w p C  s S  i +  q in  A  ð C  s ; in S  in − C  s S  i Þ  +  k r C  s ð K  D C  i − S  i Þ  +  R s ; i ð 2 Þ in which,  t   [T] is time;  x  [L] is the distance along the river axis; C  i  [ Μ c /L  3 ] is the dissolved concentration of chemical  i  in bulkwater; v [L/T]isthecross-sectionalaveragedvelocityalongtheriver;  D h  [L  2 /T] is the mechanical dispersion coef  fi cient alongthe river axis;  k b  is the sediment – water mass exchangecoef  fi cient for the dissolved species [LT − 1 ];  c  i (0) is the pore-water concentration of species  i  at the topmost layer of sediments [ Μ c /L  3 ];  P   is the wetted perimeter of the stream[L];  k r  [T − 1 ] is the mass exchange coef  fi cient betweensuspended particles and water;  C  s  [ Μ /L  3 ] is the concentrationofsuspendedparticles; K  D [L  3 /M]isthewater – soliddistributioncoef  fi cient; S  i [ Μ c /M]isthesorbedphaseconcentration; R i ,and R s, i arethesumofratesofeliminationorproductionofspecies i due to reactions for dissolved and sorbed phases, respectively; q in [L  3 T − 1 L  − 1 ]is theamountoflateralwater fl ux; C  i ,in [ Μ c /L  3 ]is the concentration of species  i  in the lateral  fl ux;  k at  [T − 1 ] isthe atmospheric exchange rate coef  fi cient;  C  i ,at  [M c /L  3 ] is thesaturation concentration for species  i  calculated using Henry'slaw;  D s  [L  2 /T] is the dispersion coef  fi cient for suspendedparticles;  Er   [ML  − 2 T − 1 ] is the sediment entrainment rate; w p  [1/T] is the deposition rate parameter;  s i (0) [Mc/M] is thesorbed concentration at the topmost layer of the bedsediments;  C  s ,in  [ Μ /L  3 ] is the concentration of suspendedparticlesinthelateralin fl ow; u f  [L/T]isthepore - watervelocityin bed sediments due to consolidation (downward de fi ned aspositive) and  θ 0  is the bed sediment porosity at the sediment – water interface, as explained in the Appendix A. In order of appearance the terms in Eq. (1) govern: local change indissolved phase concentration, advection along the river,dispersion along the river, diffusive exchange with bed porewater,sorptiveexchangewiththesuspendedparticulatephase,reaction, solution in fl ow/out fl ow contribution, exchange withtheatmosphere,andin fl owduetobedsedimentconsolidation.InorderofappearancethetermsinEq.(2)govern:localchangeintheconcentrationofparticle-associatedspecies,advectionof particle-associated species due to advection of suspendedparticles, dispersion of particle-associated species due todispersion of particles, the effect of erosion, deposition, lateralin fl ow of particle-associated contaminants, solid-water massexchange and reactions affecting particle-associated species.  2.1.2. Governing equations in the benthic sediments The one-dimensional mass balances described above arecoupled with a one-dimensional (vertical) mass balance of chemical species in the underlying bed sediments. Similar tothe governing model in the overlying water, the massexchange between water and the sediment phase is modeledvia  fi rst-order sorption – desorption, and no precipitation – dissolution reactions are yet involved. In addition, in thismodel, it is assumed that the effects of bioturbation can besimulated using a Fickian diffusive model with a modi fi eddiffusion coef  fi cient corrected in order to take into accountthe mixing due to bioturbation. Thus for the pore-water andparticle-associated phases we have respectively; ∂ ð θ c  i Þ ∂ t   =  −∂ ð θ u f  c  i Þ ∂  z  ⁎  +  ∂∂  z  ⁎  ð D m  +  D B Þ θ ∂ c  i ∂  z  ⁎  − B d k r ð K  D c  i − s i Þ  +  R i ð 3 Þ ∂ ð B d s i Þ ∂ t   =  −∂ ð B d u s s i Þ ∂  z  ⁎  +  ∂∂  z  ⁎  D B B d ∂ c  i ∂  z  ⁎  +  B d k r ð K  D c  i − s i Þ  +  R s ; i ð 4 Þ The  fi rst terms on the right-hand-side of both equationsaccount for the effect of consolidation. The boundaryconditions are as follows: ð D m  +  D B Þ θ ∂ c  i ∂  z  ⁎  =  k b ð c  i − C  i Þ  at  z  ⁎ =  z  0  ð 5 Þ ∂ c  i ∂  z   = 0 at  z  ⁎ =  z  min  ð 6 Þ and for Eq. (4): s i  j  z  ⁎ =  z  min = 0 for  J  0 b 0 erosion  ð 7 Þ Fig. 1.  Processes involved in sediment - associated contaminant transport.105  A. Massoudieh et al. / Journal of Contaminant Hydrology 112 (2010) 103 – 117 
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