Tài liệu Nonequilibrium transport and sorption of organic chemicals during aquifer remediation doc

HydrologicalSciences-Journal-des Sciences Hydrologiques, 42(2) April 1997 245 Nonequilibrium transport and sorption of organic chemicals during aquifer remediation CORS VAN DEN BRINK IWACO, Consultants for Water & Environment, PO Box 8064, 9702 KB Groningen, The Netherlands WILLEM J. ZAADNOORDIJK [WACO, Consultants for Water & Environment, PO Box 8520, 3009 AM Rotterdam, The Netherlands Abstract Aquifer remediation operations are often behind schedule. Usually, a rather sharp concentration decrease shortly after the start of an operation is followed by a situation in which hardly any concentration decrease is observed. Furthermore, the concentration increases after stopping the groundwater recovery. These phenomena are caused by so-called tailing. An important cause of tailing is the phenomenon that equilibrium is not reached for some of the transport and sorption processes involved. To predict these effects of tailing, IWACO has developed a program SORWACO, which describes the behaviour of solutes along a path line. Processes for which equilibrium is reached quickly as well as processes for which equilibrium is reached only slowly are taken into account. The program has been verified against break- through curves observed in column experiments reported in the literature. The program parameters were calibrated using the data of several experiments. The resulting set of parameter values accurately described the transport for different flow velocities. The fact that quite good results can be obtained without a lot of data from a specific site makes the program a valuable tool for the design of remediation operations. The program is especially useful when extensive input data are not available so that detailed three-dimensional or stochastic models cannot be applied. The use of the program is illustrated by means of a case study. The progress was monitored and the data show good correspondence with the predictions of the program. Transport et désorption différences de produits chimiques organiques pendant la restauration d'un aquifère Résumé Les opérations de restauration d'aquifères interviennent souvent tardivement. En général, la concentration élevée en éléments chimiques indésirables décroît rapidement dès le début de l'opération, puis intervient une période pendant laquelle la diminution de concentration est à peine observable. Parfois même, la concentration augmente à nouveau après l'arrêt du traitement des eaux souterraines. Ce phénomène est causé par un transport et une désorption retardés (tailing). Ce retard est lié au fait que l'arrêt de l'opération ne signifie pas que l'équilibre est atteint pour le transport et la désorption des éléments. Afin de prédire les effets de ce retard, IWACO a développé le programme informatique de modélisation, SORWACO, qui décrit le comportement des substances en solution le long d'un filet d'écoulement. Aussi bien les processus pour lesquels l'équilibre est atteint rapidement que ceux pour lesquels il est atteint lentement ont été pris en compte. Le programme a été vérifié par rapport aux courbes de dégradation observées sur colonnes expérimentales et décrites dans la littérature. Les paramètres du programme ont été calés en utilisant les données de plusieurs expérimentations. Les valeurs des paramètres obtenues décrivent précisément le transport pour différentes vitesses d'écoulement. Le fait que de bons résultats puissent être obtenus sur un site spécifique même en l'absence d'un grand nombre de données le concernant, fait que le programme est un outil fiable pour la conception des opérations de restauration. Le programme est en particulier utile lorsque des modèles tridimensionnels ou stochastiques détaillés ne peuvent être utilisés par manque de données de base en quantité suffisante. Dans cet article, l'utilisation du programme est Open for discussion until I October 1997 246 Cors van den Brink & Willem J. Zaadnoordijk illustrée par une étude de cas. L'avancement de l'opération a fait l'objet d'un suivi et les résultats des mesures présentent une bonne correspondance avec les prédictions faites initialement avec le programme. INTRODUCTION The design of an aquifer remediation operation is merely a case of practical experience today. The contaminant is flushed out of the soil by means of a system of injection and recovery wells. The time needed and the amount of water that has to be flushed through the soil to reach a certain required concentration are both important. The amount of water is expressed in terms of the so-called flush factor, which is equal to the ratio of the volume of this water and the volume of the pores in the soil to be flushed. The time needed and the amount of water (flush factor) are estimated based on experience with similar types of pollutants and soils. In this way, processes and para- meters which determine the course of the remediation are taken into account only indirectly. Moreover, it is not possible to gain insight in a particular situation by determining the influence of various parameters. Some important influences are: - equilibrium amount of sorption; - kinetics of sorption; - variation in flow velocities of the soil liquid caused by heterogeneities in the soil; and - kinetics of exchange between portions of the soil liquid phase with different flow velocities. IWACO has developed the program SORWACO to increase insight into the influence of the individual processes and parameters on the course of the remediation with the possibility of intermittent recovery. The program calculates the changes of the pollutant concentration along its path through the soil and as a function of time. This pathline has to be split up into a number of cells, which have fixed positions that do not change with time. The parameters may have different values for each cell (e.g. bulk density, porosity, and organic carbon content of the soil). The equilibrium of the pollutant between the groundwater and the solid phase of the soil is described by a Freundlich isotherm. This nonlinear sorption isotherm does not vary in time. Sorption takes place at the so-called "sorption sites" of the soil. Two classes of sorption sites are distinguished (Boesten, 1986; Brasseau, 1992b). The sorption sites of class 1 are continuously in equilibrium. The class 2 sorption sites are not continuously in equilibrium with the soil solution. The rate of (de)sorption at class 2 sites is driven by the shortage (or excess) of the sorbed amount relative to the concentration in the soil liquid phase, which in turn depends on the properties of the solute/soil combination and the velocity of the groundwater. When such a sorption shortage or excess exists one talks about "sorption related nonequilibrium". The soil liquid phase is divided into a fast and a slow moving portion to account for the variations in velocity that occur in a porous medium. The exchange of pollutant between these portions is driven by the concentration difference and is Nonequilibrium transport and sorption of organic chemicals during aquifer remediation 247 further determined by the extent of the slow and fast moving portions of the liquid phase, the respective velocities, and the diffusivity. A "transport related nonequilibrium" exists if the concentrations are not equal and there is exchange between the portions of the liquid phase. The program SORWACO takes both the transport and sorption related nonequilibrium into account. It calculates the concentration of the pollutant in each cell at every time step. The concentration in the water flowing out of the last cell is the concentration of the water that enters the purification or discharge system. It is possible to calculate the impact of intermittent groundwater recovery on both the decrease of the concentration during the recovery and the increase when the recovery is stopped. The groundwater flow is assumed to be steady. Changes in the flow pattern are not accounted for. When intermittent recovery is considered, it is assumed that the flow directions remain the same so that the groundwater flow pattern does not change. Only the size of the flow velocity is different. The calculations describe the behaviour of one substance. Interaction with other chemicals, like cation exchange reactions or precipitation reactions, is neglected. The time needed to reduce the concentrations to the required values and the flush-factor can be derived directly from the output of SORWACO. In this way the program can be used during the design of an aquifer remediation operation. Usually the prediction is evaluated and adjusted during the operation resulting in an improved prediction for the following periods of the operation. THEORY Recent literature shows that much has been learned about the effects of diffusion, dispersion, advection and adsorption on chemical transport in soils (van Genuchten & Wierenga, 1976; Goltz & Roberts, 1988; Ptacek & Gillham, 1992). Numerous models have been developed in attempts to describe the one-dimensional transport of chemicals. These models are important because they continuously increase insight into the basic transport mechanisms involved and, consequently, improve the capability to predict the fate in the soils of such diverse chemicals as pesticides, chlorinated hydrocarbons and heavy metals. In SORWACO, the solute transport is assumed to be one-dimensionally advective and dispersive. The conservation equation states that the change of the total mass concentration C in the system must match the gradient of the advective and dispersive flux /plus the decay R, (e.g. Bolt, 1982): where t indicates time and x the ordinate along the pathline of the groundwater flow [L]. The total amount C, the flux J, and the decay R, are given by: C = cB + pS (2) 248 Cors van den Brink & Willem J. Zaadnoordijk J = QVC<D % } (3) R,= k,c (4) in which c denotes the concentration in the soil liquid phase, 9 the volumetric soilwater content [L 3 L" 3 ], p the bulk density of the soil [M L/ 3 ], S the adsorbed concentration [M M" 1 ], v the average pore water velocity [L t" 1 ], D the dispersion coefficient and k, the decay rate. Equations (1) to (4) do not suffice for the description of contaminant transport which includes tailing. Two phenomena will be added to arrive at a set of equations that is capable of simulating properly transport with tailing: transport related non- equilibrium and sorption related nonequilibrium. Transport related nonequilibrium Equations (1) to (4) imply that single values of the velocity and of the dispersion coefficient describe the advective and dispersive transport of the entire soil liquid phase. This results in the calculation of effluent curves which are characteristically sigmoidal or symmetrical in shape. However, experimental curves often show a much earlier breakthrough and a much longer tailing. This extreme tailing does not occur only in unsaturated soils (van Genuchten & Wieringa, 1976; Boesten, 1986) but also in saturated soils (Goltz & Roberts, 1988; Ptacek & Gillham, 1992). One approach by means of which this extreme tailing can be accounted for is the introduction of regions within the soil liquid phase that have different flow velocities. Coats & Smith (1964) used a mobile and an immobile region. Leistra (1977) used a region with a low velocity instead of an immobile region. Stagnitti et al. (1993) used a larger number of regions each having a different velocity. Advective solute transport is more important in the regions with higher velocity, while the solute flux in the slower regions is mainly controlled by diffusion through those regions. The physical structure of the soil is responsible for the differences in groundwater velocity. Slow moving portions will occur, for instance, within loamy layers in a sandy aquifer. Dividing the liquid phase into a fast and slow moving portion, the conservation equation (1) can be written as: dC f dj f "àT = &•-*'•'-•'/- (5a) dt dx -Ru+Jf^, (5b) where the subscripts / and s refer to fast moving and slow moving liquid regions respectively, and J Hs is the exchange between the fast moving and slow moving liquid phase: Nonequilibrium transport and sorption of organic chemicals during aquifer remediation 249 Jf^„=k s j(c f -C s ) (6) The mass transfer coefficient, k sf , in equation (6) determines the rate of exchange between the two liquid regions. This rate is proportional to the difference in concentration between the fast and slow moving portions of the soil liquid phase. In the derivation of equations (5) and (6) no restrictions have been made on the adsorption in both regions. Thus, the adsorption around the larger pores (fast moving portion) can be different from that of the micropores (slow moving region) in SORWACO, as is the case in reality. Sorption related nonequilibrium The equations presented so far do not describe the relationship between the adsorbed concentration S and the solute concentration c. In the literature both equilibrium (Bolt, 1982) and nonequilibrium equations (Boesten, 1986; Brusseau, 1992a,b) are available for this purpose. Nonlinear equilibrium isotherms do not provide a satisfactory explanation of the asymmetrical and nonsigmoidal curves of concentra- tion vs time that are observed during groundwater remediations of organic chemicals. Further improvement can be realized by a two-site adsorption mechanism (Boesten, 1986; van den Brink, 1987). Such a mechanism accounts for the fact that the various constituents of the solid phase (e.g. soil minerals, organic matter, aluminium and iron oxides) are likely to react differently with a dissolved chemical. In the two-site sorption model, the sorption sites are divided into two fractions. Adsorption on one fraction (class 1 sites) is assumed to be instantaneous, while adsorption on the other fraction (class 2 sites) is thought to be rate limited, so that equilibrium is not reached. This is called sorption related nonequilibrium. The total adsorption, S, is equal to the sum of the amount sorbed by the class 1 sites S x and the amount sorbed by the class 2 sites S 2 : S = S,+S 2 (7) At equilibrium, the amount sorbed by both types of sites is described by the Freundlich isotherm: Si,equilibrium = F\KpC " = S] (8) ^equilibrium = Fl Kf C (9) where F l and F 2 refer to the ratio between the class 1 and class 2 sorption sites. For field application there is usually no information on the values of F, and F 2 . A value of 0.7 is then used for F,. This value is based on measurements on adsorption kinetics carried out by Boesten (1986). The parameter K F denotes the Freundlich sorption coefficient [L 3/n M" 1/n ] and n the Freundlich exponent. It is assumed that K F is the same for both the class 1 and class 2 sites because sufficient information on the sorption is not available for field sites (Brusseau, 1992b). It may be concluded from the experimental work of Boesten (1986) that the total amount of sorption sites 250 Cors van den Brink & Willem J. Zaadnoordijk exceeds the amount measured in short term sorption experiments. The practical consequence of these results is that the sum of the parameters F x and F 2 may exceed 1. This phenomenon is of great importance for remediation operations in which time-dependent desorption is one of the factors that cause tailing. The sorbed concentration at the class 1 sites, S,, can be calculated from equation (7). The amount at the class 2 sites is not directly related to the concentration in the soil liquid phase, but the rate of change is (Boesten, 1986; van den Brink, 1987): ^ 1 = k d2 (F 2 K F c l/ "-S 2 ) (10) ot where k d2 is a first-order rate coefficient and F 2 K F c v " the equilibrium amount sorbed at the class 2 sites corresponding to the current soil liquid concentration c (equation (8)). This approach, in which the sorption and transport related nonequilibrium are described explicitly, is using almost the same theoretical framework as the two- domain approach of Brusseau (1992b). A difference is the capability of SORWACO to describe the sorption by a nonlinear sorption isotherm. In addition, the soil liquid is divided into two regions with different flow velocities, instead of a mobile and an immobile region. However, the main difference consists of concepts which facilitate practical use: - SORWACO describes the transport along a pathline in three-dimensional groundwater flow (and not uniform flow); - SORWACO uses the concentrations in the soil liquid phase as input, and calculates the total mass which is usually not measured; and - parameter values from the literature work well with SORWACO (except for organic carbon content for which some measurements are usually available). In practical cases, the first goal of a field investigation is to determine the extent of the contamination. In an early stage of the investigation, not much effort is given to the collection of data that are needed to predict the duration of the aquifer remediation. In a later stage, when it has been decided to carry out a remediation, additional data can be collected to establish better input data for SORWACO. Moreover, as results of the remediation become available, they can be used to further improve the SORWACO model. IMPLEMENTATION IN THE PROGRAM SORWACO Equations have been given in the previous sections for the fast and slow moving region and the class 1 and class 2 sites separately. The way they are implemented in the program SORWACO is illustrated in Fig. 1. The two soil liquid regions are indicated by boxes as well as the solid phase. The sorptions by the class 1 and class 2 sites have been indicated by the "equilibrium sorption" and the "nonequilibrium sorption" arrows respectively. The volumetric flux, q, is related to the velocity and can be written as: Nonequilibrium transport and sorption of organic chemicals during aquifer remediation 251 f flux out fast moving portion groundwater physical exchange flux in equilibrium sorption slow moving portion groundwater t Fig. 1 Setup of SORWACO. non-equilibrium sorption equilibrium sorption q = Qv = 0yV/ + 0.vV, (11) where the subscripts/and s refer to the fast and slow moving regions respectively. The total amount C can be split into parts associated with the slow and fast moving portions of the soil liquid phase: C = C x + C f (12) The amount in the slow moving soil liquid phase is equal to (equations (2) and (7)): C, = e,c., + p(l-/)(S u +5 2 ,,) (13) where S s is the adsorbed concentration related to the slow moving region of the soil (expressed per unit mass of soil assigned to this region) and / is the mass fraction of the solid phase assigned to the fast moving region. Since only class 1 sites are associated with the fast moving soil liquid phase, the total amount in this phase is given by: C,=Q f c f + fpS if (14) where S f is the adsorbed concentrations related to the fast moving region of the soil (expressed per unit mass of soil assigned to this region). Initially, the solid phase is in equilibrium with the liquid phase: C =fi r ^.( initial ".vS- l+ p^(F 1+ F 2 )(l-/)(c v , iml (15) 252 Cors van den Brink & Willem J. Zaadnoordijk C/initial - 9/ C/initial + P K F F y /(C/initial) (16) The equations (2), (3), (4), (5a), (5b), (6), (11), (13) and (14) describe the system operation together with the initial conditions of equations (15) and (16) and boundary conditions. The concentration in the water flowing into the model on one side is equal to a specified value of the background concentration. On the other side, the water leaves the model with the concentration calculated for the final cell. The equations are converted into an iterative finite difference algorithm. The iterations are necessary since the amount sorbed by the class 1 sites depends nonlinearly on the soil liquid concentration (equation (8)). Therefore this amount is calculated by applying equations (8) and either (13) or (14) alternatively at the beginning of each time step. At the end of each time step the values of the total amounts, C s and C f , are calculated by means of equations (5a) and (5b). COMPARISON WITH COLUMN EXPERIMENTS Use of the program SORWACO was evaluated by comparing the laboratory results of column experiments from the literature with the breakthrough curves calculated by means of SORWACO. The impact of variations in pore water velocity on the nonequilibrium sorption and transport of organic chemicals was investigated by Brasseau et al . (1991) and Brusseau (1992a). In those studies, miscible displacement experiments were carried out with different organic chemicals and aquifer media having low organic carbon contents (0.02-0.39%). Four column experiments were evaluated. The experiments analysed, with respect to the type of sorbent, chemical, and (nonequilibrium) parameters, are listed in Table 1. The values of the parameters F l and F 2 have been taken from Brusseau (1992a) initially and verified during the calibration. The dispersion coefficient D has been assumed to be equal to zero. The parameters for the Lula medium were calibrated using the breakthrough curve for the low pore water velocity (5 cm h 1 ) only. Next the breakthrough curve for the high velocity (45 cm h') was predicted and matched the measured values well (Fig. 2). The calibration was carried out with a least-squares criterion. The root mean square (RMS) of the residuals was equal to 0.021 for the calibrated low Table 1 Type of sorbent, chemical and (nonequilibrium) parameters used in the column experiments (Brusseau, 1992a). Sorbent* Eustis Eustis Lula Lula Chemical" TCE TCE NAP NAP V (cm h" 1 ) 0.4 86 4 45 K F "" (ml g 1 ) 0.27 0.27 0.21 0.21 (h 1 ) 0.003 0.003 0.01 0.01 (h 1 ) 0.70 0.70 0.07 0.07 / 0.5 0.5 0.7 0.7 *", 0.5 0.5 0.7 0.7 F 2 0.5 0.5 0.3 0.3 V A l.i l.i 1.2 1.2 Lula: OC = 0.02%; sand = 91.0%; silt = 5.6%; clay = 3.4%; p = 1.52 g cm 3 ; 0 = 0.32; Eustis: OC = 0.39%; sand = 95.5%; silt = 3.2%; clay = 1.3%; p = 1.70 g cm' 3 ; 6 = 0.41. NAP = naphthalene; TCE = trichloroethene; Freundlich exponent n = 1. Nonequilibrium transport and sorption of organic chemicals during aquifer remediation 253 o O 0.8- 0.6- 0.4- 0.2- 0- J*5 È m -4 f J i _ r iH|Ljl •t^fB==l ^= =*F «" a» 4 5 f Flux Factor 10 Brusseau [1992] + v=5 cm/hr • v=45 SORWACO — v=5 cm/hr v=45 Fig. 2 Measured (Brasseau, 1992) and calculated (SORWACO) breakthrough curves for Lula medium. 0.6 o œ 0.4 0.2 - - 1 • / / 7 / i it If r ^i- »______ B8 Brusseau [1992] + v=0.4 cm/hr 4 5 6 7 Flush Factor SORWACO v=0.4 cm/hr — 8 9 10 —- v=86 Fig. 3 Measured (Brusseau, 1992) and calculated (SORWACO) breakthrough curves for Eustis medium. velocity breakthrough curve. The RMS of the residuals for the predicted values of the high velocity experiment was 0.043. For the Eustis medium, the parameters could not all be calibrated using only the low velocity experiment. However, the combined calibration of both the low and the high velocity experiment showed that they could be described with one single set of SORWACO parameters. The RMS of the residuals was 0.039 for the low and 0.016 for the high velocity experiment respectively. The breakthrough curves are given in Fig. 3. 254 Cors van den Brink & Willem J. Zaadnoordijk The calibration resulted in higher values for k dl than for k st This implies that the asymmetry of the breakthrough curves is mostly due to transport related non- equilibrium at low pore water velocities and to sorption related nonequilibrium at high velocities. The leftward shift with higher velocity also indicates nonequilibrium (Brusseau, 1992a). As could be expected from the differences in flow velocity, the leftward shift is greater for the Eustis medium than for Lula medium since the ratios of the high and low velocities in both experiments are equal to 215 and 9 respectively. The analysis of the column experiments shows that the operation of SORWACO is applicable for the description of the effects of nonequilibrium, especially during the later parts of a groundwater remediation operation. Referring to the practical use of SORWACO, it is important that the parameter values are independent of the pore water velocities, since the pore water velocity may vary along the pathline, and the recovery may be intermittent. DESCRIPTION OF A CASE STUDY Introduction SORWACO was used to predict and evaluate the course of an aquifer remediation operation at the Sappemeer gas production site (Veltkamp & Mathijssen, 1991). Fig. 4 Sappemeer gas production site in the northern part of The Netherlands. [...]... and the spreading of benzene, the contaminant to be modelled, a representative pathline to the recovery wells was selected, which is indicated in Fig 7 Nonequilibrium transport and sorption of organic chemicals during aquifer remediation 257 A / C ho r ocler ist ic paihline Fig 7 Location of the recovery wells used for the remediation Both the travel time of the groundwater and the concentration of. .. planning of the duration of the operation as well as for the scale of the purification plant It has been indicated Nonequilibrium transport and sorption of organic chemicals during aquifer remediation 263 that after the rather sharp concentration decrease over the first 400 days, the concentration decrease would be less than that evaluated with an equilibrium approach because of the (sorption- related) nonequilibrium. . .Nonequilibrium transport and sorption of organic chemicals during aquifer remediation 255 Sappemeer is a town in the northern part of The Netherlands as shown in Fig 4 Natural gas has been produced and treated at the Sappemeer site since 1966 with a design production capacity of 15 million Nm3 day'1 Due to accidental spills, the soil and groundwater at the location have... soil and groundwater quality quickly and efficiently An overview of the subsoil is presented in Fig 5 The upper 14 m consist of fine sand The groundwater table is at about 1.5 m below the surface Underneath the fine sand is a layer of fine silty sand mixed with clay with a thickness of about 15 m Below that is a layer of fine and coarse sand with a thickness of 120 m surface 1.5m ab -^7 " ~~ fine sand... development of the measured and predicted concentrations Nonequilibrhim transport and sorption of organic chemicals during aquifer remediation 259 After the calibration, predictions were made using the same parameter values in the SORWACO model The predictions showed that the benzene concentration would be reduced to approximately 10 u.g l"1 after four years of recovery, and to about 1 u.g l"1 after six and. .. value of the first order coefficient of the sorption related nonequilibrium (kdl) Thus the concentration development during the remediation was strongly influenced by the nonequilibrium process, the one which has the longest half-life time, in this case the sorption related nonequilibrium (kd2) Based on the concentration development during the remediation and a quantitative insight into the effects of nonequilibrium, ... of operation) 10month recov 2month none 3000 2month recov 10month none Fig 10 Removed fraction of benzene for alternative recovery scenarios: (a) as function of flush factor; and (b) as function of remediation time velocity of the soil liquid phase as this occurs towards the recovery wells during a remediation Both sorption and transport related nonequilibrium processes are implemented Evaluation of. .. horizontal extent of the benzene plume in the groundwater is presented in Fig 6 Most of the benzene is present between 12 and 14 m below the soil surface on top of the silty sand and clay layer (Fig 5) About 3500 m3 of soil in the unsaturated zone and the upper part of the phreatic aquifer have been contaminated with approximately 400 kg of volatile organic hydrocarbons and 4000 kg of mineral oil Approximately... surface level Tj groundwater table at -1.8m -14 m silty sand and clay -30 m fine and coarse sand -150 m below surface Fig 5 Overview of the subsoil The extent and degree of soil and groundwater contamination have been investigated in several phases of drilling, sampling and analysing From 1985 to 1990 a total of 100 boreholes were made with a hand auger ranging in depth from 2 to 5 m In addition 10... ei-a ,2 " mm Ê- ^ *- . production site in the northern part of The Netherlands. Nonequilibrium transport and sorption of organic chemicals during aquifer remediation 255 Sappemeer is. is related to the velocity and can be written as: Nonequilibrium transport and sorption of organic chemicals during aquifer remediation 251 f flux out