PREDICTION OF CADMIUM ACCUMULATION IN A HETEROGENEOUS SOIL USING A SCALED SORPTION MODEL

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1 ModelCARE 90: Calibration and Reliability in Groundwater Modelling (Proceedings of the conference held in The Hague, September 1990). IAHS Publ. no. 195, PREDICTION OF CADMIUM ACCUMULATION IN A HETEROGENEOUS SOIL USING A SCALED SORPTION MODEL ALEXANDRA E. BOEKHOLD, SJOERD E.A.T.M. VAN DER ZEE & FRANS A.M. DE HAAN Dept. of Soil Science & Plant Nutrition, Agricultural University, P.O. Box 8005, 6700 EC Wageningen, The Netherlands ABSTRACT A transport model was developed that includes the most relevant processes for cadmium accumulation in soil and accounts explicitly for effects of ph and organic carbon content. Sorption parameters for the model were estimated using batch experiments with soil from an arable field. With these parameter values model predictions of solute breakthrough were in good agreement with measurements. With the validated sorption model cadmium contents along a transect in the field were predicted using measured values of ph, organic carbon content, and cadmium concentration in soil solution. Although spatial variability of these parameters is distinct and not identical to the spatial variability of the total cadmium content of the soil, model predictions showed good agreement with the observed spatial pattern. Spatial variability of ph and organic matter content is an important factor that invokes heterogeneity of cadmium contents in this soil. NOTATION c concentration (mg I" 1 ) k Freundlich parameter (mg 1_n l n kg" 1 ) k* scaled sorption parameter (mg 1_n l n kg" 1 ) n Freundlich exponent (-) oc organic carbon content (g g" 1 %) q sorbed amount (mg kg" 1 ) r rate parameter (year" 1 ) t time (year) v water flow velocity (m year" 1 ) z depth (m) Cd T total cadmium content (mg kg" 1 ) Cd s soluble cadmium content (mg kg" 1 ) D diffusion/dispersion coefficient (m 2 year" 1 ) (H + ) proton activity (mol 1" 1 ) L column length (m) 9 volumetric water content (m 3 m" 3 ) p soil bulk density (kg m" 3 ) 211

2 272 A.E,Boekhold et al. INTRODUCTION Contaminants that enter soil may be subject to accumulation, uptake byplants, (bio)degradation, and leaching. Whereas leaching poses a threat to groundwater quality, accumulation may have an adverse effect on soil biological processes. Simulation models may be used to predict behaviour of solutes in soil. One of the main difficulties in modeling transport through soil is the heterogeneity of this medium. The impact of aggregation was examined by Raats (1984), Van Genuchten & Dalton (1986), and Rasmuson (1986). The presence of macropores can significantly influence the rate of water and solute movement. Methods that implement the effects of macropores were presented by Van Genuchten et al. (1984). An approximation of solute transport in a layered soil was formulated by Barry & Parker (1987). When a sufficiently detailed description of the transport process is derived, transport in laboratory soil columns can be explained reasonably well (Jury e_t al ). However, when such transport models are applied to field soils the behaviour of water and solutes is often not adequately described because soil physical (Biggar & Nielsen, 1976) and soil chemical (Van der Zee & Van Riemsdijk, 1987) parameters are highly variable in natural soils. In the past decade a large number of studies have addressed the problem of field heterogeneity. Methods that implement uncertainty into model predictions are proposed to approximate the actual behaviour of solutes in the field. By assuming a stochastic water flow velocity and retardation factor it was shown that field average behaviour did not correspond with behaviour predicted with average soil parameters (Van der Zee & Van Riemsdijk, 1987). Jury (1982) used the transfer function concept in which transport may be effectively described by assuming a distributed residence time in the soil. The fate of cadmium in the terrestrial environment is to a large extent determined by its chemical interactions with the soil solid phase. Cadmium sorption is an essential factor that determines the amount of cadmium available for leaching and plant uptake. Thus, it is of paramount importance to give an accurate description of the sorption process when the environmental impact of cadmium contamination of soil needs to be assessed. Additionally, variability of soil chemical parameters may have a distinct impact on field scale behaviour of cadmium. The purpose of this contribution is to validate a cadmium sorption model using column experiments and a solute transport model. The scaled sorption model is then applied to a field soil that is heterogeneous with respect to ph and organic matter content. The variability of cadmium contents of the field is predicted and compared with measured data. THEORY Sorption isotherms of cadmium in the low concentration range that is realistic for field situations appeared to be well described by the Freundlich adsorption isotherm (Garcia-Miragaya & Page, 1977, 1978; Christensen, 1984; Chardon, 1984): q - kc" (1)

Prediction of cadmium accumulation in soil 213 Many soil parameters have been reported that significantly affect the behaviour of cadmium in soil.

3 Prediction of cadmium accumulation in soil 213 Many soil parameters have been reported that significantly affect the behaviour of cadmium in soil. Soil ph appears to be the most important property that determines cadmium availability to plants (Adriano, 1986). By decreasing the ph of a soil also the amount of sorbed cadmium was decreased (Garcia-Miragaya & Page, 1978). Organic, matter is known to adsorb considerable amounts of inorganic cations (Adriano, 1986), and is therefore an important soil constituent governing cadmium sorption. Another factor is the type of exchangeable cations present (Garcia-Miragaya & Page, 1977). Calcium and also other heavy metals have been found to compete with cadmium for sorption sites (Christensen, 1984, 1987; Chardon, 1984). Since the free divalent cadmium ion is the most important species that interacts with the soil solid phase, the extent to which cadmium complexes are formed influences the sorption equilibrium. Next to dissolved organic matter also the chloride ion forms complexes with cadmium (Hahne & Kroontje, 1973). Chloride is more selective than many organic complexing agents and does not complex strongly with cations like Al 3+, Ca 2+ and Mg 2+. Chardon (1984) found that the value of parameter n (equation (1)) is approximately constant for 12 different soils and different experimental conditions. However, the value of k varied considerably. Part of the variation in k could be explained deterministically by variation in ph and calcium activity. This corresponds with findings of Christensen (1989) who observed that for linear adsorption the distribution coefficient approximately doubled for each 0.5 unit increase in ph. For the 63 different soils he used also differences in organic matter content were responsible for the variability of k. Van der Zee & Van Riemsdijk (1987) used a Freundlich sorption equation that accounts for variability of both ph and organic carbon content: q - it'oc(f*)" ' 5 c" (2) Equation (2) is used in this paper, applied to a field soil that shows significant variability in both ph and organic carbon content in the horizontal plane. The validity of the Freundlich-type representation of the sorption process was examined using column experiments. Transport of cadmium in soil is primarily controlled by convective displacement of soil water. Additionally, diffusion and dispersion processes affect the shape of the infiltrating solute front. When steady state water flow is assumed and production terms are neglected, the transport equation for one dimensional transport of reactive solutes in soil is given by: dqfc") de d 2 c àc p HK J + e = QD : - Qv (3) v àt et àz 2 K } àz Due to the nonlinearity of the sorption equation high concentrations display a higher apparent transport velocity through the soil column than low concentrations. This is the result of a decrease of the differential sorption capacity (dq/dc) when c increases, provided 0<n<l in equation (1). The spreading of the infiltrating solute front due to diffusion and dispersion is counterbalanced by this effect, which leads to the development of a relatively sharp front when a constant concentration is imposed at the column entrance that is higher than the initial concentration in the soil column. Then, the infiltrating solute

4 214 A.E.Boekhold et al. front travels with a constant propagation velocity and invariant shape. When a chemical is not adsorbed instantaneously during its transport through the soil, a kinetic sorption model may improve model predictions. To account for nonequilibrium sorption a first order rate equation is assumed: ^ - r(fcc" - q) (4) Parameter r is a rate parameter and defines to what degree local equilibrium between solution concentration and adsorbed concentration is reached. Nonequilibrium conditions may occur when water fluxes are high in the soil column. Then, the presence of immobile water may cause differences in accessibility of sorption sites in the soil (dual porosity concept). This concept is mathematically equivalent with the concept of kinetic sorption (Nkedi-Kizza et al ). A finite difference method based on the Crank-Nicholson scheme was used to solve equation (3) using the Newton-Raphson iteration method for the following initial and boundary conditions : ( = 0 0<z<L c = 0 (5a) t>0 z= 0 f èc \ lim > -vc =-uc, (5b) z-o - V z J (>0 z =,. èc hm = 0 (5c) z-l~ These conditions apply to steady state transport in a homogeneous soil column of finite length, L. Z MATERIALS AND METHODS An arable field in the "Kempen" region in the south of the Netherlands was sampled in This area has received large amounts of cadmium by atmospheric deposition during the past century due to the presence of zinc ore smelters. At 1 m intervals two hundred 0-20 cm soil samples were taken along a transect in the field. The soil samples were air dried and sieved to remove particles larger than 2 mm. The cadmium content of the soil was measured in a 0.43 M HN0 3 extract of the soil. This yielded the total amount of cadmium present, denoted as Cd T. With a 0.01 M CaCl 2 extract the cadmium concentration in the soil solution, Gd s, is approximated (Houba et al ). The ph was measured with a glass-calomel electrode in the CaCl 2 -soil suspension. Organic matter content of the soil was determined as the organic carbon content, oc, according to Kurmies (Houba et al ). Cadmium transport was examined in soil sample no. 115 from the transect. Soil columns of 0.11 m length and m diameter were percolated with a M CaCl 2 solution at a flow rate of 1 m day" 1. To obtain breakthrough within a few weeks, cadmium concentrations of 2 and 20 mg Cd l" 1 were used. Breakthrough experiments were carried out in duplicate.

5 Prediction of cadmium accumulation in soil 215 The Freundlich parameters k and n for the transport model were derived for soil sample no. 115 using a M CaCl 2 -electrolyte solution at a cadmium concentration range of 0-12 mg l" 1. Soil was shaken head over end for 24 hours to ensure sorption equilibrium. Because the cadmium concentrations in solution of the 200 field samples were measured in a 0.01 M CaCl 2 -solution, this solution was used as background electrolyte to obtain the Freundlich parameters k* and n for field-scale predictions. Sorption was measured for soil sample no. 35 and no. 115 using a cadmium concentration range of mg l" 1, which is in the same order of magnitude as measured concentrations in the field. The two soil samples differ in organic carbon content and ph. Additionally, sorption was measured at cadmium concentrations of 3-7 mg l" 1 for soil sample no. 115, to verify the validity of the Freundlich sorption parameters at cadmium concentrations higher than 0.2 mg l" 1. RESULTS AND DISCUSSION Equation (1) was fitted to the sorption data. All fitted sorption isotherms showed a goodness-of-fit of or higher. The values of k and n are different for the two soil samples and for the two electrolyte concentrations (Table 1). TABLE 1 Freundlich sorption parameters k, k*, and n, ph and organic carbon content of the two soil samples from the transect that were used in batch and column experiments. sample no. 35 sample no. 115 sample no M CaCl M CaCl M CaCl 2 n k k* ph oc However, the parameters k* and n are approximately equal for the two soil samples when the same background electrolyte concentration is used. This indicates that equation (2) gives a good description of the influence of ph and organic carbon content in this soil. The observed difference in k and n for soil sample no. 115 when a different electrolyte concentration is used (Fig. 1), can be attributed to differences in ionic strength and in calcium and chloride concentrations. Therefore, when k and n are measured using different experimental conditions they are not interchangeable. Figure 1 also shows that sorption is described very well in both the low concentration range (0-0.2 mg l" 1 ) and the high concentration range (3-7 mg l" 1 ). Thus, the Freundlich sorption isotherm gives a good description of the sorption process in the whole range of cadmium concentrations.

6 216 A.EBoekhold et al. FIG. 1 Measured (points) and isotherms for soil sample no, electrolyte concentrations concentration (mg/l) fitted (lines) sorption 115, using two different O 1 - OQ.Î D c CD u c O u 0) > CD **fsww number of pore volumes FIG. 2 Measured (in duplicate, symbols) and simulated (lines) breakthrough curves of cadmium, for c = 2 mg Cd l" 1 (a) and c = 20 mg Cd l-i (b). The values of k and n obtained with the M CaCl 2 -electrolyte were used for simulation of column scale transport, because these values correspond with experimental conditions. Figure 2 shows the measured and simulated breakthrough curves of cadmium. At the cadmium concentration of 2 mg 1 _1 the model describes measured breakthrough very well regarding the moment of first breakthrough as well as 50% breakthrough (c/c 0 =0.5).

7 Prediction of cadmium accumulation in soil 217 Further breakthrough is described reasonably. The asymmetric shape of the breakthrough curve indicates that nonequilibrium conditions prevail. This may be illustrated with the dual porosity concept. In the presence of an immobile liquid phase the water and cadmium velocity will be larger than without a stagnant water phase due to a smaller effective flow domain. This leads to early breakthrough. Diffusion of cadmium into the stagnant phase, where it will be subject to adsorption, continuously removes cadmium from the mobile region, resulting in a "tailing" of the breakthrough curve. This effect becomes more pronounced when the difference between the feed and the initial concentration is larger, as is visible in Fig. 2b, where a concentration of 20 mg 1 _1 was applied. Simulation of cadmium breakthrough at this concentration is also in agreement with measured breakthrough, which validates the assumption of Freundlich type sorption. The rate parameter r was fitted simultaneously to both breakthrough experiments, because there was no reason to expect any differences in sorption kinetics or accessibility of sorption sites between the columns. When r was fitted individually, the simulation almost perfectly matched measured breakthrough. TABLE 2 Mean, standard deviation, minimum, and maximum values of the field data. parameter mean st. dev. min max Cd T (mg kg" 1 ) Cd s (mg kg-i) oc (g g-i %) ph (-) Since the Freundlich sorption isotherm gives a good description of cadmium sorption in batch experiments as well as column experiments, this equation may also hold in the field. Because the scaled sorption model (equation (2)) yields a constant value of k* under different soil chemical circumstances, the ability of the scaled sorption model to describe and predict cadmium behaviour in an arable field was examined. Statistical information on the field data can be found in Table 2. With an average cadmium content of 3.7 mg Cd kg" 1 soil in this field, the official Dutch reference value for cadmium in soil (0.45 mg kg" 1 for this soil type; Ministry of Housing, Physical Planning, and Environment, 1987) is exceeded. Not only cadmium contents are variable, also ph, organic carbon content and cadmium concentration in the soil solution showed distinct spatial variability. The correlation between Cd T and the soil parameters ph and oc was 0.33 and 0.44 respectively. Cd s did not correlate significantly with Cd T. There was no prominent spatial relationship visible between the measured space series.

8 218 A.E.Boekhold et al. en 0 H i i i i i i i i i i i i i i i i i i i distance (m) FIG. 3 Predicted (line) and measured (points) spatial pattern of cadmium contents along the transect. The Freundlich parameters obtained using 0.01 M CaCl 2 as a background electrolyte were used to predict cadmium contents along the transect with equation (2) (Fig. 3). The scaled sorption model predicts the spatial pattern of cadmium contents rather accurate along the longest part of the transect. Apparently, cadmium behaviour in the soil studied here is mainly regulated by ph and organic carbon content. Deviations from the predicted pattern may be induced by variability in other factors that influence cadmium sorption, that are not included in the model. The goodness-of-fit is rather poor at the end of the transect, which is primarily induced by a very erratic pattern of measured ph (not depicted). These fluctuations are visible in the predicted cadmium contents, but do not show in reality. Apparently, extreme variability in ph is not transmitted to variability in cadmium contents in the field according to the scaled sorption model. CONCLUSIONS The proposed transport model for one dimensional solute transport with Freundlich type sorption that accounts for nonequilibrium sorption describes observed cadmium transport in laboratory soil columns well. Thus the Freundlich sorption equation holds for this soil. An arable soil was shown to be heterogeneous in ph, organic carbon content, and cadmium content in the horizontal plane. The extreme values may have a significant impact on the environmental effects of cadmium pollution which are not recognized when only average values are accounted for. Although the spatial patterns of ph, organic carbon content, total cadmium content, and the cadmium concentration in the soil solution are different from each other, prediction of cadmium contents along the transect with a scaled sorption model that accounts explicitly for effects

9 Prediction of cadmium accumulation in soil 219 of ph and organic carbon content is in good agreement with the measured spatial pattern. Apparently ph and organic carbon content are the main factors that govern cadmium sorption in this soil. Because these soil parameters are spatially variable, soil chemical heterogeneity is an important factor in risk assessment of cadmium in field soils. REFERENCES Adriano, D.C. (1986) Trace elements in the terrestrial environment. Springer Verlag, New York, USA. Barry, D.A. & Parker, J.G. (1987) Approximation for solute transport through porous media with flow transverse to layering. Transport in Porous Media 2, Biggar, J.W. & Nielsen, D.R. (1976) Spatial variability of leaching characteristics of a field soil. Wat. Resour. Res. 12, Chardon, W.J. (1984) Mobiliteit van cadmium in de bodem (Mobility of cadmium in soil). PhD-thesis Agricultural Univ., Wageningen, The Netherlands. Christensen, T.H. (1984) Cadmium soil sorption at low concentrations: I. Effect of time, cadmium load, ph, and calcium. Water, Air, and Soil Pollut. 21, Christensen, T.H. (1987) Cadmium soil sorption at low concentrations: V. Evidence of competition by other heavy metals. Water. Air, and Soil Pollut Christensen, T.H. (1989) Cadmium soil sorption at low concentrations: VIII. Correlation with soil parameters. Water. Air, and Soil Pollut. 44, Garcia-Miragaya, J. & Page, A.L. (1977) Influence of exchangeable cation on the sorption of trace amounts of cadmium by montmorillonite. Soil Sci. Soc. Am. J^ 41, Garcia-Miragaya, J. & Page, A.L. (1978) Sorption of trace quantities of cadmium by soils with different chemical and mineralogical composition. Water. Air, and Soil Pollut. 9_, Hahne, H.C.H. & Kroontje, W. (1973) Significance of ph and chloride concentration on behaviour of heavy metal pollutants: Mercury (II), cadmium (II), zinc (II), and lead (II). J. Environ. Qual Houba, V.J.G., Van der Lee, J.J., Novozamsky, I. & Walinga, I. (1988) Soil and plant analysis part 5. Soil analysis procedures. Agricultural Univ., Wageningen, The Netherlands. Jury, W.A. (1982) Simulation of solute transport using a transfer function model. Water Resour. Res Jury, W.A., Elabd, L.D., Clendering, L.D. &Resketo, M. (1986) Evaluation of pesticide transport screening models under field conditions. In: Evaluation of pesticides in ground water (ed. by Garner, W.Y., Honeycutt, R.C. & Nigg, H.N.). ACS Symp. Series 315, Washington DC, USA. Ministry of Housing, Physical Planning, and Environment (1987) Environmental program of the Netherlands Staatsuitgeverij, The Hague, The Netherlands. Nkedi-Kizza, P., Biggar, J.W., Selim, H.M., Van Genuchten, M.Th., Wierenga, P.J., Davidson, J.M. & Nielsen, D.R. (1984) On the equivalence of two conceptual models for describing ion exchange during transport through an aggregated oxisol. Water Resour. Res. 20,

220 A.EBoekhold et al. Raats, P.A.C. (1984) Tracing parcels of water and solutes in unsaturated zones. In: Pollutants in porous media: The unsaturated zone between soil surface and groundwater (ed.

10 220 A.EBoekhold et al. Raats, P.A.C. (1984) Tracing parcels of water and solutes in unsaturated zones. In: Pollutants in porous media: The unsaturated zone between soil surface and groundwater (ed. byyaron, B., Dagan, G. & Goldschmid J.)- Ecological Studies 47, Springer Verlag, Berlin. Rasmuson, A. (1986) Modeling of solute transport in aggregated/fractured media including diffusion into the bulk matrix. Geoderma VanderZee, S.E.A.T.M. & Van Riemsdijk, W.H. (1987) Transport of reactive solute in spatially variable soil systems. Wat. Resour. Res Van Genuchten, M. Th. & Dalton, F.N. (1986) Models for simulating salt movement in aggregated field soils. Geoderma 38, Van Genuchten, M. Th., Tang, D.H. & Guennelon, R. (1984) Some exact and approximate solutions for solute transport through large cylindrical macropores. Water Resour. Res. 20,

6.6 Solute Transport During Variably Saturated Flow Inverse Methods

6.6 Solute Transport During Variably Saturated Flow Inverse Methods JIÌÍ ŠIMæNEK AND MARTINUS TH. VAN GENUCHTEN, USDA-ARS, George E. Brown, Jr. Salinity Laboratory, Riverside, California DIEDERIK JACQUES,