Environ. Sci. Technol. 1994, 28, 2259-2266
A Model of Organic Chemical Uptake by Plants from Soil and the Atmosphere Sally Paterson and Donald Mackay’
Institute for Environmental Studies, University of Toronto, Toronto, Canada M5S 1A 4 Craig McFarlane
Environmental Research Laboratory, U S . Environmental Protection Agency, Corvallis, Oregon 97333
A three-compartment mass balance model of a plant is developed to quantify the uptake of organic chemicals from soil and the atmosphere. The compartments are as follows: root, stem, and foliage. The processes involved are diffusion and bulk flow of chemical between soil and root; transport within the plant in the phloem and transpiration streams between root, stem, and foliage; exchange between foliage and air and between soil and air; metabolism and growth. The model is applied to the uptake of Bromacil by the soybean from hydroponic solution, yielding results which compare favorably with experimental data. Illustrative applicationsto three other chemicals (2,4-D, dichlorobenzonitrile, and hexachlorobiphenyl) from soil are described showing that chemicals present in soil may reach foliage by evaporation from soil with subsequent foliar absorption and by transpiration, the proportions being determined by the chemical’s Henry’s law constant and octanol-water partition coefficient. The intent is to provide a method by which chemical concentrations in various plant tissues can be estimated from information on chemical properties, concentrations in soil and air, and plant physiology. Applications and data requirements for validation are discussed. Introduction Natural and xenobiotic organic chemicals present in soil, water, and the atmosphere may be taken up by plants. There are several incentives to develop models describing the rates of transport, transformation, and accumulation of chemicals in plants and thus concentrations in plant tissues. Plants may be used to monitor environmental levels in soils and the atmosphere; much human and wildlife exposure is by direct and indirect consumption of vegetation; plants may be used for remediating contaminated sites. It is likely that vegetation plays a significant role in determining the environmental fate of chemicals (I). Uptake and distribution has been shown to be dependent on (i) the physical-chemical properties of the chemical such as molecular weight, solubility, vapor pressure, and octanol-water partition coefficient (2-4); (ii) the characteristics of the atmosphere and the soil such as soil organic content (5) and temperature (6);and, most important, the plant species and physiology including properties such as lipid, wax, and water contents and transpiration rates. Recent models which seek to quantify these factors include those of Calamari et al. (7), which estimates partitioning of several pesticides to a biomass compartment in a generic environment; Schramm et al. (8),which predicts dynamic concentrations in spruce needles; Boersma et al. (91, which includes processes of uptake, transport, 0013-936X/94/0928-2259$04.50/0
0 1994 American Chemical Soclety
and accumulation of organic chemicals by a soybean plant from solution using a detailed compartmental model; and Bacci et al. (IO),which describes the accumulation and release kinetics of azalea leaves exposed to constant levels of organic chemicals. Davis et al. (11) have modeled the role of plants on bioremediation of contaminated soils. Paterson et al. (12,131, Trapp et al. ( 4 ) ,and Riederer (14) have developed fugacity-based models to describe the transport and distribution of chemicalsbetween soil,water, and plant tissues. In this study, we develop, apply, and discuss a model that treats chemical uptake from soil and the atmosphere, into three plant compartments-root, stem, and foliage, as shown in Figure 1. Also included are transport between air and soil, optional emissions into all compartments, advectiveinflow and outflow of chemicalin air, degradation by metabolic or other processes, and linear plant growth dilution. The model developed here is primarily designed to be fitted to experimental data in which an established plant is exposed to a chemical introduced into the soil, hydroponic solution, or atmosphere, and concentrations in plant tissues are monitored over a period of days, weeks, or months. Validation by successful fitting and prediction can, it is hoped, lead to credible expressions for partitioning, transport, and transformation processes and to confirmation of the required number and configuration of compartments. The model is not designed to be applied to perennialvegetation such as trees, which may be exposed environmentally for periods of years and which may contain substantial nonviable tissues. Ultimately, a need may arise to develop models or correlationswhich describe the concentrations in plant matter during its entire life from germination to death and decay in which there is continuous exposure to chemicals. It is also likely that seasonal changes in temperature, precipitation, transpiration, and growth rates should be included. It is suggested that these more difficult tasks can only be accomplished if credible, simpler models are available which successfully describe chemical uptake under controlled laboratory conditions. This model is thus regarded as only one modest step in what is likely to be an extended process of development, testing, and validation of chemical fate models of plant-air-soil systems. We first outline the essential features of the fugacity modeling concept, then assemble a model for the soilplant-air system, and apply it to actual experimental data for Bromacil in a soybean plant (15). The model is then applied illustratively to show that chemicals which differ in properties are likely to differ greatly in behavior. Methods by which the significant features of chemical behavior in a plant-soil-air system can be assessed and interpreted are discussed. Environ. Sci. Technol., Vol. 28, No. 13, 1994
2258
D = k,VZ (mol/Pa.h)
(4)
Mass Balance Equations. For each compartment representing the air, soil, or parts of the plant, a mass balance equation is written for amount of chemical (mol/ h):
I
in which V is the compartment volume, f is the fugacity of the chemical throughout the compartment, E is any direct chemical input to the compartment (mol/h), DI represents transport D values to the compartment from other compartments of fugacity f r , and Do represents transport and transformation D values for processes by which the chemical is removed from the compartment. If Vand Z are constant, the left side becomes VZ dfldt, but when modeling chemical fate during a period of significant plant growth, and hence growth dilution, it is desirable to separate the terms as follows: VZ dfldt
-> ---
Intermedia transport of chemical Degrading reaction j Chemical emission or discharge
Figure 1. Illustrative soil-air-plant system.
Model Development Fugacity, 2 Values, and D Values. The model is formulated using the fugacity concept (16,17). Fugacity (f,with units of pressure (Pa) is an equilibrium criterion that is linearly related to concentration C (mol/m3)through a fugacity capacity Z (mol/m3,Pa), where C is Zf. When the two phases are at equilibrium, the chemical fugacities are equal, the Zvalues being essentially “half‘ the partition coefficient (Klz) as shown below:
The Z values depend on the substance’s physicalchemical properties, temperature, and nature of the phase, especially its content of air, water, organic matter, lipids, or waxes. Z values are usually first established in the air phase and then in water and other phases employing experimental or correlated partition coefficients. Transport and transformation processes are expressed by D values (with units of mol/Pa.h), the rates being Df (mol/h). Diffusive transfer between compartments such as air and leaf employs a mass transfer coefficient (MTC) k (m/ h) and the interfacial area A (m2): D = kAZ (mol/Pa.h)
(2)
Bulk flow in a phase such as air or in a transpiration stream employs the flow rate G (m3/h) of the phase: D = GZ (mol/Pa.h)
(3)
Reaction (including metabolism) employs a first-order reaction rate constant k~ (h-l),and the whole compartment volume V (m3): 2280
Environ. Sci. Technol., Vol. 28, No, 13, 1994
+f
d(VZ)/dt = VZ dfldt
+ fDG
(6)
DGis a growth dilution D value, and the term fDG can be moved to the right of eq 5 and added to the DOterms as an apparent loss process. Compartments. For the purposes of modeling chemical uptake by plants in a relatively simple configuration, we suggest that the plant be treated as consisting of three homogeneous compartments, each of which can be assigned a different, single prevailing fugacity. These are (i) roots, Le., all plant tissues which actively exchange with the soil, (ii) leaves including petioles, and (iii) the remaining structure, which is mainly stem but could include fruit, seeds, or tubers. This is obviously a very simplistic representation, but we believe that more complex models or segmentation should be developed only when justified by demonstrating the inadequacy of simpler models. It, is noteworthy that much edible plant tissue is primarily carbohydrate, starch, or lipid designed for energy storage and formed by transport of organic matter in the phloem following photosynthesis and CQ2 fixation. There is thus direct transport from the atmosphere to such tissues. Further, not all tissues below ground are roots, potato tubers being regarded here as part of the stem. The abiotic compartments are air (which may include aerosols or particulate matter) and soil, which is normally a mixture of mineral and organic matter, air, and water, but under hydroponic conditions is entirely water. These five compartments are connected as shown in Figure 1. Chemicals can be directly introduced into all compartments, and there may be advective inflow and outflow of chemicals in air. Subscripting the compartments, air, A; soil (earth), E; root, R, stem, S; and leaves or foliage, L; with addit,ional subscripts G for growth, D for transformation reactions (degradation or metabolism), and B for advection, the overall equations are shown in Table 1. Terms such as DAEare the air to soil transport D values, with DAEnot generally equalling DEA,the soil to air value. If necessary, the model could be restructured to include more compartments, but this would require more input data. The task is to select volumes and densities for each compartment, define expressions from which Z values can be deduced from available physical-chemical property data, and define expressions and parameters for D values.
Table 1. Differential Mass Balance Equations for Compartments, the Transport D Values Being As Shown in Figure 1 inventory change c inputs - outputs emission,advection (in)
transport from soil root stem
air
VAZAdfddt = EA+ f d h + fQm V ~ dfddt E = EE + fADAE root VRZRdfddt = ER + f&ER stem VSZSdfsidt = Es leaf VLZL dfddt = EL + fADa air soil
+ fRDm + fRDm
+fdLA
+ f@SR + f@SL
Table 2. Definition of 2 Values and Volume Fractions for Soybeane air vol (ZA) fraction air (A) VAA soil (E) urn root (R) stem (S) leaf (L) VLA
4 0.2
water
VEW VRW
vsw 0.24
vol
(ZW) fraction
VLW
0.3 0.942 0.756 0.727
octanol
vol
(20) fraction VEO VRO
vso ULO
leaf
0.0015 0.01 0.01 0.03
In each case the compartment "bulk" 2 value is the sum of the phase component 2 values weighted in proportion to their volume fractions, e.g.
ZEB= V E A ~ A+ UEWZW + VE$O The phase Z values are as follows: ZA= l/RT; Zw = 1/H where H is the Henry's law constant (Pa.m3/mol); and ZO= Zw/Kow.
The set of five simultaneous linear differential equations can be solved numerically for all fugacities (and hence concentrations and amounts) as a function of time. The system of equations is essentially a 5 X 5 matrix of D values with an input vector and a compartment fugacity vector. ZValues. Expressions for estimating 2 values for air, water, soil, and plant matter are summarized in Table 2. Whereas the expressions for air, water, and soil are well established; those for plant matter are less certain. Direct experimental determination of partition coefficients (and hence 2 values) is preferred, but in the absence of such data, we hypothesize that each plant compartment can be treated as consisting of equivalent volume fractions of water, air, and octanol; these fractions not necessarily adding to 1.0. For example, plant wax may be equivalent to octanol, but carbohydrate may be equivalent to 20 3' 6 octanol and 50% water. These proportions must be determined experimentally by partitioning measurements. Suggested fractions are given in Table 2 which are derived from an analysis of correlations by Briggs and co-workers (21,Bacci and co-workers (3, IO),Paterson et al. (13),and Reiderer (14). A more detailed account of this analysis has been presented by Paterson and Mackay (18). There is a clear need to measure and correlate equilibrium partition coefficients between air, water, and plant tissues for a variety of chemicals, thus the expressions in Table 2 should be viewed as only tentative. Intermedia Transport Processes or D Values. Table 3 gives suggested equations for D values, many of which have been described previously (12, 13). Xylem and Phloem Flow. Chemical transport within the plant occurs in the xylem, which conducts the transpiration stream or sap from the roots to the stem and foliage, and in the phloem, which transports organic matter from sites of photosynthesis to growing plant tissue. D values, calculated as the product of flow rates, Gi (m3/h),
air
transport to soil root stem
+ DAE
-fA
(
-fE -fR
(
+fdLS -fs ( -fL (+DLA
leaf
reaction (D), growth (G), advection (B),(out)
+DAL +DAB) + DER + DEM) +&E +DRS + DRD+ DRG) + DSR + DSL + DSD+ DSG) + DLs + DLD+ DLG)
Table 3. Summary of Expressions for D Values transfer between soil-root
xylem flow and diffusion diffusion only xylem flow phloem flow xylem flow phloem flow
root-soil root-stem stem-root stem-foliage foliage-stem foliage-air
diffusion through cuticle diffusion through air boundary layer
air-foliage air-soil
D m =: kAAZA Dwn = kwAZw soil-air D L = l/yl/DsA + 1/(DAD+ DWD) ) air advection DBA= Dm = GBAZA suggested parameter TO = 126 h values 7~=5X10-6h kg = 1mjh kA = 0.015 m/h kw = 6 X 104m/h
diffusion through air boundary layer diffusion in soil air diffusion in soil water
and 2 values define chemical transport rates in the xylem and phloem. For calculating the flow rate in the xylem, the transpiration flow rate is best measured directly or an estimate of 5 pg/s.cm2 of foliage area (or 2 X 10-4m3/h.m2 of foliage) can be assumed (19) and combined with the total foliage area to give the xylem flow rate, Gxy. The phloem flow rate, Gph, which is slow compared to that of the xylem,is not easily measured and is less well quantified. It is assumed to be a fraction such as 5 % of the xylem flow, Le., Gph is 0.05 GxY: Bulk flow in the xylem and phloem can thus be described by D parameters: Dxy = GXTw (mol/Pa.h)
xylem
(7)
Dph = Gp,Zw (mol/Pa.h)
phloem
(8)
Soil-Root Exchange. Chemicals are assumed to enter the root mainly by bulk flow of soil pore water but also by diffusion. Transport from root to soil is assumed to be by diffusion only. As the chemical enters the root, it may be retarded at the endodermis barrier, it may partition into other root tissues, it may be metabolized,it may diffuse back to the soil, or it may leave in the transportation stream to the stem. To accommodate these processes, we suggest that the uptake D value is GxyZw or DxYplus a diffusive exchange term expressed as a fraction 4 of Dxy. An Environ. Sci. Technol., Vol. 28, No. 13, 1994
2261
arbitrary value of 0.05 was selected for 4 which allows for slow diffusion. This value is uncertain, but fortunately is relatively unimportant during uptake. The D parameters for exchange between soil (E) and root (R) are, therefore = DXy + 4DXy soil to root (Le., xylem flow and diffusion) (9)
DRE = 4Dxy
root to soil (Le., diffusion only) (10)
The effect of including this tentative diffusion term is to give an apparent retardation or “reflection” because some of the chemical which enters the root returns to the soil, although there will be net diffusion from root to soil only when the root fugacity exceeds that of the soil. There is a need to determine this diffusion rate experimentally. The model assumes that the root tissue is well mixed. Very hydrophobic chemicals such as PCBs are likely to partition to the outer surface of the root and may not penetrate to the interior, thus surface concentrations may not be well predicted, but total amounts and average concentrations should be realistic. Root-Stem-Foliage Exchange. Transfer from root to stem to foliage is treated as advective flow of sap in the xylem, resulting in the parameters DRSand DSLequaling Dxy. The reverse process from foliage to stem to root occurs in the phloem and is described by the parameters DSRand DLS,which equal Dph. Air-Leaf Exchange. Exchange of chemicals between air and leaf occurs by diffusion through the air boundary layer and then through the stomata into the interior of the leaf. It may also occur by sorption to the cuticle with subsequent slower diffusion through the cuticle. The development of numerical values for DLAand DALhas been discussed by Paterson et al. (13). Bacci et al. (3, 10) applied the simple first-order equation dCL/dt = klCA - k,CL
!11)
to experimental studies of leaf-air exchange kinetics of organic chemicals where CLand CAare the concentrations (molIm3) in the leaf and air, and kl and kz are the uptake and clearance rate constants with units of reciprocal hours. During uptake, chemical concentrations can be calculated as
C, = CAkl/k,[l - exp(-k,t)l = CABCFV[l- exp(-k,t)I (12)
A t infinite time, CL/CA is BCFv (or ZL/ZA) and kllkz and is the bioconcentration factor of the chemical concentrations in leaf and air. During clearance from an initial CLOwhen CAis zero
C, = C,, exp(-k,t)
(13)
Since the diffusive transfer parameters DLAand DALare equal, the uptake and clearance equations for a leaf of volume VL become
The rate constant kp is equivalent to DLA/VLZL (13). The uptake rate constant k l is kzBCFv or kpzL/zA or DLA/ VLZA. DLAcan be regarded as the product of volume of 2262
Environ. Sci. Technoi., ‘dol. 28. No. 13, 1994
air contacted per hour, GLA,and ZA. It follows that k1 is GLA/VLor the number of leaf volumes of air contacted per hour and is dependent on micrometeorological conditions. Using experimental results from Bacci et al. (3, 10) and Paterson et al. (13),it is suggested that DLA,the overall conductivity, can be calculated from the overall resistance 1/DLA which comprises series resistances in air 1/DA and in cuticle 1/Dc giving l/DLA= l/DA+ l/Dc
(16)
and
They then correlated k2 withKOA,the octanol-air partition coefficient for 14 chemicals giving l/kz = TO + TAKOA
(18)
T O and T A represent plant-specific transfer times or resistances in the organic (or leaf cuticle) and air phases and for azaleas have estimated values of 126 and 5 X 10-6 h, respectively. Values of Dc, DA,and DLA(and DAL)can then be calculated for each chemical as follows:
Dc = VLzL/To DA =
vLzL/
(19)
(7AKoA) = vLzLzA/ (TAZ,) = 0.05VLzA/TA since ZL= 0.052, (20) DL, = D,
= l/(l/Dc
+ l/DA)
(21)
Soil-Air Exchange. The soil is assigned a depth corresponding to the root zone, horizontal area A (mz), and composition, i.e., volume fractions of air, solids, and water and an organic carbon content foc. Diffusion from soils can take place in the air or water phase or in parallel in both. D values for this process have been developed previously (16, 17) following the approach used by Jury et al. (20) and Mackay (17). The overall D value between soil and air is expressed as = 1/(1/DsA
1/(DAD DwD))
(22)
where DSAis the D parameter for air-side boundary layer diffusion and DADand DWDare parameters for diffusive flow in soil-air and soil-water. These parameters can be calculated as DsA = kBAZA
(23)
DAD = kAAZA
(24)
= kwAZ,
(25)
D,
where kg, kA, and kw are the mass transfer coefficients or transport velocities (rn/h) associated with each process. kA and kw are calculated as the quotient of the effective diffusivity in the medium and the soil diffusion depth of some fraction, such as half of the total depth. The values are summarized in Table 3, justification being given by Jury et al. (20). Advective Flow and Aerosol Deposition. Inflow of air to the system occurs at a flow rate GBA(rn3/h)which can be estimated from an actual flow rate in controlled laboratory conditions or from wind speed under environ-
~~~~~
mental conditions. Influx of chemical of concentration CBA (mol/m3) thus occurs at a rate GBACBA
= GBPfBAZA = DBPfBA (mol/h)
(26)
The advective outflow parameter DABis equal to DBA,and A the outflow rate of chemical is D A B ~(mol/h). It is possible to include nondiffusive processes of air to soil deposition by wet and dry deposition of aerosolassociated chemical,but they are not treated in the present model. For chemicals of low vapor pressure which partition appreciably to aerosols, these processes can be more important than diffusive adsorption. Methods of estimating these D values are described by Mackay (17). It is necessary to include these processes, at least for certain chemicals, for assessing plant uptake under environmental conditions, but under laboratory conditions significant deposition is unlikely. Transformation Processes. A metabolic rate, Din, can be included for each compartment i which is equal to ViZikRi where k ~isithe first order metabolic rate constant (h-1). Other degrading reactions such as hydrolysis or photolysis may be included in an overall k ~ i .These rate constants can be determined from measured mass balances of the parent chemical or from formation rates of metabolites. Growth Dilution. The growth dilution D value is deduced as d( VZ)/dt. If the tissue composition and thus the 2 value are constant, then for a compartment growing atarate (dV/dt)/Vof0.001or 0.l%/h,D~willbe0.001VZ.
Model Application The model is first applied to the uptake of 14C-labeled Bromacil by soybean plants from a hydroponic solution. It is then applied to uptake of a series of chemicals, which vary in properties, from a solid soil under purely illustrative and hypothetical conditions. The purpose of this illustration is to show how the model may be applied in practice to treat the uptake of chemicals from soil and air and to explore the conditions which lead to foliar contamination arising from root uptake and translocation in the xylem, as distinct from evaporation from the soil with subsequent foliar absorption. In the Bromacil study (15), the plants were treated in exposure chambers as described by McFarlane and Pfleeger (21). The system consisted of two vessels, one enclosing the shoot environment and the other the root environment, in this case a hydroponic solution. The shoot and root containers were separated by a plastic plant support disk fitted to the top of the hydroponic tank. This eliminated the possibility of chemical reaching the shoot or leaves by other than root uptake and xylem translocation. Chemical was introduced to the solution by a hypodermic injection port. The total volume of solution was 6.5 L. The solution was well stirred, and oxygen content and pH were controlled. Eight plants were placed in the system, thus there was an average of 0.81 L of water/plant. Although foliage areas and transpiration rates varied from plant to plant, average values were selected of 800 cm2/plant (0.08 m2/plant) for foliage area and 7.0 X mL/(cm2.h) (1.94 pg/cm2.s) for transpiration rate. This results in a flow rate for the transpiration stream of 7.0 X 10-3 X X 800 or 5.6 X m3/(h-plant)or 5.6 mL/ (h-plant). The reported densities, water and lipid content, tissue masses in grams dry weight (g dw), and calculated
~
Table 4. Dimensions and Properties of Soybean Plant from Experiments by Trapp and McFarlane ( lgp reported
calculated
mass (gww) % % density compartment at 72 h lipid water (kg/m3) root stem foliage
44.8
7.1 22.0
1 1 3
94.2 75.6 72.7
880 920 750
volume (m3) 5.1 X 10-5 7.7 X 10-a 2.9 X 1od
mass (g dw)
2.6 1.73
6.0
Leaf area = 0.08 mZ/plant. Transpiration rate = 5.6 X 10-e ma/ (heplant). Hydroponic solution volume = 8.125 X lo4 ma/plant. g ww = grams wet weight. g dw = grams dry weight.
Table 5. PhysicalChemical Properties and 2 Values of Bromacil, 2,4-D, Dichlorobeazonitrile (DCBN), and a Hexachlorobiphenyl (6-PCB) at 20 OC Bromacil"
2,4-Db
DCBNb
6-PCBb
Physical Chemical Properties molecular 261.12 221.04 172.02 350 weight (g/mol) aqueous 700 400 20.0 0.0035 solubility (g/mS) vapor 4.1 X 106 8.5 X 106 0.07 5.0 X 10-4 pressure (Pa) log Kow 2.02 2.81 2.9 6.8 2 Values (mol/m3.Pa) air 4.04 X 10-4 4.04 X 10-4 4.04 X 10-4 4.04 X 10-4 water 65 400 21 300 1.66 0.02 soil 2.49 X lo4 17 000 1.52 97.3 root 1.15 X lo6 1.50 X lo6 14.1 1200 stem 1.09 x 105 1.18 x 106 11.1 96.7 foliage 2.53 X 106 4.28 X lo6 40.8 3790 Ref 22. Ref 23.
volumes of root, stem, and foliage tissue are summarized in Table 4. After 72 h, the distribution of V4CI-Bromacil was determined. Table 5 gives the physical-chemical properties of Bromacil and estimated Z values. Since this was a hydroponic culture, the "soil" compartment or environment surrounding the roots was assumed to be 100%water with no solid or air components. Trapp and McFarlane (15)reported solution concentrations in units of disintegrations per minute (dpm)/mL and plant tissue concentrations in units of dpm/g dw. The specific activity of Bromacil in the system was 52 800 dpm/ pg. A conversion factor of 52 800 X 261.12 X lo6 or 1.38 X 1013 dpm/mol was used, thus 1.38 X lo7 dpm/mL is equivalent to 1 m0l/m3. The mass balance for root, stem, foliage, and hydroponic solution, illustrated in Table 6, gives a total amount ranging from 27 000 to 33 700 dpm in individual plants, which exceeds the initial amount of 25 700 dpm. To satisfy this mass balance condition would require an initial concentration of 33.2-41.5 dpm/mL. The model was run to simulate an adjusted intermediate total amount of 29 500 dpm or 36.3 dpm/mL with an initial fugacity in the hydroponic solution of 36.3/(1.38 X lo7 x 65 400) or 4.0 X 10-l1 Pa. Using this initial fugacity of the hydroponic solution as input and assuming zero initial concentration in the other compartments, the model was run for 72 h. Estimated model concentrations for hydroponic solution and plant tissues at the end of 72 h are compared with experimental values in Table 6. Environ. Sci. Technol., Vol. 28, No. 13, 1994
2283
Table 6. Estimated and Reported Concentrations and Amounts of Bromacil in Hydrophonic Solution and Plant Tissues model amount concnn (dpm)
experimental amount concna (dpm)
hydroponic 36.3 29 5OOb 31.6 25 700 solution at 0 h hydroponic 22.0 17 89OC 22.2 18 000 solution at 72 h root 877 228OC 874 2 270 stem 209 36lC 350 640 foliage 1490 8 930' (1350-2130) 8 100-17 780 total 29 500 27000-33700 Concentration units of root, stem, and foliage are dpmig dw. Concentration units of hydroponic solution are dpmiml. Adjusted initial value. Calculated by the model.
The est,imated and measured concentrations are in good agreement for the hydroponic solution and the plant compartments with the exception of the stem compartment and some foliage samples which are low by a factor of approximately 2. This may be due to underestimation of the 2 values. Clearly the model gives an adequate representation of the dynamics of this chemical in the plant, showing that the dominant process is transport from hydroponic solution to foliage in the sap with relatively minor retention in the root and stem. The adequate fit does not imply that all parameters are well quantified since the results are insensitive to some quantities such as flow rate in the phloem. In the second example, which is purely a modeling exercise, the plants are assumed to be exposed to air and soil as illustrated in Figure 1with processes of diffusive exchange of chemical between air and soil included. This provides an alternate chemical route to foliage through volatilization from soil and subsequent foliar absorption. The environmental dimensions and parameters are identical to those of the first study with the exception of the root environment, which in this case is soil consisting of air, water, and solids in the volumetric ratio of 2:3:5; the solids having an organic carbon content of 0.15%. The fate of four chemicals of varying physical chemical properties is examined to show the effect of solubility, vapor pressure, and octanol-water partition coefficient on the routes of chemical transport through the system. The chemical properties and 2 values are given in Table 5. Table 7 gives the estimated fate of Bromacil, 2,4-D, dichlorobenzonitrile (DCBN),and a hexachlorobiphenyl (6-PCB) in a soybean plant after 72 h in a closed system with no advective loss of air. The initial (arbitrary) quantity of chemical in the soil ranged from 0.8 pmol for PCB to 110 pmol for 2,4-D. Fluxes and response times for transfer between compartments (deduced as VZ/D)as well as the fugacities in the various compartments are given. The results show that the main route of transport of 2,4-D is by uptake through the root to the transpiration stream where it is carried rapidly to the stem and foliage. The chemical evaporates from the foliage very slowly. There is little exchange between air and soil. This behavior is a result of the chemical's high water solubility, low vapor pressure, and low Henry's law constant. For DCBN with an intermediate value of Henry's constant, most transfer is from soil to root to stem to foliage, 2264
Environ. Sci. Technol., Vol. 28, No. 13, 1994
Table 7. Estimated Disposition of Four Chemicals in Soybean Plant after 72 h Uptake from Soil8 compartment
Bromacil
DCBN
2,4-D
PCB
initial soil soil at 72 h root at 72 h stem at 72 h foliage at 72 h air at 72 h
m 20.2 110 8.5 0.79 f 1.0 8 7000 10 m 5.5 58 4.9 0.79 f 0.27 4.2 3940 10 m 2.2 29 2.2 8x106 f 0.39 4.1 3370 1.5 X 10-9 m 0.4 4.2 0.3 6.0 x 10-9 f 0.46 3.8 3060 6.7 x 10-7 m 12.1 19 1.1 1.8 X 10-12 f 1.6 1.5 930 1.6 X 10-" m 7 X lo4 7 X lo4 4 X 10-3 8 X 10-18 f 1.5 1.4 880 1.6 X lo-" Net Fluxes at 72 h (pmol/h) 0.098 0.50 0.037 1.2 X 10-6 soil to root root to stem 0.132 0.46 ,030 1.7 X 10-'0 stem to foliage 0.137 0.45 .028 7.5 x 10-14 foliage to air 104 10-5 4 X lo"' 6.0 X 10-16 soil to air 106 2 x 10-5 9 x 10-4 1.2 x 10-6
Response Times (h) root stem foliage
15 2 400
56 9 210
68 11 135
480 000 76 200 1690
a Quantities of chemical ( m )are pmol, fugacities (f, are pPa, and fluxes are pmolih. Response times are V Z / D in h.
but there is also a contribution from evaporation from soil and subsequent foliar uptake. Bromacil, with a low Henry's constant, exhibits behavior similar to that of 2,4-D, cycling in the soil-root-stemfoliage-air direction. However, there is subsequent net transfer of chemical back to soil. The residence times in the root and stem compartments are short. There is rapid accumulation of chemical in the foliage due to its low vapor pressure and the slow foliage-air exchange rate. The relatively high Henry's law constant and hydrophobicity of 6-PCB result in contamination of the foliage mainly by the soil-air-leaf exchange route. The rate of transfer from soil to root, stem, and foliage is slow as indicated by a long residence time in each of these compartments. The residence time of 6-PCB in the air is short, and equilibrium is rapidly reached between air and foliage after evaporation from soil. In summary, the paths and patterns of accumulation of a compound in a plant species are critically dependent on chemical properties. Bromacil and 2,4-D with high water solubilities and low Henry's law constants are mainly transported through the transpiration stream. There is slow evaporation from soil and from foliage, thus foliage becomes the compartment of accumulation. These properties presumably enhance their efficacy as herbicides. Hydrophobic chemicals such as 6-PCB with a low vapor pressure but also a low water solubility, resulting in a relatively high Henry's constant, are transported slowly through the plant since transport is retarded by strong partitioning to plant tissue. The main route of transport to foliage is likely by evaporation from soil. However, this transfer is also slow, and the root is the primary compartment of accumulation. DCBN with intermediate properties follows both routes, resulting in contamination of foliage from soil evaporation as well as through the transpiration stream. It is important to recognize that assertions about the relative role of soil evaporation and transpiration as routes for foliar contamination deduced from laboratory experiments cannot necessarily be applied
to environmental conditions in which there is greater ventilation. There is an obvious need to apply the model to a range of chemicals which vary in properties and in behavior. The model’s structure is believed to be adequate to describe the range of expected behavior, but Z and D parameter values require better determination.
Discussion It is often of interest to determine the general uptake characteristicsof chemicals,for example, if contamination of foliage (especially edible foliage) occurs via the transpiration stream (i.e., xylem transport) or by evaporation from soil. A fully parametrized soil-plant-air model as described here can be inspected to provide such information by identifying the key processes, partitioning tendencies, and approximate response times. A systematic approach is suggested below. The Zvalues give a direct indication of potential relative concentration of chemicals under equilibrium conditions. The V Z group values indicate the relative capacity of each compartment for the chemical. At equilibrium, each VZ quantifies the amount of chemical to be expected in each compartment. The dominant compartment of potential partitioning can thus be identified. The D values are essentially conductivities and express the relative rates of transport and transformation. A chemical will generally follow the route of the largest D. The group ( V Z / D )is a response time and indicates the time required for the compartment in question to receive or be depleted of an appreciable fraction (1- e-l) or 73% of the ultimate quantity of chemical by the process designated by D. Long times result from large capacities ( V Z ) or slow processes (D). The data in Table 7 show that for Bromacil, 2,4-D, and DCBN an appreciable fraction of the chemical in the soil is transpired into the plant foliage in 72 h and the compartment response times are short. Most of the chemical reaches the foliage. The PCB behaves quite differently. Transfer through the plant is negligible with only slight uptake by the root. The root response time is about (a hypothetical) 50 yr, indicating that chemical entering the root is bound there for the lifetime of the plant and is not transported through the plant to any appreciable extent. Indeed, as Wild and Jones (24) have shown, most of the hydrophobic contaminant is retained in the peel. The response times of the root and stem reflect the hydrophobicity of the chemical. The response times of the foliage, which range from several days for DCBN to several months for 6-PCB, reflect the chemical’s airwater and octanol-water partition coefficients which control foliar volatilization and partitioning processes. It is thus possible to identify the fastest or most facile routes of chemical transport and how long they will take to become effective. Of particular interest are the relative rates of the soil-root-stem-foliage route compared to the soil-air-foliage route. For processes in series, the overall D value can be estimated as the reciprocal of the sum of reciprocal D values. For parallel processes, the D values can be added directly. Transport and transformation in the soil-air-plant system can thus be elucidated by examining the system properties in a manner similar to an electrical circuit in which fugacity is analogous to voltage, mass flux of chemical to current, VZ to capaci-
tance, and D values to conductivities or reciprocal resistances. Inspection of the model suggests a number of areas in which improved understanding of processes and parameter values is required. There is a need to determine and correlate the absorptive capacities or Z values or tissuewater partition coefficients for avariety of chemicals.Flow rates in the phloem and rates of exchange between soil and root are poorly quantified. I t is not clear when and how wet and dry deposition processes to foliage should be included. Metabolic rates require further evaluation. Models such as that described here are fairly complex, require considerable input data, and are probably not suitable for routine use for exposure evaluation. There is a need for simpler but rigorous models that can be used for screening purposes to predict approximate concentrations in vegetation which is in contact with contaminated air and soil.
Conclusions
A three-compartment model of chemical transport and transformation in a plant exposed to soil and air has been developed, parametrized, and applied successfully to simulate the uptake of Bromacil from hydroponic solution. Illustrative application to other chemicals suggests that the chemicals’properties, especially Henry’s law constant and octanol-water partition coefficient, play key roles in determining rates of uptake, fate, and the relative roles of transport through the xylem and evaporation from soil as routes of foliar contamination. I t is hoped that the model will be more fully tested by application to other chemicals in controlled laboratory soil and hydroponic systems and in field situations. A validated model applicable to a variety of plant species would be an asset for assessing chemical fate and especially for estimating human, wildlife, and domestic animal exposure to pesticides and environmental contaminants. Model Availability. A copy of the model in BASIC is available from the corresponding author or receipt of a 3 1/2-in. diskette and a self-addressed mailer. Acknowledgments The authors are gratefulto NSERC for financial support.
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Received for review December 21, 1993. Revised manuscript received June 28, 1994. Accepted September 8, 1994."
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Abstract published in Advance ACSAbstracts, October 15,1994.
