ENVIRONMENTAL
ASSESSMENT
FROM POTENTIAL G. B I G N O L I
OF A R S E N I C R E L E A S E D
POLLUTION
SOURCES
andE. SABBIONI
Commission of the European Communities, Joint Research Centre - lspra Establishment, Radiochemistry and Nuclear Chemistry Division, 21020 Ispra (Va) - Italy
(Received February 25, 1983) Abstract. An assessment study of the environmental pathways of arsenic released from a coal-fired power
plant (CFPP) or introduced into soil as a contaminant by phosphatic fertilizers has been carried out using a time-dependent forecasting model. The long-term predictions indicate that arsenic can be taken up by plants and that it can migrate into the groundwater system through soil layers. However, arsenic exhibits such a high degree of mobility that its retention and accumulation in biota should remain low. This fact may explain the relatively low concentrations of arsenic in environmental media as well as in groundwater systems.
I. Introduction
To meet future energy demands in the European Communities, it is envisaged that there will be increased utilization of coal for electrical energy production resulting in a mobilization of trace metals to the environment (Goetz et al., 1981). In particular, recent reports suggest that arsenic emissions from coal-fired power plants (CFPPs) may have a potential impact on terrestrial and aquatic environments (Sabbioni and Goetz, 1982; Bignoli et al., 1982). It is therefore important to predict how arsenic emitted from the stacks will be dispersed into the atmosphere, deposited on the earth's surface, accumulated in plants, enter the food chain, and migrate to groundwater (W.H.O., 1981). This type of study can be conveniently carried out by using exposure prediction models based on the systems analysis method. However, although the dynamic models are particularly useful in understanding the redistribution of trace metals in the biosphere, their application is still very difficult due to the limited input data necessary to calculate the intercompartmental transfer coefficients (Bignoli, 1980; Martin et al., 1974). In this paper the dynamic behaviour in the different environmental compartments of arsenic emitted from a hypothetical 2500 MWe CFPP has been studied with a mathematical prospective model. The input data-base for arsenic emission was drawn from estimates of trace-metal emissions for 1990 in the territory of the European Communities (Sabbioni and Goetz, 1982). In order to obtain some idea of the redistribution within the environment of arsenic released from other pollution sources, a dynamic model has also been applied to predict the impact of arsenic released in soil from the use of phosphatic fertilizers (Sabbioni and Bignoli, 1980). The long-term prediction of the increase of arsenic in the different environmental compartments suggests that atmospheric arsenic can accumulate in Environmental Monitoring and Assessment 4 (1984) 53-65. 9 1984 by D. Reidel Publishing Company.
0167-6369/84/0041-0053501.95.
G. BIGNOLI AND E. SABBIONI
54
plants and migrate through soil to groundwater systems. However, as suggested by the parameters used in the model, this element has high mobility within the terrestrial and aquatic environments in comparison with other elements such as cadmium and chromium, which leads to a low value of the accumulation index of arsenic in soil. 2. Model
Figure 1 shows the compartment model developed for this study. The various compartments of the system are interconnected by transfer coefficients K u, Kit. The factor 10 represents the input to the model of a contaminant released to the atmosphere such as arsenic emitted from the stack of a CFPP. The model considers the deposition oflo on the surface soil (I1) and on leaves of plants (13). If the contaminant enters the soil directly, as in the case of arsenic released as a contaminant from fertilizers, the input to the model at the level of root zone (Qz) is 12. The following set of differential equations represents the compartmental dynamics of the trace metal quantity (Q) considered: 0-1 = 11 + QsKs1 - QI(KI + K12)
(1)
0.2 = 12 + Q1K,2 - Q2(K25 + K23)
(2)
0-3 = R2K23 - Q3K34
(3)
0-4 = a 3 K 3 4 - Q4(K4 + K45)
(4)
0-5 = 15 + QaK45 + Q2K25 - a s ( K 2 + K s 1 ) .
(5)
This set of equations was solved by a numerical integration method implemented by the computer. The deposition onto soil and plants of the trace metals associated with particulates emitted into the atmosphere from the stack of CFPP is the fundamental parameter 10
(from source)
r ATMOSPHERE
A"
/5(deposition) Kg
(runoff=__)Ks ~ /1(deposition) (er
SURFACE LAYER Q1
(groith) kv
l k12 /,~
GRASS Qs k ~ d ROOTZONE Q2 ~ . ~ PASTURE ANDVEGETABLES chain) l kz3 k451(irrigation) I SOILSINK GBOUNOWATER O, Fig. 1. Compartmental model.
ENVIRONMENTAL ASSESSMENT OF ARSENIC
55
needed for further study of the environmental pathways of trace metals, through the terrestrial and aquatic eco-systems. This deposition can be considered as a product 09. where 09 = the deposition rate and ~ = a discrimination factor. The deposition rate 09 is defined by the following equation: 09 = V d - ~ ( x ,
0, 0)
(6)
where Va = the average deposition velocity of the particles, and ~ (total atmospheric concentration) = the integrated airborne concentration of the particulates. Vd is dependent on parameters such as surface impact due to air movement and electrostatic attraction. A value of 0.005 m s - was assumed (Wangen and Williams, 1978). The calculation of ~ requires some additional data. Trace metals emitted from the stack of a CFPP are mainly associated with the fly ash particles with a distribution
10-5
4E
&
ttl ar ttl
,'r 8,. 0 I-
_z Z 0 kz w U z 0
10-6
",r ffl >.
I-. 0
10-7
i 10 3
i
DISTANCE
Fig. 2.
=
L
L
3,10 3
FROM
I
~ I 10 4
SOURCE
(m)
I n t e g r a t e d fly a s h d i s p e r s i o n d o w n w i n d a r o u n d the C F P P , c o n s i d e r i n g P a s q u i l l ' s stability class D , s t a c k height o f 200 rn a n d a f r e q u e n c y d i s t r i b u t i o n o f w i n d s o f 0.5.
56
G. BIGNOLI AND E. SABB1ONI
dependent on the particle size of the fly ash. Therefore the definition of~ should be based on a knowledge of the fly ash dispersion in the atmosphere taking into consideration the full range of possible particle dimensions. In the present study the mathematical formulation of the atmospheric dispersion of the fly ash was derived from the generic Gaussian dispersion of the particles in the atmosphere (Van der Hoven, 1963) according to the following equation:
Z (x, y, O) -
Qf
exp
-
- exp
(7)
where Vg = the fall velocity of particles in the lower atmosphere (Chamberlain, 1953); f = the frequency distribution of the winds; the other parameters have been extensively explained in the literature (Van der Hoven, 1963). The vertical integration of the Equation (7), taking into account the particle dimensions (0-45 gin) of fly ash emitted (Sabbioni and Goetz, 1982), supplies the integrated atmosphere concentrations at ground level, ~ (x, cp), around the source of pollution. The result of the fly ash dispersion from a 2500 M W e CFPP taken as a hypothetical base case for this work, is presented in Figure 2. The discrimination factor a is a parameter which can be calculated for a trace metal when we know the total amount of the trace metal that is emitted in association with the particulates, the distribution of the trace metal as a function of the particle size, and the total fly ash emitted: a =
amount of trace metal emitted
(8)
total amount of fly ash released By combining this factor and the deposition a) of Equation (6), it is possible to calculate the dispersion of the atmospheric trace metal emission, 11 and I s, to soil and plants respectively: I, = ao~(1 - K.) 15 = aa~Kg
(9) (10)
in which K, takes into account the runoff effect transferring the element from the upper soil to surface water (Brady, 1974), and Kg the foliar interception mechanisms (Little, 1977). 2 . 1 . I N P U T S O F A R S E N I C TO T H E M O D E L ( I o A N D / 2 )
A N D D E P O S I T I O N ON SOIL A N D
PLANTS (/l AND /5)
The total arsenic emission I o from a 2500 MWe CFPP assumed for this study is 2.44 tons for the year 1990 as drawn from a recent assessment study of trace metal emission from CFPPs in the European Communities (Sabbioni and Goetz, 1982). The discrimination factor ~As for the arsenic can be estimated considering that about 2750 tons of fly ash will probably be released in 1990 (Sabbioni and Goetz, 1982): 0~As --
2"44 (t Yr- 1) -- 9 x 10 -4 2750 (t yr- ~)
(11)
ENVIRONMENTAL ASSESSMENT OF ARSENIC
57
A knowledge of 0tn~ allows us to calculate the total deposition rate value ~As (/), and consequently the deposition fractions 11 and 15 on soil and plants. Values of 1.42 mg m a yr- 1 and 0.9 mg m -2 yr- ~ and 0.5 mg m -2 yr- ~ were estimated for the three parameters. In the case of arsenic directly introduced to the soil by phosphatic fertilizer application, the input 12 to the model was 1.05 mg m - 1 yr- 1. This value is based on an annual average application of 350 kg of fertilizer per hectare containing 30 ppm of arsenic (Sabbioni and Bignoli, 1980; Bogardi et al., 1981). 2.2. C O M P A R T M E N T A L T R A N S F E R C O E F F I C I E N T S
A knowledge of the transfer coefficients K (Figure 1) is of fundamental importance for the evaluation of the migration of trace metals between the various compartments. Since the calculation of these coefficients differs for each compartment considered, quantitative knowledge of the environmental behaviour of trace metals in each compartment is required. Unfortunately these data are unavailable or incomplete, posing serious difficulties for the evaluations of the K coefficients. We were able to calculate the following K coefficients related to the migration of arsenic as shown in Figure 1: - KE5 related to the soil-to-plant concentration factor (CF) (Cataldo and Wildung, 1978) according to the following equation:
K25 =
C F Bio (kg fresh weight grass or vegetable m - 2 yr- ~)
(12)
Gs (1 - soil porosity) (kg soil m -2) Ks~ and K~ related to the mean residence time of particles deposited on leaves (Martin, 1964) and to the periodic harvesting of vegetable products or grazed by animals respectively; -Kv, transfer coefficient considering the physiological dilution of arsenic in a plant due to its periodical growth rate (Martin et al., 1974); K 1 is related to the natural erosion processes of the soil (Jackson et al., 1980); - K I 2 , K 2 3 , K 3 4 , and K4, leaching transfer coefficients for the calculation of the flux of arsenic through soil and water compartments. They depend mainly on the sorption coefficient (Ka-value) and/or on the pedological half-life (Bignoli and Sabbioni, 1981; Tyler, 1978; Tammes and De Lint, 1969; Fried, 1980; Baes and Sharp, 1983). The leaching transfer coefficients for arsenic (K) were calculated according to the following equations: -
-
Vs = Vwll + [((1 -p)lp)pKd,]
(13)
where V s -- the effective solute migration velocity (m yr- ~), Vw --- the infiltration rate of water in soil (m yr-~), p = the soil porosity, p = the bulk density of the soil (gcm 3) and Kdi -- the distribution coefficient for the solute (cm 3 g - 1). The mean residence time of a solute being leached from a soil layer may be given by: Ts = d / V s
(14)
where d = the depth from which the solute is removed via leaching (m). Combining
58
G. BIGNOLI AND E. SABBIONI
Equation (13) to (14), the first-order leaching transfer coefficient (yr- l) is given by: K=
Vw d ( l + [((1 -p)/p)pKd~]}
(15)
Equation (15) is a relatively simple approximation of an extremely complex process involving physical, chemical, and biological processes. The distribution coefficient Kd, the water infiltration velocity, bulk density, and volumetric content of soil, are the most important parameters in calculating the leaching constants. In the case of Kd especially there are many sources of variability which can influence its value (Baes and Sharp, 1983). An analysis of the literature was performed to ascertain the appropriate distribution of Kd values for arsenic and other parameters and to calculate the respective leaching coefficients (K). Tables I and II give the values of parameters considered and the values of the transfer coefficients calculated in the present study. TABLE I Parameters used in the improved soil-plant-groundwater model for Arsenic in a terrestrial environment Parameters
Surface soil bulk density, g c m - 3 Surface soil porosity Surface soil half-life, yr Pore water infiltration velocity through surface soil, m yr- 1 Soil sink bulk density, g c m - 3 Soil sink porosity Soil sink distribution coefficient, cm 3 g - 1 Pore water infiltration velocity through soil sink, m yr- 1 Groundwater bulk density, g c m - 3 Groundwater porosity
Groundwater movement, m y r -
Assigned values 1.35
0.35 6.5
40 1.5 0.2 6
References
(Brady, 1974) (Brady, 1974) (Baes etaL, 1983;Korte etaL, 1975; Tyler etaL, 1978; Tammes etaL, 1969) Beese et al., 1980; Bresler, 1973; Van de Pol etal., 1977) (Brady, 1974) (Brady, 1974) (Baes etal., 1983; Korte etal., 1975) (Beese etaL, 1980; Bresler, 1973; Van de Pol et al., 1977)
30 1.5 a 0.4 a Groundwater distribution coefficient, cm 3 g - ~ 1 ~ 10 a
a Generalized to a non site specific.
2.3. C O M P A R T M E N T A L CONCENTRATIONS
The solution of the system equations (1) to (5) represents the time dependent variation of the quantity Q; (mg) of arsenic each compartment of the model. The dynamic concentrations (C;) (ppm) of arsenic in the various soil compartments are given by the
ENVIRONMENTAL ASSESSMENT OF ARSENIC
59
equation:
Ci(t)-
O~(t) Vp X 103
(16)
assuming no internal gradient and physical discontinuities and that the flow of the trace metal is instantaneously distributed in the compartmental volume V (m3). Since the volume of a compartment is too large to assume an instantaneous distribution of the arsenic, the changing of its concentration through the volume of the compartment (i.e. deep soil column) within which it will work, can be expressed as: C,.(t) =
(17)
Qi(t) 103 ps Sto Vs(t') dt'
where s = the column section of soil (m 2) and Vs(t') dt' --- the solute movement (m). TABLE II Calculated values of the transfer coefficients for Arsenic in a terrestrial environment (Figure 1) a Transfer coefficient Kl2 (surface layer-root zone) Ke3 (root zone-soil sink) K34 (soil sink-groundwater) K45 K45 K4 K25 K25 K5 K51 KL, Kj
(irrigation by groundwater) (groundwater movement) (uptake by vegetables) (uptake by grass) (periodical harvest or consumption by animals) (translocation from leaves to soil) (plant growth) (erosion of soil)
Calculated value (y - 1) 1 0.15 0.08 0.0 0.2 b 0.4 0.0002 0.00042 1.0 18 0.18 0.00045
a Depth of the surface soil layer (Q1)= 3 cm; depth of the root zone (Q2)= 35 cm for vegetables and 15 cm for pasture grass; depth of soil sink column (Q3) = 10 m; depth of the groundwater system under soil sink column = 5 m. b When irrigation is considered, a rate of 1 m a m - 2 y r - 1 is applied.
The assumption that the dissolved arsenic in the water is in equilibrium with the arsenic adsorbed by the soil matrix, is expressed by the linear equation of the Kd:
f = Kd~R/Kd, R + 1
(18)
where f. = the fraction of arsenic adsorbed in solid phase and R = M/v, where M and v = the mass of solid phase and volume of liquid phase respectively. The effective arsenic concentration Ca(t) in groundwater was calculated as follows: C4(t) _ Q4(t) (1 - f.)
V4P4
(19)
60
G. BIGNOLI AND E. SABBIONI
where Q4 = the quantity of arsenic present in the groundwater system at time t (mg), 1:4 = the total volume of groundwater compartment (t) and P4 = the porosity of the material composing groundwater system considered. 3. Results
As shown in Figure 2, the profile of the dispersion of the fly ash emitted from a 2500 MW e CFPP assumed as the base case indicates a maximum of emission at about 3 km from the stacks. At this distance the concentration of arsenic in air at ground level has been estimated to be ~A~ '~, i.e. 9 X 10-9g m-3. The long-term effect on the arsenic level in soil due to the deposition of arsenic emission at 3 km is shown in Table III. The increase of arsenic in soil is of the order of 0.02 ppm after 40 yr (a typical life-span ofa CFPP) (Sabbioni and Goetz, 1982). This corresponds to an increase of 0.3 ~o over the typical average background level of arsenic in soil (6 ppm) (Bowen, 1966; Lisk, 1972). TABLE III Long-term amount of arsenic in soil a Time (yr)
Deposition from atmosphere (ppm)
Fertilizer use (ppm)
1 10 20 40
0.0026 0,0t6 0.017 0.018
0.0020 0.009 0.01 0.01
a In the case ofatmospheric deposition, fly ash is considered as being deposited on grass-having a depth of root zone (Q2) of 15 cm, while in a case of fertilizer directly mixed into the agricultural soil, a 35 cm depth of root zone was assumed,
Table III also reports the long-term prediction of the arsenic level in soil as a consequence of the use of phosphatic fertilizer. Amounts of arsenic corresponding to 0.01 ppm can be predicted after 40 yr of fertilizer application with a calculated increase over the background level of the order of 0.15 ~ . Figure 3 shows the dynamic results of the long-term impact of arsenic on pasture grass and vegetables as a consequence of the atmospheric deposition from the CFPP and of the use of fertilizer. The impact of arsenic on grass is higher in the case of the element emitted from the CFPP, and this corresponds to a 10~ increase over the typical present level in plants (0.5 ppm) (Cataldo and Wildung, 1978; Risby, 1979) calculated over 40yr for a continuous stack emission. The dynamic behaviour of arsenic in groundwater systems from the two pollution sources considered in comparison with the average endogenous level of arsenic (Bowen, 1977) is shown in Figure 4.
ENVIRONMENTAL ASSESSMENT OF ARSENIC
61
0.65
0.60 z
PASTURE GRASS (DUE TO COAL COMBUSTION)~ f
f
~
VEGETABLES (DUE TO FERTILIZER U S E - ~
o.~
8
~ " P R E S E N T LEVEL
0,0
[ 10
I 20
I 30
I 40
I 50
I 60
I 70
I 80
I 90
100
] 90
100
TIME (YEARS)
Fig. 3. Long-term effect of arsenic on vegetation.
2.10-3
10-3
8.10- 4
6.10-4
j
/jPRESE,'T LEVEL
4.10-4
2.10-4 10-4 0.0
II 10
Ii 20
I 30
I 40
i 50
I 60
] 70
I 80
TIME (YEARS)
Fig. 4. Long-term impact of arsenic on groundwater quality.
The following conclusions can be drawn: - a fraction of the arsenic introduced into the environment can migrate through soil layers and enter the groundwater systems. The high mobility is in agreement with the short pedological half-life (Tammes and De Lint, 1969) and the low value o f K d between arsenic and soil components (Korte et al., 1975); - the time of percolation of arsenic to groundwater is estimated to be of the order of 20 and 10 yr from the case of CFPP and fertilizer practice respectively. The higher retardation time of arsenic in soil column when the source input is related to deposition from the atmosphere as to arsenic contained in fertilizer is due to the fact that in the case of fertilizer use the arsenic contained in phosphate is directly mixed during ploughing with agricultural soil, while for arsenic deposited on soil surface the indisturbed grass root zone may be more act as an effective biological barrier for arsenic migration through soil trophic level.
62
G. B1GNOLI AND E. SABBIONI
4. Discussion An assessment of the environmental pathways of arsenic released from two potential pollution sources such as CFPP for energy production and phosphatic fertilizers has been carried out by a dynamic model based on systems analysis. This approach would appear to be particularly useful in predicting the time-dependent events of TM in the different environmental compartments such as atmospheric dispersion, soil deposition, plant accumulation and migration to groundwater (Figure 1). However, many difficulties are encountered when applying this type of model. This is essentially due to the severe lack of quantitative information concerning fundamental parameters such as the transfer coefficients for trace metal representing movement between the different compartments. To the best of our knowledge no work has been reported on the use of dynamic models to evaluate the patterns of arsenic in the biosphere and little information on this subject based on the use of equilibrium models is available. Of particular interest is a recent work of Bennett (1981 a, b)who evaluated the global situation of arsenic in equilibrium with the terrestrial and aquatic environment with the purpose of estimating the average exposure commitment to man. It must be stressed, however, that our modelling approach is not intended to arrive at a mesoscale level, and considers only a particular local situation. This explains some apparent discrepancies in the parameters assumed in our and Bennett's studies as in the case of half-life of arsenic in the compartmental soil sink. The exogenous quantity of arsenic introduced into the atmosphere from coal combustion or fertilizer use is not yet in equilibrium with the environmental arsenic and more time is required to reach this situation. In this case the pedological half-life of between 6.5 and 16yr (NRCC, 1978) employed in our study seems reasonable considering also the high mobility of arsenic in soil layers as confirmed by its low Ka-values (Korte et aL, 1975) as compared with the value of 2000 yr taken by Bennett which represents a high steady residence time for natural arsenic (Bowen, 1977). The difference of the three orders of magnitude between our own and Bennett's study is only apparent because the value used by Bennett takes into account all the possible biological and chemico-physical interactions of arsenic assumed to be in equilibrium with the terrestrial environment. The results of this study show that during the 40 yr of operation of a 2500 MWe CFPP as well as the application of phosphate fertilizers to agricultural soil, the dynamic impacts of arsenic at the various levels of the terrestrial environment do not substantially increase the typical background levels. In order to calculate the degree of mobility of arsenic in the soil matrix, a dynamic accumulation index 'a' for the trace metals seems appropriate (Zwozoziak and Zwozoziak, 1981). In the case of the arsenic emitted from CFPP, aA~ is defined as follows: aAs = Q2 - Q2, o
(20)
co~ast/dp where Q2, o and 0z = dQz/dt denote the arsenic contents in the root zone of soil at the start of CFPP operation and after t years of power plant operation respectively (ppm)
ENVIRONMENTAL ASSESSMENT OF ARSENIC
63
(Equation (2)); t --- the number of years of power plant operation; CO~Aswas calculated by formulae (6) and (11) and represents the total arsenic deposition (rag m - 2 yr-1); d = thickness (m) of soil root zone for two cases of uncultivated and cultivated land; p = the soil density (kg m - 3). The arsenic dynamic accumulation index aAs was determined from the calculation results obtained after 20 yr of power plant operation and compared with other trace metals studied (Sabbioni and Bignoli, 1980; Bignoli and Sabbioni, 1983). These results are presented in Table IV. TABLE IV Dynamic accumulationindex in soil for trace metals released for 20 yr from a 2500MW~ coal-firedpower plant Trace metal
Present concentrationlevel in s o i l (ppm)a
Accumulation index (a)
As Cd Cr b
6 0.06 40
0.12 0.60 0.75
a Based on soil backgroundlevels as generalizedfrom Bowen, 1966, and Lisk, 1972. b Total chromium(Cr § 6 + Cr +3) (Bignoliand Sabbioni, 1983). The accumulation indices for Cr and Cd are higher than the index value calculated for the dynamic movement of As. This suggests that As is characterized by a greater migration ability in comparison with Cr and Cd which are more absorbed by the root zone of soil. These results are in agreement with other in situ studied trace metals in the vicinity of a copper smelter (Zwozoziak and Zwozoziak, 1981). The high mobility of arsenic through soil layers should be responsible for the low bioavailability of this element to plants, explaining the low degree of accumulation predicted (Figure 3). The mobility of arsenic through soil should influence the groundwater quality (Figure 4). However, the concentration in this compartment should not exceed the Maximum Permissible Concentration level of 50 gg 1- 1 for the waters in the European Community (Off. J. of the European Community, 1975). In reality the movement of water through soils rarely occurs continuously at uniform flow velocity. There is a period when after the infiltration the flow is negligible but adsorption/desorption processes continue (Murali and Aylmore, 1980). A too complex mathematical system would be necessary to describe this phenomenon. We have limited our approach to simple linear adsorption isotherms. Therefore the results obtained for arsenic in plants and water are necessarily a simplified approximation of the environmental reality. This study assumed the transfer of arsenic in the different compartments to be a total amount. It does not take into account the influence of the possible chemical forms of the element. Methylarsines are released into the air from soil trophic level following biomethylation of a large quantity of methylarsenic compounds used in agriculture (NRCC, 1978). Attrep and Anirudhan, 1977, found a high concentration of arsenic in air in an area
64
G. BIGNOLI AND E, SABBIONI
polluted by arsenic from defoliants. About half of the airborne arsenic was in the organic form. However,the amount of methylarsines found in unpolluted areas or near to the stack dust from smelting operations, arsenic is predominantly in the inorganic form (Crecelius, 1974; Rosehart and Chu, 1975). From these considerations it appears that when organic arsenic reaches soils it may be biomethylated more rapidly than its inorganic forms. For these reasons and considering that the chemical form of arsenic emitted from the stack of a coal power plant or present in fertilizer phosphates is probably in an inorganic form, we have decided that the methylation mechanisms and relative parameters are at present not sufficiently well known (WHO, 1981) to be applied to a dynamic model. Since arsenic can be biotransformed into many species in the environment through the methylation process (NRCC, 1978; WHO, 1981; Wood, 1974), research is needed to investigate the behaviour of these forms in the environment. In this context a characterization of the chemical form of arsenic entering the soil as a result of atmospheric deposition or by direct release from the use of fertilizers, should receive a high degree of priority. References Attrep, M. and Anirudhan, M.: 1977, 'Atmospheric Inorganic and Organic Arsenic', Trace Subst. Environ. Health 11,365-369. Baes, C. F. and Sharp, R. D.: 1983, 'A Proposal for Estimation of Soil Leaching and Leaching Constants for Use in Assessment Models', J. Environ. Qual. 12, 17-28. Beese, F. and Wierenga, P. J.: 1980, 'Solute Transport through Soil with Adsorption and Root Water Uptake Computed with a Transient and a Constant-Flux Model', Soil Science 129(4), 245-252. Bennett, B. G.: 1981a, 'Exposure of Man to Environmental Arsenic. An Exposure Commitment Assessment', Sci. Total Environ. 20, 99-107. Bennett, B. G.: 1981b, 'The Exposure Commitment Method in Environmental Pollutant Assessment', Envir. Monit. and Assess. 1, 21-36. Bignoli, G., Sabbioni, E., and Goetz, L.: 1982, 'Implementation of a Dynamic Previsional Approach to the Study of Heavy Metals - Environmental Pollution', in International Days of Biology, Naples. Bignoli, G.: 1980, 'TOXICO: A System Analysis Model for Assessing the Behaviour and Impact upon Man of Heavy Metals Released in the Environment', EUR-Technical Note 1.07.08.80.33. Bignoli, G. and Bertozzi, G.: 1979, 'Modelling of Artificial Radioactivity Migration in the Environment: A Survey', EUR-6179. Bignoli, B. and Sabbioni, E.: 1981, 'Long-Term Prediction of the Potential Impact of Heavy Metals on Groundwater Quality as a Result of Fertilizer Use', Studies in Environmental Science 17, 857-862. Bignoli, G. and Sabbioni, E.: 1983, 'Environmental Impact of Cr Released into Biosphere from Coal Combustion in a 2500 MWe Power Plant', (to be published). Bogardi, I., Duckstein, L., and Szidarovski, F.: 1981, 'A Control Model for Phosphorous Loading Reduction under Uncertainty', Ecol. Model. 12, 83-103. Bowen, H. J. M.: 1977, 'Natural Cycles of Elements and Their Perturbation by Man', in Environment and Man, Vol. VI, Blackie Publ., London. Bowen, H. J. M.: 1966, Trace Elements in Biochemistry, Academic Press Inc., London. Brady, N. C.: 1974, The Nature and Properties of Soils, McMillan Publ. Co. Inc., New York. Bresler, E.: 1973, 'Simultaneous Transport of Solutes and Water under Transient Unsaturated Flow Conditions', Water Res. Res. 9, 975-986. Cataldo, D. A. and Wildung, R. E.: 1978, 'Soil and Plant Factors Influencing the Accumulation of Heavy Metals by Plants', Environ. Health Perspect. 27, 142-153. Chamberlain, A. C.: 1953, 'Aspects of Travel and Deposition of Aerosols and Vapour Clouds', UKAEA Report AERE (MP) R, 1261.
ENVIRONMENTALASSESSMENTOF ARSENIC
65
Crecelius, E. A.: 1974, 'The Geochemistry of Arsenic and Antimony in Puget Sound and Lake Washington', Thesis, Washington University. Fried, J. J.: 1980, 'Present Groundwater Pollution Problems', in Nuclear Techniques in Groundwater Pollution Research, Proc. of an Advisory Group Meeting, organized by the IAEA, Vienna. Goetz, L., Bignoli, G., and Sabbioni, E.: 1981, 'Mobilization of Heavy Metals from Coal-Fired Power Plants: Potential Impact on Groundwater', Studies in Envir. Sci. 17, 261-264. Jackson, R., Merrit, W. F., and Inch, K. J.: 1980, 'The Distribution Coefficient as a Geochemical Measure of the Mobility of Contaminants in a Groundwater Flow System', in Nuclear Techniques in Groundwater Pollution Research, Proc. of an Advisory Group Meeting, organized by the IAEA, Vienna. Korte, N. E., Skopp, J., Fuller, W. H., Niebla, F. E., and Alesti, B. A.: 1975, 'Trace Element Movement in Soils: Influence of Soil Physical and Chemical Properties', Soil Sci. 122, 350-359. Lisk, D. I.: 1972, Advances in Agronomy, Academic Press, New York. Martin, W. E.: 1964, 'Losses of Sr9~ Sr 89, and I TM from Fallout Contaminated Plants', Rad. Bot. 4, 275-282. Martin, W. E., Bloom, S. G., and Yorde, I. R.: 1974, 'Plutonium Transport and Dose Estimation Model', NVO-142. Murali, V. and Aylmore, A. G.: 1980, 'No-flow Equilibration and Adsorption Dynamics during Ionic Transport in Soils', Nature 283, 467-469. National Research Council Canada: 1978, 'Effect of Arsenic in the Canadian Environment', NRCC 15391, Ottawa. Official Journal of the European Communities: 1975, C214/2. Risby, T. H. (ed.): 1979, 'Ultratrace Metal Analysis in Biological Sciences and Environment', Advances in Chemistry Series, 172. Rosehart, R. G. and Chu, R.: 1975, 'Methods for Identification of Arsenic Compounds', Water, Air, and Soil Pollut. 4, 395-398. Sabbioni, E. and Goetz, L.: 1982, 'Mobilization of Heavy Metals from Fossil-Fuelled Power Plants, Potential Ecological and Biochemical Implications', EUR-6998/4. Sabbioni, E. and Bignoli, G.: 1980, 'Heavy Metals in Phosphatic Fertilizers: Potential Impact on Groundwater Quality', Europ. Appl. Research Rep. 1, 141-180. Tammes, P. M. and De Lint, M. M.: 1969, 'Leaching of As from Soil', Neth. J. Agric. Sci. 17, 128. Tyler, G.: 1978, 'Leaching Rates of Heavy Metal Ions in Forest Soil', Water, Air, and SoilPollut. 9, 137-148. Van der Hoven, I.: 1963, 'A Diffusion-Deposition Model for Particulate Effluents from Ground-Treated Nuclear Engines', Int. J. Air Water Pollut. 7, 1023. Van de Pol, R. M., Wierenga, P. J., and Nielsen, D. R.: 1977, 'Solute Movement in a Field Soil', Soil Sei. Soc. Am. J. 41, 10-13. Wangen, L. E. and Williams, M. D.: 1978, 'Elemental Deposition Downwind of a Coal-Fired Power Plant', Water, Air, and Soil Pollut. 10, 33-44. Wood, J. M.: 1974, 'Biological Cycles for Toxic Elements in the Environment', Science 183, 1049. World Health Organization: 1981, 'Environmental Health Criteria 18', ARSENIC. Zunino, H. and Martin, J. P.: 1976, 'Metal Binding Organic Macromolecules in Soil: 2. Characterization of the Maximum Binding Ability of the Macromolecules', Soil Sei. 128, 187. Zwozoziak, J. W. and Zwozoziak, A. B.: 1981, 'Trace-Metal Behaviour in the Vicinity of a Copper Smelter', Int. J. Envir. Studies 19, 35.
ASSESSMENT
FROM POTENTIAL G. B I G N O L I
OF A R S E N I C R E L E A S E D
POLLUTION
SOURCES
andE. SABBIONI
Commission of the European Communities, Joint Research Centre - lspra Establishment, Radiochemistry and Nuclear Chemistry Division, 21020 Ispra (Va) - Italy
(Received February 25, 1983) Abstract. An assessment study of the environmental pathways of arsenic released from a coal-fired power
plant (CFPP) or introduced into soil as a contaminant by phosphatic fertilizers has been carried out using a time-dependent forecasting model. The long-term predictions indicate that arsenic can be taken up by plants and that it can migrate into the groundwater system through soil layers. However, arsenic exhibits such a high degree of mobility that its retention and accumulation in biota should remain low. This fact may explain the relatively low concentrations of arsenic in environmental media as well as in groundwater systems.
I. Introduction
To meet future energy demands in the European Communities, it is envisaged that there will be increased utilization of coal for electrical energy production resulting in a mobilization of trace metals to the environment (Goetz et al., 1981). In particular, recent reports suggest that arsenic emissions from coal-fired power plants (CFPPs) may have a potential impact on terrestrial and aquatic environments (Sabbioni and Goetz, 1982; Bignoli et al., 1982). It is therefore important to predict how arsenic emitted from the stacks will be dispersed into the atmosphere, deposited on the earth's surface, accumulated in plants, enter the food chain, and migrate to groundwater (W.H.O., 1981). This type of study can be conveniently carried out by using exposure prediction models based on the systems analysis method. However, although the dynamic models are particularly useful in understanding the redistribution of trace metals in the biosphere, their application is still very difficult due to the limited input data necessary to calculate the intercompartmental transfer coefficients (Bignoli, 1980; Martin et al., 1974). In this paper the dynamic behaviour in the different environmental compartments of arsenic emitted from a hypothetical 2500 MWe CFPP has been studied with a mathematical prospective model. The input data-base for arsenic emission was drawn from estimates of trace-metal emissions for 1990 in the territory of the European Communities (Sabbioni and Goetz, 1982). In order to obtain some idea of the redistribution within the environment of arsenic released from other pollution sources, a dynamic model has also been applied to predict the impact of arsenic released in soil from the use of phosphatic fertilizers (Sabbioni and Bignoli, 1980). The long-term prediction of the increase of arsenic in the different environmental compartments suggests that atmospheric arsenic can accumulate in Environmental Monitoring and Assessment 4 (1984) 53-65. 9 1984 by D. Reidel Publishing Company.
0167-6369/84/0041-0053501.95.
G. BIGNOLI AND E. SABBIONI
54
plants and migrate through soil to groundwater systems. However, as suggested by the parameters used in the model, this element has high mobility within the terrestrial and aquatic environments in comparison with other elements such as cadmium and chromium, which leads to a low value of the accumulation index of arsenic in soil. 2. Model
Figure 1 shows the compartment model developed for this study. The various compartments of the system are interconnected by transfer coefficients K u, Kit. The factor 10 represents the input to the model of a contaminant released to the atmosphere such as arsenic emitted from the stack of a CFPP. The model considers the deposition oflo on the surface soil (I1) and on leaves of plants (13). If the contaminant enters the soil directly, as in the case of arsenic released as a contaminant from fertilizers, the input to the model at the level of root zone (Qz) is 12. The following set of differential equations represents the compartmental dynamics of the trace metal quantity (Q) considered: 0-1 = 11 + QsKs1 - QI(KI + K12)
(1)
0.2 = 12 + Q1K,2 - Q2(K25 + K23)
(2)
0-3 = R2K23 - Q3K34
(3)
0-4 = a 3 K 3 4 - Q4(K4 + K45)
(4)
0-5 = 15 + QaK45 + Q2K25 - a s ( K 2 + K s 1 ) .
(5)
This set of equations was solved by a numerical integration method implemented by the computer. The deposition onto soil and plants of the trace metals associated with particulates emitted into the atmosphere from the stack of CFPP is the fundamental parameter 10
(from source)
r ATMOSPHERE
A"
/5(deposition) Kg
(runoff=__)Ks ~ /1(deposition) (er
SURFACE LAYER Q1
(groith) kv
l k12 /,~
GRASS Qs k ~ d ROOTZONE Q2 ~ . ~ PASTURE ANDVEGETABLES chain) l kz3 k451(irrigation) I SOILSINK GBOUNOWATER O, Fig. 1. Compartmental model.
ENVIRONMENTAL ASSESSMENT OF ARSENIC
55
needed for further study of the environmental pathways of trace metals, through the terrestrial and aquatic eco-systems. This deposition can be considered as a product 09. where 09 = the deposition rate and ~ = a discrimination factor. The deposition rate 09 is defined by the following equation: 09 = V d - ~ ( x ,
0, 0)
(6)
where Va = the average deposition velocity of the particles, and ~ (total atmospheric concentration) = the integrated airborne concentration of the particulates. Vd is dependent on parameters such as surface impact due to air movement and electrostatic attraction. A value of 0.005 m s - was assumed (Wangen and Williams, 1978). The calculation of ~ requires some additional data. Trace metals emitted from the stack of a CFPP are mainly associated with the fly ash particles with a distribution
10-5
4E
&
ttl ar ttl
,'r 8,. 0 I-
_z Z 0 kz w U z 0
10-6
",r ffl >.
I-. 0
10-7
i 10 3
i
DISTANCE
Fig. 2.
=
L
L
3,10 3
FROM
I
~ I 10 4
SOURCE
(m)
I n t e g r a t e d fly a s h d i s p e r s i o n d o w n w i n d a r o u n d the C F P P , c o n s i d e r i n g P a s q u i l l ' s stability class D , s t a c k height o f 200 rn a n d a f r e q u e n c y d i s t r i b u t i o n o f w i n d s o f 0.5.
56
G. BIGNOLI AND E. SABB1ONI
dependent on the particle size of the fly ash. Therefore the definition of~ should be based on a knowledge of the fly ash dispersion in the atmosphere taking into consideration the full range of possible particle dimensions. In the present study the mathematical formulation of the atmospheric dispersion of the fly ash was derived from the generic Gaussian dispersion of the particles in the atmosphere (Van der Hoven, 1963) according to the following equation:
Z (x, y, O) -
Qf
exp
-
- exp
(7)
where Vg = the fall velocity of particles in the lower atmosphere (Chamberlain, 1953); f = the frequency distribution of the winds; the other parameters have been extensively explained in the literature (Van der Hoven, 1963). The vertical integration of the Equation (7), taking into account the particle dimensions (0-45 gin) of fly ash emitted (Sabbioni and Goetz, 1982), supplies the integrated atmosphere concentrations at ground level, ~ (x, cp), around the source of pollution. The result of the fly ash dispersion from a 2500 M W e CFPP taken as a hypothetical base case for this work, is presented in Figure 2. The discrimination factor a is a parameter which can be calculated for a trace metal when we know the total amount of the trace metal that is emitted in association with the particulates, the distribution of the trace metal as a function of the particle size, and the total fly ash emitted: a =
amount of trace metal emitted
(8)
total amount of fly ash released By combining this factor and the deposition a) of Equation (6), it is possible to calculate the dispersion of the atmospheric trace metal emission, 11 and I s, to soil and plants respectively: I, = ao~(1 - K.) 15 = aa~Kg
(9) (10)
in which K, takes into account the runoff effect transferring the element from the upper soil to surface water (Brady, 1974), and Kg the foliar interception mechanisms (Little, 1977). 2 . 1 . I N P U T S O F A R S E N I C TO T H E M O D E L ( I o A N D / 2 )
A N D D E P O S I T I O N ON SOIL A N D
PLANTS (/l AND /5)
The total arsenic emission I o from a 2500 MWe CFPP assumed for this study is 2.44 tons for the year 1990 as drawn from a recent assessment study of trace metal emission from CFPPs in the European Communities (Sabbioni and Goetz, 1982). The discrimination factor ~As for the arsenic can be estimated considering that about 2750 tons of fly ash will probably be released in 1990 (Sabbioni and Goetz, 1982): 0~As --
2"44 (t Yr- 1) -- 9 x 10 -4 2750 (t yr- ~)
(11)
ENVIRONMENTAL ASSESSMENT OF ARSENIC
57
A knowledge of 0tn~ allows us to calculate the total deposition rate value ~As (/), and consequently the deposition fractions 11 and 15 on soil and plants. Values of 1.42 mg m a yr- 1 and 0.9 mg m -2 yr- ~ and 0.5 mg m -2 yr- ~ were estimated for the three parameters. In the case of arsenic directly introduced to the soil by phosphatic fertilizer application, the input 12 to the model was 1.05 mg m - 1 yr- 1. This value is based on an annual average application of 350 kg of fertilizer per hectare containing 30 ppm of arsenic (Sabbioni and Bignoli, 1980; Bogardi et al., 1981). 2.2. C O M P A R T M E N T A L T R A N S F E R C O E F F I C I E N T S
A knowledge of the transfer coefficients K (Figure 1) is of fundamental importance for the evaluation of the migration of trace metals between the various compartments. Since the calculation of these coefficients differs for each compartment considered, quantitative knowledge of the environmental behaviour of trace metals in each compartment is required. Unfortunately these data are unavailable or incomplete, posing serious difficulties for the evaluations of the K coefficients. We were able to calculate the following K coefficients related to the migration of arsenic as shown in Figure 1: - KE5 related to the soil-to-plant concentration factor (CF) (Cataldo and Wildung, 1978) according to the following equation:
K25 =
C F Bio (kg fresh weight grass or vegetable m - 2 yr- ~)
(12)
Gs (1 - soil porosity) (kg soil m -2) Ks~ and K~ related to the mean residence time of particles deposited on leaves (Martin, 1964) and to the periodic harvesting of vegetable products or grazed by animals respectively; -Kv, transfer coefficient considering the physiological dilution of arsenic in a plant due to its periodical growth rate (Martin et al., 1974); K 1 is related to the natural erosion processes of the soil (Jackson et al., 1980); - K I 2 , K 2 3 , K 3 4 , and K4, leaching transfer coefficients for the calculation of the flux of arsenic through soil and water compartments. They depend mainly on the sorption coefficient (Ka-value) and/or on the pedological half-life (Bignoli and Sabbioni, 1981; Tyler, 1978; Tammes and De Lint, 1969; Fried, 1980; Baes and Sharp, 1983). The leaching transfer coefficients for arsenic (K) were calculated according to the following equations: -
-
Vs = Vwll + [((1 -p)lp)pKd,]
(13)
where V s -- the effective solute migration velocity (m yr- ~), Vw --- the infiltration rate of water in soil (m yr-~), p = the soil porosity, p = the bulk density of the soil (gcm 3) and Kdi -- the distribution coefficient for the solute (cm 3 g - 1). The mean residence time of a solute being leached from a soil layer may be given by: Ts = d / V s
(14)
where d = the depth from which the solute is removed via leaching (m). Combining
58
G. BIGNOLI AND E. SABBIONI
Equation (13) to (14), the first-order leaching transfer coefficient (yr- l) is given by: K=
Vw d ( l + [((1 -p)/p)pKd~]}
(15)
Equation (15) is a relatively simple approximation of an extremely complex process involving physical, chemical, and biological processes. The distribution coefficient Kd, the water infiltration velocity, bulk density, and volumetric content of soil, are the most important parameters in calculating the leaching constants. In the case of Kd especially there are many sources of variability which can influence its value (Baes and Sharp, 1983). An analysis of the literature was performed to ascertain the appropriate distribution of Kd values for arsenic and other parameters and to calculate the respective leaching coefficients (K). Tables I and II give the values of parameters considered and the values of the transfer coefficients calculated in the present study. TABLE I Parameters used in the improved soil-plant-groundwater model for Arsenic in a terrestrial environment Parameters
Surface soil bulk density, g c m - 3 Surface soil porosity Surface soil half-life, yr Pore water infiltration velocity through surface soil, m yr- 1 Soil sink bulk density, g c m - 3 Soil sink porosity Soil sink distribution coefficient, cm 3 g - 1 Pore water infiltration velocity through soil sink, m yr- 1 Groundwater bulk density, g c m - 3 Groundwater porosity
Groundwater movement, m y r -
Assigned values 1.35
0.35 6.5
40 1.5 0.2 6
References
(Brady, 1974) (Brady, 1974) (Baes etaL, 1983;Korte etaL, 1975; Tyler etaL, 1978; Tammes etaL, 1969) Beese et al., 1980; Bresler, 1973; Van de Pol etal., 1977) (Brady, 1974) (Brady, 1974) (Baes etal., 1983; Korte etal., 1975) (Beese etaL, 1980; Bresler, 1973; Van de Pol et al., 1977)
30 1.5 a 0.4 a Groundwater distribution coefficient, cm 3 g - ~ 1 ~ 10 a
a Generalized to a non site specific.
2.3. C O M P A R T M E N T A L CONCENTRATIONS
The solution of the system equations (1) to (5) represents the time dependent variation of the quantity Q; (mg) of arsenic each compartment of the model. The dynamic concentrations (C;) (ppm) of arsenic in the various soil compartments are given by the
ENVIRONMENTAL ASSESSMENT OF ARSENIC
59
equation:
Ci(t)-
O~(t) Vp X 103
(16)
assuming no internal gradient and physical discontinuities and that the flow of the trace metal is instantaneously distributed in the compartmental volume V (m3). Since the volume of a compartment is too large to assume an instantaneous distribution of the arsenic, the changing of its concentration through the volume of the compartment (i.e. deep soil column) within which it will work, can be expressed as: C,.(t) =
(17)
Qi(t) 103 ps Sto Vs(t') dt'
where s = the column section of soil (m 2) and Vs(t') dt' --- the solute movement (m). TABLE II Calculated values of the transfer coefficients for Arsenic in a terrestrial environment (Figure 1) a Transfer coefficient Kl2 (surface layer-root zone) Ke3 (root zone-soil sink) K34 (soil sink-groundwater) K45 K45 K4 K25 K25 K5 K51 KL, Kj
(irrigation by groundwater) (groundwater movement) (uptake by vegetables) (uptake by grass) (periodical harvest or consumption by animals) (translocation from leaves to soil) (plant growth) (erosion of soil)
Calculated value (y - 1) 1 0.15 0.08 0.0 0.2 b 0.4 0.0002 0.00042 1.0 18 0.18 0.00045
a Depth of the surface soil layer (Q1)= 3 cm; depth of the root zone (Q2)= 35 cm for vegetables and 15 cm for pasture grass; depth of soil sink column (Q3) = 10 m; depth of the groundwater system under soil sink column = 5 m. b When irrigation is considered, a rate of 1 m a m - 2 y r - 1 is applied.
The assumption that the dissolved arsenic in the water is in equilibrium with the arsenic adsorbed by the soil matrix, is expressed by the linear equation of the Kd:
f = Kd~R/Kd, R + 1
(18)
where f. = the fraction of arsenic adsorbed in solid phase and R = M/v, where M and v = the mass of solid phase and volume of liquid phase respectively. The effective arsenic concentration Ca(t) in groundwater was calculated as follows: C4(t) _ Q4(t) (1 - f.)
V4P4
(19)
60
G. BIGNOLI AND E. SABBIONI
where Q4 = the quantity of arsenic present in the groundwater system at time t (mg), 1:4 = the total volume of groundwater compartment (t) and P4 = the porosity of the material composing groundwater system considered. 3. Results
As shown in Figure 2, the profile of the dispersion of the fly ash emitted from a 2500 MW e CFPP assumed as the base case indicates a maximum of emission at about 3 km from the stacks. At this distance the concentration of arsenic in air at ground level has been estimated to be ~A~ '~, i.e. 9 X 10-9g m-3. The long-term effect on the arsenic level in soil due to the deposition of arsenic emission at 3 km is shown in Table III. The increase of arsenic in soil is of the order of 0.02 ppm after 40 yr (a typical life-span ofa CFPP) (Sabbioni and Goetz, 1982). This corresponds to an increase of 0.3 ~o over the typical average background level of arsenic in soil (6 ppm) (Bowen, 1966; Lisk, 1972). TABLE III Long-term amount of arsenic in soil a Time (yr)
Deposition from atmosphere (ppm)
Fertilizer use (ppm)
1 10 20 40
0.0026 0,0t6 0.017 0.018
0.0020 0.009 0.01 0.01
a In the case ofatmospheric deposition, fly ash is considered as being deposited on grass-having a depth of root zone (Q2) of 15 cm, while in a case of fertilizer directly mixed into the agricultural soil, a 35 cm depth of root zone was assumed,
Table III also reports the long-term prediction of the arsenic level in soil as a consequence of the use of phosphatic fertilizer. Amounts of arsenic corresponding to 0.01 ppm can be predicted after 40 yr of fertilizer application with a calculated increase over the background level of the order of 0.15 ~ . Figure 3 shows the dynamic results of the long-term impact of arsenic on pasture grass and vegetables as a consequence of the atmospheric deposition from the CFPP and of the use of fertilizer. The impact of arsenic on grass is higher in the case of the element emitted from the CFPP, and this corresponds to a 10~ increase over the typical present level in plants (0.5 ppm) (Cataldo and Wildung, 1978; Risby, 1979) calculated over 40yr for a continuous stack emission. The dynamic behaviour of arsenic in groundwater systems from the two pollution sources considered in comparison with the average endogenous level of arsenic (Bowen, 1977) is shown in Figure 4.
ENVIRONMENTAL ASSESSMENT OF ARSENIC
61
0.65
0.60 z
PASTURE GRASS (DUE TO COAL COMBUSTION)~ f
f
~
VEGETABLES (DUE TO FERTILIZER U S E - ~
o.~
8
~ " P R E S E N T LEVEL
0,0
[ 10
I 20
I 30
I 40
I 50
I 60
I 70
I 80
I 90
100
] 90
100
TIME (YEARS)
Fig. 3. Long-term effect of arsenic on vegetation.
2.10-3
10-3
8.10- 4
6.10-4
j
/jPRESE,'T LEVEL
4.10-4
2.10-4 10-4 0.0
II 10
Ii 20
I 30
I 40
i 50
I 60
] 70
I 80
TIME (YEARS)
Fig. 4. Long-term impact of arsenic on groundwater quality.
The following conclusions can be drawn: - a fraction of the arsenic introduced into the environment can migrate through soil layers and enter the groundwater systems. The high mobility is in agreement with the short pedological half-life (Tammes and De Lint, 1969) and the low value o f K d between arsenic and soil components (Korte et al., 1975); - the time of percolation of arsenic to groundwater is estimated to be of the order of 20 and 10 yr from the case of CFPP and fertilizer practice respectively. The higher retardation time of arsenic in soil column when the source input is related to deposition from the atmosphere as to arsenic contained in fertilizer is due to the fact that in the case of fertilizer use the arsenic contained in phosphate is directly mixed during ploughing with agricultural soil, while for arsenic deposited on soil surface the indisturbed grass root zone may be more act as an effective biological barrier for arsenic migration through soil trophic level.
62
G. B1GNOLI AND E. SABBIONI
4. Discussion An assessment of the environmental pathways of arsenic released from two potential pollution sources such as CFPP for energy production and phosphatic fertilizers has been carried out by a dynamic model based on systems analysis. This approach would appear to be particularly useful in predicting the time-dependent events of TM in the different environmental compartments such as atmospheric dispersion, soil deposition, plant accumulation and migration to groundwater (Figure 1). However, many difficulties are encountered when applying this type of model. This is essentially due to the severe lack of quantitative information concerning fundamental parameters such as the transfer coefficients for trace metal representing movement between the different compartments. To the best of our knowledge no work has been reported on the use of dynamic models to evaluate the patterns of arsenic in the biosphere and little information on this subject based on the use of equilibrium models is available. Of particular interest is a recent work of Bennett (1981 a, b)who evaluated the global situation of arsenic in equilibrium with the terrestrial and aquatic environment with the purpose of estimating the average exposure commitment to man. It must be stressed, however, that our modelling approach is not intended to arrive at a mesoscale level, and considers only a particular local situation. This explains some apparent discrepancies in the parameters assumed in our and Bennett's studies as in the case of half-life of arsenic in the compartmental soil sink. The exogenous quantity of arsenic introduced into the atmosphere from coal combustion or fertilizer use is not yet in equilibrium with the environmental arsenic and more time is required to reach this situation. In this case the pedological half-life of between 6.5 and 16yr (NRCC, 1978) employed in our study seems reasonable considering also the high mobility of arsenic in soil layers as confirmed by its low Ka-values (Korte et aL, 1975) as compared with the value of 2000 yr taken by Bennett which represents a high steady residence time for natural arsenic (Bowen, 1977). The difference of the three orders of magnitude between our own and Bennett's study is only apparent because the value used by Bennett takes into account all the possible biological and chemico-physical interactions of arsenic assumed to be in equilibrium with the terrestrial environment. The results of this study show that during the 40 yr of operation of a 2500 MWe CFPP as well as the application of phosphate fertilizers to agricultural soil, the dynamic impacts of arsenic at the various levels of the terrestrial environment do not substantially increase the typical background levels. In order to calculate the degree of mobility of arsenic in the soil matrix, a dynamic accumulation index 'a' for the trace metals seems appropriate (Zwozoziak and Zwozoziak, 1981). In the case of the arsenic emitted from CFPP, aA~ is defined as follows: aAs = Q2 - Q2, o
(20)
co~ast/dp where Q2, o and 0z = dQz/dt denote the arsenic contents in the root zone of soil at the start of CFPP operation and after t years of power plant operation respectively (ppm)
ENVIRONMENTAL ASSESSMENT OF ARSENIC
63
(Equation (2)); t --- the number of years of power plant operation; CO~Aswas calculated by formulae (6) and (11) and represents the total arsenic deposition (rag m - 2 yr-1); d = thickness (m) of soil root zone for two cases of uncultivated and cultivated land; p = the soil density (kg m - 3). The arsenic dynamic accumulation index aAs was determined from the calculation results obtained after 20 yr of power plant operation and compared with other trace metals studied (Sabbioni and Bignoli, 1980; Bignoli and Sabbioni, 1983). These results are presented in Table IV. TABLE IV Dynamic accumulationindex in soil for trace metals released for 20 yr from a 2500MW~ coal-firedpower plant Trace metal
Present concentrationlevel in s o i l (ppm)a
Accumulation index (a)
As Cd Cr b
6 0.06 40
0.12 0.60 0.75
a Based on soil backgroundlevels as generalizedfrom Bowen, 1966, and Lisk, 1972. b Total chromium(Cr § 6 + Cr +3) (Bignoliand Sabbioni, 1983). The accumulation indices for Cr and Cd are higher than the index value calculated for the dynamic movement of As. This suggests that As is characterized by a greater migration ability in comparison with Cr and Cd which are more absorbed by the root zone of soil. These results are in agreement with other in situ studied trace metals in the vicinity of a copper smelter (Zwozoziak and Zwozoziak, 1981). The high mobility of arsenic through soil layers should be responsible for the low bioavailability of this element to plants, explaining the low degree of accumulation predicted (Figure 3). The mobility of arsenic through soil should influence the groundwater quality (Figure 4). However, the concentration in this compartment should not exceed the Maximum Permissible Concentration level of 50 gg 1- 1 for the waters in the European Community (Off. J. of the European Community, 1975). In reality the movement of water through soils rarely occurs continuously at uniform flow velocity. There is a period when after the infiltration the flow is negligible but adsorption/desorption processes continue (Murali and Aylmore, 1980). A too complex mathematical system would be necessary to describe this phenomenon. We have limited our approach to simple linear adsorption isotherms. Therefore the results obtained for arsenic in plants and water are necessarily a simplified approximation of the environmental reality. This study assumed the transfer of arsenic in the different compartments to be a total amount. It does not take into account the influence of the possible chemical forms of the element. Methylarsines are released into the air from soil trophic level following biomethylation of a large quantity of methylarsenic compounds used in agriculture (NRCC, 1978). Attrep and Anirudhan, 1977, found a high concentration of arsenic in air in an area
64
G. BIGNOLI AND E, SABBIONI
polluted by arsenic from defoliants. About half of the airborne arsenic was in the organic form. However,the amount of methylarsines found in unpolluted areas or near to the stack dust from smelting operations, arsenic is predominantly in the inorganic form (Crecelius, 1974; Rosehart and Chu, 1975). From these considerations it appears that when organic arsenic reaches soils it may be biomethylated more rapidly than its inorganic forms. For these reasons and considering that the chemical form of arsenic emitted from the stack of a coal power plant or present in fertilizer phosphates is probably in an inorganic form, we have decided that the methylation mechanisms and relative parameters are at present not sufficiently well known (WHO, 1981) to be applied to a dynamic model. Since arsenic can be biotransformed into many species in the environment through the methylation process (NRCC, 1978; WHO, 1981; Wood, 1974), research is needed to investigate the behaviour of these forms in the environment. In this context a characterization of the chemical form of arsenic entering the soil as a result of atmospheric deposition or by direct release from the use of fertilizers, should receive a high degree of priority. References Attrep, M. and Anirudhan, M.: 1977, 'Atmospheric Inorganic and Organic Arsenic', Trace Subst. Environ. Health 11,365-369. Baes, C. F. and Sharp, R. D.: 1983, 'A Proposal for Estimation of Soil Leaching and Leaching Constants for Use in Assessment Models', J. Environ. Qual. 12, 17-28. Beese, F. and Wierenga, P. J.: 1980, 'Solute Transport through Soil with Adsorption and Root Water Uptake Computed with a Transient and a Constant-Flux Model', Soil Science 129(4), 245-252. Bennett, B. G.: 1981a, 'Exposure of Man to Environmental Arsenic. An Exposure Commitment Assessment', Sci. Total Environ. 20, 99-107. Bennett, B. G.: 1981b, 'The Exposure Commitment Method in Environmental Pollutant Assessment', Envir. Monit. and Assess. 1, 21-36. Bignoli, G., Sabbioni, E., and Goetz, L.: 1982, 'Implementation of a Dynamic Previsional Approach to the Study of Heavy Metals - Environmental Pollution', in International Days of Biology, Naples. Bignoli, G.: 1980, 'TOXICO: A System Analysis Model for Assessing the Behaviour and Impact upon Man of Heavy Metals Released in the Environment', EUR-Technical Note 1.07.08.80.33. Bignoli, G. and Bertozzi, G.: 1979, 'Modelling of Artificial Radioactivity Migration in the Environment: A Survey', EUR-6179. Bignoli, B. and Sabbioni, E.: 1981, 'Long-Term Prediction of the Potential Impact of Heavy Metals on Groundwater Quality as a Result of Fertilizer Use', Studies in Environmental Science 17, 857-862. Bignoli, G. and Sabbioni, E.: 1983, 'Environmental Impact of Cr Released into Biosphere from Coal Combustion in a 2500 MWe Power Plant', (to be published). Bogardi, I., Duckstein, L., and Szidarovski, F.: 1981, 'A Control Model for Phosphorous Loading Reduction under Uncertainty', Ecol. Model. 12, 83-103. Bowen, H. J. M.: 1977, 'Natural Cycles of Elements and Their Perturbation by Man', in Environment and Man, Vol. VI, Blackie Publ., London. Bowen, H. J. M.: 1966, Trace Elements in Biochemistry, Academic Press Inc., London. Brady, N. C.: 1974, The Nature and Properties of Soils, McMillan Publ. Co. Inc., New York. Bresler, E.: 1973, 'Simultaneous Transport of Solutes and Water under Transient Unsaturated Flow Conditions', Water Res. Res. 9, 975-986. Cataldo, D. A. and Wildung, R. E.: 1978, 'Soil and Plant Factors Influencing the Accumulation of Heavy Metals by Plants', Environ. Health Perspect. 27, 142-153. Chamberlain, A. C.: 1953, 'Aspects of Travel and Deposition of Aerosols and Vapour Clouds', UKAEA Report AERE (MP) R, 1261.
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Crecelius, E. A.: 1974, 'The Geochemistry of Arsenic and Antimony in Puget Sound and Lake Washington', Thesis, Washington University. Fried, J. J.: 1980, 'Present Groundwater Pollution Problems', in Nuclear Techniques in Groundwater Pollution Research, Proc. of an Advisory Group Meeting, organized by the IAEA, Vienna. Goetz, L., Bignoli, G., and Sabbioni, E.: 1981, 'Mobilization of Heavy Metals from Coal-Fired Power Plants: Potential Impact on Groundwater', Studies in Envir. Sci. 17, 261-264. Jackson, R., Merrit, W. F., and Inch, K. J.: 1980, 'The Distribution Coefficient as a Geochemical Measure of the Mobility of Contaminants in a Groundwater Flow System', in Nuclear Techniques in Groundwater Pollution Research, Proc. of an Advisory Group Meeting, organized by the IAEA, Vienna. Korte, N. E., Skopp, J., Fuller, W. H., Niebla, F. E., and Alesti, B. A.: 1975, 'Trace Element Movement in Soils: Influence of Soil Physical and Chemical Properties', Soil Sci. 122, 350-359. Lisk, D. I.: 1972, Advances in Agronomy, Academic Press, New York. Martin, W. E.: 1964, 'Losses of Sr9~ Sr 89, and I TM from Fallout Contaminated Plants', Rad. Bot. 4, 275-282. Martin, W. E., Bloom, S. G., and Yorde, I. R.: 1974, 'Plutonium Transport and Dose Estimation Model', NVO-142. Murali, V. and Aylmore, A. G.: 1980, 'No-flow Equilibration and Adsorption Dynamics during Ionic Transport in Soils', Nature 283, 467-469. National Research Council Canada: 1978, 'Effect of Arsenic in the Canadian Environment', NRCC 15391, Ottawa. Official Journal of the European Communities: 1975, C214/2. Risby, T. H. (ed.): 1979, 'Ultratrace Metal Analysis in Biological Sciences and Environment', Advances in Chemistry Series, 172. Rosehart, R. G. and Chu, R.: 1975, 'Methods for Identification of Arsenic Compounds', Water, Air, and Soil Pollut. 4, 395-398. Sabbioni, E. and Goetz, L.: 1982, 'Mobilization of Heavy Metals from Fossil-Fuelled Power Plants, Potential Ecological and Biochemical Implications', EUR-6998/4. Sabbioni, E. and Bignoli, G.: 1980, 'Heavy Metals in Phosphatic Fertilizers: Potential Impact on Groundwater Quality', Europ. Appl. Research Rep. 1, 141-180. Tammes, P. M. and De Lint, M. M.: 1969, 'Leaching of As from Soil', Neth. J. Agric. Sci. 17, 128. Tyler, G.: 1978, 'Leaching Rates of Heavy Metal Ions in Forest Soil', Water, Air, and SoilPollut. 9, 137-148. Van der Hoven, I.: 1963, 'A Diffusion-Deposition Model for Particulate Effluents from Ground-Treated Nuclear Engines', Int. J. Air Water Pollut. 7, 1023. Van de Pol, R. M., Wierenga, P. J., and Nielsen, D. R.: 1977, 'Solute Movement in a Field Soil', Soil Sei. Soc. Am. J. 41, 10-13. Wangen, L. E. and Williams, M. D.: 1978, 'Elemental Deposition Downwind of a Coal-Fired Power Plant', Water, Air, and Soil Pollut. 10, 33-44. Wood, J. M.: 1974, 'Biological Cycles for Toxic Elements in the Environment', Science 183, 1049. World Health Organization: 1981, 'Environmental Health Criteria 18', ARSENIC. Zunino, H. and Martin, J. P.: 1976, 'Metal Binding Organic Macromolecules in Soil: 2. Characterization of the Maximum Binding Ability of the Macromolecules', Soil Sei. 128, 187. Zwozoziak, J. W. and Zwozoziak, A. B.: 1981, 'Trace-Metal Behaviour in the Vicinity of a Copper Smelter', Int. J. Envir. Studies 19, 35.
