Environ. Sci. Technol. 1990, 24, 1490-1497
A Field Comparison of Several Methods for Measuring Pesticide Evaporation Rates from Soil Michael S. MaJewski,*,+Dwight E. Glotfeity,' Kyaw Tha Paw U,* and James N. Seiber'
Department of Environmental Toxicology and Department of Land, Air, and Water Resources, University of California, Davis, California 95616, and Environmental Chemistry Laboratory, Natural Resources Institute, USDA-ARS, Beltsville, Maryland 20705 Several methods for determining pesticide evaporative flux from fallow soil were evaluated in a side-by-side field study. Four compounds were applied to the surface as an emulsifiable concentrate. Statistical analysis of the results showed no significant difference at the 95% confidence level in flux values between the methods. The profile techniques used included the aerodynamic, eddy correlation, and energy balance methods and an integrated horizontal flux method. A theoretical profile shape method based on predictions of a trajectory simulation model was also used. The results indicate that each method yields comparable results when used over fallow soil under moderate wind conditions. The simpler theoretical profile shape method was shown to be reliable in determining evaporative fluxes, providing more rapid final results at an overall reduced expense, when compared with profile techniques. However, under low wind conditions, the eddy correlation and energy budget-Bowen ratio methods resulted in more reliable measurements. Introduction
Pesticides represent a major group of organic compounds that are deliberately released into the environment by man. What happens to them after they are released is of major current concern. Knowledge concerning their volatilization rates is necessary to fully understand the environmental fate of these chemicals. With increasing concern over environmental quality, including contamination of ambient air, the future may bring regulations restricting emission rates into the lower atmosphere from sources such as agricultural fields for both new and existing pesticides. If such regulations are adopted, accurate, reliable, fast, and inexpensive field-testing methodologies will be needed. This paper examines existing methodologies for determining the postapplication volatilization flux of pesticides from fallow soil and discusses their advantages and shortcomings. The methods included (1)an aerodynamic (AD) vertical profile technique using the ThornthwaiteHolzman (TH) equation ( I ) utilizing two different atmospheric stability description functions, (2) a bulktransfer technique using energy balance-Bowen ratio (EB) and eddy correlation (EC) methodologies, (3) an integrated horizontal flux (IHF) method, and (4)a theoretical profile shape (TPS) method based on a trajectory simulation model of turbulent dispersion. The compounds chlorpyrifos, diazinon, lindane, and nitrapyrin were applied to the soil surface as an emulsifiable concentrate in a side-by-side plot field study. In atmospheric theory, the transfer of any conservative entity, i.e., water vapor (w), heat (h), momentum (m), or chemical vapor (p), from a surface to the atmosphere is * Present address: USDA-ARS. 'Department of Environmental Toxicology, University of California. USDA-ARS. 9 Department of Land, Air, and Water Resources, University of California.
*
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Environ. Sci. Technol., Vol. 24, No. 10, 1990
governed primarily by the atmospheric turbulence generated by wind moving over the surface. These vertical fluxes can be expressed as flux gradient equations analogous to eq 1for sensible heat, where H is heat flux density
H = p,Cd(h(dT/dz)
(1)
(W m-2),pa is the air density, C, is the specific heat of dry air at constant pressure, Kh is the turbulent exchange coefficient for sensible heat, and d T / d z is the vertical temperature gradient. Equation 1 is appropriate when the gradient and flux are measured a t a height sufficiently above the surface roughness elements. All fluxes characterized by equations similar to 1 have corresponding K values. During neutral atmospheric conditions all of these K terms are assumed to be equivalent (2,3). If the relationship between vertical flux and vertical gradient is known for one conservative property, it is known for all of them. However, the assumption that K , = K , = Kh has been shown not to be the case during most of the day (4-7), and it is also possible that the K values may differ for each pesticide (8). Since the purpose of this experiment was to use existing atmospheric theories to simplify the process of measuring pesticide evaporative flux, it was assumed that Kh = K p ,where K , is the exchange coefficient of each pesticide. The resulting equation used to calculate pesticide flux is
P =-Kh(d~/d~)
(24
where P is the vertical pesticide flux density (pg m-2 h-l) and dc/dz is the measured vertical pesticide vapor gradient (pg m-3 m-l). By convention the negative sign indicates vapor movement away from the surface. M e t h o d s Description
Aerodynamic (AD). The most frequently used field technique for determining pesticide evaporative flux is the aerodynamic method utilizing the Thornthwaite-Holzman equation, which is based on the logarithmic wind profile (9). Prior studies have used the convention that the stability correction function for momentum (4,) is equal to that of the pesticide vapor (@p) (10, 11). Atmospheric science literature has shown that &, is not equivalent to that of heat or water vapor (4,12-15) and, by inference, that of pesticide vapor (4J. In this study the expression 4m4pwas utilized, resulting in the equation
were k is von KBrmbn's constant (dimensionless, -0.4), AC (pg m-3) and Aii (m s-l) are the average pesticide air concentration and average horizontal wind speed differences, respectively, between heights z1and z 2 (m). The 4 functions are described by several empirical expressions (12, 13, 15, 16). Pesticide fluxes were calculated by using the stability expressions described by Dyer and Bradley (15) (eqs 3a and b) where z / L = Ri during unstable
0013-936X/90/0924-1490$02.50/0
0 1990 American Chemical Society
4m= (1 - 282 /L)4.25 r$p
= (1- 1 4 ~ / L ) + , ~
(34 (3b)
conditions (Ri < 0) (4,13,14,17,181 with k = 0.40. These flux values were then compared to those calculated by using the expressions of Pruitt et al. (12) (eqs 4a and b), c$m $p
= (1- 16Ri)-0.333
= 0.885 (1 - 22Ri)4.40
(44 (4b)
with k = 0.42. Ri is a gradient Richardson number, an atmospheric stability parameter described by eq 5, with (5)
g the gravitational acceleration, T the ambient temperature (degrees Kelvin), and aT/dz and aula2 the temperature and wind speed gradients, respectively. The aerodynamic method using the Dyer and Bradley (15) functions is identified as (AD-DB) and using those of Pruitt et al. (12) as (AD-P). The micrometeorological requirements for the T H aerodynamic method include a large, uniformly surfaced area with similar land surrounding it. An upwind distance (fetch) of a t least 100 times the height of the instruments is the generally used rule of thumb (19, 20). This fetch requirement ensures that the boundary layer in which the fluxes are being determined has the same characteristic as the adjacent underlying surface, and that the fluxes are constant with height. Energy Balance (EB). The energy balance-Bowen ratio method is based upon the energy budget described by eq 6, where Rn is net radiation, G is soil heat flux, LE Rn + G + LE + H = 0 (6) is latent heat flux, and H is sensible heat flux. Additional energy fluxes such as photosynthesis and advection are usually negligible. Rn and G are measured directly while H and LE are determined indirectly through the estimation of the Bowen ratio (0) (21). The Bowen ratio is the ratio of sensible to latent heat flux and is proportional to the ratio of air temperature (aT/dz) and water vapor pressure (ae/az) gradients as described in eq 7, where X p = H/LE i= h(aT/ae) (7) is the thermodynamic value of the psychrometric constant. is calculated by substituting eq 7 into The heat flux (H) 6:
H = (Rn + G)P/(l
+ p)
(8)
Pesticide flux is then calculated from eq 2a by using the value (assuming Kh = K,) determined from the calculated H, the measured temperature gradient, and eq 1. The EB method calculates fluxes from gradient measurements of air and dew point temperatures, along with net radiation and soil heat flux by using fairly sophisticated instrumentation described in detail elsewhere (22). Measuring gradients imply the field size considerations are the same as for the AD-TH method. Eddy Correlation (EC). In atmospheric turbulent flow, all entities exhibit short-term fluctuations about their mean value. The instantaneous value consists of a timeaveraged mean component and a fluctuating component:
Kh
w=lB+w'
(9)
where w is an instantaneous property (in this example vertical wind velocity), a is the time-averaged mean property, and w'is the instantaneous deviation from the
mean. The mean vertical flux (F)of an entity can be expressed as a product of the vertical wind velocity (w), and the volumetric content (u) of the entity and its mass:
-
F = p,w'u'
(10) The overbar denotes the time average of the instantaneous covariance of w and u. On the basis of eq 10, the fluxes for heat (H) and pesticide vapor (P)can be expressed as
-
H = -p,C,wT'
-
(11)
P = -paw%' (12) where w', T', and c'are the instantaneous deviations from the mean vertical wind velocity, air temperature, and pesticide air concentration, respectively. While this method has been in use for some time for measuring momentum, heat flux, and water vapor (23-27), the sensors needed to detect pesticide concentration a t the required frequencies (10-20 Hz) have not yet been developed. In this experiment the EC technique was used to measure heat flux (H)with a sonic anemometer and a fine wire thermocouple. With the assumption that Kh = Kp the pesticide flux was calculated by using eq 2a. The same field size requirements are also needed as for the AD and EB methods. Integrated Horizontal Flux (IHF). This method is a time-averaged mass balance technique in which the flux is determined by use of eq 13 where X is the upwind p= 1 cu dz (13)
xo
distance to the leading edge of the source, and ii and are the averaged wind speed and air concentration at height z (28). This method requires a uniformly surfaced source, and a uniform source strength (as is the case for all vertical flux determination methods), but is independent of any K-similarity considerations and does not require atmospheric stability corrections. Also, the long upwind fetch required in the AD, EB, and EC methods is not critical with the IHF method, but the actual fetch distance must be known. Theoretical Profile Shape (TPS). The theoretical profile shape method for determining evaporative fluxes is based on a two-dimensional trajectory simulation model that has been shown to be in good agreement with observed vertical profiles of horizontal flux a t the center of a circular source (29). This method differs from the others in that gradient measurements of meteorological parameters and pesticide vapor concentration are not necessary. Also, a large experimental source area is not needed. The TPS method requires a circular plot (r I 50 m) with a single measurement of horizontal wind speed and pesticide air concentration taken above the center of the source plot. The measurement height (ZINST) is a function of the radius and surface roughness of the source and is nearly independent of atmospheric stability considerations. The source strength is calculated, after the air concentration and wind speed at ZINST have been determined, with eq 14, where Fz(0) is the actual unit area vertical flux
(a ,x)measured F*(O)=
9
(14)
rate (pg cm-2 d),(iii?)m""d is the product of the measured average wind speed and air concentration, and 9 is the normalized horizontal flux predicted by the model. This method has been demonstrated with volatile compounds such as ammonia (3&33) as well as for compounds of low volatility such as pesticides (34). The theory and Environ. Sci. Technol., Vol. 24, No. 10, 1990
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Table 1. Instrument Type and Placement above the Surface variable
sensor type
height, cm
wind speed horizontal
rotating cup anemometefl vertical sonic anamometed temp thermisterf pesticide concn XAD-4 resin trap' differential temp aspirated thermopyle* copperconstantan vapor pressure dew point hygrometei net radiation Fritschen-type net radiometee soil heat flux heat flux plates'
20 40 75' 80 150 100 50 20 35 55 750 90' 150 3ff 8W lod 9od
IOd 9od
lood 2-4d
FIgure 1. Experimental plot layout and instrument location at Davis, CA. A, meteorological instrumentation; 6, high-volume air sampler; C, pesticide air concentration gradient mast; E, Bowen ratio instrumentation; S. sonic anemometer;L, the aercdynamiclprofileplot; CS and CN. the TPS plots.
"TPS plots ZINST. Not used in AD plot 'Not used in TPS plots. 'Aerodynamic instrumentation. dEnergy balance instrumentation. 'C. W. Thornthwaite Assoc., Elmer, NJ. fCampbell Scientific CA-27, CA-107, Logan, UT. 'Rohm-Haas, Philadelphia, PA. "Similar to that described hy Lourence et al. (46). 'General Eastern, Dew-10, Watertown, MS. jQualimetrics, Inc., Model
active ingredient (ai.) ha-l between 0730 and Os00 h (PST) on 11September 1985. Separate, precalibrated spray rigs were used for the large and small plots. Instrumentation. Air Sampling. Immediately following the end of the pesticide application, the meteorological instrumentation and air sampling masts were set up in the center of each of the three plots (Figure 1, Table I). Air sampling was done during daylight hours on 11-14, 21, and 28 September 1985. The vapor traps consisted of either Teflon cups (4 cm i.d. X 12.5 cm length) or aluminum cups (3.2 cm i.d. X 12.6 cm length) packed with XAD-4,20-50 mesh, macroreticular polymeric resin (Rohm-Haas, Philadelphia, PA), precleaned by a multiple-extraction method (36). Each cup was filled with 40 mL of resin, which was exchanged for fresh material at the end of each sampling period. The circular plots were sampled at five heights in order to better illustrate the concentration profiles over these short-fetched plots even though only the ZINST height (75 cm) was needed for the TPS calculation. A universal high-volume air sampling pump (BGI Inc., Waltham, MA), modified with a five-inlet manifold and connected to the sampling cups via Tygon tubing, aspirated each cup at approximately 40 L min+. Air flow rates were measured at the beginning of each sampling period. A detailed description of the field preparation, layout, and measurement equipment has been reported elsewhere (34). Gradient air concentration measurements at each plot were taken for 1-h intervals, with one 0.5-h lag between
3032.
some applications are described in detail elsewhere (34, 35).
Materials and Methods Field Site. The field experiment was conducted during late August and September 1985, on a field located on the campus of the University of California, Davis. It consisted of a side-by-side comparison of the flux measurement techniques described above. The experimental site was a 4-ha fallow field that was disked and floated, and approximately 5 cm of water was applied by overhead sprinklers 8 days before the beginning of the experiment. The profile methods (AD, EB, EC, and IHF) utilized a 100 m X 100 m (1ha) plot (L) while the TPS method used duplicate circular plots (CS and CN) of 20-m radius (0.12 ha each) (Figure 1). The layout took into consideration the typical prevailing wind patterns to minimize interplot pesticide drift. Pesticide Application. Four compounds, chlorpyrifhs phosphoro[O,O-diethyl 0-(3,5,6-trichloro-2-pyridinyl) thioate], diazinon [0,0-diethyl-0-(2-isopropyl-6-methyl4-pyrimidinyl) phosphorothioate], lindane (y-hexachlorocyclohexane), and nitrapyrin [2-chloro-6-(trichloromethyl)pyridine] with vapor pressures of 0.002, 0.014, 0.008, and 0.280 P a at 25 "C, respectively, were applied as a mixture to the soil surface at a rate of -1.5 kg of
Table 11. Sampling Period Averages for Time, Wind Speed (m 8.') at 1 m ( u ) , Wind Direction (dir) with Standard Deviation (SD), IHF Upwind Fetch ( X ) (m), Atmospheric Stability Ri, (&&-',and Differential Air Temperature (Del. T ) during September 1985 at Davis, CA (4&p
Sept date
time,# h
ri
dir' (SD)
X
Ri
AD-P
AD-DB
Del. P '
11
1045 1215 1345 1515 1645 0715 0930 1100 0930 1200 1430 0930 1200
3.71 4.23 4.74 6.87 6.83 0.84 1.24 1.26 2.06 1.38 2.55 2.11 1.46
155 (3) 163 (5) 157 (9) 179 (3) 178 (5) 126 (2) 168 (16) 317 (43) 288 (23) 157 (44) 86 (9) 169 (30) 159 (61)
55 52 55 50 50 62 51 68 53 55 50 51 53
-0.046 -0.050 -0.046 -0.014 -0.009 4.054 4.457 4.451 -0.072 4.709 4.072 -0.035 -0.554
1.793 1.844 1.801 1.351 1.265 1.901 5.983 5.935 2.132 8.035 2.133 1.651 6.802
1.510 1.549 1.516 1.167 1.102 1.595 4.895 4.855 1.777 6.588 1.777 1.399 5.569
4.802 -1.136 -1.236 4,977
12 13 14
'Midpoint sampling time. 'N = ,'O E = 90°, S = 180', 1492 Environ. Sci. Technol.. Vol. 24. No. 10, 1990
W = 270'. 'Between 30 and 80 em.
4,600 4,061 4.327 4.401 4.283 4.584 4.440 -0.172 4.539
sampling periods, for the first 2 days. The remaining sampling days had 2-h sampling intervals with one 0.5-h lag between periods (Table TI). A calibrated high-volume air sampler (Bendix, Baltimore, MD) charged with 100 mL of XAD-4 resin was used to monitor the area between plots for interplot drift (Figure 1). Each resin sample was placed into a clean glass jar with a Teflon-lined lid a t the end of each sampling period. These samples were then stored on dry ice until transferred to a freezer (-20 "C) for storage until analyzed. Analytical Procedures Air Sample Extraction. Each resin sample was warmed to room temperature before being extracted. Ethyl acetate (60 mL) (Baker Resi-Grade, Phillipsburg, NJ) was added to the sample, which was then agitated for 30 min with a rotary shaker. The solvent was decanted off and the extraction repeated twice more. The three aliquots were combined and evaporated to -4 mL on a steam bath, using a Kuderna-Danish evaporative concentrator with a three-ball Snyder column. The samples were then passed through 25-mm hydrophobic filters, 0.45-pm pore size (Gelman, Ann Arbor, MI), and then either further concentrated or diluted as needed for gas chromatographic analysis. All samples were extracted within 6 months of completing the field experiment. While no actual storage stability studies were done, the samples were kept frozen at -20 "C until analyzed and no chemical decomposition was expected. The recovery/extraction efficiency studies gave results between 91 and 99% for each compound (34). Gas Chromatographic Conditions. Chlorpyrifos and diazinon were analyzed on a Hewlett-Packard 5890 gas chromatograph (GC) using a flame photometric detector (phosphorus filter). The oven temperature was 210 "C with the injector and detector both at 250 "C. The carrier gas was helium a t 19 mL min-'. Lindane and nitrapyrin were analyzed on a Hewlett-Packard 5730A GC using a 63Nielectron capture detector. The oven temperature was 190 "C with the injector and detector a t 250 and 300 "C, respectively. The carrier gas was argon/methane (95:5) a t 20 mL min-l. The column in each instrument was a 30-m, fused-silica, DB-5 megabore (J&W Scientific, Rancho Cordova, CA). These methods are similar to those described by Wehner et al. (36). Analytical reference standards were obtained from the US.EPA Pesticide and Industrial Chemical Repository, Las Vegas, NV. Statistical Evaluation. The flux density values from each method were compared to those of the AD-P method to determine if any differed significantly. In addition, each method was compared to the others to determine if any significant differences existed between methods. A paired t test (LSD) was used to compare the variable flux a t the 95% confidence interval ( p > 0.05). The following assumptions were made while performing the statistical evaluations: The method effect did not interact with time; the locations a t which the measurements were made did not introduce a confounding effect; and the circular plot fluxes were treated as duplicate measurements. Results and Discussion
Pesticide Volatilization. The concentration of pesticide in air dropped off dramatically during the first few hours after sunrise for the first 4 sampling days. By the third or fourth period (midday) pesticide concentration in air was usually less than the measurement detection limit of 1.0, 0.3, 10, and 10 ng m-3 for chlorpyrifos (CP), diazinon (DZ), lindane (LD), and nitrapyrin (NP), respectively. No pesticides were detected in the air on the
4
2.30 2.20
1 40
\
4
1 30 120
I
OW
100
300 Concentration(mg m
200
4M)
500
6 00
3)
Flgure 2. Aerodynamic plot air concentration profile for nitrapyrin. Period 1-A, 11 September 1985. 2.30
--E E
2.20
j
2.10
-
I
2.w-
0)
1.90-
.-p a
5
1.80 1.70 1.60-
s"
1.50 -
1.30 1.40 8.L"
4 I
0.00
1.00
2.00 3.00 4.00 Concentration(mg m-3)
5.00
6.00
Figure 3. TPS plot (CN) air concentration profile for nitrapyrin. Period 1-A, 11 September 1985.
fifth and sixth sampling days after application in any air sampling periods. Irrigation water was not added to the field during the course of the experiment, and consequently, the top few millimeters became very dry during daylight hours. This surface drying apparently inhibited the volatilization process almost completely within the first few hours after sunrise. Heavy dew formed during the early morning hours of 12 September and light dew formed in the morning of 14 September. Dew formation and the consequent wetting of the soil surface coincided with high early morning flux values (Table 111). In contrast, flux rates were very low on the morning of 13 September, when no dew formed. The increased flux during the third period (1430 h) on 13 September coincided with a very light rainfall lasting approximately 20 min. This rainfall did not thoroughly wet the soil. Pesticide volatility is well-known to depend on soil moisture (37, 38). The air concentration profiles of each compound applied to the aerodynamic plot (L, Figure 1)were generally loglinear with height (Figure 2). The concentration values used to calculate fluxes with the aerodynamic and bulk methods were within the linear portion of each profile. The IHF upwind fetch distance ( X )was calculated trigonometrically from the averaged wind direction for the time period and the minimum fetch distance of 50 m. The other profile methods do not take into consideration the upwind fetch in the flux calculations except in the initial field requirements. In contrast, the smaller circular plot concentration profiles were generally curvilinear (Figure 3). This curvature was most pronounced at high wind speeds, with the profiles becoming more linear during periods of light wind Environ. Sci. Technol., Vol. 24, No. 10, 1990
1493
Table 111. Flux valueso for the Four Test Compounds Calculated by Using Six Methods during September 1985 at Davis, CA Sept date 11
12 13 14
11
12
13 14
11
12
13 14
11
12
13 14
TPS
time,b
h
AD-P
AD-DB
EC
IHF'
CN
cs
AVGC
Nitrapyrin 1242 1141 151 235 60 68 24 67 d 223 d 3960 694 828 77 131 248 126 21 20 219 66 d 662 d 35
1210 265 127 118 333 1339 247 59 464 33 177 780 31
1476 d d d 785 1408 540 141 147 34 123 454 41
673
785 1141 46 1 167 327 43 125 410 46
1075 d d d 785 1274 500 154 237 39 124 432 44
Lindane 472 187 75 62 90 1854 648 318 156 33 75 799 63
478 205 84 90 127 501 195 91 542 40 183 911 40
763 d d d 170 670 421 250 189 46 133 500 41
382 d d d 163 565 347 252 367 48 99 482 50
572 d d d 166 617 384 251 278 47 116 491 46
EB
1045 1215 1345 1515 1645 0715 0930 1100 0930 1200 1430 0930 1200
1163 285 93 92 307 2495 675 84 180 28 91 409 25
973 238 78 77 259 2085 563 70 150 24 76 343 21
1045 1215 1345 1515 1645 0715 0930 1100 0930 1200 1430 0930 1200
479 226 102 86 125 1064 527 203 224 35 104 494 45
401 189 85 72 105 889 440 170 187 29 87 414 37
1045 1215 1345 1515 1645 0715 0930 1100 0930 1200 1430 0930 1200
5.7 2.9 1.1 0.6 0.5 32.8 4.6 0.5
