Environ. Sci. Technol. 2000, 34, 3079-3085
Atmospheric Transport, Deposition, and Fate of Triazine Herbicides and Their Metabolites in Pristine Areas at Isle Royale National Park E . M . T H U R M A N * ,† A N D ARON E. CROMWELL‡ U.S. Geological Survey, 4821 Quail Crest Place, Lawrence, Kansas 66049-3839, and 811 East 11th, Lawrence, Kansas 66044
Trace concentrations of triazine herbicides, used in the Midwestern United States, are being transported atmospherically hundreds of kilometers and deposited by precipitation onto pristine areas, such as Isle Royale National Park (Lake Superior). Atrazine, deethylatrazine, deisopropylatrazine, and cyanazine were detected in Isle Royale rainfall from mid-May to early July (1992-1994) at concentrations of less than 0.005 to 1.8 µg/L. Analysis of predominant wind direction indicated that the herbicides originated from the upper Midwestern United States. The annual mass of herbicides deposited by rainfall varied between years, from 13.4 µg/m2/yr for 1992, 3.7 µg/m2/yr for 1993, and 54 µg/m2/yr for 1994. Atrazine and deethylatrazine were found also in concentrations of less than 5-22 ng/L in lakes across Isle Royale. Concentrations of atrazine in the surface layer of the lakes increased during deposition periods and decreased later in the year. The fate of triazines in shallow lakes suggests faster degradation and shorter half-lives, while deeper lakes have residence times for atrazine that may exceed 10 years.
Introduction In the past 50 years, farmers have improved corn yields by using herbicides, which are applied extensively to row crops in the Midwestern United States. For example, the six most frequently used herbicides in the United States at the time of the study were atrazine, cyanazine, EPTC, metolachlor, trifluralin, and 2,4-D (1). Only one of these herbicides, trifluralin, is not applied to corn (Zea mays L.) (2, 3). Furthermore, the cultivation of corn alone accounts for about one-half of the total national herbicide use in the United States (2, 3). Atrazine (2-chloro-4-(ethylamino)-6-isopropylamine-s-triazine) is the most frequently applied herbicide on corn in the United States, with almost 30 million kg of atrazine applied annually. Once applied, atrazine undergoes rapid chemical and biological degradation, with a half-life in soil of ∼60 days (4). Atrazine loss from soil to surface water is approximately 1% per year (5-11). Furthermore, atrazine is removed from fields through atmospheric transport (11, 12-23). A study by Glotfelty et al. (22) found that volatilization loss of atrazine from fields was approximately 2% of the applied mass, and * Corresponding author phone: (785)832-3559; fax: (785)832-3500; e-mail:
[email protected]. † U.S. Geological Survey. ‡ 811 E. 11th, Lawrence, KS. 10.1021/es000995l Not subject to U.S. Copyright. Publ. 2000 Am. Chem. Soc. Published on Web 06/20/2000
a study by Goolsby et al. (21) found that about 0.6% of atrazine applied was deposited by precipitation on the Midwestern United States. Thus, the mass of atrazine removed due to volatilization is approximately one-half the loss of atrazine due to surface runoff on a regional scale. Despite this important pathway of atrazine removal from soil, much less is known of the fate and transport of atrazine in the atmosphere compared to surface- and groundwater pathways. Atrazine reaches the atmosphere through at least three pathways: volatization, particle transport, and physical drift. Numerous studies indicate that volatilization of atrazine is a major pathway of transport to the atmosphere (12-22). Volatilization amounts are highest immediately after application, decreasing logarithmically thereafter (13, 14). Environmental conditions that increase volatilization rate include warm temperatures, wind, and soil moisture (1214). The second pathway for atrazine transport to the atmosphere is particle movement. Particle movement occurs when either soil containing atrazine or atrazine itself is picked up by wind and blown into the atmosphere (24). The third pathway is physical drift that can occur as the liquid pesticide mixture is being sprayed on a field without incorporation. Once in the atmosphere, atrazine is transported by low-level winds. Several studies have identified atrazine in rainfall (11, 12-15, 23). Near the source of the compound, concentrations of atrazine in rainfall have been measured at 1-3 µg/L (14, 15, 21), and in a study by Nations and Hallberg (18), rainfall collected adjacent to a corn field contained as much as 40 µg/L atrazine, but this is unusual. Atrazine concentrations in rainfall decrease with increasing distance from its source due to atmospheric dilution, removal by wet and dry deposition, and possibly photochemical degradation (22, 25). Despite these processes, studies have detected atrazine being transported hundreds of kilometers (11, 21-23). Although some work has been done concerning the longrange transport of atrazine and its subsequent deposition onto pristine areas, little is known about the flux of atrazine and its subsequent fate in a pristine area. With this goal in mind, a 3-year study (1992-1994) was conducted at Isle Royale National Park in Lake Superior (United States). The objectives of this study were (1) to identify herbicides in the rainfall of a pristine area and to quantify the mass of these contaminants being deposited per unit area, (2) to survey pristine lakes for the accumulation and degradation of atmospheric herbicides, and finally (3) to determine the transport and origin of herbicides being deposited onto pristine areas using air-parcel, back-trajectory analysis.
Experimental Procedures Study Area and Sampling. The study area was Isle Royale National Park, MI, a 575-km2 island located in Lake Superior near the United States-Canada border (Figure 1). The park is protected from excessive human influence by its Congressional designation as “Wilderness Area” and its United Nations designation as an "International Biosphere Reserve”. Isle Royale has never had any history of herbicide application and, being an island, is isolated from surface and subsurface flow from herbicide-contaminated areas. Two data-collection sites were established at Isle Royale. The sites were located near two lakessLake Richie and Wallace Lake (Figure 1). Samples were collected at these sites in 1992, 1993, and 1994. An additional data-collection site was established in 1994 at Davidson Island (Figure 1). VOL. 34, NO. 15, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3079
FIGURE 1. Index map and location of Isle Royale National Park (Michigan) in Lake Superior. Bulk precipitation (wet plus dry fall) was collected using low-density polypropylene funnels connected to 4-L baked amber-glass bottles. Within 12 h after each rain, bulk precipitation was transferred from the collectors into either 125-mL or 1-L baked amber-glass bottles. The collectors then were washed in detergent and deionized water before being 3080
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 15, 2000
used again. In 1992, three of these collectors were placed at each of the two initial data-collection sites. Samples were collected during approximately eight different rainfall events between May 23 and July 16, 1992. In 1993, the number of rain collectors was increased to six at each site. Four rainfall events between May 13 and June 17 were sampled in 1993.
In 1994, to accommodate a new ultrasensitive 1-L analytical method, the number of collectors at each of the three sites was increased to 10 to collect 1 L or more of rainfall during small events (1-2 mm of rain). In 1994, samples were collected during 11 rainfall events between May 15 and August 27. Rainwater samples were filtered through 0.45-mm glassfiber filters (Gelman, Ann Arbor, MI) and stored at 5 °C until analyzed. Soil water was collected in suction-cup lysimeters in 1992 only. The polyvinyl-chloride lysimeters had porous porcelain cups at their base. Previous studies have shown that triazine compounds are not sorbed to these surfaces. Twelve lysimeters were installed near Wallace Lake, and six lysimeters were located near Lake Richie. Lysimeters at both sites were paired at depths of 15 and 30 cm. The lysimeters were evacuated with a hand pump after each rainfall event in 1992, and the following day, the water in the lysimeters was pumped into 125-mL baked amber-glass bottles. Precleaned Teflon tubing was used for the transfer. Surface water was collected and analyzed to determine the extent of herbicide contamination in the watershed. Samples from the lakes were collected in 125-mL or 1-L baked amber-glass bottles at a depth of 0.5 m. Several lakes across Isle Royale were sampled in August of 1992, 1993, and 1994. In the summer of 1994, surface water was collected at frequent intervals from three lakes as well as from several beaver ponds along streams on the interior of the island. Analysis by GC/MS. Methanol, ethyl acetate, and acetonitrile (Fisher, Springfield, NJ) were the pesticide-grade solvents used. The solid-phase extraction (SPE) cartridges that were used for SPE-ELISA analyses and 100-mL GC/MS analyses contained 360 mg of 40-µm C18-bonded silica (SepPak from Waters-Millipore, Milford, MA). The SPE cartridges used for 1-L GC/MS analyses were ENVI-Carb from Supelco (Bellefonte, PA). These cartridges contained 250 mg of Carbopak-B. Herbicide standards were prepared in methanol, and deuterated atrazine-d5 (U.S. Environmental Protection Agency, Cincinnati, OH) was used as an internal standard for GC/MS analysis. Solid-phase extraction by C18 cartridges was carried out using a Waters Millilab workstation (Milford, MD). Cartridges were cleaned sequentially with 2 mL of methanol, 6 mL of ethyl acetate, 2 mL of methanol, and 2 mL of distilled water. Next, 100 mL of sample was passed through the cartridge at a flow rate of 10 mL/min. The cartridges then were eluted with 3 mL of ethyl acetate followed by a transfer step to remove the ethyl acetate (top layer) from the residual water (bottom layer) in the eluate. The ethyl acetate then was evaporated to approximately 80 µL for GC/MS analysis. This method is described in detail in Thurman et al. (26) and Meyer et al. (27). Solid-phase extraction by Carbopak-B was carried out by using a vacuum manifold (Supelco, Bellefonte, PA) to pass liquids through the cartridge. The cartridges were cleaned sequentially using 3 mL each of ethyl acetate, 20:80 acetonitrile:methanol, and distilled water. One liter (1 L) of water was aspirated through the cleaned Carbopak-B cartridge by connecting a Teflon tube from the cartridge to a water sample. After passing the sample through the cartridge, the cartridge was dried by passing air through the cartridge for several hours. This drying step eliminated residual water in the cartridge, which could complicate GC/MS analysis. After drying, the cartridges were eluted with 20:80 acetonitrile: methanol. The eluates then were evaporated to approximately 80 µL for analysis by GC/MS. The Carbopak-B GC/MS method detection limit was 5 ng/L for alachlor, atrazine, DEA, and metolachlor based on a signal-to-noise ratio of 10:1 using at least two selected ions. Also DI water blanks were analyzed between samples to ensure that carryover of trace amounts of herbicide did not occur.
A GC/MS procedure was used for the C18 and Carbopak-B preparations. The analyses were performed using a HewlettPackard Model 5890A (Palo Alto, CA) with a mass selective detector (MSD). A 12-m HP-1 fused-silica capillary column (Palo Alto, CA) was used for the separation of analytes. The column was coated with a 0.33-µm methyl silicone film and had an internal diameter of 0.2 mm. Operating conditions from both methods were identical to those described by Meyer et al. (27). Analysis by ELISA. Atrazine ELISA was performed on rainwater samples to screen concentrations to choose the correct volume of sample and extraction material to use for GC/MS analysis. Some of the samples were analyzed onsite at Isle Royale using a combination of SPE and ELISA (SPEELISA), as well as in the U.S. Geological Survey laboratory in Lawrence, KS. The SPE-ELISA method is described in detail by Aga and Thurman (28). The ELISA analysis was performed with Ra-PID assay kits (Ohmicron Corp., Newtown, PA). This immunoassay kit uses polyclonal antibodies coated on paramagnetic beads. The samples were analyzed according to the directions supplied with the kits. The optical densities were read using a RPA-1 photometric analyzer (Ohmicron, Newtown, PA), which also calculated the analyte concentrations on the basis of a linear regression of linear/logit-transformed data. The calibration standards for atrazine were prepared at concentrations of 0, 0.1, 1.0, and 5.0 µg/L using 20/80 (% v/v) methanol/water solvent, the same solvent used to reconstitute the analyte. When performed at Isle Royale, a standard of 0, 10, or 25 ng/L atrazine separated every sample processed through SPE to ensure the quality of the onsite data.
Results and Discussion Herbicides in Rainfall. Figure 2 shows the mean concentration of atrazine detected in Isle Royale rainfall in 1992 and 1994. Atrazine was detected as soon as sampling began (midMay) and was present until early July. Typical atrazine concentrations ranged from 0.1 to 0.5 µg/L. The highest mean concentration was 1.8 ( 0.2 µg/L, occurring in samples collected on June 3, 1992; however, it was associated with only 1 mm of rainfall and 1.8 µg/m2 of atrazine was deposited. Considerably more deposition occurred on June 17, 1992, when the mean concentration of atrazine was 0.25 µg/L and the amount of atrazine deposited was 4 µg/m2 because of increased precipitation. The deposition of herbicides was seasonal, with maximum concentrations occurring during the first week of June, and maximum deposition occurring during the month of June, which corresponds to a 0-4-week post herbicide application season in the middle to northern Corn Belt in the Midwestern United States, such as Northern Iowa, Minnesota, and Wisconsin (2, 3). Also occurring in the rainfall during this time was the triazine herbicide, cyanazine, which had the highest mean concentration of ∼0.5 µg/L in samples collected on June 5, 1994 (Figure 3). Cyanazine concentrations followed the same patterns as that of atrazine, with detections in 1994 as early as mid-May. Unfortunately, it was not possible to sample prior to mid-May because the park was not open and transport to the island was not possible. The chloroacetanilide herbicides, alachlor and metolachlor, were not detected in any of the samples from 1992 through 1994, even with a detection limit of 5 ng/L. This result indicates that these herbicides, although volatile and used extensively in the Corn Belt, are not being transported long distances, whereas the triazine herbicides, atrazine and cyanazine, are more longlived in the atmosphere. The longer lived nature of the triazine herbicides is also evident in two metabolites of atrazine, deethylatrazine (DEA) and deisopropylatrazine (DIA). The highest mean DEA VOL. 34, NO. 15, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3081
FIGURE 2. Mean atrazine, deethylatrazine (DEA), and deisopropylatrazine (DIA) concentrations in rainfall at Isle Royale National Park for 1992 and 1994, determined by GC/MS. concentration was 0.3 µg/L, occurring in samples collected on June 6, 1992 (Figure 2). The highest mean DIA concentration was 0.2 µg/L, also occurring in samples collected on June 6, 1992. DIA was not detected in 1993 rainfall and was found in the lowest concentrations of the four triazine compounds detected in 1992 and 1994. Again, these early June maximum herbicide concentrations occurred during the period from application to 4 weeks following application of herbicides in the Corn Belt. Considering that the atmospheric transport of triazine compounds is greatest during the several weeks of application (12-14), it is not surprising that this was the time that triazine concentrations also were at a maximum in rainfall. Table 1 shows the mass deposition of atrazine, DEA, DIA, cyanazine, and the chloroacetanilide herbicides (alachlor and metolachlor) for 1992-1994. Although 1992 was the year during which the highest mean atrazine concentrations during a rainfall event (1.8 µg/L) were observed, 1994 had the greatest atrazine deposition rate (compare Figure 2 and 3082
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 15, 2000
Table 1). The highest concentrations and the greatest mass were deposited in late May and early June 1994. Table 1 indicates that atrazine had the largest mass deposited on Isle Royale in 1994 (12.8 kg/yr), with a deposition rate of 22.3 µg/m2/yr compared to 9.2 µg/m2/yr for 1992. The 1994 rate is approximately 0.02% of the application rate on corn fields, hundreds of kilometers away, or a dilution greater than 10 000 times compared to field-application rates of 1 to 2 g/m2. Because of the considerable dilution of atrazine in transport, it is not thought that the concentrations deposited on Isle Royale are of toxicological or herbicidal concern (29). Nor is it likely that these low concentrations could accumulate in soil to toxic concentrations, given a typical half-life in soil of ∼60 days for atrazine and 30 days for cyanazine (4). The deposition rate of cyanazine has increased from 0.5, to 0.8, to 15.6 µg/m2/yr from 1992 to 1994 (Table 1). In 1994, it was the second most-deposited triazine compound, with a rate that is 70% of the deposition rate of atrazine. Cyanazine
FIGURE 3. Mean concentrations of atrazine and metabolites, cyanazine, and chloroacetanilide herbicides (alachlor and metolachlor) in rainfall at Isle Royale National Park during 1994 determined by GC/MS.
TABLE 1. Amount and Rate of Herbicide Deposition on Isle Royale for 1992-1994a
compound
year
atrazine 1992 DEA DIA cyanazine chloroacetanilide herbicides NDb total DAR ) 0.3 DIA/DEA ) 0.31 atrazine 1993 DEA DIA cyanazine chloroacetanilide herbicides NDb total DAR ) 0.45 DIA/DEA ) not calcd atrazine DEA DIA cyanazine chlorocetanilide herbicides total DAR ) 0.41 DIA/DEA ) 0.77
1994
rate of amt of deposition deposition (kg) (µg/m2/yr) 5.3 1.6 0.5 0.3
9.2 2.8 0.9 0.5
7.7
13.4
1.2 0.5 NDb 0.5
2.0 0.9
2.2
3.7
12.8 5.2 4.0 9.0
22.3 9.1 7.0 15.6
31.0
54.0
0.8
NDb
a Also included are deethylatrazine/atrazine ratios (DAR) and the DIA (deisopropylatrazine) to DEA (deethylatrazine) ratios (DIA/DEA) for deposition on Isle Royale. b ND, not detected.
usage has increased in the Corn Belt from 10 million kg in 1992 to 15 million kg in 1993 (2, 3); these data suggest that the increased concentrations of cyanazine in rainfall may be due to increased application of cyanazine in the Corn Belt, especially in nearby Minnesota. Furthermore, cyanazine use is sometimes greater than atrazine use in the northern Corn Belt because its rate of decomposition in soil is faster than that of atrazine and carryover of residue from one year to the next is minimized. For example, Minnesota used 0.7 million kg of atrazine, 1.3 million kg of cyanazine, 1.1 million kg of metolachlor, and 1.8 million kg of alachlor in 1990 (1).
The amounts of atrazine, DEA, DIA, and cyanazine deposited on Isle Royale varied considerably from year to year. Of the 3 years for which data were collected, 1994 year had the greatest mass of herbicides deposited on Isle Royale with an estimated 31 kg of triazine compounds deposited on the island (Table 1). In 1992, an estimated 7.7 kg of triazine compounds were deposited, whereas in 1993 only an estimated 2.2 kg of triazine compounds were deposited. The low deposition rates for triazine compounds in 1993 are curious because nearly as much atrazine was applied in the Corn Belt in 1993 as in 1992 (1, 2). This decrease in deposition rates could be due to several factors. First, rainfall was collected for only 1 month, mid-May to mid-June, which is the peak herbicide deposition time. According to 1992 and 1994 data, approximately 85% of atrazine should have been deposited during the month sampled in 1993. An additional reason for the low herbicide deposition rates in 1993 could be the unusually high rainfall in the Midwest, which resulted in extensive flooding. Frequent and intense rain in the Midwest could have scavenged atrazine out of the atmosphere before it could be transported to Isle Royale. Richards et al. (20) as well as Nations and Hallberg (18) report that atrazine is rapidly scavenged out of the atmosphere during rainfall events. In a recent study by Goolsby et al. (21), rainwater samples from across the Midwest were analyzed for herbicides, and deposition maps for the Corn Belt were extrapolated to estimate the annual deposition of atrazine onto Lake Superior. The estimates extrapolated for Isle Royale were 10 kg of atrazine for 1990 and 2 kg for 1991 (21). These estimates correspond well with this study’s deposition of atrazine for 2 of the 3 years followings7.7 kg in 1992, 2.2 kg in 1993, and 12.8 kg of atrazine deposited in 1994 (Table 1). The concentrations of herbicides in Midwestern rainfall (21) were similar to concentrations of herbicides found in rainfall across Isle Royale. Although individual samples associated with small amounts of rainfall occasionally yielded concentrations of more than 1 µg/L, typical concentrations during May and June throughout the Midwest were between 0.2 and 0.4 µg/L (21). These findings were consistent with the concentrations found in rainfall on Isle Royale during the same time period (Figure 2). VOL. 34, NO. 15, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3083
The major difference between herbicides detected in the rainfall from the Corn Belt and the rainfall from Isle Royale was that alachlor and metolachlor were found in rainwater samples from the Corn Belt, whereas these compounds were absent in rainwater samples from Isle Royale. Data from the study by Goolsby et al. (21) indicate that in the Corn Belt the percentage of alachlor deposited is about 40% less than the amount of atrazine that is deposited. Furthermore, Goolsby’s (21) study also indicates that alachlor is deposited closer to the Corn Belt source than atrazine is and that considerably less alachlor is deposited on the Great Lakes. Alachlor and metolachlor also have been detected in other rainfall studies (18, 20). The absence of alachlor and metolachlor in rainfall from Isle Royale, coupled with Goolsby’s (21) data on alachlor deposition patterns, suggests that chloroacetanilide herbicides, such as alachlor and metolachlor, are degraded during long-range atmospheric transport. Other evidence for degradation in transport pertains to the atrazine metabolite, DEA. Its concentration in the rainfall on Isle Royale was approximately 40% of the concentration of atrazine, with an average ratio of DEA to atrazine (called the DAR, deethylatrazine-to-atrazine ratio) of ∼0.4 (Table 1). This average ratio is considerably greater than the values that occur in streams of the Midwestern United States during the same time period, with DARs of approximately 0.2 (30). The elevated DARs in precipitation are most likely indicative of photodegradation of atrazine in the atmosphere (31). The fact that atrazine degrades to DEA by photooxidative processes has been shown by Pelizzetti et al. (25). Furthermore, Goolsby et al. (21) found a median DAR of 0.5 for nearly 2200 precipitation samples from throughout the Midcontinent, which is also consistent with atrazine degradation to DEA in the atmosphere. The atrazine metabolite, DIA, was found in rainfall from Isle Royale in 1992 and in 1994 (Table 1). However, the deposition was considerably greater during 1994 with a ratio of 0.77 for DIA to DEA (Table 1). The DIA-to-DEA ratio was only 0.31 in 1992 when cyanazine deposition was low and is consistent with published data on soil studies for the decomposition of atrazine to deethylatrazine and deisopropylatrazine (30). This large increase in DIA also correlates with the marked increase in cyanazine deposition from 0.5 to 15.6 µg/m2/yr, comparing 1992 to 1994. It has been found that cyanazine will degrade to DIA, and cyanazine is an important source of DIA in surface water of the Midwestern United States (30). Thus, it appears that the DIA in rainfall could be a result of both degradation of atrazine and cyanazine during atmospheric transport. It is possible also that DEA and DIA are volatilizing from the field rather than degrading in the atmosphere. However, the increased water solubility of both DEA (∼900 mg/L) and DIA (∼3000 mg/L) compared to atrazine (33 mg/L) suggests that the Henry’s Law constants are much less for the metabolites than for the parent compound, atrazine. Thus, the volatile losses of atrazine would exceed the losses of the metabolites with the result that the average DAR value in rainfall would be much less than the 0.4 that is found. Air-Parcel Back Trajectories. The source of pollutants deposited onto an area can be determined by examining the source of surface winds. This is accomplished by projecting wind vectors backward from the destination in 3-h steps. This procedure is known as air-parcel (or geostrophic) back trajectory and was developed by David Yap of the Air Resources Branch of the Ontario Ministry of the Environment (32, 33). Mr. Yap generously calculated the source regions of surface winds for several deposition periods at Isle Royale. These calculations show the direction of air movement in the 0-1000-m altitude during the past 48 h and are not accurate for air above the 1000-m altitude. Typically, winds are slightly more westerly at greater altitudes over Isle Royale. 3084
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 15, 2000
TABLE 2. Concentrations of Atrazine and DEA in Surface-Water Samples from Isle Royale for 1991-1994a atrazine (ng/L) surface-water body
1991
1992
1993
1994
Chickenbone Lake Lake Desor Feldtmann Lake Intermediate Lake Lake LeSage Lake Livermore Lake Richie Siskiwit Lake Wallace Lake Lake Whittlesey Wood Lake beaver ponds (av of 5 ponds)
b b b b b b 8.7 (s) 16 (d)
