Chapter 15
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Atmospheric Transport and Deposition, an Additional Input Pathway for Atrazine to Surface Waters
1
2
23,
Dorothea F. Rawn, Thor H. J. Halldorson, and Derek C. G. Muir 1
Department of Soil Science, University of Manitoba, Winnipeg, Manitoba R3T 2N2 Canada Department of Fisheries and Oceans, Freshwater Institute, 501 University Crescent, Winnipeg, Manitoba, R3T 2N6, Canada
2
This paper reviews the current literature observations of atrazine in precipitation and ambient air and evaluates the importance of atmospheric deposition to surface waters using measurements of atrazine in water, air and precipitation at two locations where this chemical is not used. Precipitation was found to be an important route of entry into aquatic systems, particularly during periods when extensive regional use of atrazine occurs. Gas exchange and dry deposition were estimated using established methods and gas exchange was found to be the primary dry process of atrazine deposition to surface waters. Surface runoff losses from treated fields and groundwater contamination have been traditionally viewed as the main sources of the triazine herbicides to rivers (7). There has been little attention paid, until recently, to the role of atmospheric contributions to surface waters by current use herbicides, particularly in agricultural regions. In the most comprehensive study to date, Goolsby et al. (2) have measured atrazine in precipitation samples from both agricultural and non-agricultural regions of the US. Maximum atrazine concentrations in precipitation samples collected through midwestem and northeastern US were observed in May and June during 1990 and 1991 (3). Atrazine deposition via precipitation and gas exchange was examined in the Chesapeake Bay region (4), based on bulk rain and ambient air levels. From this and previous studies, Glotfelty et al. (4) concluded that seasonal high atrazine concentrations in surface waters resultedfromrunoff contamination, whereas atmospheric deposition was responsible for low-level, widespread contamination year round. Although the triazines have been detected in precipitation and air, there has been little research to examine dry deposition and gas exchange of these chemicals (Figure 1), or the relative importance of each of these pathways to surface waters. The objective of this paper is to review existing 1Current Address: National Water Research Institute, Environment Canada, Burlington, Ontario L7R 4A7 Canada 158
©1998 American Chemical Society In Triazine Herbicides: Risk Assessment; Ballantine, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
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159 information on atrazine in precipitation and air, and to evaluate the importance of atmospheric deposition pathways. Triazines are relatively non-volatile (Table 1), which may be the reason why volatilization losses and atmospheric transport of these chemicals have not been studied as extensively as surface runoff losses. Atrazine losses in the vapor phase from treated fields in a southern California study were found to be 0.16% during the first 3 days of the experiment (5). Atrazine losses through volatilization from treated fields in Maryland, however, were found to be 2.4% (6). Surface runoff losses of pesticides are considered catastrophic if they exceed 2% of applied levels in a single event (7). In 1989, 1.7% of the atrazine applied in the US Midwest was estimated to have been transported to the Gulf of Mexico (8). Thus, overall atrazine losses via volatilization may be similar to runoff losses. Table I: Physical/Chemical Properties related to atmospheric behaviour of triazines. Chemical
Molecular Weighf g-mol' 1
Water Solubility mg-L(T°C)
3
1
Subcooled Liquid Vapor Pressure V P , mPa
Melting Point °C 3
Henry's Law Constant Pa-m /mol 0
3
b
L
Atrazine Simazine Terbutryn
215.7 201.7 241.4
33 (25) 6.2 (25) 25 (20)
3
1.22 χ 10"
174
4
2.52 χ 10'
3
6.23 χ 10"
6
225-7
9.59 χ 10"
3
104-5
2.70 χ 10'
2.99 χ 10
3
\9) "Calculated using the relationship: ( V P
liquid
/VP
solid
) = exp[6.81 (T
M e l t i n g p o i n t
/T-l)] (35)
V0) Seasonal trends of atrazine in precipitation have been found, generally corresponding to application times (77). Goolsby and coworkers (2) found temporal patterns of atrazine in rainwater samples fit closely to the trends in water previously observed for the Mississippi River. The highest observed concentrations were detected in samples collected during application times from regions where greatest atrazine use occurred. Atrazine was found infrequently in areas where it is not used, such as Maine and parts of Michigan. When detected in these areas, it was at much lower concentrations than observed in high use regions (2). Recent atrazine measurements in several European countries also show seasonal trends in precipitation and evidence of long range transport. Although atrazine use was reduced in Switzerland in 1988, detectable levels of atrazine were found in precipitation samples collected during 1988 and 1989 (72) (Table II). Detectable levels of atrazine were found in approximately 25% of the precipitation samples collected in Germany, both prior to, and following, the ban on atrazine use in 1991 (75). Atrazine was detected in precipitation samples at four locations in Northern Germany, however concentrations were not found to be significantly different between agricultural, urban and coastal sites (14) and no seasonal or spatial differences in atrazine concentrations were observed. Atrazine was not registered for use in Germany throughout the duration of the latter study. Maximum atrazine concentrations in rainwater samples collected near an Italian
In Triazine Herbicides: Risk Assessment; Ballantine, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
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forest were 1.9 Mg/L, with temporal trends reflecting local high use intervals (75). Combined wet and dryfall concentrations of atrazine were found at maximum levels in early spring in a rural area of France (16). In Paris, however, the highest observed levels were found during sampling in June, 1991 (16). Atrazine residues in Norwegian rain samples collected during 1993 were detectable during early to mid-May sampling times (7 7), four years following its removal from the market in Norway. In all cases, highest concentrations observed in precipitation samples reflected local or regional atrazine application.
Table II: Recent observations of atrazine in precipitation. Reference
Concentration Range (ng/L)
Duration of Study
Maximum Concentration Observed
Ontario, Canada
