Blue Water Navy Vietnam Veterans and Agent Orange Exposure (2011) / Chapter Skim
Currently Skimming:
4 FATE AND TRANSPORT OF HERBICIDES USED IN VIETNAM
4 FATE AND TRANSPORT OF HERBICIDES USED IN VIETNAM
Pages 61-86
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 61... ...
This chapter lays out the committee's understanding of the fate of Agent Orange after its environmental release as described in Chapter 3, with special attention to TCDD transport to and fate in coastal waters and atmospheric drift, and to modeling efforts to estimate environmental concentrations of TCDD. Because the relative importance of fate pathways is chemical specific, the discussion is restricted to TCDD that reached the environment through spraying and other releases of Agent Orange.
|
From page 62... ...
Furthermore, large uncertainty would accompany any attempt to model overall TCDD fate by modeling emissions from individual spray paths on which data are available and scaling them up to hundreds of miles of coastline and coastal waters. The committee explored the variability and uncertainty of several of the many parameters that could be used for assessing the fate and transport of TCDD in the environment, as described in the sections below.
|
From page 63... ...
tends to affect mostly the southern delta region. The direction of predominant monsoonal winds follows the axis of the South China Sea, which gives the wind a maximum fetch that produces more powerful currents and larger waves during the fully developed monsoon season.
|
From page 64... ...
(2006) investigated river-coastal water flows of the Mekong Delta by developing a local coastal ocean model based on the Princeton Ocean Model of Blumberg and Mellor (1987)
|
From page 65... ...
On the basis of the presence or absence of TCDD, it might be possible to determine whether military personnel in Vietnam, particularly Blue Water Navy veterans, had the opportunity for exposure to TCDD and at what concentrations.
|
From page 66... ...
66 BLUE WATER NAVY VIETNAM VETERANS AND AGENT ORANGE EXPOSURE FIGURE 4-1 Environmental fate and transport processes for Agent Orange and TCDD.
|
From page 67... ...
Data on model evaluation specific to Vietnam are not available. This is a critical need because to the committee's knowledge such models have not been used to estimate the fate of Agent Orange in Vietnam, although there was some simulation of Agent Orange fate in tropical conditions at Elgin Air Force Base in Florida during the Vietnam War (Young, 2009)
|
From page 68... ...
. The committee concluded that using models to estimate likely concentrations of TCDD in the Vietnamese environment to which Blue Water Navy personnel would be exposed is not possible because of the lack of data with which to assign values to model parameters.
|
From page 69... ...
With a range of input values intended to reproduce conditions during Agent Orange spraying and drift over open water -- for example, 46–76 m release height, 1–6.2 m/s wind speed, 25–30°C, 90% relative humidity, stable atmospheric conditions, ASAE medium–coarse droplet distribution, and 25 L/ha spray volume rate -- the AGDISP model estimated that more than 90% of the Agent Orange sprayed was deposited within 100–200 m of the spray path when it was sprayed from a height of 46 and 76 m, respectively. However, a wind of 6.2 m/s, or about 12 knots, the maximum at which spray missions were conducted, could increase the spray drift so that 90–95% of the amount sprayed
|
From page 70... ...
Thus, the daily timing of the sorties favored sea-breeze conditions, which would minimize spray drift over coastal waters. Atmospheric fate The 13% of the Agent Orange not deposited on the forest canopy would be subject to spray drift (Figure 4-1[9]
|
From page 71... ...
Wet deposition or washout due to gasphase and particle-phase washout is considered the least likely of the deposition mechanisms because most spraying occurred during dry conditions; however, washout would be an efficient mechanism for removing TCDD and other compounds from air. Thus, the committee concluded that with atmospheric deposition, there was an opportunity for herbicides applied by aerial spraying in Vietnam to enter inland waters of Vietnam and the coastal waters that received the discharge from those waters in addition to herbicide that was applied directly to water.
|
From page 72... ...
measured the rate of photodegradation of Agent Orange that contained TCDD at 15 ppm applied to excised leaves of a rubber plant when exposed to sunlight for a half life of about 2 h. They measured a slower photodegradation rate (half life of about 5– 6 h)
|
From page 73... ...
It is also important to note that most experiments and calculations of photodegradation are conducted to simulate environmental conditions at about 40°N latitude; at this latitude, ultraviolet radiation from the sun is less intense than that at 16°N latitude, where Vietnam is. Deposition of TCDD from Canopy to Soil TCDD could deposit on soil directly from Agent Orange spraying, by the falling of sprayed foliage, by dry deposition of eroded leaf waxes, and by leaf washoff during precipitation (Figure 4-1[4]
|
From page 74... ...
Young (1983) reported that 99.9% of the TCDD applied in herbicides at Elgin Air Force Base in Florida was photodegraded during and immediately after application.
|
From page 75... ...
Although it is logical to conclude that some fraction of TCDD and other herbicides applied in heavily sprayed areas would reach inland and coastal waters from soils, particularly as the forest is denuded, the amount of TCDD that entered water courses would be subject to enormous dilution from river flows. Quinh et al.
|
From page 76... ...
Fate of TCDD in Marine Coastal Waters As TCDD entered the coastal waters of Vietnam as a result of spray drift and atmospheric deposition (Figure 4-1[A]
|
From page 77... ...
at 24°C using water from Lake Mendota and Lake Wingra in Wisconsin. It is expected that the half-life of TCCD in Vietnamese coastal waters, if they were relatively clear, would be shorter than that because of the higher intensity of ultraviolet radiation hitting Vietnamese water than that at 40°N.
|
From page 78... ...
As noted in the discussion above, the committee's conclusions are based on a theoretical understanding of fate processes of TCDD. Field studies of TCDD have been undertaken in temperate zones or with systems set up to simulate conditions that prevailed during the Vietnam War, such as the experiments conducted at Elgin Air Force Base in Florida.
|
From page 79... ...
CONCLUSIONS The committee reviewed the fate of Agent Orange and TCDD in particular in the Vietnamese environment as a result of spray applications during the Vietnam War. The discussion is entirely qualitative because the committee was not able to quantify any component of the fate processes discussed.
|
From page 80... ...
Atmospheric deposition of TCDD on coastal waters from spray drift would have been greatly diluted in these waters. Given the paucity of information and the variability and uncertainty in the available information, the committee concludes that it is not possible to estimate the likely concentrations of TCDD in marine waters and air at the time of the Vietnam War.
|
From page 81... ...
Journal of Environmental Science and Technology 36:4831-4845. Crosby, D
|
From page 82... ...
2008. The utility of proximity-based herbicide exposure assessment in epidemiologic studies of Vietnam veterans.
|
From page 83... ...
Environmental Science & Technology 26(3)
|
From page 84... ...
Presentation to the IOM Committee on Making the Best Use of the Agent Orange Exposure Reconstruction Model.
|
From page 85... ...
Environmental Science & Technology 35(3)
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.