Assessing pesticide wet deposition risk within a small agricultural watershed in the Southeastern Coastal Plain (USA)☆
Graphical abstract
Introduction
Evaporation from soil, plant, and other treated surfaces results in pesticide transfer in the gas phase into the atmosphere. As a process this is commonly termed volatilization and can occur during or post-application. The topic has been investigated for > 50 years and periodically reviewed (Abbott et al., 1965, Bedos et al., 2002, Majewski and Capel, 1995, Spencer et al., 1973, Van Den Berg et al., 1998). Studies have indicated that post application volatilization losses span a broad range, from 0.2% to > 90% of the amount applied (Barbash, 2007, Van Pul et al., 1999). The magnitude of losses was linked to pesticide physical chemical properties including: vapor pressure; Henry's Law constants; potential to sorb to soil, plant, and other treated surfaces; and diffusion coefficients. Volatilization losses were also linked to climatic conditions, soil water status, mode of application including spray characteristics and formulation, plant uptake, and management practices, such as soil incorporation and conservation tillage (Alletto et al., 2009, Barbash, 2007, Bedos et al., 2002, Bedos et al., 2010, Woodrow et al., 1997).
Once in the atmosphere, gas phase pesticides are subject to advective and diffusive transport, photochemical oxidation and degradation, sorption on aerosols and particulate matter, and wet and dry deposition. Transport distances from areas of application and emission vary from meters to 1000s of kilometers depending on rates of atmospheric degradation and depositional fluxes (Van Pul et al., 1999).
Most studies that have evaluated deposition have focussed on collection and analysis of rain. Generally this is relatively simple compared to dry deposition measurements that involve both pesticides in the gas phase and those sorbed to particulate matter and aerosols. Wania et al. (1998), have described the process of pesticide scavenging from air by rainfall. Equilibrium partitioning between vapor-phase pesticides and raindrops is typically assumed, with Henry's Law constants and temperature governing the process.
Like pesticide volatilization, the occurrence and deposition of pesticides in rain has been the focus of a very large number of published investigations spanning more than 5 decades. They have included reports of detection of legacy pesticides, like DDT, as well as numerous products in current use (Wheatley and Hardman, 1965, Dubus et al., 2000). Monitoring was conducted in remote areas, approximately 1000 km away from pesticide sources, at stations at intermediate distance (1 to 100 km), and within areas < 1 km where the pesticides were applied (Dubus et al., 2000, Potter et al., 2014a, Van Dijk and Guicherit, 1999).
In some studies measured deposition was related to the mass of pesticides used in contributing areas. For example, Vogel et al. (2008), reported that total pesticide wet deposition measurements made in 4 agricultural watersheds ranged from 0.06 to 1.73% of the amount applied. The highest deposition rate was in the local-scale area where the pesticides were applied to agricultural fields. This value is comparable to surface runoff rates that are commonly observed, ≈ 1% of the amount applied (Wauchope et al., 1995). Potter et al. (2014a) estimated that about 0.1% of the insecticide endosulfan applied to farm fields in Southern Florida was deposited locally in rainfall. In another investigation that linked surface runoff to volatilization, Gish et al. (2011), found that the volatilization mass loss of the herbicides atrazine and metolachlor when applied preemergence to bare soils was 2 to > 130 and 10 and > 150 times the mass loss in runoff; however deposition was not measured.
Findings that volatilization of some active ingredients may greatly exceed runoff rates and that wet deposition of pesticides at least at the local scale may be comparable, suggests that pesticide volatilization and wet deposition may need to be considered in risk assessments of pesticide use. The human and ecological risks of pesticide volatilization and deposition were discussed in detail at an international symposium in 1998 (Guicherit et al., 1999). The symposium included recommendations for approaches that may be used to incorporate volatilization and deposition into regulatory risk assessments (Bakker et al., 1999, Gilbert, 1999). However these processes are not currently an integral part of risk assessment processes. For example, in USA pesticide release into the atmosphere is evaluated, but the focus is on drift and post-volatilization transport of fumigants (USEPA, 2016a). Wet deposition is not considered.
We measured wet deposition of 14 current-use and 2 legacy pesticide active ingredients and 1 degradate within a small farm scale (123 ha) watershed in south central Georgia (USA) for 3 years. The watershed is intensively farmed with > 50% of the land in mixed crop production (Lowrance et al., 2007). We hypothesized that frequent rainfall during growing seasons and high rates of pesticide use in the watershed would contribute to relatively high rates of pesticide wet deposition. Wet deposition measurements were used to support first-tier risk assessments and to identify the upper bounds of pesticide wet deposition in watersheds in the region.
Section snippets
Study area and sample collection location
The study was conducted within a 123-ha drainage basin near Tifton, GA (Fig. 1) that forms a headwater for a stream flowing into the Little River, a tributary of the Suwannee River. The bowl-like basin is typical of low-order streams in this landscape with dense riparian forests on stream banks and well-drained soils in uplands that are intensively cropped (Lowrance et al., 2007). In 2007–09, land cover was vegetable and row crops (50%) and adjacent grassed areas (6%), mixed deciduous and
Detection and concentration in rain samples
Two fungicides, 8 herbicides, 5 insecticides, and tribufos, an active ingredient used for cotton defoliation (tribufos), were targeted in the analyses (Table 1). Tests related to the insecticide, endosulfan, included its α- and β-isomers, and their primary soil degradate, endosulfan sulfate. Estimated use of the targeted compounds within the watershed indicated that, on a mass basis, they represented 61% of conventional pesticides used during the 3-yr study period with chlorothalonil,
Disclosure statement
All authors hereby certify that they have no actual or potential conflict of interest including any financial, personal or other relationships with other people or organizations within three (3) years of beginning the work submitted that could inappropriately influence (bias) their work. The work is a U.S. Government product.
Acknowledgements
Numerous field and technical staff, students, and volunteers contributed to success of the project. We especially thank Margie Whittle, Sally Belflower, and Rex Blanchett and our USDA-ARS colleague, Cathleen Hapeman, for use of the rain sampler.
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