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Leveraging high spatiotemporal resolution data of pesticides applied to agricultural fields in California to identify toxicity reduction opportunities [1]
['Nicol Parker', 'Bren School Of Environmental Science', 'Management', 'University Of California', 'Santa Barbara', 'Ca', 'United States Of America', 'Ashley Larsen', 'Priyanka Banerjee', 'Arturo A. Keller']
Date: 2023-08
The Environmental Release Tool has two platforms: a web application for California and a desktop version for all study areas in the United States, which offer different advantages. The web-based tool, available on any internet-accessible device, summarizes applied toxicity in seconds and provides a simpler user interface. The offline tool offers a high degree of customization, more detailed information, and custom simulations. To assist experts and non-experts, the desktop and web tools were built in RStudio [46] version 1.4. The development environment accommodates full customization of the tool’s code for experts and the ability to run unique simulations for non-experts via editing spreadsheet files in Google Sheets and clicking a start button.
This tool does not quantify fate or exposure but rather illustrates the location and amount of applied toxicity [47] for designing toxicity reduction strategies and planning monitoring campaigns by identifying areas where higher toxicity is released in the environment, and its sources. Although the ERT is a spatial tool designed for large extents, the tool works best to understand sources of pesticide exposures for species with a small habitat range. However, for organisms whose activities are more widespread and who have less direct contact with environmental compartments where pesticides are most likely to persist, the location of applied toxicity may be less useful for understanding sources of potential exposure.
Watersheds in the ERT are delineated using the Watershed Boundary Dataset [ 48 ], a data product of the United States Geological Survey. Each watershed is assigned a hydrologic unit code (HUC), which is based on the hydrologic connectivity and scale of the watershed. Watersheds with shorter HUCs, such as HUC 2-digit codes, are large watersheds encompassing hundreds of thousands of square kilometers, while longer HUCs such as HUC 8-digit codes (HUC8) represent subwatersheds of the shorter digit codes (e.g., HUC2). The assignment of pesticide use data to watersheds of various spatial extents is facilitated by the tool (see S1 Text ).
To enable evaluations of the variability of pesticide toxicity over large extents, the tool summarizes pesticide applications and toxicity by watershed. The data is summarized by watershed, and applications sites as well as pesticides within since pesticide losses via runoff and eroded sediments share a common outlet. Summarizing applied toxicity by watersheds is important to conceptualize areas that share common hydrologic routes for pesticide transport. Though the Environmental Release Tool does not simulate loss processes, it is the first stage of development of the Pesticide Mitigation Prioritization Model, and the product of the second stage of development is a companion, mechanistic fate and transport tool where loss processes are simulated.
2.2 Sources
To evaluate the spatiotemporal distribution of pesticides, the ERT benefits from the ability to autoload daily pesticide use report data in California from statewide agricultural applicators [33]. The tool internally hosts the data, and using an autoload script, aggregates data for the area of interest to the user, which watersheds or counties may define. Where counties are used, the tool automatically aggregates data to watersheds in the county. For other pesticide input options (e.g., manual inputs or for analyses of other land uses or regions), see S1 Text.
The amount of pesticide applied on application sites (e.g., a specific crop) in California is substantial, millions of pounds for widely cultivated crops, and as high as ~40 million for almonds [49]. To assist efficient analyses, the ERT extracts pesticide usage data for California from CDPR Pesticide Use Reports [33] by active ingredient (AI) and for the 432 agricultural site types for the study area of interest to the user. These reports record daily applications at the County Meridian Township Range Section (referred to as Section) spatial scale (2.6 km2). For Sections where pesticide use data is reported that overlaps multiple watersheds, the area fraction of overlap is used to weight the mass of AI applied. Notably, urban applications were not included in the autoload feature. The reports do not include household applications, and for professional urban applications are recorded at the county level and at a monthly time-step, which cannot be allocated to a specific watershed or date.
For evaluating pesticide sources of toxicity, ERT facilitates the summarization of similar AIs. This feature is useful because many AIs have a similar chemical make-up (e.g., isomers or are produced in several forms, including acids, salts, amines, and esters), but have no or limited toxicity data for the various AI forms. Provided that AI forms can have very different effect concentrations, where possible, the user should provide AI form-specific toxicity. To accommodate specific endpoints where available, but to enable simplification of tool outputs, unique toxicity endpoints are accepted and calculated for pesticides within a user-defined pesticide group, and the group ID reports the group’s total applied toxicity in tool output. In this investigation, we considered AIs detected (2014–2018) within California’s surface waters with available toxicity data (n = 151). From the CDPR’s Pesticide Use Reports, 290 forms of the AIs were observed (e.g., 12 unique esters and 15 salts of 2,4-D).
In addition to pesticide sources of applied toxicity, a key feature of ERT is the ability to preserve information relating to application site types. However, too many application sites make the interpretation of results difficult. The tool thus enables users to group similar application sites (e.g., alfalfa and alfalfa-grass mixture) by assigning the same ID to multiple site types. By default, 432 agricultural application site types from Pesticide Use Reports are simplified to 116 based on the similarity of the crops. Groupings can be viewed and modified in the tool input file for application sites.
To identify pesticide toxicity reduction targets, the ERT quantifies applied pesticide toxicity. Applied toxicity refers to the mass of pesticide applied to an area with the potential to do harm [50]. The applied toxicity for the ith pesticide in the jth watershed is calculable from applications to the kth site type and toxic endpoint of the mth taxon of interest as: Eq (1) Where TI is the Toxicity Index (kg-m3/kg), M (kg) is the mass of applied AI, and T (kg/m3) is the adverse health-effect concentration of concern (e.g., the lethal concentration of fifty percent of the test organism population) for the species or taxonomic groups of interest. Within a simulation, the tool is suitable for quantifying the applied toxicity to taxa within the same compartment, not across environmental compartments, because variation in the transport of pesticides based on physicochemical properties is not simulated. The tool illustrates applied toxicity within the soil compartment or available for transport to the compartment of interest. While the transport of pesticides from the application site is sensitive to their physicochemical properties [51], property correlation to surface water detection frequencies has been demonstrated to the more robust for pesticide sales data than physicochemical properties in a monitoring campaign of 72 pesticides of diverse properties in over 100 streams [52]. Though this approach is not suitable for risk assessments, it facilitates an understanding of where mitigation opportunities exist [53] without data requirements and uncertainty of fate and transport models over large extents [26, 27, 54].
Our investigation considers the applied toxicity of pesticides for fish, as well as aquatic invertebrates, nonvascular plants, and vascular plants. Toxicity endpoints employed were acute values from the United States Environmental Protection Agency (USEPA) Aquatic Life Benchmarks Database [55]. The USEPA derives Benchmarks from the concentration at which fifty percent of a species sample in single-dose laboratory investigations experience severe effects derived from mortality endpoints or, for plants, significant changes in growth/biomass (LC50 or EC50). A genera endpoint is then calculated based upon a 0.05 cumulative probability of toxicity for represented species, which typically reflects the most sensitive species within the taxonomic group. For fish and invertebrates, the USEPA calculates the final acute value as the product of the taxonomic group endpoint multiplied by a safety factor of 0.5 and does not adjust plants. Where no toxicity endpoints were reported for the pesticide in the Aquatic Life Benchmark database (n = 10), the Pesticide Properties DataBase [56] acute toxicity endpoints were employed, and unverified data were excluded.
The first applied toxicity index reported by ERT for pesticides, sites, and watersheds is the Relative Toxicity Index (RTI) (kg-m3/kg-m2). The index weights the toxicity of the ith applied pesticide by the size of the application area within the jth watershed as: Eq (2) where A (m2) is the area affected by pesticide applications.
To estimate the areas affected by pesticide applications, agricultural land use datasets are used. In California, the Pesticide Use Reports can be used to retrieve the impacted area. However, there are known inaccuracies. The planted area is often recorded for all the grower’s land; although reported for a specific crop, and fields are subject to multiple crop rotations within a year. For the applied area, multiple applications are typical for a crop that renders the net-application area unknown. Due to these concerns, alternative land use datasets were evaluated for use [57–59].
After reviewing several datasets, the California Department of Water Resources land use surveys [60] were found to be the most accurate with a median accuracy of 97.5% and positional quality of 8m. However, a limitation of the dataset, as well as the others, is that it provides fewer site types (43) compared to Pesticide Use Reports (432). Using this dataset to determine the affected area of specific application site types would require highly reducing the resolution of pesticide source data. Attempts to recategorize crops to fit available land use data did not obtain reliable results. As a result, we chose to consider the affected area to be all agricultural land in the California Department of Water Resources dataset. The representation of the affected compartment to all agricultural land was deemed appropriate because only 5% of agricultural fields in California employ organic cultivation practices [61] and use non-synthetic pesticides recorded in use reports.
As the quantification of the affected compartment area is frequently limited, and the fraction of a watershed subject to pesticide application is highly variable, we provide a second applied toxicity index independent of area, the Net Toxicity Index (NTI). The NTI is a relative rank toxicity index to determine if the applied toxicity is greater than what is typical for the ith pesticide in the jth watershed. As our reference of what is typical, we calculate for the study area the 50th percentile (perc 50 ) of the applied toxicity for any applied pesticide (pst) in watershed (w). The NTI is calculable from the TI of the ith pesticide in the jth watershed as: Eq (3)
The NTI approach can quickly identify applied toxicity above typical levels in the study extent. For example, if the 50th percentile of the applied toxicity of pesticides to a watershed in the study area is applications of imidacloprid in the San Joaquin Watershed, (e.g., 1000 TI), to calculate the NTI, the TI of the ith pesticide and the jth watershed of interest is divided by 1000 TI. Using this approach, pesticide applications within a watershed over the simulation period with an NTI greater than unity have applied toxicity above the 50th percentile. This normalization provides a unitless applied toxicity index that does not affect the relative rank of the applied toxicity for pesticides, sites, or watersheds, and can identify effective toxicity reduction targets specific to the study area.
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