Estimating simazine treated area in watersheds based on annual stream loads

Authors: LERCH, ROBERT; WILLETT, CAMMY - UNIVERSITY OF MISSOURI

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/20/2021
Publication Date: 6/24/2021
Citation: Lerch, R.N., Willett, C.D. 2021. Estimating simazine treated area in watersheds based on annual stream loads. Journal of Environmental Quality. 50: 1184-1195. https://doi.org/10.1002/jeq2.20257.
DOI: https://doi.org/10.1002/jeq2.20257

Interpretive Summary: For the majority of the United States, there are currently no reporting requirements for pesticide applications and areas treated, and existing national pesticide usage data are insufficient for estimating pesticide treated crop areas within specific watersheds. The objective of this research was to evaluate an approach for estimating simazine usage on corn based on its transport to streams of the Salt River Basin (SRB) of Missouri, USA. Seven streams representing large watersheds (~50,000 to ~290,000 acres) were sampled from 2005 to 2011 and one watershed, Long Branch, and three sub-watershed streams were sampled from 2012 to 2017. Because data were lacking for treated area simazine losses (i.e., pounds of simazine lost per acre of corn treated), we used atrazine treated area losses, which have been well characterized, as a substitute for simazine. Simazine treated corn area (in acres) was then calculated as simazine lost in the streams from each watershed, in pounds, divided by the treated area atrazine loss, in pounds per acre. From 2005 to 2010, the results showed that an estimated 3.8 to 31% of the corn in the SRB watersheds was treated with simazine. Long Branch and its sub-watersheds had the greatest simazine usage, with >20% of corn area treated. Compared to national estimates for the SRB, this approach estimated simazine usage that was generally 10 to 100 times greater and showed far greater differences in usage among watersheds. These results demonstrated that this broadly applicable method is a significant improvement over existing national data for estimating pesticide usage in watersheds. This research benefits federal and state agencies that apply pesticide usage data for estimating exposure to humans, via drinking water, and to aquatic ecosystems.

Technical Abstract: Existing data in the United States are insufficient for estimating pesticide treated crop areas at the watershed-scale. The objective of this research was to evaluate an approach for estimating simazine usage on corn based on its transport to streams of the Salt River Basin (SRB) of Missouri, USA. Annual loads of total simazine and atrazine (parent + metabolites) were quantified for seven SRB watersheds from 2005 to 2017. Simazine treated corn area was computed as the total simazine load (g) divided by total atrazine load (g/ha), on a treated area basis; atrazine was used as surrogate in the absence of treated area simazine load data. From 2005 to 2010, an estimated 3.8 to 31% of the corn area within SRB watersheds was treated with simazine, and four of six watersheds had <10% of corn treated. In contrast, Long Br (2005 to 2017) and its sub-watersheds (2012 to 2017) had >20% of corn area treated with simazine. Key sources of variation in treated area estimates included extremely dry years with little simazine transport and disparities between spring-applied atrazine and fall-applied simazine transport. However, compared to national estimates for the SRB, this approach estimated simazine usage that was generally 1-2 orders of magnitude greater and showed far more spatial and temporal variation among watersheds. These results demonstrated that this broadly applicable output-based method is a significant improvement over existing input-based national data for estimating pesticide usage in watersheds.