Chloro-triazine transport to streams - evaluating methods for partitioning deisopropylatrazine sources

Authors: Lerch, Robert; WILLETT, CAMMY - University Of Arkansas

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/12/2019
Publication Date: 8/18/2019
Citation: Lerch, R.N., Willett, C.D. 2019. Chloro-triazine transport to streams - evaluating methods for partitioning deisopropylatrazine sources. Science of the Total Environment. 697:133931. https://doi.org/10.1016/j.scitotenv.2019.133931.
DOI: https://doi.org/10.1016/j.scitotenv.2019.133931

Interpretive Summary: Streams in the Salt River Basin of northeastern Missouri, USA, have some of the highest herbicide levels of Corn Belt streams. The corn herbicide, atrazine, and two of its breakdown products - known as DEA and DIA - are among the most commonly detected agricultural chemicals in Salt River Basin streams. Their concentrations are highest in the spring and early summer following atrazine application to cropland. Since 2005, fall application of simazine has increased within the Salt River Basin, resulting in high concentrations of simazine and its breakdown products during the winter and early spring in some streams. Triazine herbicides, such as atrazine and simazine, are applied to control broadlead weeds in grassy crops, such as corn and grain sorghum, and they both degrade in soil to form the DIA breakdown product. Therefore, dual application of two herbicides that both form DIA increases concerns associated with the toxic effects of these compounds on human and aquatic life. It also created the challenge of distinguishing the sources of DIA present in streams. The objective of this research was to develop and evaluate methods that could distinguish the amount of DIA from each parent herbicide – i.e., DIA from atrazine (DIA-ATR) and DIA from simazine (DIA-SIM). Three methods were developed; two were based on field runoff concentrations of atrazine, DIA, and DEA from atrazine treated fields; and one was based on the ratio of simazine:atrazine concentrations in stream water from 7 streams within the Salt River Basin. A series of extensive statistical analyses showed that the method based on the ratio of simazine:atrazine concentrations provided the most accurate estimates of DIA-SIM and DIA-ATR in streams. This method was then used to demonstrate that DIA-SIM had peak concentrations from mid-November to April each year while DIA-ATR peak concentrations occur from May to June each year. Dual season triazine herbicide applications in a watershed increased the period of high DIA concentrations from ~3 to ~8 months per year. Distinguishing DIA parent sources extends our current understanding of triazine herbicide transport to streams, leading to more accurate risk assessments, detailed knowledge of DIA transport, and improved watershed-scale estimates of triazine herbicides and their breakdown products in streams.

Technical Abstract: Streams in the Salt River Basin (SRB) of northeastern Missouri, USA, have been chronically contaminated by atrazine and metabolites, with annual peak concentrations and loads occurring from spring to early summer. Since 2005, increased fall-applied simazine has introduced a second chloro-triazine herbicide that degrades to deisopropylatrazine (DIA). Distinguishing DIA parent sources would extend current understanding of chloro-triazine transport, leading to more accurate risk assessments, detailed knowledge of DIA transport, and improved watershed-scale load estimates of chloro-triazines. Therefore, a method is needed to partition DIA between its two parent sources – i.e., DIA derived from atrazine (DIA-ATR) and that from simazine (DIA-SIM). The objectives of this study were to evaluate proposed methods for DIA partitioning, and to apply the most effective method to estimate DIA-ATR and DIA-SIM concentrations and loads. Three DIA partition methods were developed and statistically evaluated: 1) edge-of-field (EOF) based on DIA and deethylatrazine (DEA) runoff concentrations from atrazine treated fields; 2) DIA:DEA concentration ratios (D2R) in runoff from atrazine treated fields; and 3) concentration ratios of simazine:atrazine (SAR) in streams. Stream samples were collected year-round at 7 SRB stream sites from 2005 to 2010 and daily, quarterly, and annual concentrations and loads of atrazine, DEA, DIA, and simazine computed. The SAR method was superior to EOF and D2R in its ability to estimate concentrations and loads of DIA-SIM and DIA-ATR that were more accurate and highly correlated to observed concentrations and loads of simazine, atrazine, and DIA. The SAR method results demonstrated the differences in DIA-SIM and DIA-ATR transport timing, with peak DIA-SIM transport occurring from mid-Nov to Apr and peak DIA-ATR transport from May to Jun. Dual season triazine applications within a watershed substantially increased the period of high chloro-triazine concentrations in streams from ~3 to ~8 months/yr.