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

Streams in the Salt River Basin (SRB) of northeastern Missouri, USA, have been chronically contaminated by atrazine and metabolites, with peak annual transport occurring from spring to early summer. Since 2005, increased fall-applied simazine has introduced a second chloro-triazine herbicide that degrades to deisopropylatrazine (DIA), creating the need for a method to partition DIA between its two parent sources - i.e., DIA derived from atrazine (DIA_ATR) and that from simazine (DIA_SIM). Distinguishing DIA parent sources would extend current understanding of chloro-triazine transport, leading to more accurate risk assessments and improved watershed-scale load estimates. 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) concentrations in runoff 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 transport 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, potentially increasing the risk of toxicity to aquatic ecosystems.