The applicant’s planned diquat concentration in treated waters was 0.37 parts per million (or 370 ug/L), during the herbicide treatment. Therefore, a Day 3 post-treatment sample yielding a diquat concentration of 100 ug/L reveals diquat dissipated in the water column, as sampled in the treatment area.
In regard to the report identifying pass or fail (limit listed as 20 ug/L), this concentration is based on the US EPA’s drinking water standard for diquat. The NH Dept. of Environmental Services has also established 20 ug/L as the ambient groundwater quality standard for diquat (Env-Or 603.03). The Day 7 post-treatment sampling results yielded a diquat concentration below this level, and water use restrictions have expired (as conditioned in special permit SP-258, based on the Tribune label requirements).
Water Quality Protection
Division of Pesticide Control
New Hampshire Department of Agriculture, Markets, & Food