Effects of tilling no-till soil on losses of atrazine and glyphosate to runoff water under variable intensity simulated rainfall

Herbicides released through agricultural activities to surface waters and drinking water systems represent a risk to human and environmental health, as well as a cost to municipalities for removal. This study focuses on the viability of glyphosate tolerant cropping systems as an alternative to atrazine-based systems, and the impact of tilling historically no-till ground on the runoff pollution potential of these systems. Variable intensity field rainfall simulations were performed on 2 m long × 1 m wide plots within a field in first-year disk and harrow following no-till (CT), and within a long-term no-tilled (NT) field, both treated with atrazine and glyphosate according to label. Rainfall sequence was: 50 mm h⁻¹ for 50 min followed by 75 mm h⁻¹ for 15 min, 25 mm h⁻¹ for 15 min, and 100 mm h⁻¹ for 15 min. Runoff was collected at regular time intervals during two simulated rainfall events and analyzed for herbicide concentration, sediment content, and volume. Maximum glyphosate concentration in runoff was 233 μg L⁻¹ for NT and 180 μg L⁻¹ for CT (approximately 33% and 26% of the maximum contaminant limit (MCL) for glyphosate (700 μg L⁻¹), respectively, while maximum atrazine concentrations in runoff was 303 μg L⁻¹ for NT and 79 μg L⁻¹ for CT (approximately 100 times and 26 times the atrazine MCL (3 μg L⁻¹)). Atrazine concentration and loading were significantly higher in runoff from NT plots than from CT plots, whereas glyphosate concentration and loading were impacted by tillage treatment to a much lesser degree. Results suggest that glyphosate-based weed management may represent a lower drinking water risk than atrazine-based weed management, especially in NT systems.