Sorption, mineralization and mobility of N-(phosphonomethyl)glycine (glyphosate) in five different types of gravel

Sorption, mineralization and mobility of glyphosate were studied in six substrates: the five types of gravel most frequently used as surfacing in Denmark and a sandy agricultural soil from Simmelkær that served as a reference soil. Cumulative mineralization of methyl- ¹⁴C]glyphosate in batch studies was highest in coarse gravel, amounting to 14% after 4 days at 30°C and 32% after 31 days. Mineralization was slowest in the sandy reference soil, amounting to only 2% after 31 days. The adsorption coefficient (K _d) of glyphosate in gravel ranged from 62 to 164 litre kg ^-1, while that in the sandy reference soil was 410 litre kg ^-1. The results indicate that the relatively low K _d in gravel allows a relatively high rate of glyphosate mineralization by the biomass. When K _d is high, in contrast, mineralization is slow. Lowering the temperature to 10°C decreased mineralization by 50% in one of two gravels. The leaching of glyphosate was screened in simple columns of gravel or soil in which precipitation events (20mm over a 2-h period) were simulated on three occasions, starting either immediately after or 2 days after application of glyphosate. [ ¹⁴C]Glyphosate was applied as a tracer mixed with the commercial product Roundup® Garden at the recommended rate of 2.4 kg glyphosate ha ^-1, equivalent to 1 μg ^-1 soil. The highest concentration of [ ¹⁴C] compounds (expressed in terms of glyphosate concentration) in leachate from the columns exceeded 1300 μg litre ^-1, and was detected in rounded gravel after the first rain event. No glyphosate was detected in leachate from the sandy reference soil.