Potential for aerobic isoproturon biodegradation and sorption in the unsaturated and saturated zones of a chalk aquifer

The potential fate and behaviour of the herbicide isoproturon, under aerobic conditions, was studied in soil, chalk and groundwater from two sites on an unconfined aquifer in Hampshire, UK. A small but significant sorption potential for isoproturon was noted in the upper chalk, suggesting that some retardation would take place in transport through the chalk. The degradation potential of the samples was studied using laboratory microcosms. Very little degradation potential appeared to exist for isoproturon in the unsaturated zone of upper chalk 3 m below the soil surface. Wide variations in degradation rates between samples from the same depth was noted. A degradation potential was noted in the chalk from shallow depths under laboratory microcosm conditions at a pesticide concentration of 100 μg l−1. Of the two sites examined, the most rapid and consistent degradation observed was associated with the groundwater rather than the chalk in the saturated zone. No significant isoproturon ring mineralisation occurred in the chalk or groundwater samples, implying that where isoproturon degradation does occur a by-product containing the phenyl ring will persist. Isoproturon degradation potential was not directly related to the moisture content, total organic carbon, ability to metabolise acetate, or number of viable bacteria present in the sample.