Penetration of herbicides to groundwater in an unconfined chalk aquifer following normal soil applications

The persistence and penetration of the herbicides isoproturon and chlorotol uron in an unconfined chalk aquifer has been monitored over a 4-year period through soil sampling, shallow coring and groundwater monitoring. Chloroto luron was applied on plots as a marker compound, having never been used pre viously on that, or surrounding fields. The fieldsite had a 5 degrees slope with soil depths of 0.5 to 1.5 m and a water table between 20 and 5 m from the soil surface. Where the water table was deepest (9-20 m below surface (mbs)) little or no positive herbicide detections were made. However, where the water table was at only 4-5 mbs, a regular pesticide signal of around 0.1 mug/l for isoproturon and chlorotoluron could be distinguished. Over th e winter recharge period automatic borehole samplers revealed a series of s hort-lived peaks of isoproturon and chlorotoluron reaching up to 0.8 mug/l. This is consistent with a preferential flow mechanism operating at this pa rticular part of the field. Such peaks were occurring over 2 years after th e last application of these compounds. Shallow coring failed to uncover any significant pesticide pulse moving through the deep unsaturated zone matri x at the fieldsite. (C) 2001 Elsevier Science B.V. All rights reserved.