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

Abstract

The persistence and penetration of the herbicides isoproturon and chlorotoluron in an unconfined chalk aquifer has been monitored over a 4-year period through soil sampling, shallow coring and groundwater monitoring. Chlorotoluron was applied on plots as a marker compound, having never been used previously on that, or surrounding fields. The fieldsite had a 5 degree 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 microg/l for isoproturon and chlorotoluron could be distinguished. Over the winter recharge period automatic borehole samplers revealed a series of short-lived peaks of isoproturon and chlorotoluron reaching up to 0.8 microg/l. This is consistent with a preferential flow mechanism operating at this particular part of the field. Such peaks were occurring over 2 years after the last application of these compounds. Shallow coring failed to uncover any significant pesticide pulse moving through the deep unsaturated zone matrix at the fieldsite.

The significance of colloids in the transport of pesticides through Chalk

“…However, in a groundwater sample taken from depth no such association with colloids is observed. This is despite the previous observations of Johnson et al (2001) and Gooddy et al (2001) which strongly suggested that pesticide concentrations in the shallow groundwater were controlled, at least in part, by the association of pesticides with colloidal material. This infers that either our method was too insensitive analytically to detect pesticides bound to colloids in the groundwater, or that the pesticide present was in a free (i.e.…”

“…It would appears therefore that the previous observations of Gooddy et al (2001) and Johnson et al (2001) pertaining to the likelihood colloidal transport of pesticides were correct. The fact that they were found at the study site was a function of the shallow depth from which the samples were taken.…”

“…Although no direct field measurements were made, the workers hypothesized that the isoproturon had moved through the soil and unsaturated zone by a preferential flow route sorbed to soil colloids. A related study by Johnson et al (2001) found even more compelling evidence for colloidal transport of pesticides. In this study high concentrations of the herbicide isoproturon were found in pumped shallow Chalk groundwater following intensive rainfall events for three successive years after the herbicide was last applied.…”

“…Recently the occurrence of both pesticides and their metabolites within groundwater have been identified (Kolpin et al, 1996(Kolpin et al, , 2004Gooddy et al, 2002Gooddy et al, , 2005Johnson et al, 2001;Postle et al, 2004). Pesticides in groundwater have been found to originate from arable land (e.g.…”

A field study to assess the degradation and transport of diuron and its metabolites in a calcareous soil

“…In the United States, a considerable body of work exists relating to the occurrence of both parent compound pesticides and their metabolites in aquifers (Thurman et al, 1992;Baker et al, 1993;Lawrence et al, 1993;Kolpin et al, 1996;Kolpin et al, 1998;Kolpin et al, 2000a;Kolpin et al, 2000b). In the UK the number of field studies has been limited (Clark et al, 1991;Gooddy et al, 2001;Johnson et al, 2001) with researchers often concentrating on laboratory studies of transport and degradation potential (Johnson et al, 1998;Besien et al, 2000;.…”