Point and non-point source leaching of pesticides in a till groundwater catchment

1. Introduction

Scope of investigation

The objective of the project is to assess the risk of pesticide leaching from controlled application of pesticides in an orchard and from an associated spill, where pesticides and equipment for spraying have been handled in an extensive period. Furthermore, the objective is to evaluate the leaching in a clayey till groundwater catchment to the underlying aquifer and the influence of fractures in the till as transport paths for pesticide leaching.

Pesticide contamination of drinking water supply wells, streams and shallow groundwater has been identified as an increasing problem in Europe and elsewhere (Fielding et al., 1991, U.S. Environmental Protection Agency, 1992 and GEUS 1995).

Sources of pesticide

leaching

In Denmark more than 99% of the drinking water comes from ground water. Several sources of contamination may contribute to the pesticide findings in the groundwater. Leaching from controlled application on farm land (diffuse source) has been considered to be the main source of contamination. Another important source may be point source spills, which have resulted from the handling of pesticides in association with spraying. However, examples of the relative important of different sources have not previously been reported in denmark.

Contaminant leaching in clayey tills

An other main issue of the investigation is the influence of fractures on the pesticide transport in clayey till deposits. 40-50% of the Danish groundwater is covered by Quaternary deposits of glacial clay till, typically 10-40 m in thickness. Recent investigations in Denmark (Jørgensen 1990, Fredericia 1990, Jørgensen and Fredericia 1992, Jørgensen 1995, Hindsby et.al., 1996, Jørgensen and Spliid 1997, Jørgensen and McKay in review, Jørgensen et.al., in review, Thorsen et.al., in review and Broholm et.al., (a,b) in review), and in Canada (D'Astous et.al., 1989, Rusland et.al., 1991, McKay et.al., 1993), have shown that contaminants can migrate at significantly faster rates due to fractures in shallow tills than in the massive unfractured till matrix which is typically characterized by very low hydraulic conductivities. In the reported project, hydrogeological emphasis is on the influence of deep fractures on transport, as evaluated by laboratory experiment, field observations and the distribution of tritium in the geological profile.