Diseño de investigación

A Field and Modeling Study to Determine Pesticide Occurrence in a Public Water Supply in Northern England, UK
by Marianne E. Stuart, Daren C. Gooddy, Andrew G. Hughes, and Christopher R. Jackson

Abstract
In the UK, increasing numbers of pesticides are being detected in ground water as monitoring programs are developed and analytic techniques are refined. This study describes theinvestigation of pesticides at a site on the Triassic Sandstone aquifer of northern England. Drilling and pore water sampling demonstrated the presence of agricultural herbicides in the unsaturated zone of the aquifer close to a public supply well site at which elevated pesticide concentrations had been detected in the abstracted ground water. These were similar to the compounds detected in the supply wells,but there was no evidence for some of these having been used at the nearby farm. Modeling of water movement to wells showed that individual ground water flow subcatchments were retained during a monthly duty cycle and should be well-defined. Detecting and quantifying the sources of pesticide pollution of ground water was unsuccessful, probably due to local land use and aquifer heterogeneity. Thisremains difficult even where the overall catchment is small (1-km radius) and there are apparently few possible point sources. Even with sophisticated flow models, there is still a high degree of uncertainty in modeling pesticide movement, and treatment of ground water may be the only viable option for water utilities.

Introduction
In the UK, increasing numbers of pesticides are being detectedin ground water as monitoring programs are developed and analytic techniques are refined (Gooddy et al. 2001; Environment Agency 2002). In the Doncaster area in northern England, an upturn in the frequency and magnitude of trace detections of pesticide concentrations was found in blended water drawn from the Triassic Sherwood Sandstone aquifer for public supply from the late 1990s onward (Knapp2005). The local water utility, Yorkshire Water, initiated monitoring of raw water from individual wells in the wellfield for a wide range of pesticides. Following the wet autumn and winter of 2000 to 2001, water levels in many UK aquifers rose to exceptionally high levels and remained high for extended periods of time. Associated with this rise, increased concentrations of diffuse pollutants inground water were found. Against this national background, pesticide detections in the Doncaster wellfield became both more frequent and at higher concentrations (Knapp 2005). In particular, at one multiwell site (BP) concentrations of the herbicide bentazone in one of the wells became a cause for concern with > 0.1 lg/L

Copyright ª 2006 British Geological Survey, NERC Journal compilation ª 2006National Ground Water Association.
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(European drinking water maximum allowable concentration [MAC]) being detected on several occasions and total pesticide concentrations approaching 0.5 lg/L. Concentrations in a second well at the site also increased considerably, with significant concentrations of the agricultural herbicides bentazone, isoproturon, and mecoprop as well as an increasedrange of other herbicides and fungicides. The site was taken out of distribution to maintain supply quality. The amount of pesticide being abstracted and the observed sharp variations in concentration suggested that it was unlikely to be coming wholly or even dominantly from diffuse sources. Water abstracted from this site is anaerobic, with very low nitrate concentration and significantconcentrations of iron and manganese, consistent with ground water being locally confined, so the route for transport of pesticides to the well was unclear. This study describes an investigation conducted at the site with the objectives of identifying the source and scale of the contamination to allow evaluation of strategies to reduce the impact of this contamination on ground water abstracted for public...