Diffuse pollutants such as pesticides pose a significant threat to groundwater quality. Following the wet autumn and winter of 2000–2001 an upturn in trace pesticide concentrations was detected in blended water drawn from part of the Triassic Sandstone aquifer. A groundwater sampling and monitoring programme was undertaken to assess whether this increase would continue. Over a period of 18 months, 190 groundwater samples were collected from the upper part of the aquifer and analysed for 40 pesticides and for nitrate. A total of 25 pesticides were found of which 16 were detected more than once. Positive pesticide detections were in excess of the EU maximum admissible concentration (MAC) for individual substances of 0.1 μg l−1 on 33% of occasions. The most commonly detected pesticides generally have higher agricultural application rates and/or relatively greater solubilities. Very high concentrations of mecoprop (up to 7.1 μg l−1) were consistently found at two of the sampling sites. Analysis of mecoprop enantiomers suggested more than one source for this contaminant. High concentrations of atrazine (up to 1.4 μg l−1) were also detected at three sites and high concentrations of isoproturon (1.2 μg l−1) were found where very high mecoprop concentrations were also present. Nitrate concentrations exceed the EU MAC of 11.3 mg l−1 N at four sites. The spatial and temporal distribution of pesticides obtained from the monitoring network shows no clear trends for prediction of future concentrations. Nitrate-N concentrations and pesticide detections show no clear relationship, suggesting different source, transport or degradation pathways.

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