Development of a screening tool to assess the temporal risk of pesticides leaching to groundwater using the source, target, vector approach. An Irish case study for shallow groundwater.

During this study, a groundwater screening tool was developed to assess the temporal risk of groundwater contamination from the use of pesticides. It is based on a source, vector, target approach. The method utilised in this study uses a semi-quantitative probabilistic risk assessment where the input parameters were classified and assigned a relative score from 1 to 5 (i.e. 1 = no risk and 5 = high risk). The model was parameterised by using national data and calibrated with 2 years of national pesticide groundwater monitoring data. After calibration, two specific sites were selected for model validation. Based on the presence of the source, vector and target, the evaluation indicated that the temporal risk is site specific (i.e. May to December for the country model, June to September for the Oak Park site and September for the Castledockrell site). A sensitivity analysis performed on the national scale revealed that the groundwater vulnerability category (gv), the clay content (cc%), the persistence of pesticides in soil (DT50) and the rainfall represented by wet day (wd) were the most important parameters that affected model predictions (correlation coefficients of 0.54, -0.39, 0.35 and 0.31, respectively), highlighting the importance of soil hydrogeological conditions, soil type and rainfall in influencing water model predictions. The model developed can help to identify the temporal risk from pesticides to groundwater and guide regulators in highlighting at-risk periods, therefore allowing more focused monitoring programmes.