Pesticides from point sources are released and transported in a different manner compared to those of diffusion sources. For example, accidental spills during tank filling may result in extremely high but localised concentrations of pesticides in water. Consequently, the point source may show a sharp concentration breakthrough curve. In addition, the point sources may be active randomly; therefore, the pesticide concentration can differ greatly over time.
Thorling et al. (2015) had identified such patterns by analysing long-term monitoring data of pesticides in groundwater in Denmark. They, then, proposed a set of indicators and threshold values to identify influx of point sources (Table 7.3). Several regions in Denmark has implemented this protocol since 2018.
Table 7.3: Indicators and threshold values for pesticide point source (Thorling et al. 2015)
|Numbers of pesticide||>4 over the detection limit or >2 over the water quality limit|
|Maximum concentration||>1 µg/L for individual pesticide|
|Temporal variation in concentrations||>1 order of magnitude|
|Spatial variations in concentrations||Any difference in 100 m distance|
|Decay rate of pesticide concentration in water||>0.01 µg/L/yr|
This set of pesticides indicators and threshold-values may be applicable only for Denmark and for systems that are similar to Denmark (i. e. matrix-flow dominating the groundwater system). For the surface water system, different indicators and thresholds will be required. For instance, the most obvious signal of point source input to the surface water is high pesticide concentrations at low discharge (Holvoet et al., 2007).