Occurrence and behavior of pesticides in rainwater, roof runoff, and artificial stormwater infiltration

To prevent overloading of sewer systems and to ensure sufficient recharging of the groundwater underneath sealed urban areas, collection and artificial infiltration of roof runoff water has become very popular in many countries including Switzerland. However, there is still a considerable lack of knowledge concerning the quality of roof runoff, particularly with respect to the presence of pesticides. In this work, the occurrence and the temporal variations in concentration in rainwater and in roof runoff from different types of roofs (i.e., clay tile roofs, polyester roofs, flat gravel roofs) were determined for the most important members of three widely used classes of pesticides (i.e., triazines, acetamides, phenoxy acids). It is shown that in rain and roof runoff, maximum pesticide concentrations originating primarily from agricultural use occurred during and right after the application periods. Maximum average concentrations for single rain events and total loads per year were, for example, for atrazine, 903 ng/L and 13 900 ng/(m(2) year); for alachlor, 191 ng/L and 5900 ng/(m(2) year); and for R-dichlorprop, 106 ng/L and 5100 ng/(m(2) year). Further, the study reveals that a major portion of the compounds washed out from the atmosphere may actually reach the groundwater, particularly if the roof runoff is infiltrated directly into highly permeable zones of the subsurface. Nevertheless, although in some cases European Union and Swiss drinking water standards (100 ng/L) were not always met in rain and roof runoff waters, for the three compound classes investigated, the groundwater contamination potential of the pesticides originating from the atmosphere can be considered of equal or even smaller importance as compared to their direct use in agriculture. The investigations also show that leaching of pesticides used as construction chemicals on roofs, that is, as roof protection agents in sealings used on Rat gravel roofs, may be a much more significant source of organic pollutants present in roof runoff (see also our following paper in this issue).