Laboratory-based experiments to determine the impacts of applying glyphosate-containing herbicide onto a model porous paving rig

Recent legislation has promoted the use of Sustainable Drainage Systems (SUDS) in individual houses (Future Water, October 2008) and new build housing estates (Floods and Water Management Act). These pieces of legislation will integrate SUDS devices such as Porous Paving Systems (PPS) into housing estates where their management will fall to the individual householder. There are currently no guidelines for the application of herbicides to remove weeds growing on the PPS surface. Whilst it is well known that herbicides break down fairly readily in the environment, particularly when in contact with soil, it is not known how the application of herbicide will affect the ability of the PPS to break down hydrocarbons or trap particulate-associated pollutants. Research carried out at Coventry University (e.g. Coupe et al., 2003) showed that biofilms are central to this water quality role and that they are adversely affected by herbicide addition. This paper reports on small-scale laboratory-based experiments, where used engine oil was added to the surface of PPS rigs containing geotextile on which a mature biofilm had been allowed to develop. Glyphosate-containing herbicide (GCH) was then applied, followed by simulated rainfall, causing three related impacts which allowed pollutants such as hydrocarbon and heavy metals to be released in the PPS effluent. Firstly, the herbicide reduced the micro-ecology of the biofilm virtually to zero immediately post-application of the GCH, thus completely compromising its ability to deal with any pollutants. Secondly, because of the removal of the biofilm, the geotextile’s role in acting as a hydrobrake enabled the pollutants carried in the water percolating through the structure to be pushed through more effectively. Lastly, the surfactant passed through the test rigs, appearing as a foam in the sample bottles. It is believed that it mobilised pollutants already present in the rig structure, allowing them to pass out of the rig in the effluent.