Animal Waste, Water Quality and Human Health

242Animal Waste, Water Quality and Human Healthsurvive temporarily in the moist and shady environment of a VBS and beremobilised in subsequent rainfall events. The FIO attenuation figures aresimilar to the much more extensive data available on the retention of sediments.In a review of over 40 studies covering 79 sites, Liu, Zhang et al. (2008) reportmedian log 10 trapping efficiencies for both RBSs and VFSs (on slopes) of 0.87,with lower/upper quartiles of 0.63/1.15 and 0.62/1.52, respectively, and theresults of regression analyses that reveal a log 10 sediment reduction of 1.32 forthe most effective combination of buffer strip width (10 m) and gradient (5.14°).Inevitably, the efficiency of a VBS will vary according to local, site-specificfactors such as slope angle (both upslope and within the VBS), the sizedistribution of transported particles, and soil and vegetation characteristics withinthe VBS. The key VBS design variable is strip width (i.e. length of flow). Datarelating to FIOs are very limited and while several studies have reported increasesin attenuation with wider strips (e.g. Young, Huntrods et al. 1980, Collins, Rosset al. 2002, Goel, Rudra et al. 2004, Liu, Zhang et al. 2008, Hay 2006, Tate,Atwill et al. 2006), others have found no clear relationship with width (e.g. Entry,Hubbard et al. 2000, Núñez-Delgado, López-Periago et al. 2002). The underlyingimportance of buffer width is, however, clearly confirmed by the statisticallysignificant correlation (p < 0.001) between sediment removal and log n bufferwidth reported in an analysis of published data from 79 sites (Liu, Zhang et al.2008). The latter study also revealed that VBS efficiency peaks at a slope angle of5.26°. Below this optimal angle, diminishing lateral flow rates limit sedimenttrapping, whilst at steeper gradients sedimentation is more likely to becompromised by increased channelization of flow and shorter retention times.Attenuation rates for an individual VBS will also vary temporally, depending onfactors such as antecedent weather conditions and the magnitude of runoff events.Several studies have reported reduced attenuation of FIOs and/or pathogens attimes of greater runoff/higher rainfall intensity (e.g. Collins 2004, Trask, Kalitaet al. 2004, Tate, Atwill et al. 2006).Because of the inadequacy of the FIO/pathogen database, the few models thathave been developed for microbial attenuation within VBSs rely on the adaptationof models developed for other parameters such sediments, nutrients and pesticides.While laboratory experiments have demonstrated that rates of nutrient and FIOattenuation are quite closely correlated (e.g. Stout 2005), erosion-based sedimentmodels are more commonly used. Examples include VFSMOD developed byMuñoz-Carpena, Parsons et al. (1999) to model runoff and sediment transportwithin VFSs, and the Universal Soil Loss Equation (USLE), which is used bythe US Natural Resources Conservation Service to set standards for bufferwidth:catchment area ratios for different R factors (rainfall amounts andintensities).