Animal Waste, Water Quality and Human Health

244Animal Waste, Water Quality and Human Healththat lack a well-developed structure will be effective in filtering microbes (e.g.certain allophonic and volcanic soils (McLeod, Aislabie et al. 2001).6.6 BMPs TO ATTENUATE PATHOGENCONCENTRATIONS WITHIN WATERCOURSESOnce pathogens/FIOs enter a watercourse there will some degree of naturalattenuation as a result of sedimentation of particle-attached microbes, die-offthrough exposure to sunlight and predation. Two BMPs are currently employedto increase the effectiveness of these attenuation processes:Grassed waterways (“swales”). Grassed waterways function very much asVBSs (Liu, Zhang et al. 2008). They may be either natural features on slopes,along which surface runoff may occur following rainfall (i.e. a naturalextension of the drainage network), or constructed features, such as thoseconnecting different components of a water treatment system. Contaminatedrunoff from cattle tracks might also be diverted through grassed waterwaysbefore entering a permanent watercourse. In their review of published data Liu,Zhang et al. (2008) report a median log 10 sediment attenuation rate of 1.15 forthree grassed waterway sites.In-stream ponds (see: Table 6.2, Section F). Large bodies of water are veryeffective in the attenuation of FIOs. Crowther et al. (2010), for example,report high-flow GM FC and EN concentrations of 8.3 × 10 1 and 1.6 × 10 1 cfu100 ml −1 , respectively, in effluent waters from five upland lakes and reservoirsin England and Wales, compared with GM inputs from areas dominated byrough grazing of 8.6 × 10 3 and 1.2 × 10 3 cfu 100 ml −1 , respectively (Kay,Crowther et al. 2008) – that is, attenuation rates of c. 2.00 log 10 . For smallerponds on streams, rates of attenuation are likely to be generally lower, andquite variable, depending on local factors such as the ratio of flow/pondvolume, retention time and water depth, and again there are relatively fewpublished data. Vinten, Sym et al. (2008), for example, report a 1.05 log 10attenuation in GM FC concentration during rainfall events through a small, 0.6ha, in-stream pond (a disused reservoir) in a dairy farming-dominatedcatchment in Scotland. Attenuation of FS was much less (0.20 log 10 ), which islikely attributable to inputs from nesting/roosting birds. Somewhat higher ratesof attenuation have been reported where river water from a semi-urbanisedarable catchment in SE England is temporarily impounded behind a tidalsluice gate. In the case of GM EC, for example, the log 10 attenuation ratesreported are 1.79 during dry weather conditions and 1.60 under high-flowconditions following rainfall.