Animal Waste, Water Quality and Human Health

Transport of livestock-derived pathogens within catchments 1996.2.2 Processes of microbial attenuationOnce excreted from the digestive tracts of animals, the numbers of pathogens andassociated FIOs tend to diminish as a result of natural die-off through acombination of aging, in an environment which is generally unfavourable formicrobial re-growth; exposure to adverse environmental conditions (UV-light,desiccation, fluctuating temperatures); and predation. Within catchments, theground surface and soils effectively act as a labile pollutant ‘store’ within whichpathogen strength diminishes through die-off, leading to a natural reduction inpathogen delivery to receiving waters. Opportunities for die-off are increased byprocesses that promote the retention of microbes on the ground surface or withinsoils, notably the filtering effect of vegetation on surface runoff throughentrapment and deposition (as a result of reduced flow velocities) of largermineral and organic particles to which microbes might be attached, and throughabsorption by soils as a result of infiltration. Sedimentation, particularly ofparticle-attached microbes, and die-off also occur in ponds and alongwatercourses, though in the latter case the bed sediments and surviving microbeswill tend to be remobilised when the bed is disturbed (Muirhead, Davies-Colleyet al. 2004, Jamieson, Joy et al. 2005, Wilkinson, Kay et al. 2006), for example,in response to increased water velocity following rainfall or physical disturbanceat fording points (Davies-Colley 2004). The various BMPs designed to attenuatemicrobial fluxes within catchments effectively replicate and enhance theseprocesses.6.3 LIMITATIONS OF THE EXISTING EVIDENCE BASEMost studies, including those of analogous natural sewage treatment systems(examples of which are included in the present database), have only monitoredFIOs, principally total coliforms (TC), faecal coliforms (FC), Escherichia coli(EC), faecal streptococci (FS) and enterococci (EN). These are generally mucheasier and less expensive to determine than specific pathogens (Vymazal 2005).The evidence base for livestock-derived pathogens such as Escherichia coliO157, Campylobacter (both bacterial) and Cryptosporidium spp. (protozoan)remains extremely limited. While broad underlying relationships might beanticipated between concentrations of FIOs and pathogens, there are fewcomparative data on the survival and transport of livestock-derived FIOs andpathogens (Pachepsky, Sadeghi et al. 2006). For example, Reinoso et al. (2008)report an absence of correlation between FIOs and pathogens in the context of aconstructed wetland (CW) – with different organisms being retained andeliminated at different rates. Also, while data on the effectiveness of natural