Animal Waste, Water Quality and Human Health

Transport of livestock-derived pathogens within catchments 243In a recent study the Chemicals, Runoff and Erosion from AgriculturalManagement Systems (CREAMS) model (Cooper, Smith et al. 1992) forsediment attenuation has been adapted for FIOs, and applied to the major soilgroups of New Zealand (MAF) 2006. In this study particular attention wasgiven to the influence of particle size, with free-floating bacteria being assumedto behave as clay particles, and attached microbes as larger particles (i.e. siltsand sands). Although a lack of understanding of the form in which microbes aretransported in surface runoff limits the veracity with which the model might beapplied in specific circumstances, the model outcomes highlight some of the keyfactors affecting attenuation rates. The results generated optimum RBS widths(taking into account the amount of land given over to the buffer) and associatedefficiencies for the following combinations of site characteristics: slope, soildrainage characteristics and degree of bacterial attachment. Overall, themodelled rates of FIO attenuation range from 0.10–1.30 log 10 , with wider bufferstrips needed on steeper slopes, where soil infiltration rates are lower, and/orwhere a low proportion of bacteria are attached to particles. The modelpresented does not take into account very small microbes (e.g. viruses oftypically 25–350 nm) for which entrapment efficiencies are likely to be lowerthan for those microbes modelled, nor of the possibility of the survival andre-mobilisation of microbes during subsequent runoff events. The model isbased long-term average rainfall conditions (efficiencies are likely to be lowerduring events of higher intensity). Despite these limitations, the model providesa good basis for developing design guidelines.Since the function of the VBS is to filter out pollutants from surface runoff,the measure is likely to be relatively ineffective in two circumstances (MAF)2006. First, on steeper slopes there is likely to be greater convergence ofsurface (and subsurface) flow, resulting in channelized flow with greatervelocity (cf. diffuse sheet flow), much of which will pass rapidly through andhave little contact with the VBS (Collins 2005). Where channelization doesnot occur, Atwill, Hou et al. (2002) found in soil-box experiments that forVBSs with slopes of 3–11° attenuation may increase with gradient as a resultof the reduced depth of flow. Secondly, where the soils have a high infiltrationcapacity, then some flow will again bypass the VBS. Where vertical flow israpid, for example in well-structured clay soils, quite high proportions ofmicrobes are likely to be carried down through the soil and, ultimately, intoadjacent watercourses. For example, Collins (2004) reports considerablevolumes of subsurface flow bypassing RBS study plots on Hamilton clay loamsoils in New Zealand, and Núñez-Delgado, López-Periago et al. (2002) foundquite high concentrations of FIOs, especially FS (>1.1 × 10 3 MPN 100 ml −1 ),in soil waters at depths of 31–51 cm within a VBS. In contrast, porous soils