Animal Waste, Water Quality and Human Health

Comparative risk analysis 375some uses in food safety risk assessment, in this Chapter we seek to translate thatmodel into a format appropriate for use in microbial water quality risk assessmentand present it for evaluation.It is not intended nor inferred that the model presented can provide accurateestimates of recreational waterborne risks under all circumstances and scenarios.Rather, it can provide quick screening of relative risk, and the effects of multiplefactors in combination on overall risk. It is also intended to illustrate anapproach that could make risk assessments for recreational waters moreaccessible and also has great utility in teaching the principles and philosophy ofquantitative risk assessment.However, the model clearly has limitations. For example, while the model issuperficially simple to use, it relies on a relatively high level of knowledge of thewatershed being considered to be able to answer the questions appropriately. Ifanswers to the questions posed are inappropriate, the relative risk estimates fromthe model will be unreliable (in other words: “garbage in – garbage out”). Whilethe logic inherent in the model is essentially correct, the weightings used forresponses to the answers may not be appropriate in all situations and this couldlead to unrealistic or illogical conclusions in some circumstances. Furthermore,the model only considers one source of faecal contamination at a time when, inmany circumstances, there will be multiple sources of contamination. Nonetheless,the model could be used to estimate which source represents the greatest risk byassessing each source separately, or assessing the combined risk from multiplesources.Users should be aware of the uses and limitations of the model. Such limitationsand caveats were discussed in detail by Ross and Sumner (2002) in relation tothe food safety risk assessment model and most apply equally to the modelpresented here.10.4.2 Model structure and interfaceEvaluation of the health risk from a water source requires knowledge of thestrength (“load”) of the hazard, and an understanding of the modification ofpathogen numbers together with the characteristics of the “transport” (Goss &Richards, 2008) and routes of exposure. The structure of the decision toolcorresponds to that generally accepted paradigm (i.e. load, transport, exposure)and can be considered as three banks of questions corresponding to those threeaspects of risk.The model attempts to consider the collective contribution of many factors tothe overall risk to the public exposed to bodies of water, whether due to