Animal Waste, Water Quality and Human Health

376Animal Waste, Water Quality and Human Healthrecreation, domestic needs (e.g., clothes washing in developing nations; foodgathering) or employment (e.g. irrigation farmers, fishermen).The model is implemented in spreadsheet software and uses an approach similarto the microbial food safety risk model of Ross & Sumner (2002). The benefits ofthe use of spreadsheet software are that it:• Allows automation of the calculations required to estimate the risk,facilitating a quick exploration of the effect of different assumptions by theuser;• Is widely available and used, that is, users do not need special training nor tohave access to specialized software.Users are presented with a series of sixteen questions and asked to select from a listof possible answers to those questions. Figure 10.2 shows the ‘user interface’ of thetool. Question 1 relates to the animals that are the source of the contamination toestimate the severity of the microbial hazard. Questions 2 to 5 are used toestimate the pathogen “load”. Questions 6 to 11 relate to mobilisation andtransport of the pathogens to the recreational water body being assessed.Questions 12 to 16 relate to exposure to the water body being assessed.The model requires users to provide answers based on ‘average’ situations, notextreme or unusual circumstances, for the purposes of estimating relative risk.However, users could potentially use the model to estimate the relative riskincrease, or decrease, due to unusual circumstances that may be of interest orrelevance for water safety management. 6In its current form the model is limited to consideration of faecal contaminationof recreational water by farmed cattle, sheep, pigs or poultry.10.4.3 Model logicThe overall principal of the tool is that the answers that are chosen by the user foreach of the qualitative Questions 1 to 16 are assigned numeric values. The numericvalues are assumptions about the relative risk contribution of the alternativeanswers offered for each question. Those values can then be used in calculationsto generate estimates of relative risk.6Note, however, that in the model if only a single value is changed, the predicted change in risk will innearly all cases simply reflect the difference in “weight” applied to the subjective answers offered tothe user. The weights are a very simplified measure of relative risk contribution from each factor andchanging the answer to one question will not usually generate a reliable estimate of the increasedrelative risk, because the weights used are, for most questions, arbitrary. The benefit of the modelis to assess the influence on relative risk of simultaneous changes in multiple risk-affecting factors.