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Predicting Bathing Water Quality
Vinten et al. (2004) devised a spreadsheet model to predict E. coli in bathing water
by accounting for inactivation, sedimentation, transport and mixing in the Irvine
Beach/River Irvine catchment system for various loading in the catchment. We have
modified this model so FS levels in bathing waters could be predicted rather than
E. coli, using changes to key parameter values, summarised in Table 1. The relative
levels in animal faeces and inactivation factors show considerable variation in the
literature, so these figures should only be considered indicative at this stage.
Table 1:
Correction factors used with the model of Vinten et al. (2004) to
predict FS concentrations in bathing water instead of E. coli
Parameter
Correction factor for
FS/E. coli
References
Levels in animal faeces 5.6 Sinton et al., 1993 Reddy
et al., 1981
Half lives in soil and water 0.7 Sherer et al., 1992 in
Merrilees, 2004
Bathing water quality predictions were made for current and 25–95% reduction in
livestock loadings. Summer discharge percentiles were calculated using 14 years
(1989-2002) of River Irvine data.
Bathing Water Quality and Risk of Illness
A dose–response function between the concentration of FS in bathing water and the
probability of contracting gastro-enteritis was determined by Kay et al. (1994) and
Wyer et al. (1999), based on risk of contracting illness from bathing water quality
determined by the World Health Organization using a probability density function
of bathing water quality throughout Europe (WHO, 2001). This relationship has data
for up to 12% probability of infection, and these data were fitted to an exponential
function (eq. 1) to predict risk of illness, including extrapolation where necessary
beyond the data range.
-
Risk of illness = 1-
e 0. 000218c
(1)
Where c = concentration of FS in bathing water (cfu/100mL).
This equation was linked to model simulations of FS in bathing water to give the risk
of contracting gastro-enteritis as a function of faecal loading and river discharge
(Step 2 in Figure 1).
Economic Valuation of Reduced Risk of Illness
Consumer willingness to pay (WTP) for a reduction in risk of illness resulting from
swimming in contaminated waters was determined using a benefit transfer of a
contingent valuation used in a previous study in England and Wales (EFTEC, 2002).
The EFTEC study determined consumers’ WTP for a reduction in risk of illness from
swimming in contaminated bathing water. Valuations from the study indicate that
respondents are willing to pay between £1.10 and £2.00 per household per year
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