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Figure 3 shows that risk of illness is insignificant at all FS input rates below the
75 percentile flow. During 95 percentile flow, faecal loading needs to be reduced
by 95% to give a risk of illness < 20%. These figures depend on the reliability
of the extrapolation used in eq. (1) for risk of illness, and improvement of the
epidemiological database for such modelling should be a research priority. There
is also little information on the levels of FS in animals faeces, and literature data on
inactivation suggest longer survival of FS than E. coli in soil (Cools et al., 2001) but
higher sensitivity to sunlight (Sinton et al., 2002).
100%
75 th 80 th 85 th 90 th
95 th Percentile
80%
Risk of Illness (%) .
60%
40%
20%
0%
C urrent loading 25% reduction
50% reduction 75% reduction
80% reduction 85% reduction
90% reduction 95% reduction
0 1 2 3 4 5 6 7 8 9 10 11 12
Daily C atchment Discharge (mm/day)
Figure 3: Discharge and related risk of illness
The WTP for avoidance of illness of BW improvement can now be compared with the
costs of implementing BMPs to attempt to control diffuse pollution. Recent work has
estimated the annual cost of installing BMPs on two farms in SW Scotland to be £3 k
per farm plus £50 k capital cost (Merrilees, 2004). If we assume an annualised capital
cost of £10 k, and that across the River Irvine catchment there are about 70 farms,
total annualised mitigation costs are (3 + 10) x 70 = £910 k. From Table 4 the required
reduction of FS loading achieved needs to be > 95% to obtain a benefit/cost ratio
which exceeds 1. BMPs to assist in bathing water quality improvements include
measures such as stock reduction, fencing, buffer strips, constructed wetlands,
slurry storage, cattle access routes, and separating clean and dirty water. In practice
achieving > 50% efficacy may be very difficult, particularly considering that BMP
performance during high intensity flow events is critical. Dickson et al. (2005) found a
40% reduction in high flow faecal indicator loadings where BMPs had been installed.
If the entire Irvine catchment installed these practices, bathing waters may still pose
a high risk of illness after storm events.
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