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Each
threshold is based on the prescribed standards set
in
the California Department of Health Service's Beach
Bathing Water Standards. The magnitude of the water
quality threshold exceedance and laboratory variability
was addressed by the inclusion of standard deviations
in setting the thresholds. The standard deviations
used were developed during the 1998 laboratory intercalibration
study led by the Southern California Coastal Water
Research Project and the Orange County Sanitation
Districts
that involved over 20 shoreline water quality monitoring
agencies in Southern California.
The
number of points subtracted from 100 for total coliform,
fecal coliform, and enterococcus are: 6 points for bacterial
densities falling in group one (threshold minus one
standard deviation or T - 1 s.d.), 18 points for group
two (T + 1 s.d.), and 24 points for group three (indicator
densities > T + 1 s.d.). The point system for total
to fecal ratio is: 7 points for group one, 21 points
for group two, 35 points for group three, and 42 points
for group four (very high health risk). Exceedance of
the total to fecal ratio threshold leads to lower grades
because exposure to water with low ratios causes an
even higher incidence of a variety of adverse health
effects relative to the health risk associated with
the other bacterial indicators.
These
points are added to obtain a subtotal for that week.
The point subtotal for the most current week's worth
of data is multiplied by 1.5 in order to give it more
weight. Then the points from the previous three weeks
are added in for an overall point total. The total number
of points for the 28-day period is divided by the average
number of samples collected in a week. This number is
then subtracted from the original 100 points to obtain
a grand total from which a letter grade is derived.
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