TABLE 3.

Assumptions used in risk calculations

ParameterSymbolUnitDistributionReference
Fraction of organisms ingested that initiate infection (r value)
    GiardiargUnitlessEmpirical (2.5th = 0.009798; 12.5th = 0.01336; 25th = 0.01584; 50th = 0.0198; 75th = 0.02466; 87.5th = 0.02845; 97.5th = 0.03582)44; calculated based on likelihood confidence intervals
    CryptosporidiumrcUnitlessEmpirical (5th = 0.0074; 25th = 0.0227; 50th = 0.0539; 75th = 0.1197; 95th = 0.3044)62
Incidental sediment ingestion rateIRsedmg/dayTriangularSelected based on a
    RecreatorMinimum = 25, mode = 100, maximum = 200    review of sediment
    VisitorMinimum = 25, mode = 50, maximum = 95    ingestion rates
    Homeless personMinimum = 25, mode = 200, maximum = 350    provided in regulatory guidance and recent health assessments
Exposure frequencyEFDays/yrTriangularSelected based on a
    RecreatorMinimum = 1, mode = 12, maximum = 95    review of sediment
    VisitorMinimum = 1, mode = 2, maximum = 12    ingestion rates
    Homeless personMinimum = 1, mode = 24, maximum = 150    provided in regulatory guidance and recent health assessments. Maximum number of days limited by meteorological factors such as precipitation and temperature
Incidental water ingestion rate, swimming (for bacterial calculations)IRswimml/dayTriangular (minimum = 10, mode = 16, maximum = 100)20