TABLE 4.

Assumptions used in risk calculations

ParameterSymbolUnitDistributionNote(s)
Concn
    Fecal Streptococcus
        IECCsMPN/100 mlNormal (mean = 2,988, SD = 785)Distribution of mean determined using central limit theorem because n > 30
        SaybrookCFU/100 mlDistribution of mean could not be determined because n = 2
    Enterococcus
        IECCeMPN/100 mlNormal (mean = 2,762, SD = 784)Distribution of mean determined using central limit theorem because n > 30
        SaybrookCFU/100 mlDistribution of mean could not be determined because n = 2
    Giardia
        Saybrook, CSOCgCysts/mlDistribution of mean could not be determined because n = 1
        Saybrook, downstream of CSODistribution of mean could not be determined because n = 1
GiardiargNo unitEmpirical (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)Rose et al. (26); calculated based on likelihood confidence intervals
Water ingestionIRhourlyml/h
    Recreator50 ml/h; incidental ingestion rate during swimming
    Visitor10 ml/h; adults/adolescents—surface water during wading
    Homeless person50 ml/h; incidental ingestion rate during swimming
Time spent outdoors at riverTMinutes/dayEmpirical (5th = 20; 25th = 60; 50th = 150; 75th = 320; 90th = 511; 95th = 615)USEPA, 1997, Table 15-110 (31)
Fraction of time at river spent in waterF%
    RecreatorAssumed 20%
    VisitorAssumed 20%
    Homeless personAssumed 40%
Incidental water ingestion rate implicit in AWQCa dose-response relationship, swimmingIRAWQCml/dayTriangular (minimum = 10; mode = 16; maximum = 100)Ingestion rate used to account for uncertainty in incidental ingestion of water inherent in AWQC dose-response equation for incidental ingestion of water during swimming (Dufour et al., 2006 [4])
Incidental water ingestion rateIRdailyml/dayIRdaily = IRhourly × F × (T/60)
    RecreatorCalculated
    VisitorCalculated
    Homeless personCalculated
Exposure frequencyEFDays/yrTriangularMaximum no. of days limited by
    RecreatorMinimum = 1, mode = 12, maximum = 95    meteorological factors such as
    VisitorMinimum = 1, mode = 2, maximum = 12    precipitation and temp
    Homeless personMinimum = 1, mode = 24, maximum = 150
  • a AWQC, ambient water quality criteria.