TABLE 4.
Assumptions used in risk calculations
| Parameter | Symbol | Unit | Distribution | Note(s) |
|---|---|---|---|---|
| Concn | ||||
| Fecal Streptococcus | ||||
| IEC | Cs | MPN/100 ml | Normal (mean = 2,988, SD = 785) | Distribution of mean determined using central limit theorem because n > 30 |
| Saybrook | CFU/100 ml | Distribution of mean could not be determined because n = 2 | ||
| Enterococcus | ||||
| IEC | Ce | MPN/100 ml | Normal (mean = 2,762, SD = 784) | Distribution of mean determined using central limit theorem because n > 30 |
| Saybrook | CFU/100 ml | Distribution of mean could not be determined because n = 2 | ||
| Giardia | ||||
| Saybrook, CSO | Cg | Cysts/ml | Distribution of mean could not be determined because n = 1 | |
| Saybrook, downstream of CSO | Distribution of mean could not be determined because n = 1 | |||
| Giardia | rg | No unit | Empirical (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 ingestion | IRhourly | ml/h | ||
| Recreator | 50 ml/h; incidental ingestion rate during swimming | |||
| Visitor | 10 ml/h; adults/adolescents—surface water during wading | |||
| Homeless person | 50 ml/h; incidental ingestion rate during swimming | |||
| Time spent outdoors at river | T | Minutes/day | Empirical (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 water | F | % | ||
| Recreator | Assumed 20% | |||
| Visitor | Assumed 20% | |||
| Homeless person | Assumed 40% | |||
| Incidental water ingestion rate implicit in AWQCa dose-response relationship, swimming | IRAWQC | ml/day | Triangular (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 rate | IRdaily | ml/day | IRdaily = IRhourly × F × (T/60) | |
| Recreator | Calculated | |||
| Visitor | Calculated | |||
| Homeless person | Calculated | |||
| Exposure frequency | EF | Days/yr | Triangular | Maximum no. of days limited by |
| Recreator | Minimum = 1, mode = 12, maximum = 95 | meteorological factors such as | ||
| Visitor | Minimum = 1, mode = 2, maximum = 12 | precipitation and temp | ||
| Homeless person | Minimum = 1, mode = 24, maximum = 150 |
a
AWQC, ambient water quality criteria.