Water Quality Management - River Dynamics and Integrated River Management - page 602

have advocated microbial human health risk assessment to determine the true threats to human health for
cases when the indicator-based standards are difficult to achieve, such as in the case of surface runoff
with E. coli levels greater than 10,000 CFU/100 mL but with no human sources documented by Bower et
al. (2005). If the bacterial contamination does not pose a substantial risk even if the applicable standard is
not met, is it worthwhile to apply extraordinary means to meet the standard if the change in risk is small?
For example, human health risk assessment was recommended to assess the true extent of pathogen pollution
before extraordinary means would be applied to reduce fecal coliform contamination in the recent
Southeastern Wisconsin Water Quality Management Plan Update (SEWRPC, 2007).
Risk assessment methods for human health were first proposed in 1989 by the USEPA (1989) for
guidance assessment for the Superfund program. Microbial risk assessment methods have been reviewed
by the U.S. National Research Council (1994) and the Water Environment Research Foundation (2004),
summarized in a book by Haas et al. (1999), and incorporated into the World Health Organization (WHO)
Guidelines for Safe Recreational Waters (WHO, 2003). Further, many researchers and agencies are calling
for microbial risk assessment before extraordinary measures are taken to meet indicator bacteria standards
(Pitt, 2004; SEWRPC, 2007).
The steps in a microbial human-health risk assessment are as follows (National Research Council,
1994; USEPA, 1989):
(1) Hazard identification: Description of human health effects of the particular hazard
(2) Exposure assessment: Determination of the relevant pathways and nature of the exposed population
along with quantitative estimates of the level of exposure.
(3) Dose-response assessment: Characterization of the relation between administered dose and incidence
of health effects.
(4) Risk characterization: Integration of the information from the previous steps in order to estimate
the magnitude of risks and to evaluate the variability and uncertainty.
Hazard identification —Recreational use of a water body may expose individuals through incidental
ingestion, dermal, and inhalation pathways to disease-causing bacteria, viruses, and protozoa within the
waters. For recreation, the most common illness is gastrointestinal upset (nausea, vomiting, and diarrhea),
usually of moderate intensity and short duration. These illnesses were the focus of the epidemiological
studies used to develop the USEPA 1986 Ambient Water Quality Criteria for Bacteria and they have been
hazards considered in previous microbial human-health risk assessments (e.g., Geosyntec, 2008).
Exposure assessment —This assessment evaluates the duration, frequency, and magnitude of pathogen
exposure by one or more pathways. The exposure parameters used in these assessments include incidental
ingestion rates and exposure duration. These parameters typically are defined by probability distributions
so that a probabilistic risk assessment is done. These parameters need to be evaluated for each exposure
pathway. The pathways through which recreators are exposed to pathogens are incidental ingestion,
inhalation, and eye or dermal contact. These pathways need to be evaluated for each recreational use of
the water body. Recreational uses may include—Primary Contact: swimming, wading, jet skiing, and
kayaking; and secondary contact: fishing, canoeing, and pleasure boating. For example, Fig. 9.22 shows
the incidental ingestion rate and Fig. 9.23 shows the exposure duration distribution for canoeists on the
Chicago Waterway System (Geosyntec, 2008).
The exposure assessment also needs to consider Secondary Attack Rates, which are the rates at which
people who are infected through contact with water then infect other people. That is, the number of family
members and possibly others that may be potentially exposed to illness by a person infected while recreating
on the water body of interest must be determined and considered in the assessment.
Dose-response assessment —The dose-response is the mathematical relationship between the dose of
a pathogenic organism and the probability of infection or illness in exposed persons. Biologically plausible