Conducting Subsurface Environmental Investigations in Urban Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 133

Mn, Ni, Pb, and Zn, asbestos, nitrate and nitrite, or organic chemicals. The toxicities of
organic chemicals are difficult to fully assess. We need to take into account not only the
exposure level, but also the duration of exposure (Yong 2002).
Exposure pathways fall into three broad categories: inhalation, dermal adsorption, and
ingestion (USEPA 1989). Any chemical can affect the human body, with the outcome
depending on several factors:
• The nature and toxicity of the chemical itself
• The concentration of the chemical
• The distribution of the chemical at a particular site
• The chemical's route of entry into the human body (inhalation, dermal adsorption,
or ingestion)
• The length of the exposure
• Individual human susceptibility
Receptors implies biotic receptors, but can also pertain to the physical land environment,
including surface water and groundwater (see Section 4.2.8, Natural Resource Damage
Assessment). Consequently, the nature and extent of the threat of a particular pollutant
will not only depend on the nature and distribution of the pollutant but also the target
that is threatened.
Risk assessment is accomplished using a four step process:
1. Hazard identification—answers this question: Does exposure to a chemical or
agent cause an increase in the incidence of an adverse health effect (e.g., cancer or
birth defects)?
2. Dose-response assessment—quantitative characterization of the relationship
between the dose of a chemical or agent and the incidence of an adverse health
effect.
3. Exposure assessment—evaluation of the intensity, frequency, duration, and routes
of exposure to the chemical or agent.
4. Risk characterization—estimation of the potential incidence of a health effect, cal-
culated by obtaining information from the dose-response assessment, along with
information from the exposure assessment.
The outcome of a risk assessment usually produces a hazard quotient, which is the result
of evaluating each chemical of concern for potential carcinogenic risk and chronic health
risk. USEPA (2002) currently has an acceptable carcinogenic risk of 1 in a 100,000 (1 × 10 −5 )
cancer incidence, with values under 1 being an acceptable noncarcinogenic chronic health
risk. However, many risk assessments evaluate cumulative risk. For instance, there may
be 10 different chemicals present, and one chemical alone may not result in an unaccept-
able risk, but exposure to more than one may result in an unacceptable exposure. In most
circumstances, this is an acceptable methodology, especially if contaminants overlap.
USEPA and most states have published guidelines for conducting ecological and human
risk assessments, and these address whether a cumulative risk evaluation is warranted.
A review of federal, local, and state requirements is recommended before conducting an
ecological or human health risk assessment.