A Risk Assessment is a decision-making tool used to evaluate the likelihood of an unwanted event. An unwanted event could be someone getting cancer because of exposure to contaminants or it could be great numbers of fish dying because of pollution being dumped into a stream. Specially trained individuals called risk assessors evaluate how threatening a hazardous waste is to human health and the environment. In order to evaluate the potential for exposure to hazards and determine the likelihood for adverse effects on humans or the environment, risk assessors consider four questions:
1. Hazard Identification – What contaminants exist at the site?
2. Exposure – How are people exposed to them?
3. Toxicity Assessment – How dangerous could the contaminants be to human health?
4. Exposure and Toxicity Assessment – What contaminant concentrations are safe?
Reliable risk-assessment methods provide the information necessary to discriminate between important and trivial threats. They also help people set priorities and allocate resources responsibly. The selection of site-remediation alternatives involves the interaction of regulators with the community. A key challenge at the end of a risk assessment is to present the risk in a way that is meaningful and clear to the public.
The risks of getting cancer from exposure to site waste are expressed in probabilities. According to U.S. Environmental Protection Agency (EPA) guidelines, a person shouldn't be exposed to radiation or a cancer-causing chemical if exposure increases that person's lifetime cancer risk by more than one chance in 1,000,000 (or by 0.000001). Therefore, cleanup actions are selected primarily on how well they protect human health and the environment and on how well they meet safety requirements. In comparison, almost one in three people will get cancer from other causes.
A Hazard Index (HI) is used to estimate potential toxic risk from chemicals that don't cause cancer. The HI takes into account likely exposure and the toxicity of different chemicals and elements. If the HI at a site is 1.0 or more, this means that a person's health could be affected.
Scientists evaluate the effects of pollutants on an environment and its inhabitants by examining the exposure pathways. A pathway is a route by which hazardous materials may come into contact with people.
Although external contact with radiation can cause damage by irradiating the body from the outside, the primary hazard FUSRAP material poses is from material taken internally. Pollutants may reach people directly if they inhale or ingest contaminated air, water or soil. Exposure is also possible via secondary pathways like a food chain. As a simplified example, dust released from a production stack settles onto a field and is mixed into the soil. Plants growing there absorb the pollutant through their roots and into the edible portions of the plant. People who eat the plants (or who eat the animals that ate the plants) might then be exposed to the pollutant.
The actual route that pollutants take can be very complex, and the quantity of material that may eventually reach people can be very small. Not all materials released settle immediately; some will be washed out by rain and enter surface water or groundwater. Of the portion that does fall, not all will fall on the field, and of what does fall on the field, not all will be absorbed into plants. This process of dilution and separation alters what reaches the plant and the fruit of the plant until it is a small fraction of the initial release.
Certain plants, animals and soils are more likely to concentrate specific pollutants and are, therefore, important points in pathways to be sampled. However, pathways frequently overlap, and it is difficult to trace pollutants precisely. Environmental sampling and analysis are performed to detect the presence and concentration of pollutants.
Assessing the St. Louis Sites
The St. Louis Sites were used to process and/or store uranium for the nation's nuclear defense program. Byproduct materials contained radioactive radium, thorium and uranium, as well as processing chemicals. A Baseline Risk Assessment, which evaluated the Missouri sites in 1993, describes the potential risks to human health and the environment. The study found that if nothing were done to clean up the site and the future land use became residential, the public could be exposed to unacceptable cancer and toxic risks.
This chart compares the carcinogenic risk at St. Louis Sites to the leading mortality risks in the United States.
Carcinogenic risks from chemicals at the St. Louis Sites were within the acceptable EPA range. However, the sites could pose carcinogenic risks from chemicals to people who live directly on the properties for long periods of time. Without cleanup, carcinogenic chemicals and elements could potentially affect human health and the environment.
Although both radioactive and nonradioactive materials can reach people through the same pathways, the pathway scenarios studied at the St. Louis Sites focus on radioactive materials as the primary hazard to human health. Uranium and its daughter products, thorium and radium, are the primary contaminants of concern. The two major pathways under study at the St. Louis Sites are air and liquid pathways.
The air pathway at the St. Louis Sites includes airborne contamination from storage sites and buildings. Dust from construction and remediation activities, waste handling and wind erosion are also important potential sources. The form and chemical makeup of the contaminants influence how they are dispersed into the environment. For example, fine particles and gases may be breathed in, while larger, heavier particles tend to settle rapidly. Chemical properties determine whether the pollutant will dissolve in water, be absorbed by plants and animals, or settle in sediments and soils. Airborne pollutants are subject to weather conditions. Wind speed and direction as well as rainfall and temperature are important factors in predicting how pollutants are distributed in the environment.
Protective coverings are in place at SLAPS and HISS to limit the air-exposure pathways and reduce health risks.
The liquid pathway examined at the St. Louis Sites includes all releases that could carry waterborne pollutants, including radioactive materials. The first step in monitoring this pathway is to sample the effluent streams as they leave all contaminated sites. Types and concentrations of pollutants in these streams provide the first estimate of the potential dose that could be delivered via the liquid pathway. Some pollutants in the liquid effluent may be carried along as suspended solids, which eventually settle out as sediment in the streambed. Other pollutants dissolve in water and may be absorbed by plants and ingested by animals. Fish sampling can show how pollutants are absorbed by aquatic animals and can predict how much radioactive material could reach people if they ate the affected fish. Fish are often used as biological indicators, as their bodies concentrate certain pollutants, resulting in biological effects. Fish sampling helps to develop an evaluation of long-term contamination.
Groundwater is also an important component of the liquid pathway because it is the source of water for many homes and farms in the St. Louis area. Extensive well sampling on the St. Louis Sites and in the surrounding area provides information about the status of area groundwater. By sampling groundwater in many locations and at many different depths, scientists can determine the extent of contamination.
To help limit the potential for radioactive materials to move off-site by the liquid pathway, run-off water at the St. Louis Airport Site has been diverted to a Sedimentation Basin. Protective coverings at the Hazelwood Interim Storage Site minimize erosion by wind and water.
The effects of exposure are unpredictable. If a person comes in direct contact with a contaminant by touching, eating, breathing or drinking a pollutant-bearing substance, he is exposed. Variable factors, however, affect the severity of the adverse health effect. Primarily, these variable factors include the concentration of the contaminant, the frequency and duration of exposure, the number of contaminants and the pathways of exposure. An individual's own characteristics (age, sex, nutritional/health status, lifestyle, etc.) also influence the results of exposure to a contaminant.
Exposure pathways at the St. Louis Sites have specific standards and guidelines set by federal and state regulators that define the allowable dose limits. Health guidelines provide a way for public-health administrators to compare estimated exposures with concentrations of contaminants in the soil, air, water and food that people contact. The relative risk for cancer and other illnesses is determined, and guidelines and limits are set according to established legislation.