4.3 Risk assessment of contaminated site
           Risk assessment or hazard assessment is required to decide the extent of contaminant remediation required for a particular site. The factors influencing risk assessment are:
Toxicity
            A material is deemed toxic when it produces detrimental effects on biological tissues or associated process when organisms are exposed to concentration above some prescribed level. Acute toxicity is the effect that occurs immediately after exposure where as chronic toxicity deals with long term effects. It is expressed as mass unit of toxicant dose per unit mass of receiving organism. It must be noted that concentration is an important factor while deciding toxicity. Only when a contaminant crosses a particular concentration, it becomes toxic. If the concentration is within the prescribed limit then no remediation need to be performed. Only those site which have toxic level of contaminant concentration needs remediation. For example, toxic contamination level leading to cancer becomes the basis for some of the site clean-up programs.   
            Test protocols such as toxicity characteristics leaching procedure (TCLP) (Method 1311, EPA) have been developed for extraction of chemicals from wastes to verify whether the concentration is within the prescribed toxicity limit. TCLP is designed to determine the mobility of both organic and inorganic analytes present in liquid, solid, and multiphase wastes. Several regulatory agencies such as central pollution control board (CPCB), India, United States Environmental Protection Agency (USEPA) have prescribed toxic concentration levels for various chemicals that get leached from the waste samples by conducting TCLP. In some cases, multiple extractions from the wastes become necessary. For performing TCLP appropriate extraction fluid need to be used. Glacial acetic acid mixed with water is used as the extraction fluid. In some cases sodium hydroxide is also added. For detailed procedure, readers are advised to refer to Method 1311, EPA.
Reactivity
            It is the tendency to interact chemically with other substances. These interactions become hazardous when it results in explosive reaction with water and/or other substances and generate toxic gases.
Corrosivity
            Corrosive contaminants degrade materials such as cells and tissues and remove matter. It is defined as the ability of contaminant to deteriorate the biological matter. Strong acids, bases, oxidants, dehydrating agents are corrosive. pH < 2 or pH > 12.5 is considered as highly corrosive. Substances that corrode steel at a rate of 6.35 mm/year is also considered hazardous.

Ignitability
            It is the ease with which substance can burn. The temperature at which the mixture of chemicals, vapour and air ignite is called the flash point of chemical substances. Contaminants are classified as hazardous if it is easily ingnitable or its flash point is low.
            Based on the above four factors the risk associated with a particular site is determined by specifying maximum acceptable risk using risk estimation equations (Reddi and Inyang 2000). Risk assessment provides a numerical quantification of the probability of harm from hazardous or toxic contamination. Risk management uses this input of risk assessment in deciding how much regulation and corrective measure need to be taken. The corrective action is mostly the practice of remediation of the contaminated site. The maximum possible concentration that could lead to the maximum acceptable risk is back calculated. If the level of concentration at a particular site is greater than the maximum possible concentration, then it requires remediation. This approach would clearly indicate the extent of remediation required for the contaminated site. Appropriate remediation scheme is then selected to bring the concentration level much less than the maximum possible concentration. Since risk assessment and risk management is a very broad topic, it is difficult to discuss the mathematical formulation in this course. Interested readers are requested to go through additional literature (USEPA 1989; Asante-Duah 1996; Mohamed and Antia 1998).