Contaminants, Fate and Transport, and Risk Assessment
Contaminant classes
Know the major classes: petroleum hydrocarbons (BTEX, TPH fractions), chlorinated solvents (PCE, TCE, TCA, vinyl chloride), metals (lead, arsenic, chromium), pesticides, PCBs, and emerging contaminants such as PFAS. Each has characteristic sources and behaviors. Chlorinated solvents are denser than water and sink as DNAPLs; petroleum products are lighter and float as LNAPLs.
Fate and transport
Key processes include advection, dispersion, sorption, volatilization, biodegradation, and dilution. Contaminant properties matter: solubility, volatility, and sorption coefficient. Hydrogeology matters: permeability, hydraulic gradient, and preferential pathways. Vapor intrusion is a pathway from soil gas or groundwater into buildings, driven by pressure differences. Natural attenuation includes biodegradation, dispersion, and sorption, and must be monitored.
Risk assessment
Risk assessment has four steps: hazard identification, dose-response assessment, exposure assessment, and risk characterization. Carcinogens are evaluated with a slope factor and lifetime excess cancer risk; noncarcinogens use a reference dose and a hazard quotient. Exposure pathways include ingestion, dermal contact, and inhalation. Risk depends on both toxicity and exposure, so a highly toxic contaminant with no pathway may pose little risk.
Traps
Do not confuse LNAPL and DNAPL behavior. Do not assume a contaminant will migrate far; sorption and degradation can limit it. Risk is not the same as hazard. Screening levels are not cleanup standards in every jurisdiction.
Rule of thumb
To predict movement, ask: what is the phase (dissolved, vapor, free product), what is the geology, and what is the pathway?