Section 11: Surface Water Pathway - Likelihood of Release
Structurally, the surface water pathway is the most complex HRS pathway, but the LR × WC × T equation still holds true.
The pathway is divided into two components. One component is the threat to targets from overland flow of hazardous substances and from flooding. This is the component that is almost always used. The other component is the threat from the release of hazardous substances to ground water and the subsequent discharge of contaminated ground water to surface water. This component is rarely used. It is designed as a special case and has been applied, for instance, to two old coal gas facilities where coal tar was buried close to a river. We will deal with this component separately in a later session. Since it parallels the overland flow/flood component, we will be able to deal with it briefly.
The overland flow/flood component evaluates LR by surface water overland flow or by flooding. PR by overland flow and by flood are added together to get the LR value.
The LR is the same for all three threats within the component, but WC and T are different for each threat. This is because there is an interaction between WC and target types (e.g., toxicity for human targets, ecotoxicity for sensitive environments).
The three threats describe the types of targets that might be impacted by a site through surface water. The three threat scores are added, with the sum capped at 100.