Hydrologically sensitive areas: Variable source area hydrology implications for water quality risk assessment

A conceptual approach to assessing water quality risk in the context of var iable source area hydrology was developed and applied to the New York City (NYC) water supply watersheds. The term hydrologically sensitive area (HSA) was used to refer to areas in a watershed especially prone to generating r unoff that are, therefore, potentially susceptible to transporting contamin ants to perennial surface water bodies. As an example, a cost benefit metho d for quantifying HSAs was developed and applied such that water quality pr otection was balanced with agriculture needs for NYC watersheds. In accorda nce with the variable source area concept, the spatial extent of HSAs vary throughout the year. On an annual average, approximately 10% of the watersh ed is designated HSA and about 20% of the total annual runoff originates on these HSAs. This study is meant to illustrate an approach to dealing with water quality risk assessment.