Nutrient Criteria Development in Pennsylvania Establishing a Periphyton Endpoint

Since 2000, EPA has urged the states to develop numeric water quality criteria for nutrients in rivers and streams. While many states have expended significant resources attempting to develop these standards, the development of scientifically defensible numeric criteria for nutrients has remained elusive. The lack of such standards has not prevented a proliferation of TMDLs to address nutrient-related impairments. Historically, such actions targeted compliance with dissolved oxygen standards as the basis to regulate nutrients. More recently, nutrient TMDLs have been established to control nuisance plant growth, particularly that of benthic algae (periphyton), through implementation of narrative criteria. The science supporting these approaches is a work in progress. Most recently, EPA has employed a new approach, abandoning the periphyton endpoint, a focusing on benthic invertebrates. The merit of this approach is still being debated.