A Quest for Appropriate Water Quality Metrics to Support a Nutrient TMDL Allocation.

A nutrient TMDL allocation project has been undertaken for the Assabet River, a eutrophic river system located in eastern Massachusetts. The river receives excessive nutrient loadings, primarily from Publicly-Owned Treatment Works (POTWs), and extensive quantities of floating and rooted vegetation inhabit the river system during the summer season. The Assabet was placed in the 303(d) list for violations of the numeric water quality standard for dissolved oxygen concentration and for the narrative water quality standard for nuisance aquatic vegetation. The Assabet River nutrient TMDL allocation project was funded by MA Department of Environmental Protection (MA DEP) and US Army Corps of Engineers (US ACE) and included a field investigation and a water quality modeling application. A major focus of this investigation was the definition of water quality goals for the TMDL effort.The TMDL allocation process requires establishment of metrics to quantify water quality improvements and designate endpoints for acceptable water quality conditions. For the Assabet River nutrient TMDL allocation project, a set of potential endpoint metrics including dissolved oxygen concentration, phosphorus concentration, nitrogen concentration, chlorophyll-a, and total biomass, were evaluated. The ideal water quality endpoint metric was identified as one that would meet all requirements of the TMDL allocation process. Specifically, an ideal water quality endpoint metric should be: (1) enforceable by law; (2) indicative of ambient water quality; (3) of ecological and/or anthropogenic significance; (4) measurable in the field; (5) predictable using a water quality model; and (5) of stakeholder concern. All of the available metrics for evaluating the eutrophic Assabet River system were found to meet some, but not all of these characteristics. No single endpoint metric was identified that satisfied all requirements. A stakeholder group including United States Environmental Protection Agency, US ACE, MA DEP, POTW operators, and watershed organization personnel worked together to select appropriate water quality metrics. Ultimately, a set of dissolved oxygen concentration, phosphorus concentration, and biomass endpoint metrics was selected as appropriate for the Assabet River system.This paper provides a discussion of the strengths and weaknesses of each potential water quality metric for supporting nutrient TMDL allocations with references to the Assabet River application. The impact of confounding factors to TMDL allocation such as limitations on available water quality data and models and the importance of broad narrative water quality standards are also discussed.