Volume Analysis for Attainability of Water Quality Criteria for Three Loading Constituents

This paper presents a method to analyze the outputs of an estuarine water quality model to determine the attainability of water quality criteria for designated-use-areas in the Chesapeake Bay estuary using different levels of reduction of nitrogen, phosphorus, and sediment loads. From a set of water quality model scenarios with different nitrogen (N), phosphorus (P) and sediment (S) loads, a relationship between a water quality index (W) and the loading-constituents N-P-S, i.e., W = f(N, P, S), in a designated-use-area, can be established through regression. In a designated-use-area, we may obtain a percentage of non-attainment (or violation, V) of a water quality criterion accumulated over time and water-volume versus the entire simulated time and volume for a model scenario. From a set of model scenarios, a relationship between W and V, i.e., W = g(V), can be established. The value of the water quality index (Wo) equivalent to near-zero violation (Vo) of the criteria can be projected, i.e., Wo = g(Vo). Using W = Wo as a constraint for the function W = f(N, P, S), we can plot a 3-dimensional surface of minimum water quality attainment, Wo, versus N, P, and S loads for the designated-use-area. This surface divides the 3-dimensional N-P-S "loading volume" into regions of attainment and non-attainment of the water quality criterion in the designated-use-area. This plot can be used to analyze tradeoffs among N-P-S load reductions that still ensure attainment of the water quality criteria, providing useful information for optimal nutrient and sediment controls in water quality management.