Assessment of Nitrogen and Phosphorus Control Trade-Offs Using a Water Quality Model with a Response Surface Method

Abstract

Excessive nutrient loads to the Chesapeake Bay cause violations of the new dissolved oxygen water quality standard established to protect the Bay’s living resources. Reducing nitrogen and phosphorus loads is necessary to achieve the dissolved oxygen standard. Based on a set of water quality model runs, a response surface method to establish a function of dissolved oxygen (DO) versus total nitrogen (TN) and total phosphorus (TP) loads was used, which plots as a three-dimensional surface. For a specific criterion for DO, i.e., achievement of the DO standard, a curve of DO versus TN and TP loads that meets the DO criterion can be isolated. Each of the paired TN and TP loads on this trade-off curve results in an equivalent level of DO, but usually at different nutrient reduction costs. This paper explores cost-effective alternatives in nutrient reduction to achieve the DO water quality standard in the deep water designated use of Segment CB4, which is the last and most difficult region for achievement of DO standards in the Chesapeake. This paper analyzes DO response surface plots and nitrogen–phosphorus trade-off curves. The effects of nutrient limitation on algal growth, water clarity, and DO concentrations in two different nitrogen and phosphorus load scenarios are examined to understand the responses of water quality to nitrogen and phosphorus trades.