Assessment of Nitrogen and Phosphorus Control Tradeoffs Using A Water Quality Model with A Response Surface Method

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, we used a response surface method to establish a function of dissolved oxygen (DO) versus total nitrogen (TN) and total phosphorus (TP) loads, which plots as a 3-D surface. For a specific criterion for DO, i.e., achievement of the DO standard, a curve of DO versus TN and TP loads that meet the DO criterion can be isolated. Each of the paired TN and TP loads on this tradeoff 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 tradeoff 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. 1 Senior Research Scientist, University of Maryland Center for Environmental Science, 410 Severn Avenue Annapolis, MD 21403, USA. Phone: (410) 267-5744, e-mail: [email protected] 2 Modeling Coordinator, US Environmental Protection Agency, Chesapeake Bay Program, Annapolis, MD 21403, USA. e-mail: [email protected] CE Database subject headings: Water quality; Nutrient loads, Equivalence; Dissolved oxygen; Chesapeake Bay.