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The Chatfield Basin comprises that portion of the South Platte River watershed tributary to Chatfield Reservoir and is located just southwest of Denver, Colorado. Although average algal concentrations in Chatfield Reservoir are among the lowest in Denver metropolitan area reservoirs, algal blooms have been observed there in recent years and the rapid urbanization occurring in portions of the watershed have caused concern for future water quality in the Reservoir. Phosphorous has been regarded as the principal nutrient of concern.

Whether phosphorous should be removed primarily from point sources (PS's), nonpoint sources (NPS's) or some combination of both, as well as the total maximum annual load (TMAL) that will support beneficial uses have been significant management issues in the Chatfield Basin. NPDES permit limits for several of the six wastewater treatment plants (WWTPs) were set at 0.2 mg/l of total phosphorous, a level most dischargers found financially and technologically difficult to meet. Indeed, only one discharger has been able to afford the phosphorous removal processes to meet this limit. To further complicate matters, a recent study concluded that the vast majority of phosphorous reaching the Reservoir was from NPS, rather than PS, origins and this finding raised the question of whether NPS, PS, or some combination of both should be the focus of additional phosphorous reduction activities. The Water Quality Control Commission agreed to relax total phosphorous effluent limits to 1.0 mg/l in part so that this question could be addressed before significantly more monies were spent on additional PS controls.

This study funded by the U.S. EPA, examined the PS/NPS phosphorous load allocation issue from a cost-effective perspective. Mathematical optimization methods were used to develop:

The cost function representing minimum PS treatment costs as a function of total annual phosphorous load removed among the six WWTPs,

The cost function representing minimum NPS treatment costs as a function of total annual phosphorous load removed among NPS's and

Based on these two minimum cost functions, the minimum cost load allocation between PS's and NPS's as a function of total annual phosphorous load removed.

Once the TMAL has been established, this minimum cost load allocation function can be used to meet that TMAL in a cost-effective manner. This paper emphasizes the use of mathematical optimization methods for cost-effective TMDLs. It was one of two case studies presented in the 1999 EPA publication, Protocol for Developing Nutrient TMDLs (EPA, 1999).

Document Type: Research Article


Publication date: 2003-01-01

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  • Proceedings of the Water Environment Federation is an archive of papers published in the proceedings of the annual Water Environment Federation® Technical Exhibition and Conference (WEFTEC® ) and specialty conferences held since the year 2000. These proceedings are not peer reviewed.

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