DEVELOPMENT OF PATHOGEN AND DISSOLVED OXYGEN TMDLS FOR A COMPLEX, URBAN WATERSHED.
Abstract:The Louisville and Jefferson County Metropolitan Sewer District (MSD) is developing Total Maximum Daily Loads (TMDLs) for three sub-basins of Beargrass Creek, Jefferson County, Kentucky. These sub-basins have been on the State of Kentucky's 303(d) Listing of Impaired Water Bodies since the early 1990's for pathogens and dissolved oxygen/organic enrichment and do not meet the Designated-Use Criteria for Primary Contact Recreation and Aquatic Life. TMDLs must be completed by September 2005. MSD is taking a unique approach to TMDL development, implementation and stakeholder involvement.
To support the development of pathogen and dissolved oxygen TMDLs, a three-year monitoring and modeling effort was undertaken. The monitoring effort consisted of 14, in-stream, “continuous” monitors collecting water temperature, pH, dissolved oxygen (DO), dissolved oxygen saturation (% DO), and specific conductance at 15-minute intervals. Ammonia data were also collected using continuous monitors at some locations. “Continuous” (5-minute interval) stream flow data was monitored at three United States Geological Survey (USGS) gauges. Discontinuous data was collected during dry and wet weather conditions. Sampling occurred within the stream and combined sewer system during dry weather. During storm events, sampling occurred in-stream, at combined sewer overflow locations, and from runoff from specific land uses. The modeling approach included the development of the Beargrass Creek Water Quality Model (BCWQM). The BCWQM was built by integrating HSPF and XPSWMM to continuously simulate flow, DO, phosphorus, nitrogen, solids and fecal coliform in the South Fork and Middle Fork of Beargrass Creek. HSPF was used to simulate runoff (volume and quality) and in-stream flow and water quality at 1-hour intervals throughout the watershed. Runoff was distributed to combined sewer overflow (CSO) and storm water systems in the heavily urbanized downstream portion of the watershed. XPSWMM was used to simulate flow and quality within the combined sewer system (CSS) and overflow events. HSPF utilized XPSWMM outputs to generate flow and water quality simulations in the CSS portion of the watershed.
The continuous monitoring data provided significant advantages for HSPF development and calibration. Hourly DO simulations were improved by using continuous water temperature and pH data as model inputs. Hourly simulations proved essential to understanding the diurnal changes in water quality, especially DO and in accurately simulating storm events. Water quality calibration was accomplished using residuals analysis to compare simulated and observed values. Typically only flow calibrations are accomplished this way because water quality data are collected using grab samples. This calibration method is possible because of the availability of hourly water quality and stream flow observations. A recent literature review indicates that the BCWQM pioneered HSPF-SWMM integration for continuous simulation modeling.
To date, the BCWQM has been used to simulate the effects of Real-time Controls (RTCs) on the CSS and in-stream flow and water quality. The final BCWQM is intended be a water quality management tool used to simulate and compare the effectiveness of various water quality improvement strategies. This paper describes the development and calibration of the HSPF portion of the BCWQM and the expected refinements to support the TMDLs.
Document Type: Research Article
Publication date: January 1, 2003
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