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Development of Site-Specific Water Quality Standards for the Murderkill River

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TMDLs for the Murderkill River watershed were completed by the Delaware Department of Natural Resources and Environmental Control (DNREC) and were approved by the USEPA Region 3 in 2001 (amended in 2005). The watershed modeling framework developed to establish these TMDLs for dissolved oxygen (DO), nutrients and bacteria was challenged by a major discharger to the river and resulted in an agreement to jointly:

Form a study group to plan and implement a comprehensive monitoring effort for quantifying the impact of tidal marshes and other natural resources on water quality in tidal portions of the Murderkill River;

Utilize the information obtained to develop a state-of-the-science watershed and water quality model for the river; and

Develop site-specific water quality criteria for DO and nutrients if warranted by the study results. The study group includes representatives from DNREC, Kent County, University of Delaware, University of Maryland, USGS and other researchers. (

The extensive monitoring program designed for the study included: tidal salt marsh sampling (nutrient dynamics, temperature, LIDAR); algal production studies (carbon production); sediment flux measurements (SOD and nutrients); and project specific tidal and water quality monitoring. These new monitoring efforts were designed to support calibration of the watershed and tidal hydrodynamic/water quality models (HSPF, ECOM/RCA) and also to quantify the interaction of the extensive tidal salt marshes with water quality in the main stem of the Murderkill River. The marsh monitoring program was designed to estimate: whether the marshes are a nutrient source or sink during different seasons; the loading of particulate organic matter to the main stem river and its contribution to sediment oxygen demand (SOD) and nutrient fluxes; and the importance of marsh storage volume on the tidal circulation in the river.

The tidal modeling includes wetting and drying of the tidal salt marshes, nutrient uptake/loading associated with the marshes, a sediment flux model to properly account for sediment oxygen demand and nutrient fluxes in the river, and linkage to the upstream watershed model to provide nonpoint source (NPS) loadings to the tidal river. Model calibration includes comparisons to water column monitoring data but also to measured SOD and nutrient fluxes, and algal production data. After model calibration and validation is complete, the model will be used to determine if existing water quality standards can be met or if background, naturally occurring loads to the system prevent attainment of the DO standard. If the monitoring and modeling results suggest that background, natural sources to the river prevent attainment; site-specific water quality standards will be developed.

This paper provides an overview of past monitoring/modeling studies of the Murderkill River, a brief summary of the TMDL history, description of the Murderkill River Study Group and its current activities in the river, and the modeling being completed to support the effort. This study provides a good example of TMDL development (and potentially site-specific standard development) through the collective efforts of both regulatory and regulated entities.
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Keywords: Coastal watershed; dissolved oxygen; nutrients; sediment oxygen demand; site-specific water quality criteria; tidal salt marsh

Document Type: Research Article

Publication date: 2009-01-01

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