APPLICATION OF A WATERSHED ASSESSMENT PROTOCOL FOR ROCK CREEK IN THE DISTRICT OF COLUMBIA ALLOWING TMDL DEVELOPMENT IN THE ABSENCE OF EXTENSIVE DATA

Like many of the watersheds in the United States, Rock Creek has signs of impairment, but little is known about the causes of this impairment. This work applied a watershed assessment protocol to categorize the potential causes of impairment in the watershed, leading to targeted monitoring. With the use of this assessment and targeted monitoring, the District of Columbia can now develop a TMDL implementation plan to improve the water quality of the Rock Creek watershed.The Rock Creek watershed covers approximately 76 square miles of heavily urbanized area in central Maryland and the northwest portion of the District of Columbia (DC). The stream runs primarily through an urban park landscape for 33 miles from Laytonsville, Maryland to the Potomac River. The study area of interest is located in the DC portion of the watershed. It consists of approximately 18.1 square miles of watershed, which is over 50 percent parkland, and contains nine miles of Rock Creek. Bioassessment surveys showed impairment in the watershed, and Rock Creek has been listed for organics, metals, and bacteria in the District of Columbia 303(d) listing. Before application of the watershed assessment protocol and targeted monitoring, the causes of impairment in Rock Creek were unknown.Using a watershed assessment protocol developed under an EPA grant for Kentucky Sanitation District No. 1, a detailed investigation of Rock Creek was completed. Following the protocol, known impairments were reviewed, sources of impairment were characterized, and cause/effect mechanisms were evaluated. This led to the development of source rankings that prioritized the constituents of concern within the Rock Creek watershed. With these rankings, a list of targeted monitoring opportunities was prepared.A monitoring priority determined from the assessment protocol was the generation of acceptable cadmium and copper data for the water column of Rock Creek. This included the collection of both dissolved and total metals. The data will assist in determining how copper and cadmium contribute to the impairment of Rock Creek. Existing cadmium and copper data was analyzed using detection limits that exceed both the acute and chronic class C water quality standard in the District of Columbia. Additionally, total suspended solids (TSS) data was collected for use in modeling cause and effect relationships in Rock Creek.The use of a watershed assessment protocol and targeted monitoring can greatly improve the process of characterizing a watershed. This work is unique in that it provides a stepwise approach that can be applied allowing watershed management efforts to follow a given process with limited data. It should have wide applicability for TMDLs and other restoration sites when extensive data is not available. Using the case of Rock Creek in the District of Columbia as an example, this approach was followed leading to a more specific list of causes of impairment in Rock Creek. With this information, the District of Columbia can now move forward, correcting specific problems in the watershed.