NITRATE TMDL DEVELOPMENT: THE MUDDY CREEK/DRY RIVER CASE STUDY Teresa B. Culver Kathryn A. Neeley Shaw L. Yu Harry X. Zhang, Andrew L. Potts Troy R. Naperala Civil Engineering Department University of Virginia In 1972, Section 303(d) of the Clean Water Act (CWA) estuaries, which includes approximately 475,000 established the regulatory concept of a Total Maximum kilometers (300,000 miles) of river and shoreline Daily Load (TMDL) as the maximum loading rate of a (USEPA, 2001). The USEPA suggests that states plan pollutant that a receiving water can assimilate without to complete the TMDL’s with a maximum planning resultant water quality impairments with respect to the time frame of 13 years (Perciasepe, 1997). Given that applicable water quality standards. The CWA specified there are typically hundreds of impaired waterbodies per that the watershed-level TMDL approach should be state, the effort required to meet these timelines is used to systematically manage both point and non-point enormous. Furthermore, in many states, court orders or source pollution. However, it was not until the 1990’s consent decrees now specify the rate at which TMDL’s after a series of legal actions that the TMDL program must be established (USEPA, 2001). In addition, active has been actively pursued at federal and state levels. and effective community involvement is expected in a The TMDL concept has now grown into a TMDL project, so modeling analysis should be made comprehensive surface water management approach. A intelligible to community members. thorough description and guidance for the TMDL program can be found at the U.S. Environmental There are many opportunities for researchers to Protection Agency web page (USEPA, 2001). contribute to the development of the state-of-the-art for TMDL analyses. In fact, in Virginia, universities have While the systems approach of the TMDL program may had a direct role in the TMDL program with university help to initiate an era of sustainable watershed representatives serving as the primary technical analysts management, the program presents many challenges for for ten of the first twenty TMDL’s in the state the water resource management community. For (VADEQ, 2001). As an example, this work briefly instance, computer modeling of watershed hydrology, presents the Muddy Creek/Dry River nitrate TMDL water quality, and load allocations is typically necessary case study and then discusses the relationship between to address required components of a TMDL, such as the University research efforts and the TMDL program. spatial and temporal variability. Although the science Full details of the nitrate TMDL study can be found in and tools of watershed modeling are expanding rapidly Culver et al. (2000a). (e.g., Vieux, 1991; Devantier & Feldman, 1993; Hornberger & Boyer, 1995; Sample et al., 2001), there THE MUDDY CREEK/DRY RIVER CASE STUDY is still a desperate need for practical tools and approaches to facilitate modeling of the diverse range of Background watersheds and water quality problems to be addressed by the TMDL program. Not only must the analyses be The Muddy Creek/Dry River watershed is located in accurate while coping with typically scarce data, but Rockingham County in northwestern Virginia (Figure they must also be completed rapidly. The USEPA 1). Sections of Muddy Creek, Dry River and North expects timely completion of about 40,000 TMDL's for River are designated for public drinking water use over 20,000 impaired river segments, lakes and because they are less than 8 kilometers (5 miles) 5 upstream of the intakes for water treatment plants on the Muddy Creek/Dry River Nitrate TMDL North River (VADEQ, 1998). The U.S. EPA water quality standard for nitrate, also adopted by Virginia, in Muddy Creek generally flows south to its confluence the portions of Muddy Creek, Dry River, and North with Dry River, which joins the North River River designated for drinking water is 10 mg/l nitrate- approximately 3.63 kilometers (2.25 miles) farther to nitrogen (VADEQ, 1998). This nitrate standard is the south (Figure 1). The North River discharges to the intended to be protective of human health, especially for South Fork of the Shenandoah River, a tributary of the infants who are especially susceptible to high levels of Potomac River that eventually flows into the nitrate intake and may develop methemoglobinemia Chesapeake Bay. The land area of the Muddy Creek (“blue-baby” disease), a potentially fatal blood disorder watershed is approximately 8,106 hectares (20,030 (USEPA, 1996; USEPA, 1998b). acres), with forest (34 percent) and agriculture (61 percent) as the primary land uses (Culver et al., 2000a). An 11.35 kilometers (7.04 mile) reach of Muddy Creek, The Upper Dry River watershed is approximately Dry River and the North River was added to the state 18,960 hectares (46,850 acres), with over 99 percent of 1998 303(d) list after a preliminary modeling study (Yu the land forested, while the Lower Dry River watershed & Barnes 1998) suggested that violations of the nitrate is approximately 4,120 hectares (10,180 acres) with 30 standard could occur in the listed reach due to a percent forested and 62 percent agricultural lands combination of point and non-point sources. The state (Culver et al., 2000a). The intensive agriculture of this water monitoring program also found that three of 75 watershed helps to give Rockingham County the highest water samples in the listed reach of Muddy Creek poultry and dairy production levels in Virginia collected between September 1993 and October 1999 (VADEQ, 1997). To date fecal coliform TMDL’s have violated the nitrate water quality standard (Culver et al., been developed for both the Muddy Creek (Muddy 2000a). The highest concentration observed was 13.5 Creek TMDL Establishment Workgroup (MCTEW ) mg/L nitrate-nitrogen (Culver et al., 2000a). These 1999), and Dry River watersheds (Virginia Tech, 2000), violations of water quality standard within the reach and a nitrate TMDL was developed for the Muddy protected for drinking water use were measured at a Creek/Dry River area (Culver et al., 2000a). All three sampling location on Muddy Creek (Virginia State TMDL’s have been approved by the USEPA (VADEQ, Water Control Board monitoring station 1BMDD000.4), 2001). just above its confluence with the Dry River (Figure 1). No water quality violations were observed on the Dry The Muddy Creek/Dry River watershed was subdivided River or North River (Culver et al., 2000a). In addition into eleven subwatersheds. The Muddy Creek and Dry to the surface water, three recent studies (Shenandoah River watersheds contained eight and three Valley Soil and Water Conservation District 1995; Ross subwatersheds, respectively (Figure 2). The study area 1999; Culver et al., 2000b) have found elevated nitrate was divided to allow for spatial variation of nitrogen levels in the karst aquifer below the Muddy Creek/Dry loading throughout the watershed and to allow the River watershed. Through these studies, a total of 152 relative contribution of sources to each stream segment ground water samples were collected from private wells to be determined. Subwatershed delineation was based between 1994 and 2000. The average and standard on a topographic analysis of the region and past work deviation of the nitrate-nitrogen concentration in the completed by the Virginia Department of Conservation samples was 12.01±13.18 mg/L, with 51 percent of the and Recreation. In addition, nitrogen non-point source samples over the drinking water standard of 10 mg/L loads differed between the Muddy Creek and Dry River nitrate-nitrogen. watersheds due to variations in farm management practices. No subdivisions were imposed on the Upper TMDL development in the Muddy Creek has laid Dry River watershed due to its homogeneity; it is almost important groundwork for future TMDL development in completely forested. Virginia. Virginia’s first and second TMDL’s (VADEQ, 2001) to be approved by the USEPA were the The water quality/quantity model, Hydrologic Muddy Creek fecal coliform TMDL (MCTEW, 1999) Simulation Program-FORTRAN (HSPF) version 11.0 and the Muddy Creek/Dry River nitrate TMDL (Culver (Bicknell et al., 1997), was used to predict stream flow, et al., 2000a), respectively. Furthermore, the Muddy in-stream water quality and the significance of nitrogen Creek/Dry River nitrate TMDL was Virginia’s first sources. HSPF was selected because of its ability to TMDL to be approved with significant contributions simulate both nonpoint and point source loads, as well from both point and non-point sources. All other as the flow and transport of pollutants in each stream approved TMDL’s in Virginia have focused on fecal reach. In addition, HSPF is able to assess in-stream coliform impairments. water quality response to changes in flow, season, and load (Bicknell et al., 1997). While HSPF is a 6 component of the USEPA watershed model, Better from the point source was based on monitoring records Assessment Science Integrating Point and Nonpoint for its discharge permit, although the load from the plant Sources, or BASINS (USEPA, 1998), the nitrogen was highly variable in both flow and concentration and chemical cycle is not supported within the BASINS the monitoring record was sparse compared to the modeling framework. Thus HSPF was used outside of modeling requirements. The point sources, septic tanks the BASINS modeling system for the nitrate TMDL. and cattle in the stream were modeled as direct discharges along each stream reach. Although the The basis for the hydrological calibration was the septic tank load was assumed constant, the point source coliform bacteria TMDL for Muddy Creek (MCTEW, load and the loads from cattle in the stream varied over 1999). In the coliform bacteria study, BASINS time. (USEPA, 1998) was calibrated to the continuously recording U.S.