Characterizing Hysteretic Water Quality in Southern Appalachian Streams

Water quality in mountain streams of the southern Appalachians varies seasonally and with storms. In an effort to validate Total Maximum Daily Loads (TMDLs) for sediment in the Chattooga River Watershed (NE Georgia, NW South Carolina, and SW North Carolina), we studied four tributary streams over an eighteen-month period. Two of the streams had completely forested watersheds; one stream was a benchmark with exceptional water quality for the purposes of TMDL establishment while the other was impaired by excessive sedimentation. The third stream, with a completely forested watershed, was adjacent to a gravel road. This stream was threatened by excessive sedimentation. The fourth, while mostly forested, had residential development and gravel roads in its riparian corridor. This stream was also threatened by excessive sedimentation. We measured stream flow continuously and sampled Total Suspended Solids (TSS) to characterize the hydrology and water quality of these streams during baseflow and storm flow conditions. TSS data on the benchmark stream and a forested stream exhibited strong hysteresis and were elevated on the rising limbs of hydrographs and declined rapidly on the recession limbs. While there was weak hysteresis apparent in the constituent concentrations and loadings of the impaired streams, it was not statistically significant. Thus, we could not simply characterize loadings with typical constituent vs. discharge rating curves. We filtered TSS and discharge data into rising and recession limb data based upon hydrograph slope and analyzed the data separately. We constructed a series of rating curves based upon hydrograph thresholds that allow us to predict loadings as a function of hydrograph dynamics. This modified approach facilitates the establishment of TMDLs because the hydrograph threshold-rating curves can be used to directly link loading rates to discharge frequency and duration relationships. Introduction The Chattooga River is a federally designated Wild and Scenic River that drains 450 square kilometers of the Blue Ridge Ecosystem in the southern Appalachian Mountains of NE Georgia, NW South Carolina, and SW North Carolina (Figure 1). The United States Environmental Protection Agency (EPA) (2001) identified numerous tributaries to the Chattooga that are impaired and many additional kilometers of streams whose aquatic health is threatened by excessive sedimentation. The EPA consequently established TMDLs for several sedimentimpaired streams in the Chattooga Watershed (EPA, 2001). Due to difficulties stemming from the interpretation of Georgia’s water quality standards, the characterization of stream sedimentation, and judicially imposed time limitations for the establishment of TMDL’s, the EPA adopted a phased approach to developing TMDLs for the Chattooga River Watershed. The EPA has published these TMDLs (2001) stating that they will be revised in 2004. The Chattooga River Watershed is also one of twelve USDA Forest Service Large Scale Watershed Restoration projects. The Forest Service has been working to improve water quality and watershed health throughout the watershed, 68% of which is National Forest lands. Van Lear, et al (1995) estimated that more than 80% of the sedimentation threat to streams in the Chattooga River watershed is caused by unpaved gravel roads. One of the primary actions is the reduction of runoff, erosion, and sedimentation from forest roads through road closure and reconstruction. As part of this work, we monitored water quality and streamflow on four tributaries of the Chattooga River. Site Description We instrumented and monitored stream flow and water quality on four tributaries of the Chattooga River (Table 1). Each of these streams had previously been evaluated by the EPA to determine if the streams were supporting designated uses in compliance with Georgia State water quality standards (EPA, 2001). With the exception of a small portion of the Reed Mill watershed, all of the watersheds have been managed as part of the Chattahoochee National Forest by USDA Forest Service since the early 1930's. Aside from the small portion of private land in Reed Mill watershed, the forests of the watersheds are intact. The only significant site disturbances at the time of this study were the use and maintenance of forest roads. Table 1: Summary of characteristics for study streams. The ability of each stream to support its designated uses is indicated by 303 (d) list status. Stream 303 (d) Status Watershed Size (km) Mean Elevation (m) Mean Slope (%) Aspect Streamflow record (mos.) Roach Mill Impaired 0.8 712 16 ESE 18 Reed Mill Threatened 4.4 700 14 S 18 Addie Branch Unlisted 5.6 925 19 ENE 15 Pounding Mill Threatened 1.3 706 14 ESE 13 Roach Mill has been identified by the EPA as an impaired stream due to sediment impairment of its biological community and habitat (EPA, 2001, page 7). The site location used to gather TMDL data was adjacent to a paved county highway immediately downstream of private residential and agricultural land. We were unable to install sampling equipment at this location and had to install it upstream, on national forest land. There were no roads or development upstream of this site. Watershed land use is 100% forested (26% deciduous, 24% coniferous, 50% mixed). Reed Mill is generally the most turbid of the study streams. Three percent of its watershed is privately owned. This land, adjacent to the stream, is a mixture of agricultural and residential and accessed by numerous gravel roads. The EPA listed Reed Mill as being threatened by sedimentation. The remaining 97% of the watershed is forested (17% deciduous, 37% coniferous, 43% mixed). Addie Branch creek is the most remote watershed. It is highest in elevation (exposing it to greater precipitation), steepest and the most northerly facing watershed in this study; conditions that typically generate more runoff in