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.