Baseflow water quality and storm hysteresis variability in urban headwaters

Urban development has been nearly ubiquitous worldwide in the past decades. This is especially true in the metro-Atlanta region, as its population has grown considerably in recent years. As a result, headwater streams in the area have undergone significant water quality and hydrological changes. Such low-order streams have been shown to hold a pronounced importance for solute dynamics on the watershed scale in both undeveloped and urbanized catchments. As such, understanding these processes and how they relate to land use changes in the urban environment is important for monitoring and remediation in these systems. This study attempts to evaluate spatiotemporal heterogeneity of major ions at baseflow in a highly urbanized watershed (the Upper South River Watershed, South Atlanta, GA), as well as to evaluate the hysteretic concentration-discharge (c-Q) responses of solutes on the storm event scale. An intensive field sampling campaign was carried out during summer and fall 2020, with baseflow water quality samples taken from 18 sites a total of 15 times. Baseflow samples were analyzed for major ions. Additionally, specific conductivity (SPC), pH, and other parameters were measured in the field. We also performed storm event sampling for 12 events across three sites in the watershed, all of which were collocated with USGS gages to utilize 15-minute discharge measurements. Baseflow results showed relatively high levels of spatial variability across the headwater sites. However, higher-order sites showed a more homogenous chemistry. In terms of land use, we found that NO3- was the only ion correlated with urban impervious coverage. In contrast with the observed headwater heterogeneity, storm event c-Q responses in the South River Watershed showed a similar response with regard to sites, solutes, and events. Most loops showed clockwise rotation and dilutive slopes. This suggests that the South River Watershed is a source-limited system, with stream concentrations diluted by runoff. Knowledge that hydrochemical variability is attenuated downstream in this catchment while storm responses remain homogenous could have important implications for assessing monitoring locations in similar systems.