Relationship between urban land use variation and base and stormflow water quality in a highly urbanized watershed, Atlanta, GA

Abstract:

Urbanization has significantly altered many natural systems, including low-order streams. Degradation of urban headwaters ultimately affects the larger, downstream water bodies to which these low-order streams contribute. In Atlanta, GA, increases in urban development (and associated impervious surface coverage) have caused well-documented hydraulic and hydrochemical disturbance over the past 100 years. The changes associated with this high level of impervious cover are compounded by the presence of combined sewer overflows (CSOs). Given the history of development in Atlanta and its rapid continuing growth, understanding both sources and transport mechanisms for contaminants on the watershed scale is crucial for future mitigation efforts. The objectives of this study are: (1) to characterize variation in baseflow solute loading, export, and retention in headwater streams of the South River Watershed (South Atlanta, GA, USA), a predominately Black watershed, and relate it to differences in land use, as well as to (2) examine export dynamics of various solutes during storm events. Baseflow samples were collected at 18 nested sites in the headwaters of the South River during summer and fall 2020, while storm samples were collected at three of these sites. Samples were analyzed for major ions and nutrients. Preliminary baseflow results suggest substantial variability among sites, with particularly high concentrations observed at a site downstream from a CSO facility. During storms, concentration-discharge hysteresis loops were generated for major ions and specific conductivity. Preliminary storm results have shown the dominance of a dilution or “source-limited” response for specific conductivity and most major ions, indicating flushing of surrounding landscapes may play a minimal role in contaminant transport. Additionally, nearly all loops exhibit a predominately clockwise rotational pattern, which may suggest that groundwater represents the component of stormflow with the highest concentrations, not runoff. These results highlight the complicated interactions between natural hydrology and urban land use and the high range of variability that can be seen within a single headwater catchment, which complicates mitigation efforts to address environmental justice issues around water quality.