Stability in headwater chemical signatures across a dynamic flow regime in a highly urbanized Piedmont catchment

Abstract

AbstractUrbanization has substantially altered low‐order streams, which show systematic degradation in water quality. Studying the spatiotemporal character of catchment scale solute dynamics is a crucial part of understanding the hydrology of urban catchments. The objectives of our study were to characterize spatial and temporal variability of solutes in headwater streams of the Upper South River, Atlanta, GA, and to examine relationships between solute concentrations and stream discharge during storm events across the watershed. Synoptic sampling was performed at 18 nested sites in the watershed during summer and fall 2020. Storm samples were also collected at three of these sites, all collocated with USGS stream gages. Samples were analysed for major ions including Ca2+, Mg2+, Na+, K+, Cl−, SO42−, and NO3−. Synoptic sampling showed an unexpectedly high level of spatial stability over time at all first‐order sites, meaning that the relative ranks of concentrations among headwater sites remain consistent over time. This is even more unexpected considering the extremely flashy and dynamic nature of the highly urbanized watershed. Contrary to stormflow studies in other settings, examination of concentration‐discharge relationships showed a homogeneous response among storms, sites, and solutes, with most hysteresis loops exhibiting a dilutive response with clockwise rotation. This indicates source‐limited behaviour for solutes in this system, where groundwater represents the highest‐concentration component of stormflow. Taken together, baseflow and stormflow solute data show that this system retains consistent headwater signals across a highly dynamic flow regime. Furthermore, such stability drives a consistent solute signature at the watershed outlet. This type of information may prove useful for understanding sources and flow paths that drive urban stream geochemistry, particularly where budget constraints limit the number of locations where instrumentation may be installed.