Characterizing Storm Event Concentration-Discharge Relationships in Urban Watersheds Using High-Frequency Data

Concentration-discharge (C-Q) relationships offer valuable insight into solute mobilization and transport in watersheds, in part, by highlighting the relative contributions of transport pathways, e.g., overland and subsurface flows. More C-Q studies have been performed on forested and agricultural watersheds than urban watersheds even though stormwater infrastructure and impervious cover increase hydrologic connectivity. We examined storm event C-Q relationships for two small urban watersheds (<6.5 km²) in the Baltimore, Maryland USA metropolitan area: West Branch Herring Run (HERR), a partially-channelized stream, and Plumtree Run (PTR), an unchannelized stream. Both watersheds are underlain by crystalline silicate bedrock, have similar impervious surface cover, and receive inputs of road salt during winter months. Patterns in C-Q hysteresis were characterized based on slope and rotational direction for 10 chemical parameters, including specific conductance (SC) and concentrations of dissolved silica and major ions (HCO₃^-, Cl^-, NO₃^-, SO₄^2-, Na⁺, K⁺, Ca²⁺, Mg²⁺). Clockwise hysteresis patterns were observed at HERR while PTR predominantly exhibited anticlockwise patterns. The prevalence of clockwise patterns at HERR implies that solute transport occurs more rapidly than at PTR and may be attributable to differences in stormwater drainage infrastructure and/or basin morphology. SC and solute concentrations generally exhibited positive (flushing) patterns during smaller magnitude storm events (Q < 1.42 m³/s) in both watersheds. Negative (dilution) patterns were observed during larger magnitude storm events. First-flush pulses of ions were commonly observed in both watersheds even when dilution was the overall trend. Concentration peaks occurred later on the rising limb at PTR. At HERR, strong similarities were observed between the hysteresis patterns of K⁺, Mg²⁺, Ca²⁺, and SO₄^2- and those of Na⁺ and Cl^-, which suggest that, in addition to Na⁺ and Cl^-, faster hydrological flow paths can deliver many ions related to road salt during winter events. Concentrations of dissolved silica, HCO₃^-, and NO₃^- exhibited only dilution patterns, suggesting groundwater sources. Ongoing research is being performed to investigate potential seasonal variations in C-Q hysteresis patterns.