356-3 Groundwater Contributions Dominate for Export of Most Solutes in Urban Streams near Baltimore, MD

Abstract:

Decades of research have documented increased stream discharge in urban watersheds due to runoff from impervious surfaces with accompanying increases in erosion. Additionally, solute concentrations are commonly found to be elevated in urban streams over nearby forested streams. However, much less is known about patterns of solute export from urban streams, including how export is partitioned between quick flow (e.g., overland flow) and slow flow (predominantly groundwater). We investigated solute concentrations and export in four small watersheds (<7 sq km) in the Mid-Atlantic of the United States near Baltimore, Maryland. In a mostly forested (>70%) watershed with ~1% impervious surface cover (ISC) and two urban watersheds with ~25% ISC, we collected biweekly water samples, stormwater samples, and high-frequency data with specific conductance (SpC) probes. In a fourth watershed that was forested with no ISC, we collected biweekly samples. Discharge data came from US Geological Survey gaging stations. High-frequency concentrations were calculated using relationships with SpC or discharge (depending on the solute) and then were used to calculate export. We used a digital recursive filter to separate quick and slow flow contributions. 

The two urban watersheds had much higher flashiness than the (mostly) forested watershed with solutes typically exhibiting strong dilution as discharge increased (except for dissolved organic carbon). Baseflow export of water was 85% of total discharge in the forested watersheds and ~45–55% in the urban watersheds. In the mostly forested watershed, solute export at baseflow was ~85%, matching baseflow water export. By contrast, in the urban watersheds, baseflow solute export was much higher than baseflow water export as a percentage of total export, e.g., ~70% for chloride and dissolved silica and ~65% for alkalinity. Higher concentrations of most solutes in the urban watersheds resulted in larger export from these watersheds than the forested watersheds. The urban watersheds also had a much higher percentage of total chloride export (~70%) during winter months (Dec–Mar) than water export (~38%); export of other solutes was more similar to water export though with some variability. Our results indicate that groundwater was the predominant contributor to export of most solutes in small urban watersheds even though only ~50% of water in these watersheds was exported at baseflow.

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