Urban Stream Syndrome & Pollutant Transport
Another way I have studied freshwater resources is by investigating pollutant transport and surface water-groundwater interactions in urban streams. Urban streams are threatened globally due to increased demand for potable water, widespread pollution, species introductions, and land-use change. When investigating urban stream hydrology and biogeochemistry, I have been primarily motivated by informing management practices.
Using concentration-discharge relationships to understand surface water-groundwater interactions in tributaries of the Potomac River
In 2014, I started researching three tributaries of the Potomac River in Northern Virginia (only 32 km from Washington, DC; Fig. 1) as part of a project funded by the Bureau of Land Management. The population in the surrounding area of these streams had been increasing for years, with a 15% increase between 2010 and 2017 resulting in higher levels of development within the largest stream's (Giles Run) watershed. The Bureau of Land Management wanted to better design management practices within the streams' watersheds based on pollutants entering these streams.
For this project, I helped sample surface water in each stream every week for two non-consecutive years (2014–2015 and 2016–2017) with the hyporheic zone also sampled during the second year. We measured many variables we assumed would be related to urbanization including: nutrients (nitrite + nitrate, and phosphate), salts (sulfate and specific conductance, a composite measurement of all ions in the water), metals (Al, Ba, Ca, Cr, K, Fe, Mg, Mn, Ni, and Sr), turbidity, pH, temperature, and discharge.
Groundwater-surface water interactions primarily occur in the hyporheic zone, which is the region directly beneath the streambed where groundwater and surface water mix. This zone is critical for nutrient cycling (e.g., denitrification occurs here) and it also sustains populations of aquatic organisms by limiting changes in temperature that occur in surface water and providing shelter for organisms from predators in surface water.
Fig. 1. The image above displays the three streams we studied within Meadowood Special Recreational Management Area (SRMA) in Northern VA. The different colors correspond to land-cover properties such as forested or developed. Giles Run had the highest levels of development within its watershed (66%), while South Branch had only moderate levels of development (32%), and Thompson Creek had very little (14%). White dots indicate the lowest accessible point within each stream's watershed, which is where weekly sampling occurred over two non-consecutive years in 2014-2015 and 2016-2017. Black dots indicate additional sampling points within each stream system to identify any spatial variability. These streams are tributaries of the Potomac River, which ultimately empties into Chesapeake Bay in Maryland.
Our research questions for this project include:
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Are there significant differences in surface water or subsurface water (measured in the hyporheic zone) quality among these three streams?
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What patterns in water quality & stream discharge appear over time in the three streams? Do concentrations of pollutants rise in the winter and fall in the summer? What about stream flow? How do those patterns vary by stream and by surface or subsurface water?
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How do C–Q relationships vary among different water quality parameters and among the three streams? Which pollutants are transport-limited versus source-limited? Do any exhibit chemostatic behavior?
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Are different water quality variables more likely to be coupled (e.g., transported together from the same natural/anthropogenic source) in either surface or subsurface water, and if so, how does this vary by stream?
Sampling for this project was completed in 2017. Our findings were published in the open-access journal Water in February 2021 (you can read about the publishing process in one of my blog posts).
Funding acknowledgements:
Research on tributaries of the Potomac River was in collaboration with my undergraduate advisor, Dr. Karen Knee, and Jacob C. Melone. This research was supported by a National Conservation Lands System grant to Karen L. Knee (Award #L14AC00102) and by the College of Arts & Sciences Buell Scholarship at American University (Washington DC).
Want to learn more? Check out my blog posts for updates.
