Lea Pollack, Christof Zweifel, Ben Janke, Jacques Finlay, Emilie Snell-Rood, Cara Santelli
University of Minnesota
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Introduction
We are investigating contamination levels in stormwater ponds across the Minneapolis-Saint Paul Metropolitan area to understand how and where ponds accumulate pollutants like heavy metals and microplastics.
We hypothesize that stormwater ponds could act as pollution hotspots in urban freshwater networks.
Research Questions
- What determines variation in lead accumulation across urban stormwater ponds?
- Where is lead accumulating within the pond?
- Is lead correlated with other contaminants, like microplastics, within pond sediments?
A photograph shows a duckweed-covered pond surrounded by trees with two people in a canoe at the center of the pond.
Methods
We sampled 38 sites, focusing on older areas within the Twin Cities Metropolitan Area (Fig 1).
Figure 1: Map of sample sites. Sites are indicated on a map of the Twin Cities Metropolitan Area, with their associated parcel ages.
Our group uses pXRF to screen for lead in the ponds which allows for rapid results on sediment bound lead.
We assessed relative concentrations of microplastics from surface sediment (top 10 cm) using oil extraction.
A photograph shows a person in mud boots and safety vest holding a sediment core.
A photograph shows two people in a canoe, on a pond, surrounded by duckweed.
A photograph shows a person boring a hole through the surface of a frozen pond.
Results and Discussion
Lead is correlated with development age in sediments. Ponds in older areas were more likely to have higher sediment lead, suggesting that legacy environmental inputs like lead are a major source of sediment lead (Fig 2). Figure 2: Relationship between sediment lead and parcel age. A scatter plot displays mean built year on the x-axis, beginning with 1930 and ending with 1980. Surface lead is indicated on the y-axis.
Lead in sediment does not correlate with lead in bottom water, suggesting that lead is not leaching from sediment within the ponds, sediment resuspension is the main concern (Fig 4). Figure 4: Relationship between water and sediment lead. A scatter plot displays surface sediment lead in parts per billion, on the x-axis, ranging from 0 to 400. The y-axis displays bottom water lead. Only one data point is above the chronic aquatic hazard level.
Lead concentrations increase with sediment depth, suggesting that less lead is currently being deposited than in the past, likely a result of policy changes (Fig 3). Figure 3: Relationship between sediment depth and lead. A scatter plot displays sediment depth on the x-axis, starting with 0 and ending with 40. Lead, in parts per million, is displayed on the y-axis.
Preliminary data indicates that microplastics are correlated with lead in the top 10 cm of sediment (Fig 5). Figure 5: Preliminary data showing relationship between sediment lead and microplastics concentration estimates. A scatter plot displays sediment lead in parts per million on the x-axis, and microplastic concentration on the y-axis.
Acknowledgements
We thank Nic Jelinski and Nora Pearson for advice while designing this experiment. We also thank Zoe Culshaw-Klein, Tristina Ting, Camryn Sharpe, Alex Nygen, Alexandria Wenger, Jackson Peterson, Ian Coffman, Mitchell Pariseau, Claire Bass, Caroline Castellon, Jenna Duncan and Jake Wiebe for assisting in the field and laboratory. We thank the City of Minneapolis, Minneapolis Parks and Recreation, Minnesota Department of Transportation, Saint Paul Parks and Recreation, the City of Roseville, the City of Bloomington, Falcon Heights, Capitol Region Watershed District, and the University of Minnesota for providing access to stormwater ponds for research sampling.
This work was funded by the Minnesota Futures Program of the University of Minnesota, the University of Minnesota College of Science and Engineering Fellowship, and the National Science Foundation Long Term Ecological Research Program. Further funding for this project was provided by the Minnesota Environment and Natural Resources Trust Fund as recommended by the Legislative-Citizen Commission on Minnesota Resources (LCCMR).
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