Predicting lead hotspots in urban stormwater ponds across the Twin Cities Metro

Stormwater ponds play an important role in urban food webs. Critically these same ponds could also serve as pollution hotspots since stormwater ponds can act as local concentrators of urban runoff. One such contaminant of concern is lead, which remains a significant issue for human and ecosystem health in the United States despite regulatory bans on its use in paint and gasoline imposed in the 1970s. Despite high levels of lead in some urban stormwater, little is known about the distribution of lead in urban aquatic ecosystems. Therefore, we characterized lead within the sediment, water column, and surrounding soil of stormwater ponds across the Minneapolis-Saint Paul Metro Area. We hypothesized that lead would be highest in ponds that receive runoff from landscapes with older construction (i.e., legacy leaded paint and gasoline), have high traffic volume (i.e., legacy leaded gasoline), and areas with low impervious surface cover (i.e., increased mobilization of contaminated soil). Moreover, we hypothesized that stormwater ponds capture lead within sediments, with more dissolved lead at the bottom of the water column, where it would interact with lead containing sediments. Across ponds, we found that age of parcel development where the pond was located was the strongest predictor of surface sediment lead content. Within pond sediments, we found that lead concentrations increased with depth below the sediment surface, which is unsurprising since depth is likely correlated with time. The strongest predictor of surface water lead concentration was the strength of pond stratification, while the strongest predictor of bottom water lead concentration was pond duckweed cover and water conductivity. Water column oxygen concentrations varied across ponds yet were not important in determining dissolved lead within the water column. Importantly, lead within pond water remained quite low despite elevated sediment lead levels. These findings confirm that stormwater ponds can act as one source of environmental lead remediation by capturing lead within sediments under a wide range of environmental conditions. Our results suggest relatively low lead release from ponds to downstream areas, indicating that ponds generally serve as sinks, not sources within the urban lead cycle.