The short version
When rain washes over roads, roofs and lawns, it picks up leaves, soil, algae and other organic material. Once that material reaches a river or bay, bacteria break it down and use up oxygen that fish and shellfish need. Researchers sampled runoff from 16 developed areas around Myrtle Beach, South Carolina. Stormwater used up about three times as much oxygen as the river water it flowed into. The first rush of runoff in a storm was the worst, and retention ponds built to treat stormwater often added algae instead of removing pollution.
What the researchers did
Low oxygen is the main water quality problem along the South Carolina coast, including in the Waccamaw River. Kelly McCabe and colleagues at the University of South Carolina and North Carolina State University set out to learn how much stormwater contributes. They measured “biochemical oxygen demand” (BOD), a standard test of how much oxygen bacteria use as they break down the organic material in a water sample.
- They sampled runoff from 16 urban and suburban drainage areas: 10 ordinary ditches, 3 stormwater retention ponds and 3 constructed stormwater wetlands. For comparison, they also sampled 4 streams draining natural forests and wetlands.
- Automatic samplers collected water during 17 rainstorms in the summers of 2018 and 2019, both as water levels rose and as they fell.
- In the lab, they measured oxygen use over 5 and 28 days, along with carbon, nitrogen, chlorophyll (a measure of algae) and chemical fingerprints showing where the organic material came from.
What they found
- Runoff is a real oxygen drain. Across 108 urban samples, 5-day BOD averaged 2.92 mg/L, about three times the average for the Waccamaw River that receives it.
- The first flush matters most. Samples taken as water was rising had significantly higher oxygen demand than samples taken as it fell, and they carried the most particles of organic matter.
- Treatment ponds didn’t help with this problem. Ponds and wetlands slowed down the flood pulse, but their outflow had about the same oxygen demand as untreated ditches. Water leaving ponds was rich in algae. Among the pond and wetland sites, algae levels explained 75% of the variation in oxygen demand.
- Urban organic matter breaks down faster. Runoff from forests had about the same oxygen demand as urban runoff but more total carbon. About 20% of the organic carbon in urban runoff was used up in 28 days, compared with 12% for forest runoff.
- Particles are the key target. Oxygen demand tracked particles of organic matter and algae more closely than the larger pool of dissolved material.
What it means for Waquoit Bay today
On Cape Cod, most of the nitrogen reaching Waquoit Bay travels slowly through groundwater, mainly from septic systems. Stormwater is a smaller piece, and conditions here differ from South Carolina’s blackwater rivers. Still, this study shows that what goes down a storm drain doesn’t disappear. Leaves, grass clippings and soil washed off pavement feed the bacteria that pull oxygen out of the water. That adds to the stress on a bay that is already overloaded with nutrients.
The authors recommend focusing on the particles carried off in the first flush of a storm. They conclude that managers should focus on reducing stormwater POM exports
(McCabe et al., 2021), meaning particulate organic matter. Keeping storm drains clear of yard waste and debris, as CPWB’s storm drain adoption project encourages, is one simple way residents can help. The finding that retention ponds can export algae is also worth keeping in mind when towns design stormwater fixes.
Read the paper
McCabe, K. M., Smith, E. M., Lang, S. Q., Osburn, C. L., & Benitez-Nelson, C. R. (2021). Particulate and Dissolved Organic Matter in Stormwater Runoff Influences Oxygen Demand in Urbanized Headwater Catchments. Environmental Science & Technology, 55(2), 952–961. https://doi.org/10.1021/acs.est.0c04502
License: © 2021 The Authors. Published by the American Chemical Society as an open access article under the ACS AuthorChoice Creative Commons Attribution-NonCommercial-NoDerivatives (CC BY-NC-ND) license, which permits non-commercial sharing. The PDF below is reproduced unmodified from the publisher for non-commercial, educational use.
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