Part of the new infrastructure bill that recently passed includes actions to mitigate and clean up runoff pollution. Runoff pollution was included in this bill because it’s a big problem.
Fecal bacteria, nitrogen and phosphorus, oil and petroleum products, and road salt are just some of the contaminants being swept into our rivers and lakes.
In this article, we are going to go over some of the problem areas in the nation and some recent research to help clean up the problem.
Problem Areas Around the Nation
As more and more urban areas are developed, the amount of runoff produced increases significantly. The Chesapeake Bay Foundation estimates that the amount of rain that turns into runoff goes from 10 to 20 percent in natural areas to 100 percent in urban areas.
To get an idea for the magnitude, here are a few examples of problem areas around the nation. In Minnesota, another 305 streams, lakes and rivers were recently added a growing list of impaired waters. This list is now up to around 3,000 bodies of water.
Bodies of water added to this list, managed by the Minnesota Pollution Control Agency, tested very high in at least one contaminant that can harm humans, kill aquatic life, or make fish unsafe to eat. Much of these contaminants reach the water through runoff.
Over on the other coast, the organization San Diego Coastkeeper published a list of the 10 worst runoff locations in San Diego. This shocking list includes many locations with an open channel carrying parking lot runoff straight into the beach. These beaches are frequently visited by unaware swimmers.
Treatments and Mitigation
Researchers have speculated that urban forest can be used as stormwater management tools. Up to this point, we have been unsure of particulars – mainly how much runoff is stopped by urban trees.
Urban trees tend to be in single trees versus a patch in the forest. Also, cities tend to hotter and dryer, which likely impacts the runoff reduction potential.
This gap limits the current use of urban trees in stormwater runoff management strategies.
University of Maryland researchers wanted to close that gap by understanding the details. They looked at three situations – “single trees over turfgrass and a cluster of trees over turfgrass in Montgomery County, and a closed canopy forest with a leaf litter layer in Baltimore”
The researchers built and used sap flux sensors. These sensors gave them a better image of how much groundwater is used by trees.
The study found that single trees had higher transpiration rate and were more responsive to climate influences. Thanks to this study, we can now correctly integrate urban trees into stormwater green infrastructure networks.
Another method that is showing promise is grass barriers. Grass barriers are already used to reduced sediment, but researchers in Northeast China wanted to see if it could remove diffuse pollution.
The study monitored two types of grass barriers, one from alfalfa and the other of native grass, during natural rainfall evens during 2012 to 2015.
The study found that “more than 50% of the nutrient loads could be removed by the alfalfa barrier”. At the same time, the alfalfa reduced the volume of runoff by 50%-60% depending on the thickness of the grass barrier.
The study concluded that the type of grass matters, as the native grass didn’t perform nearly as well as the alfalfa. More research is needed to determine the best slope and barrier width.