Sustainable methods for reducing pollutants in stormwater runoff are being developed and tested by Ming-Han Li, associate department head at Texas A&M’s Department of Landscape Architecture and Urban Planning, with the help of $1.8 million in grant funds.
Among those efforts, Li heads a 5-year, $565,364 project examining bioretention, a “green” water filtering process advocated by the U.S. Environmental Protection Agency, to clean stormwater runoff in urban and suburban landscapes.
Bioretention uses various layers of vegetation, soil and gravel to filter pollutants by physical, chemical and biological means; plant roots take out pollutants such as nitrogen and phosphorus, while soil absorbs metals such as zinc, lead and pathogens such as E. coli.
“The results will provide fundamental knowledge needed for designing bioretention systems that can effectively remove pathogens from highway stormwater runoff in semi-arid regions like Texas …this will be a unique contribution to the field,” said Li. His collaborators on the project include two environmental engineers and an environmental scientist with a planning background.
To determine an ideal configuration of plants and soil, the team built five pilot bioretention units on state highway 6 in Bryan planted or seeded with either shrubs, grass species specified for Texas highway applications, Texas native grasses or Bermudagrass, and a unit without vegetation to serve as a control subject. Runoff with known levels of pollutants were injected into the units and, to mimic highway conditions, were allowed to operate without weed control.
While plants’ roots are necessary for pollutant removal, they also decrease the water’s detention time, reducing the cleaning that runoff receives from soil. Li and his research collaborators are determining how to optimize performance between roots and detention time — while dealing with fire ants, whose elaborate tunnels become an expressway for water.
“This research bridges disciplinary gaps,” said Li. “We’re combining landscape architecture and engineering disciplines to eventually create aesthetically pleasing, environmentally conscious and functional designs,” he said.
A three-year, $736,293 Texas Department of Transportation-funded study that will ultimately provide TxDOT with guidance on complying with changing federal runoff regulations is pairing Li as a co-principal investigator with researchers from TTI, the University of Texas and Texas Tech University.
The U.S. Environmental Protection Agency has drawn up new, tighter construction site stormwater runoff regulations taking effect in 2013. The new rules will apply to all sites disturbing 20 or more acres of land at a time. In 2014, the limits will apply to construction sites disturbing 10 or more acres of land.
Since TxDOT's construction activities are considered a source of pollution in terms of stormwater quality, its sites must also be compliant with EPA regulations, said Li.
"Because turbidity, or opacity, has never been a concern for stormwater before, TxDOT is seeking research-based solutions for meeting this soon-to-be-implemented rule," he said.
The research team is conducting experiments to:
• determine typical turbidity of runoff collected by TxDOT's drainage structures at construction sites,
• collect performance data on innovative erosion and sediment control measures, such as bioretention, that might be expected to achieve the discharge standard, and
• produce guidance measures and sampling protocols for TxDOT to negotiate with the Texas Commission on Environmental Quality in the development of statewide monitoring/sampling procedures.
In low-slope regions such as Houston, the Texas’ gulf coast and high plains, traditional methods to estimate water runoff characteristics in highway designs can yield unreliable results.
Li’s 2009-2011 project, undertaken with $464,000 in funding from TTI, employed literature, data, modeling and physical experiments to determine when alternate approaches should be considered and provided guidance on what approaches are appropriate in low-slope situations.
“Uncertainty in these cases often results in costly overdesign or underdesign and liability for damage,” wrote Li and his collaborators. Identification of dimensionless slopes where traditional estimating techniques become suspect, they said, is important so analysts can apply appropriate methods based on locale.
Li collaborated with researchers from Texas Tech, the Texas Water Science Center and R.O. Anderson Engineering.