Bioretention For Highway Stormwater Quality Improvement in Texas

The bioretention in this research exceeded these requirements and actually removed more pollutants by absorption than sand or clay.

What's the Problem?

Water runoff from highways carries pollutants which can cause serious damage to the local environment. The Texas Department of Transportation (TxDOT) formulated a plan in 2003 that would use different best managing practices (BMP) to counteract the effects of runoff. However, this plan did not involve bioretention, a BMP that uses plant fibers and engineered soil to absorb pollutants and requires minimum maintenance. Bioretention is a proven method in the removal of pollutants, but bioretention tests have only been performed in the northern states or in Australia, where the climate and soil types are different than those found in Texas.

What's the Solution?

Texas A&M Transportation Institute (TTI) researchers performed a bioretention study to develop a plan for use in TxDOT districts, running tests on the effectiveness of bioretention both in a laboratory and in a real-world scenario. Along with testing different soil and plant types, researchers looked at how internal water storage (IWS) affects bioretention. Including IWS in the bioretention plan can provide a constant source of water during hot Texas summers and decrease the stress plant roots undergo during droughts. After the tests, researchers recommended increasing the drainage area to increase pollutant removal.

How Was the Study Done?

The lab test used synthetic runoff to mimic pollutant concentrations in Texas highway runoff. Researchers tested for hydraulic and water quality performance. The field test was conducted where State Highway 6 meets State Highway 21. The watershed for drainage included water from the highway and from a nearby gas station. Due to the drought in Texas summers, the test included both synthetic and natural rainfall runoff. Researchers looked at how different soil and vegetation types affect bioretention. They also studied bioretention cells with and without IWS, examining the effects of increasing water amounts.

Conclusions

Texas requires that BMPs remove 80 percent of the pollutants from runoff. The bioretention in this research exceeded these requirements and actually removed more pollutants by absorption than sand or clay, two other common BMPs. IWS improved the water quality and performance of bioretention. Bioretention is a beneficial alternative to other BMPs because of its efficiency and long-term positive effects on the environment. Financially, bioretention is more cost effective than other BMPs. Due to bioretention’s reliance on plant fiber to absorb pollutants, researchers recommend further research to find the best plant and soil combinations.