Double Duty: Can Roadside Ditches Catch and Clean Water?

Excessive nutrients, particularly nitrate and phosphorous, are a growing concern in groundwater aquifers and springs across Florida. This causes serious environmental issues like eutrophication and degradation of groundwater quality.

The Florida Department of Transportation (FDOT) and researchers at the University of Central Florida (UCF) are experimenting with creative ways to remove excess nutrients from water using an unlikely tool – the common roadside ditch.

Ditches, also known as swales when water infiltrates into the ground, run alongside most of Florida’s roadways. Their main purpose is to catch and divert rainwater from the road, keeping drivers safe. However, they also collect water from surrounding properties. As such, swales are an intermediate step in runoff water’s journey from ground level to underground aquifers and springs.

There are naturally occurring microbes in the ground that actually consume nitrates and other nutrients, removing them from water before it filters into groundwater. For instance, they help with waste removal in home septic systems.

But can these microbes be used to remove nutrients from runoff water in swales? The answer is yes, under certain conditions. The goal of UCF’s research was to create those conditions.

“We know the effectiveness of microbes to clean water,” said Dr. Ni-bin Chang, Director of UCF’s Stormwater Management Academy and a Principle Investigator for the project. “But in Florida, our sandy soil is not naturally conducive to harboring enough of them to do the job.”

A Home for Microbes

For the right microbes to grow in the right amounts, they need enough porous surface area and consistent moisture to form colonies on soil particles, called a biofilm. For soil without those qualities, this can be achieved by amending it with specially-mixed materials called Biosorption Activated Media, or BAM.

An effective BAM has several traits:

  • Good adsorption and absorption properties
  • A long life expectancy
  • High surface area
  • Ease of water filtration
  • Low maintenance (reasonably non-degradable or easily rejuvenated)

BAMs have been used to remove nutrients in runoff areas like wet detention ponds and basins to much success, but had not been tested in linear roadside swales. So, the team tested two BAMs in a swale near Fanning Springs, Florida – wood chips and a product created by UCF called Bold & Gold™ (B&G).

Left: Bold & Gold™, Right: wood chips

The wood chip mixture contained both small chips and wood shavings, about 1/16-to-1 inch. B&G is a custom BAM, made of clay, tire crumb, and sand. Variations have been used in a number of unconventional applications, like green roofs and water supply filters. The tests in this study compared the effectiveness of B&G versus wood chips to provide enough moisture retention and surface area for a biofilm to grow and microbes to remove pollutants in the swale.

The location around Fanning Springs was chosen because the watershed has a variety of land uses, including residential, a dairy farm, a wastewater treatment plant, and agricultural fields.

The test was conducted in a 600 ft. linear ditch. Half of the ditch (300 ft.) was filled with the woodchip mixture, the other with B&G. To study the impacts of media depth on nitrogen removal, the woodchips were installed in three 100-ft. long sections at depths of 2, 3, and 4 ft. respectively. The B&G section was divided into two 150-ft segments and the material was placed at 1 and 2 ft. respectively.

Lysimeters were placed at several depths along the entire ditch to measure water quality as it progressed through each BAM. A groundwater well with a solar-powered pump was also installed to distribute water across both sections of the ditch so researchers could study the effectiveness of the BAMs for treating groundwater as well as storm runoff.


“We wanted to know if a swale could do double duty,” said Dr. Martin Wanielista, UCF Professor Emeritus and Co-Principle Investigator. “That way the ditches could clean water almost constantly, during and between storm events.”

And the Winner is…

Over the six-month study period, B&G was significantly more effective at removing total nitrogen from both groundwater and storm runoff. This was due to a number of factors, but the key was the structural composition of the B&G versus wood chips.

B&G is a uniform material, with relatively fine grains compared to wood chips. Therefore, the B&G could be calibrated to provide the crucial surface area that microbes need to form a biofilm but hold water long enough to maintain the moisture levels that the microbes also need. The B&G performed well during large storm runoff periods as well as dry periods when only filtering groundwater.

The wood chips, while they were able to maintain a biofilm under certain circumstances, were not able to do so consistently in the unpredictable conditions of the field study. In some cases, nutrient levels even rose after passing through the chips.

“The wood chips, because of their variable size and larger air spaces between pieces, simply let water pass through too quickly to be cleaned well,” said Catherine Earp, FDOT Drainage Design Engineer and Project Manager of the study. “They could not handle the wide variations of water flow in natural conditions.”

An Untapped Resource

The possibilities for BAM-lined roadside swales to remove nutrients from water are significant. Land scarcity within the right-of-way of many roadways makes using linear ditches economically attractive, as agencies do not need to purchase new land. Ditches also require much less footprint relative to large basins to achieve removal efficiency, and the BAM can be incorporated in the swales with minimal construction impact on traffic or adjacent properties.

“Stormwater is a relatively untapped resource when it comes to meeting today’s freshwater demand,” Wanielista said. “If properly managed, it could provide another valuable source for integrated water management in Florida.”

“BAM-lined linear ditches have great potential,” Earp said. “FDOT already owns the swales, the material can be installed cost-effectively and with little impact, and the swales can remediate groundwater during non-storm periods and also treat runoff during storms. We are excited by the possibilities.”

For Further Reading

BDV24-977-14 – Comparative Nitrogen and Pesticide Removal with Sorption Media in Linear Ditch for Groundwater and Stormwater Treatment
Research in Progress

BDV24-977-20 – Optimal Design of Stormwater Basins with Bio-Sorption Activated Media (BAM) in Karst Environments – Phase II: Field Testing of BMPs
Research in Progress