Bioretention

Initial Research at the University of Maryland

This work consisted of laboratory column studies, box bioretention prototype studies, and field studies of existing bioretention facilities.  It was completed in 1999.  Three manuscripts have been published describing this work.

Small Box Studies 

A small bioretention study box was constructed 107 cm long by 76 cm wide with a depth to hold up to 61 cm of materials, plus a 15 cm freeboard. Two sets of perforated PVC pipes were installed laterally in this box. The upper set had a diameter of 1.3 cm and was 18 cm below the media surface. The second set contained three 2.2 cm dia. pipes at the bottom of the box. A thin gravel layer was packed around each pipe. The box was filled with sandy loam soil and it was topped with a 2.5-cm layer of mulch. Six Creeping juniper plants with branches 13-18 cm long were installed in this box.

Large Box Studies

A large bioretention prototype was 305 cm long by 152 cm wide with a depth to hold up to 91 cm of materials, plus a 15 cm freeboard. This box had 3 sets of perforated PVC laterals. The upper set was 25 cm below the mulch surface, containing two 2.2 cm diameter pipes. Three 2.2 dia. laterals placed 56 cm below the surface made up the middle set. The lower set was at the bottom of the box; six pipes with diameters of 3.2 cm were employed. Gravel, soil, and mulch were added, as with the smaller box. This box was planted with twelve small Creeping juniper plants with branches 13-18 cm long and twelve large Creeping juniper plants having branches up to 38 cm long.

The synthetic runoff contained µg/L concentrations of lead, copper and zinc, and mg/L concentrations of phosphorus, ammonia-N, nitrate-N, and organic-N, at pH 7.0.

At selected time intervals, infiltrated water samples were collected from the lateral ports. The bottom ports were always open; the upper ports were opened only for sampling. From each experiment samples were collected in polyethylene bottles for metals and nutrients analysis. After collection, samples were refrigerated until they were analyzed. Three identical experiments each were performed with the small and with the large box. Additional experiments were performed under different conditions.

Davis, A.P., Shokouhian, M., Sharma, H. and Minami, C., “Laboratory Study of Biological Retention (Bioretention) for Urban Storm Water Management,” Water Environ. Res., 73(1), 5-14 (2001).  

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Field Studies

Greenbelt, MD, 1997 

The first experiment was at a facility that was constructed in 1992 at a shopping mall parking lot in Greenbelt, MD. These bioretention systems were covered with about 5 cm of mulch and held a thick growth of grasses (90-120 cm tall) mixed with a few shrubs and small trees. A 15-cm diameter perforated PVC pipe was located at a depth of 114 cm in the facility to collect infiltrated runoff. The outlet of this pipe opens into a manhole which feeds into a large storm sewer pipe.

An area within the facility, 2.2 m x 2.2 m, near the manhole was boxed off with sandbags.  Using a calibrated pump and 200-L plastic drums, a synthetic runoff was dispersed throughout the boxed-off area at a hydraulic loading of 4.1 cm/hr over a six hour duration.  The total volume applied was 1000 L. 

Fifteen minutes after runoff application, the first flow into the manhole was observed, indicating a high infiltration rate for this facility. Initially, the flow was mostly coming from cracks in the manhole concrete around the PVC pipe, but later flow also occurred through the pipe itself. The surface of the bioretention area became almost completely wet, but no pooling occurred throughout the entire application period. Approximately 15 minutes after the termination of the experiment, the drainage from the PVC pipe slowed; by ˝ hr it was reduced to a trickle, and by 1 hour, flow was not noticeable.

Samples were collected from the PVC underdrain in plastic bottles and transported to the University of Maryland Environmental Engineering Laboratory for analysis. 

Landover, MD, 1999

The second experiment was completed June 1999 at a County facility in Landover, Maryland. This system had been installed about one year earlier, being retrofit into an existing curbside inlet at a parking island. The media consisted of 50% construction sand, 20-30% leaf mulch, and 20-30% topsoil.  A 15-cm T-shaped underdrain runs the span of the entire facility, branching to the inlet 127 cm below the facility surface, to allow discharge to freely enter the storm drainage system. Bare mulch made up most of the surface, with some grasses, bushes, and trees. An area 2.1 m by 2.4 m was cordoned off for runoff application in the center of the south facility.

Again, synthetic runoff was dispersed throughout the boxed-off area at a hydraulic loading of 4.1 cm/hr over a six hour duration.  The runoff contained µg/L concentrations of lead, copper and zinc, and mg/L concentrations of phosphorus, nitrate-N, and organic-N, at pH 7.0. 

Flow in this facility did not originate from the underdrain, but entered through a joint in the storm drain inlet. Artifacts resulting from having to collect the samples from the storm drain joints, instead of having the runoff exit from the perforated underdrain are not entirely known.

Infiltration was rapid during the test and ponding of the applied runoff never occurred.  Periodic samples were taken in plastic bottles and transported to the Environmental Engineering Laboratory.

High concentrations of sodium and chloride were noted in the effluent, likely from the washout of salt accumulated in the facility from the previous winter.  Also, a significant moderation of water temperature was noted from infiltration through the facility. 

At Landover, total lead was removed significantly, at about 70±23% (16 µg/L effluent); total copper was removed to a lesser extent (43±11%). Zinc was also reduced with an average removal of 64±42%. Effluent dissolved zinc averaged 390 µg/L. Both copper and lead concentrations were significantly reduced by the treatment at Landover, but not to the extent found at Greenbelt

The total phosphorous removal at Greenbelt was 65±8%. Effluent phosphorus levels were fairly constant over the sampling period. Removal of TKN was also constant at about 52±7%. Ammonium removal was excellent, averaging 92±7%; the mean effluent ammonium concentration was 0.22 mg/L as N. The removal for nitrate was poor, at only 16±6%. This was not unexpected.

Effluent nutrient concentrations were observed to be below the input in all cases at Landover.  Removal for phosphorus was 87±2%, excellent removal, with effluent concentrations just above 0.1 mg/L P. This was better than the removal at Greenbelt. TKN removal was 67±9% and nitrate removal averaged 15±12%. Both showed some variation with time. These results are both comparable to those found at Greenbelt. Ammonium was not added to the runoff in the Landover study. In contrast to the metals, very similar nutrient removal efficiencies were found between the two facilities.

Pollutant Removal Summary Table for Field Studies

Combined Studies-Lead

A plot showing average lead removals for each depth is shown to the left.  Error bars represent ± the standard deviation for data collected over the application times.   Results show excellent agreement among the various laboratory experiments. As well, excellent agreement was found between the laboratory box studies and the Greenbelt field data, even though the materials of design were not the same. Removals were all greater than 90% and variations were small. Nonetheless, results from the Landover study did not demonstrate the same degree of metals removal.  There were several differences between the Landover and Greenbelt facilities, including differences in bioretention media, which could be responsible for the removal efficiency variations.

Companion studies have illustrated the importance of the mulch layer for metals uptake/sorption in bioretention. Accordingly, a shallow bioretention facility with several cm of mulch may be adequate for substantial removal of heavy metals from storm water runoff.

Combined Studies-Phosphorus

As expected, the overall scatter of the phosphorus data is greater than for lead.  Nonetheless, very good agreement was noted among the laboratory data and the two field studies. In fact, the scatter among the various system scales was well within that exhibited by the three boxes that employed the same media. The phosphorus removals showed very good agreement among all types of experimentation. Better removal resulted from deeper bioretention through about 61 cm depth. At this point, the removal plateaued at about 70-85%. Most soils have a significant capacity to adsorb phosphorus at neutral pH and adsorption was likely the dominant phosphorus uptake mechanism.

Davis, A.P., Shokouhian, M., Sharma, H., Minami, C., and Winogradoff, D. "Water Quality Improvement through Bioretention: Lead, Copper, and Zinc," Water Environ. Res., 75(1), 73-82 (2003).

Davis, A.P., Shokouhian, M., Sharma, H., and Minami, C. “Water Quality Improvement through Bioretention Media: Nitrogen and Phosphorus Removal,” Water Environ. Res., 78(3), 284-293 (2006).

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Bioretention Installations