In many of our watersheds, runoff from impervious surfaces leads to both high hydraulic loads and excess nutrients in our surface waters. We’ve found numerous ways to dampen the high flow events but removing the pollutants has proven more difficult. One option is to pass the runoff through an engineered bioretention system designed to create a number of removal mechanisms; however, these have often had disappointing efficiencies and sometimes become sources themselves. Two recent studies have suggested modifications to standard designs that can make these systems work much better.
Lopez-Ponnada et al. conducted a unique study of a rain garden experiment two years after it was installed in South Florida.1 Two cells were established adjacent to a building and received mainly roof runoff for two years before data collection. One cell used a standard infiltration design with 30 cm of sand, a 5 cm pea gravel layer and 30 cm of limestone gravel and a drainage pipe. The modified cell had a 30 cm layer of wood chips and pea gravel (1:2) below the sand layer, plus an internal water storage zone of 30 cm (upturned drainage pipe). Synthetic runoff containing different forms of nitrogen (N) and ground oak leaves (C source) were introduced at different rates and the effluent was sampled for removal rates. After 14 tests, five plants were established in the cells and the next season the same simulated events were conducted. Weighted average total N removal was about 50 percent for the standard system; however, the modified system reduced it significantly another 25 percent. The modified system also removed more ammonia-N and nitrite+nitrate N than the standard system. Dissolved organic carbon, however, was not changed by either system. Both systems had higher removal rates at the lower loading rates tested, and the authors suggested that a storage system for high flow events might enhance the effectiveness of these devices. Plants may have improved system performance, but the data did not clearly establish this effect.
When bioretention systems are also intended to provide aesthetic value through landscape plantings, it is important that the plants can survive dry periods between storms. Unfortunately, the tradeoff between high infiltration rates and plant-available soil moisture can result in high plant stress during dry periods. The e ect of alternative media in these systems on plant survival was the subject of a study in Ohio.2
Three large bioretention cells were established adjacent to a parking lot that provided runoff for the study. The conventional cell used a topsoil-sand blend (84 percent sand, 4 percent clay) mixed with compost (12 percent by volume) at a 55 cm depth on top of a 45 cm gravel drainage layer. A second cell used a proprietary mix of expanded shale, pine fines and compost, and the third had the site soil mixed with expanded shale at a 2:3 ratio, both cells with an expanded shale gravel drainage layer. For all three, the drainpipe was upturned to create a 75 cm storage zone. Nine species of ornamental plants were established in the same pattern on all cells. Parking lot runoff was evenly split between the three cells. Soil moisture and plant survival were monitored for two and a half seasons. The conventional cell plant survival rate was less than 50 percent due to high moisture stress levels that were present for more than 50 percent of the growing seasons. The expanded shale (shale heated to high temperatures) mixes held more plant-available moisture, with the soil:expanded shale mix having 22 percent mortality and the expanded shale/pine fines/compost mixture only losing 3 percent of the plants. The authors observed that all three bioretention cells performed well hydraulically, draining within 24 hours of storm events, and would be considered successful installations.
(1) Lopez-Ponnada, E. V., T. J. Lynn, S. J. Ergas, J. R. Mihelcic. Long-term field performance of a conventional and modified bioretention system for removing dissolved nitrogen species in stormwater runoff. Water Research 170 (2020) https://doi.org/10.1016/j.watres.2019.115336
(2) Funai, J. T., and P. Kupec. 2019. Evaluation of three soil blends to improve ornamental plant performance and maintain engineering metrics in bioremediating rain gardens. Water Air Soil Pollut 230:3 https://doi.org/10.1007/s11270-018-4049-x