Bioretention for Stormwater Quality Improvement in Texas: Pilot Experiments (0-5949-2)

This report summarizes the results of pilot-scale bioretention experiments. Five steel boxes of 6 ft (L) x 6 ft (W) x 4 ft (D) were constructed, each of which has a different type of vegetation: (1) shrubs, (2) grass species in Texas Department of Transportation (TxDOT) Bryan District seed mix, (3) native grasses, (4) Bermudagrass, and (5) no vegetation as the control. Vegetation was given 14 months to establish before testing. Synthetic runoff containing predetermined pollutants with target concentrations was used. The results indicate that pilot bioretentions effectively removed zinc (Zn), lead (Pb), total suspended solids (TSS), and ammonia-nitrogen (NH³-N), total nitrogen (TN), and total phosphorus (TP). Vegetation plays an important role on NO³-N and TN removals due to root uptake and the denitrification processes in root zone. However, vegetation could negatively affect the water quality if the soil infiltration rate is significantly increased by its root system. This effect was specifically obvious on the TSS removal, in which the control box with the longest detention time had much better performance than the vegetated boxes. The results suggest that bioretention is useful to treat stormwater runoff from TxDOT highways, but the design specifications developed in other states should be revised to reflect Texas' unique climate and environmental conditions. Challenges and learned lessons are described in the report.