A Study of Hydraulic and Nutrient Retention Dynamics in Vegetated and Non-Vegetated Bioretention Mesocosms

Bioretention systems are stormwater treatment devices installed to remove both dissolved and particulate pollutants. As stormwater percolates through the bioretention system, dissolved pollutants are removed from solution by chemical and biological processes. In our study, 20 bioretention mesocosms (10 with loamy sand and 10 with sandy loam, half with and half without vegetation) were used to investigate hydraulic behavior and dissolved nutrient retention. The mesocosms were dosed with 120L to 160L synthetic stormwater comprising 0.8 mg-l^–1 TP and 4.8 mg-l^–1 TN over 3h to 5h. The infiltration and percolation rates in the sand was rapid (>15 cm-h^–1), while rates in the loam were much less (2–3 cm-h^–1). Retention time in the sand was 1.5h while that in the loam was well over 10h. The effluent was collected in 150L containers. Composite samples were taken after 24h in the sand and 48h in the loam. Average TP retention in the vegetated loam mesocosms was over 90%, compared to 75% in non-vegetated (barren) mesocosms. TP retention in the sand was very high (average 90%), regardless of treatment. TN retention increased to 80% in the vegetated loam mesocosms from 43% in barren loam mesocosms. TN retention in the sand was less overall, with retention increasing to 41% in the vegetated loam mesocosms from 26% in barren mesocosms. Increasing retention time in the sand mesocosms increased retention of nitrogen to 61%, nearly a 50% increase in retention performance. Nutrient retention persisted even under very high nutrient loads. While the loam is initially a less effective media by itself, the presence of vegetation substantially improves TP and TN retention. TN retention performance is improved by increasing in retention time.