Nutrient retention in vegetated and nonvegetated bioretention mesocosms

Thirty well-established 240L bioretention mesocosms were used to investigate retention of dissolved nutrients by bioretention systems. Ten mesocosms were comprised of 80 cm sandy loam, ten of 80 cm loamy sand, and ten of pea gravel with 20 cm of loamy sand. Half were vegetated with shrubs/grasses, while the other half had no vegetation (barren). In the first part of our study, the loam and sand mesocosms were dosed with synthetic storm water comprising 0.8 mg L-1 total phosphorus (TP) and 4.8 mg L-1 total nitrogen (TN). TP retention in the vegetated loam was 91% compared to 73% in the barren, and TN retention was 81% compared to 41% in the barren loam. TP retention was 86-88% in the sand treatments, while TN retention in the vegetated sand was 64%, compared to 30% in the barren. In the second part of our study, all 30 mesocosms were loaded weekly with 45 cm of tertiary effluent with high nutrient loads (22.3 m year(-1) hydraulic load at a flow-weighted average of 4.5 mg L-1 TP and 4.8 mg L-1 TN, or 1,012 kg ha(-1) year(-1) TP and 1,073 kg ha(-1) year(-1) TN). After 50 weeks of loading, cumulative TP retention was 92% in the vegetated loam, 67% in the sand, and 44% in the vegetated gravel. However, TP retention by barren media was 56% in the loam, 39% in the sand, and 14% in the gravel. Cumulative TN retention was 76% in the vegetated loam, 51% in the sand, and 40% in the vegetated gravel. In contrast, maximum TN removal by barren media was 18% in the loam. The increase in TP retention by vegetated systems substantially exceeds phosphorus uptake rates for plants, suggesting that other processes are involved. The increase in TN retention by vegetated systems also exceeds nitrogen uptake rates for plants, suggesting that denitrification is involved.