Nutrient Retention in Mature Vegetated Bioretention Systems under Elevated Nutrient Loads

30 well-established 240L bioretention mesocosms were used to investigate retention of dissolved nutrients by bioretention systems. 10 mesocosms were comprised of 80cm sandy loam, 10 of 80cm loamy sand, and 10 of pea gravel with 20cm of loamy sand. Half were vegetated with shrubs/grasses, while the other had no vegetation (barren). In our study, the mesocosms were loaded weekly with 45cm of tertiary effluent with high nutrient loads (23.2 m-yr^–1 hydraulic load at a flow-weighted average of 4.8 mg-1^–1 TP and 4.8 mg-1^–1 TN, or 1,090 kg-ha^–1-yr^–1 TP and 1,093 kg-ha^–1-yr^–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, 38% in the sand, and 15% in the gravel. Cumulative TN retention was 79% in the vegetated loam, 52% in the sand, and 43% in the vegetated gravel. In contrast, maximum TN removal by barren media was 21% 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.