Twenty pilot scale mesocosms at the Puyallup low impact development research center test pollutant removal capability of bioretention soil mixes. The mesocosms are plastic tanks approximately 2 meters in diameter and 1.25 meters tall. Five treatments (four replications each) consist of various soil mixes are dosed with stormwater containing known concentrations of pollutants. The treatments contain the following soil mixes (by volume):
- MIX 15: 60% sandy aggregate, 15% compost, 15% shredded bark and 10% water treatment residuals.
- MIX 20: 80% sandy aggregate and 20% compost.
- MIX 30: 60% sandy aggregate, 30% compost and 10% water treatment residuals.
- MIX 40: 60% sandy aggregate and 40% compost.
All treatments are planted with the same plant palette consisting of red osier dogwood (cornus sericea "Kelseyi") and Deschampsia cespitosa and Deschampsia cespitosa "Northern Lights".
The facility collects stormwater from roads and rooftops in a large cistern (approximately 11,356 liters) allowing for the application of stormwater at various hydraulic and pollutant loads. During storms stormwater is distributed by gravity from the cisterns through weirs at known volumes to the mesocosms. Each mesocosm has an under-drain and a two stage control structure to control hydraulic residence time, saturated zone and ponding depth. Tipping bucket flow gauges (Hydrological Services TBL1) measure flow continuously at the under-drains.
Between storms, dead storage in the tanks can be used to mix synthetic stormwater mixtures and dose the mesocosms with desired volume and pollutant concentrations. The flexibility built into the collection and delivery system will also allow for the application of various pollutants of concern as scientists and agencies in the region determine research needs.
Previous research suggests that bioretention systems excel at managing most metals, hydrocarbons and sediment, but have shown variable nutrient removal capability. The low compost and water treatment residual mixes will attempt to improve nutrient management while retaining the excellent pollutant removal characteristics for other constituents. Bacteria capture is also being evaluated.
Some of the specific questions assessed in the mesocosm study include:
- What is the ability of the soils to remove pollutants such as metals, oil, grease, pesticides, bacteria, nitrogen, and phosphorus.
- What is the long-term capability of the soils to remove pollutants while supporting healthy plants and soil biota.
- How does water move through the soils and can a more detailed understanding of this soil water behavior significantly refine our ability to model the flow control performance of bioretention.
- What are the long-term pollutant concentrations in bioretention soils.