Why Maintenance of a Bioretention is Critical
Bioretentions attempt to replicate the natural filtration process of pollutants by using a combination of soil, plants, and microbes to treat stormwater before it is discharged. However, because bioretentions do not occur naturally maintenance is essential to maximizing the effectiveness of the filtration over time. Additionally, maintenance increases the lifespan of the facility, and improves aesthetics, and property value.
Maintaining Ground Cover
Groundcover is an essential part of both the interior of the bioretention and the surrounding perimeter, though each uses different types of ground covers. The interior of the bioretention should consist of a variety of trees, shrubs, and perennials which must achieve a total of 80% coverage for the bioretention. The plants in the bioretention provide a myriad of benefits to the system including: minimizing erosion, enhancing infiltration, containing sediment, regulating temperature, controlling weeds, filtering pollutants, building soil, phytoremediation, and aesthetic enhancement. Similarly, the area immediately surrounding the bioretention should be grass covered in order to maintain soil stability and keep sediment from running off into the bioretention.
Sediment accumulation within a bioretention can quickly coat the permeable bioretention soil and limit water infiltration. Limited water infiltration causes less effective pollutant treatment and greater chance of rutting or rip-rap getting blown out of place from higher water flows. Simply removing sediment as it accumulates is usually enough to maintain appropriate infiltration/treatment levels.
Biosoil is an engineered mix of sand, soil, and compost which is tailored to the geographic location of the bioretention and the plants within it. The soil has to provide a sufficient infiltration rate as well as support the health of plants and microorganisms. The infiltration/percolation rate of the bioretention must be sufficient enough so that surface water ponding can be prevented over a 48 hour period. In regards to the compost component of the soil, mature leaf compost performs better than manure or yard waste because its low nutrient content prevent nutrient overloading. The proper soil composition will help the bioretention: remove 95 to 98% of the metals (Cd, Zn, Pb, etc) often found in stormwater runoff, reduce total nitrogen by 40%, nitrate-nitrogen by 15 to 75%, and phosphorus by as much as 65%
If the area immediately surrounding the inlet/outlet is beginning to erode it should be addressed as soon as possible. Initially minor erosion doesn’t appear to be a serious issue however if left unchecked it can rapidly result in costly repairs. As the soil around the inlet/outlet is washed away deep channels form around and below the inlet/outlet structures that diminish the structural integrity of the inlet/outlet.