Bioretention Systems


A manufacturing facility in Old Hickory, TN received a Notice of Violation (NOV) stating that their 0.75-acre bio retention system was out of compliance with the City of Nashville. Northstar was called to inspect the system and determine the best course of action to resolve the issues identified by the City of Nashville.


At the time of Northstar’s free consultation, the bioretention system was completely overgrown with invasive vegetation. This was causing each bioretention to hold water longer than the required infiltration period designated by the City of Nashville. The dense root systems of the invasive vegetation had formed a mat-like system under the surface of the infiltration media which was causing water to pool up instead of allowing it to infiltrate through the bio soil. This increased the chance of flooding and created a breeding ground for mosquitos.


Northstar developed a scope of work and timeline to bring the system back into compliance, which was approved by the City of Nashville as well as the facility manager. Northstar and their Land Clearing Services began the rehabilitation of this system by removing all of the invasive vegetation, a majority of which were cattails. Compaction in a bio-retention system can create other issues with infiltration. Because of this, all vegetation was removed by hand, in an effort to keep heavy equipment from compacting the infiltration media. The inlets to each bio-retention were armored with rip rap and river rock in order to slow down the incoming water. This helps disperse the water into the entire bio-retention basin and prevents erosion from occurring. The remainder of the exposed bio soil was covered with triple shredded hardwood mulch at a minimum depth of 3 inches. 14 (1-year-old) sycamore trees were also planted throughout the bio-retention system in order to achieve the required canopy coverage of 80% as required by the City of Nashville. The bio-retention system is now functioning properly and infiltrating as designed. Since the conclusion of the rehabilitation project, Northstar has been conducting regular maintenance on the bio-retention system and it is continuing to function and infiltration properly.