Infiltration trenches and basins are designed to infiltrate runoff and remove pollutants from the surface water stream through attenuation in soil or media or transport into underlying groundwater at concentrations below drinking water standards. It is difficult to assess the performance of these BMPs, although considering only potential impacts to surface waters, a properly functioning infiltration system is considered to be highly performing.
Performance of an infiltration BMP is determined by the length of time needed for captured water to infiltrate. This time is called the drawdown time or period of inundation. The drawdown time is typically 48 hours, meaning water captured by an infiltration BMP should completely infiltrate into the underlying soil or media within 48 hours.
Note: experience has demonstrated that, although the drawdown period is 48 hours, there is often some residual water pooled in the infiltration practice after 48 hours. This residual water may be associated with reduced head, water gathered in depressions within the practice, water trapped by vegetation, and so on. The drawdown period is therefore defined as the time from the high water level in the practice to 1 to 2 inches above the bottom of the facility. This criterion was established to provide the following: wet-dry cycling between rainfall events; unsuitable mosquito breeding habitat; suitable habitat for vegetation; aerobic conditions; and storage for back-to-back precipitation events. This time period has also been called the period of inundation.
An online manual for assessing BMP treatment performance was developed in 2010 by Andrew Erickson, Peter Weiss, and John Gulliver from the University of Minnesota and St. Anthony Falls Hydraulic Laboratory. The manual advises on a four-level process to assess the performance of a Best Management Practice.
Level 1 activities do not produce numerical performance data that could be used to obtain a stormwater management credit. BMP owners and operators who are interested in using data obtained from Levels 2 and 3 should consult with the MPCA or other regulatory agency to determine if the results are appropriate for credit calculations. Level 4, Monitoring, is the method most frequently used for assessment of the performance of a BMP.
Use these links to obtain detailed information on the following topics related to BMP performance monitoring:
Additional information on designing a monitoring network and performing field monitoring are found at this link.