Item 16.16 of the Minnesota Pollution Control Agency (MPCA) Construction Stormwater General Permit (CGP) states "This permit prohibits permittees from constructing infiltration systems in areas where soil infiltration rates (including amended soils) are field measured at more than 8.3 inches per hour unless they amend soils to slow the infiltration rate below 8.3 inches per hour".
Item 16.11 states "For design purposes, permittees must divide field measured infiltration rates by 2 as a safety factor or permittees can use soil-boring results with the infiltration rate chart in the Minnesota Stormwater Manual to determine design infiltration rates. When soil borings indicate type A soils, permittees should perform field measurements to verify the rate is not above 8.3 inches per hour. This permit prohibits infiltration if the field measured infiltration rate is above 8.3 inches per hour".
The primary concerns with infiltration rates above 8.3 inches per hour are (1) a diminished ability to attenuate pollutants due to the relatively short contact time between the soil and infiltrating stormwater and (2) a higher potential for rapid contaminant transport to groundwater systems (e.g., in the event of chemical spills).
To address these concerns, the MPCA CGP allows for amending soils to slow the infiltration rate to an acceptable level. The approach to amending soils typically involves seeking to accomplish one of two basic objectives: to physically decrease the infiltration rate or to increase pollutant attenuation capacity.
Although slowing infiltration rates to less than 8.3 inches per hour via soil amendments can allow for installation of infiltration systems that help meet permit water quality volume Vwq retention requirements, it also has the potential to create problematic conditions. One of the most common complications of amending soil is decreasing the infiltration rate so much that it becomes unacceptably slow. This is often caused by the introduction of fine-grained materials which become clogged in the native soil. Subsequent consequences of clogging the soil may include failure to meet 48 hour drawdown time requirements and killing vegetation that was not intended for prolonged inundation. Another complication is the potential for the soil amendment to serve as a pollutant source. For example, certain amendment media such as compost can export soluble phosphorus in higher concentrations than the incoming stormwater runoff, thus contributing to increased phosphorus loading.
Designers developing a soil amendment plan to slow the infiltration rate below 8.3 inches per hour should seek to physically decrease the infiltration rate or to increase pollutant attenuation capacity while taking steps to ensure common complications are prevented.
The first step in developing a soil amendment plan involves understanding the baseline conditions of the native soils where the amendment will be performed, including the following.
Soil samples should be taken in close proximity to the infiltration test locations.
Depending on the site-specific conditions determined from the first step, the soil amendment plan should define one or more of the following.
Compost and topsoil are the most commonly used soil amendment media. Minnesota’s specifications for use of these materials should be followed:
When calculating the volume of soil amendment material needed, the following should be taken into consideration.
Implementation of the soil amendment construction procedure needs to ensure the appropriate volume of amendment is used and that the mixing process results in a consistent, heterogeneous media across the entire site to the proper depth (typically 12 to 18 inches). The mixing process can be accomplished by either:
The specified construction procedure must also ensure that common complications are prevented, and may include the following.
The soil amendment plan should specify post-soil amendment infiltration testing, which is critical to ensuring the amended soil performs as expected (that the new infiltration rates are not too high, too low, or uneven throughout the site). In addition to ensuring the amendment plan and implementation was successful, all infiltration areas that are part of the permanent stormwater management system must be tested for infiltration rates after they are completed in order to submit the NPDES Notice of Termination. It is HIGHLY RECOMMENDED that all infiltration areas are tested prior to project close out, even if an NPDES permit is not required. MnDOT projects require at least five tests per acre of infiltration area and a minimum of five tests per infiltration area. Infiltration rates shall meet or exceed double the design rate assumed. The test results from a MnDOT project must be submitted to MnDOT.
If the above procedure is followed, we recommend one of the following: