Using Infiltration as Stormwater BMP - Regulatory Issues

Infiltration is used as a stormwater control or Best Management Practice (BMP) in many areas. However, infiltrating run-off has some unique regulatory issues. First, in some States, infiltrating stormwater is a viable means to eliminate a discharge that would otherwise require an NPDES discharge permit. However, in most cases, the system would need to infiltrate all run-off from even the largest rain events. In addition, seasonal changes must also be considered; all run-off would need be collected and infiltrated even during frozen conditions. Some States, such as Minnesota, regulate groundwater as a "water of the State", and therefore can regulate discharges to the groundwater using infiltration.

The Minnesota Pollution Control Agency (MPCA) has been drafting regulations relating to infiltration of stormwater at sites regulated under the industrial stormwater discharge general permit. These requirements are still in revision, however as of November 2008 the format for regulation of infiltration as a stormwater control has included:

• Design criteria for amount of run-off to be infiltrated
• Rate at which infiltration must occur after a storm event
• Pretreatment requirements for run-off prior to discharge to designed infiltration basin
• Stormwater monitoring requirements and standards ("benchmarks") for run-off prior to discharge to designed infiltration basin
• Operation and maintenance requirements.

The draft MPCA infiltration requirements also include several prohibitions, where infiltration can not be used to manage stormwater. These include:

• Certain industrial sectors are prohibited from using infiltration BMPs;
• Certain sensitive areas where groundwater is vulnerable to contamination, including karst areas, and well head protection areas.

Finally, under some circumstances the disposal of stormwater run-off using a designed infiltration system can be further regulated as a Class V Injection Well. This will require further permitting and associated regulation.

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