Protecting Public Health and the Environment.

IPDES Storm Water Permitting

DEQ requires permit coverage of storm water discharges according to the applicable federal requirements. DEQ will apply the regulatory criteria in IDAPA to determine when a general permit versus an individual permit should be issued. Three categories of storm water discharges are covered in the IPDES Program: construction, industrial, and municipal activities.

Authority for issuing storm water permits will transfer to DEQ on July 1, 2020.


Storm water discharges from construction activities can significantly impact water quality. As storm water flows over a construction site, it can pick up pollutants like sediment, debris, and chemicals. These pollutants may then be transported to nearby storm sewer systems or directly into rivers, reservoirs, or lakes, and their tributaries.

The IPDES storm water program requires permits for discharges from construction activities that disturb one or more acres, and discharges from smaller sites that are part of a larger common plan of development or sale. Construction storm water permits include effluent limits for erosion and sediment control, pollution prevention, and site stabilization from the Construction and Development Effluent Guidelines and Standards (40 CFR 450).


Materials and equipment handling and storage activities at industrial facilities are often exposed to the weather. Runoff from rain or snow fall that comes in contact with these activities may pick up pollutants and transport them either directly or indirectly to a surface water body.

Industrial activities subgrouped under 40 CFR 122.26 (b)(14), excluding construction, will be regulated under the existing multisector general permit (MSGP), unless due to ineligibility, an individual or alternate general permit is required. State use of the MSGP will allow all regulated industries to be covered under a general permit for discharges of storm water runoff. DEQ will evaluate compliance and effectiveness of this permit, along with experience of other states’ NPDES programs (e.g., Oregon and Washington) to determine whether general permits for one or more specific industrial sectors is appropriate. When authority for the storm water program is transferred to DEQ and upon expiration of the MSGP, DEQ will reissue the general permit to include Idaho-specific requirements and conditions.


MS4s are regulated as either large/medium (Phase I) or small (Phase II) systems. There are currently 16 individual permits for MS4 coverage: one for large/medium systems and 15 for small systems. DEQ will evaluate other small MS4s for designation as regulated MS4s based on the IPDES Designation Criteria and Selection Process for Small Municipal Separate Storm Sewer Systems, as well as criteria and models developed by other states and EPA Region 10. EPA issued a general permit in 2016 to cover Phase II MS4 discharges. DEQ will likely continue with the general permit for Phase II MS4 discharges upon approval of the storm water component of the IPDES Program. Similarly, the single Phase I MS4 permit will be reissued upon its expiration.