This guidance is intended to clarify the erosion and sediment control requirements of the NPDES/SDS Construction Stormwater General Permit for construction sites in former crop production areas or on sites that incorporate crop production on portions of the site after construction activity has temporarily or permanently ceased. The guidance also is intended to clarify the use of the federal exclusion for agricultural soil-disturbing activities in these situations.
Under federal and state law, construction activity that disturbs one or more acres of land, or less than one acre of land that is part of a larger plan of development greater than one acre, must obtain a Minnesota National Pollutant Discharge Elimination System/State Disposal System (NPDES/SDS) Construction Stormwater General Permit (permit). The permit is required under the federal Clean Water Act that regulates pollutant discharges from activities that are considered point sources. Construction sites have been determined to be “point sources” under the Clean Water Act. Therefore, qualifying construction sites must obtain the permit and use certain Best Management Practices (BMPs) to minimize pollutants such as sediment and other contaminants from reaching surface waters during construction.
Construction site requirements
The permit specifies that both temporary and permanent erosion prevention and sediment control BMPs must be implemented to control stormwater discharges during the construction activity. Multiple temporary erosion prevention and sediment controls such as perimeter controls, mulch, erosion control blankets, riprap and inlet protection must be used during construction to keep sediment on site until permanent vegetative cover is established. These permit requirements apply to all construction projects.
No agricultural activity exclusions to permitted sites
The difference between excluded agricultural activities and non-excluded construction activities that disturb soil creates confusion regarding the application of construction stormwater permit requirements. This is a particular concern for sites that intend to use crop production as erosion prevention or sites in agricultural areas that will be returned to crop production. However, the federal exclusion does not apply to crop production activities in permitted construction sites. The sites must still comply with the permit requirements.
Construction activity that requires permit coverage must comply with the temporary erosion and sediment control requirements specified in the permit even if the site is located on agricultural land. Conversely, growing crops on the permitted construction site does not create an exemption from the permit erosion and sediment control requirements. The site is required to comply with the permit stabilization timeline of 14 days (or 7 days for sites within one mile of special or impaired waters) on any portion of the site where land disturbance has temporarily or permanently ceased. Planting a row crop on the site is not likely to satisfy the temporary stabilization requirement. However, if the site is intended to be returned to agricultural use after construction, returning the site to cropland once construction is complete will satisfy the permanent stabilization requirement.
Examples where the entire site or portions of the site might be returned to its agricultural use after construction include:
- Transmission lines
- Wind farms; or
- Soil borrow sites.
Until work is completed and agricultural activity resumes, the site must be in compliance with all the temporary erosion prevention and sediment control requirements of the permit.
Permitted sites: Row crops are not a temporary stabilization method
The permit requires that soils are stabilized within 14 days of temporarily or permanently ceasing land-disturbing activity on any portion of the site. Sites that discharge to a special or impaired water located within a mile of the site must be stabilized within 7 days. Temporary soil stabilization usually consists of mulches or erosion mats to prevent erosion until the site is vegetated or permanently covered with asphalt or concrete.
Crop vegetation can take many weeks and even months to reach a plant size or development stage that can minimize, impede or prevent erosion. Failing to provide adequate temporary cover within the permit timeline leaves the site subject to erosion into stormwater infrastructure and nearby surface waters, leading to noncompliance with the permit. After harvest, soils on the site may be left exposed again for several months. Crop residue may be insufficient as a temporary cover to prevent soil erosion during this time. Sediment discharges resulting from improperly stabilizing the site or not stabilizing the site within permit time frames could result in enforceable violations of the permit and/or water quality regulations.
Crop production on construction sites may also result in runoff containing fertilizers, pesticides and crop residue along with the additional sediment inputs from large exposed areas draining to stormwater infrastructure, such as sediment ponds. This may result in increased maintenance or premature failure of these systems and potential water quality impacts to downstream receiving waters.
If portions of the site will remain open for an extended period of time until the construction can be completed, then dense perennial crops such as grasses or alfalfa may be used as a temporary or permanent soil stabilization method. Perennial crops can be used provided temporary mulch is also applied at the time of planting to meet the soil stabilization timelines until the crop provides adequate soil cover. In this case, permit coverage is maintained and permittees are responsible to ensure the site remains stabilized and compliant with the permit until construction resumes.
Additional information regarding permitting requirements for agricultural-related construction sites: